SEARCH

SEARCH BY CITATION

Keywords:

  • Angiotensin II;
  • vasodilatation;
  • blood pressure;
  • cardiac and vascular remodelling;
  • fibrosis;
  • AT2 receptor
Abbreviations:
AT1R

angiotensin II type 1 receptor

AT2R

angiotensin II type 2 receptor

ACE

angiotensin-converting enzyme

BP

blood pressure

NO

nitric oxide

RAS

renin–angiotensin system

SHR

spontaneously hypertensive rats

VSMC

vascular smooth muscle cells

WKY

Wistar–Kyoto

It is widely accepted that the angiotensin II type 1 receptor (AT1R) accounts for the majority of cardiovascular effects evoked by angiotensin II (Ang II), such as contraction/pressor activity and growth-promoting effects leading to cardiac and vascular hypertrophy. However, there has been an increasing body of evidence that indicates that the angiotensin II type 2 receptor (AT2R) may exert pharmacological actions per se as well as play a role in pathophysiological processes. In particular, it has been suggested that the AT2R may exert beneficial vasodilator and antigrowth effects, as well as contribute to the efficacy of AT1R antagonists (see reviews by Matsubara, 1998; Horiuchi et al., 1999a; de Gasparo & Siragy, 1999; Unger, 1999; de Gasparo et al., 2000; Gallinat et al., 2000; Henrion et al., 2001).

Therefore, for the purposes of the current review, we have updated the status of such work, particularly in light of some recent data suggesting that the AT2R, in fact, causes opposite effects, for example, cardiac growth-promoting effects (Senbonmatsu et al., 2000; Ichihara et al., 2001). In addition, we have critically reviewed whether or not experimental outcomes are consistent with the hypothesis that AT2R may contribute to the therapeutic effects of AT1R antagonists (de Gasparo et al., 2000; Carey et al., 2001a; Siragy, 2002).

Distribution of AT2 receptors

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

Ang II mediates its biological actions by binding to distinct membrane-bound receptors and consequently activating multiple intracellular pathways. Two major Ang II receptor subtypes have been identified and cloned as AT1R and AT2R (de Gasparo et al., 2000). Ang II receptors have been localised throughout the vasculature, heart, kidneys, adrenals, nervous and endocrine systems. However, there is different anatomical distribution and expression of the AT1R/AT2R as well as differences in signalling pathways and function.

In foetal tissue, AT2R is the predominant subtype expressed, although this situation is rapidly reversed after birth with the AT1R becoming the dominant subtype in the adult (Matsubara, 1998; Horiuchi et al., 1999a; de Gasparo et al., 2000). While there is a relatively lower expression of the AT2R in adult tissue, AT2R predominates at particular sites including uterus, ovary, adrenal medulla as well as in distinct areas of the brain (Zhuo et al., 1995; de Gasparo et al., 2000; Roulston et al., 2003). The distribution of AT2R in tissues relevant to the cardiovascular system is briefly considered below.

Kidney

AT2Rs are detected in adult kidney, although extent and location varies considerably depending on techniques used. Autoradiographic studies using nonselective AT ligands with selective AT1R and AT2R displacing agents (Zhuo et al., 1995) were less able to detect AT2R levels than AT2R identified by selective AT2R autoradiography using CGP42112 (Cao et al., 2000). Likewise, there are distinctions between AT2R mRNA and immunohistochemical studies (Ozono et al., 1997; Miyata et al., 1999). Generally, AT2R mRNA and protein were distributed throughout tubular and vascular segments of the renal cortex and medulla, although results were more equivocal for the glomerulus (Ozono et al., 1997; Miyata et al., 1999; Cao et al., 2000). AT2Rs are also developmentally regulated with greater AT2R expression observed in foetal kidney (Ciuffo et al., 1993; Shanmugam et al., 1995; Ozono et al., 1997).

One aspect of renal AT2R function that has received attention is its role in pressure natriuresis. AT2R may play a role in pressure natriuresis, thereby opposing the antinatriuretic effects of AT1R activation, since the AT2R antagonist PD123319 decreased urinary sodium excretion in renal hypertensive rats while valsartan exerted opposite effects (Siragy & Carey, 1999). This natriuretic effect of AT2R was confirmed in AT2R knockout mice in which pressure natriuresis was inhibited (Siragy et al., 1999a; Gross et al., 2000). However, the exact nature of AT2R involvement in this field of research is somewhat unclear since others reported that AT2R stimulation attenuated pressure natriuresis (Lo et al., 1995; Liu et al., 1999).

Sodium depletion is reported to upregulate renal AT2R (Ozono et al., 1997), whereas AT2R was downregulated only in the ischaemic kidney from 2-kidney, 1-clip rats (Wang et al., 1999). These contrasting effects are likely to be model-specific since both situations result in a heightened RAS; furthermore, Ang II infusion per se did not alter renal AT2R expression (Wang et al., 1999). The AT2R was also decreased in kidneys of SHR-SP compared with WKY rats, and growth-factor-dependent induction of AT2R occurred in cultured mesangial cells from WKY rats but not from SHR-SP (Goto et al., 2002). In addition, there was a marked increase in AT2R expression in rats with renal failure (Bautista et al., 2001).

Vascular

The common misconception that AT2R do not exist in appreciable amounts in vasculature is slowly changing. AT2Rs are actually located in many different vessel types, albeit at low (but functional) levels. Indeed, early reports of AT2R comprising approximately 30–40% of AT receptors in rat aorta (Chang & Lotti, 1991; Viswanathan et al., 1991) were largely ignored until a functional vasodilator role of AT2R began to emerge. Subsequently, AT2Rs have been detected in vessels such as mesenteric (Matrougui et al., 1999, 2000; Touyz et al., 1999) and uterine (Cox & Cohen, 1996; Burrell & Lumbers, 1997; McMullen et al., 1999) arteries. AT2R in vasculature is also developmentally regulated (Viswanathan et al., 1991; Nakajima et al., 1995), whereas the AT1R is expressed at a relatively constant level throughout life (de Gasparo et al., 2000).

AT2R mRNA expression and Ang II receptor autoradiography have also provided evidence for AT2R in kidney vasculature (Zhuo et al., 1995, 1996; Matsubara, 1998; Miyata et al., 1999). Indeed, AT2R predominate in the adventitia of the human renal artery and arcuate and interlobar arteries (Goldfarb et al., 1994; Zhuo et al., 1996), or in vascular smooth muscle cells of such vessels (Grone et al., 1992), although others did not detect AT2R in human kidney (Sechi et al., 1992a). AT2Rs are also present in endothelial cells and vascular smooth muscle cells in small resistance arteries obtained from rats (Nora et al., 1998; Matrougui et al., 1999), and AT2Rs have recently been detected in mouse coronary arteries (Akishita et al., 2000a; Wu et al., 2002).

Various pathologies can affect AT2R levels in vasculature. AT2Rs are increased in skin during wound healing (Kimura et al., 1992; Viswanathan & Saavedra, 1992). Balloon injury to rat carotid arteries resulted in detectable AT2R mRNA in vessel wall, which was otherwise below the limit of detection in uninjured vessels (Nakajima et al., 1995). Likewise, an inflammatory cuff model caused re-expression of AT2R in media/neointima of mouse femoral artery (Akishita et al., 2000a). The AT2R can also be regulated in the vasculature by Ang II itself in a heterogeneous manner, since chronic infusions of this peptide have been reported to decrease AT2R expression in sheep uterine arteries (McMullen et al., 2001), but increase AT2R expression in rat mesenteric arteries (Bonnet et al., 2001). In addition, there is an increase in AT1R expression in VSMC from AT2R knockout mice (Tanaka et al., 1999), whereas overexpression of AT2R in vasculature of mice does not alter the level of expression of the AT1R (Tsutsumi et al., 1999). There is greater expression of vascular AT2R in young SHR (Touyz et al., 1999) and adult SHR (Otsuka et al., 1998) compared with WKY rats.

Heart

Both receptor subtypes exist in the heart although, in most animal studies, the AT2R is the minority subtype (Chang & Lotti, 1991; Baker et al., 1992; Sechi et al., 1992b; Suzuki et al., 1993; Wang et al., 1998; Busche et al., 2000). AT2Rs are expressed at low levels in adult rat cardiomyocytes (Busche et al., 2000) but are increased in both absolute levels and relative to AT1R in hypertrophied rat and failing hamster hearts (Lopez et al., 1994; Ohkubo et al., 1997; Bartunek et al., 1999). AT2R was increased in SHR heart compared with WKY in one study (Makino et al., 1999) but not in another by the same group (Makino et al., 1997). AT2R increased within 1 day after myocardial infarction (Nio et al., 1995). Moreover, studies using single-cell reverse transcriptase–polymerase chain reaction have shown that the proportion of rat cardiomyocytes expressing AT2R increased from a basal state of 10% to approximately 50% 1 week after myocardial infarction (Busche et al., 2000). By contrast, ischaemia and reperfusion decreased AT2R mRNA and protein acutely in isolated working rat heart (Xu et al., 2000).

Although animal studies indicate that the AT1R is the major binding site in adult hearts, the AT2R gains particular prominence in human heart. In both normal noninfarcted or hypertrophied human hearts, there is a predominance of AT2R binding sites in the myocardium (Brink et al., 1996; Matsubara, 1998; Wharton et al., 1998; de Gasparo et al., 2000). Even in studies that indicate that the AT2R is not the major subtype, there were approximately equal proportions of both AT2R and AT1R in nonfailing human hearts (Tsutsumi et al., 1998). In most clinical reports, AT1R density tends to decrease with cardiac dysfunction (Regitz-Zagrosek et al., 1995; Rogg et al., 1996; Asano et al., 1997; Haywood et al., 1997; Tsutsumi et al., 1998; Goette et al., 2000), whereas AT2R density may be decreased (Regitz-Zagrosek et al., 1995; Matsumoto et al., 2000), increased (Rogg et al., 1996; Tsutsumi et al., 1998; Goette et al., 2000) or unchanged (Asano et al., 1997; Haywood et al., 1997) with increasing cardiac dysfunction. Discrepancies between studies are hardly surprising given the different types and severity of heart failure examined, together with a range of detection methods to identify AT2R including ligand binding, autoradiography, mRNA expression and immunohistochemistry. Indeed, these data generally favour an increase in the ratio of AT2R/AT1R in human heart. Of those studies that have examined cellular localisation, the AT2R was mainly localised, using autoradiography, on fibroblasts at sites of fibrosis (Brink et al., 1996; Tsutsumi et al., 1998; Wharton et al., 1998). However, in immunohistochemical studies using less-diseased cardiac tissue, AT2R was confined to myocytes, not fibroblasts, in atrial tissue obtained from patients undergoing coronary artery bypass graft surgery (Matsumoto et al., 2000) or in myocardium of 4-week-old rats (Wang et al., 1998). Experimental data in failing myopathic hamster heart are consistent with AT2R upregulation in fibrotic regions (Ohkubo et al., 1997).

AT2R signalling

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

Kinase/phosphatase crosstalk

Numerous studies have revealed that the signal transduction mechanisms associated with AT2R activation are appreciably different to those linked with AT1R coupling. Moreover, it is becoming increasingly accepted that activation of AT2R in various cell lines results in the stimulation of protein phosphatases, which directly inhibit the protein kinase pathways (and hence growth-promoting function) associated with AT1R (Horiuchi et al., 1999a).

In PC12W cells expressing only AT2R, Ang II rapidly induces activation of protein tyrosine phosphatase (PTPase), which then causes dephosphorylation (and hence inactivation) of tyrosine residues; an effect that is abolished by general PTPase inhibitors such as the vanadate compounds (Bottari et al., 1992; Brechler et al., 1994). These findings have been extended to various other cell lines, including N1E-115 neuroblastoma cells (Nahmias et al., 1995), nondifferentiated NG108-15 cells (Buisson et al., 1995) and R3T3 fibroblasts (Tsuzuki et al., 1996a, 1996b).

More recently, attempts have been made to elucidate the specific PTPases involved in AT2R activation. In PC12W cells (Yamada et al., 1996; Horiuchi et al., 1997) and R3T3 cells (Yamada et al., 1996), for example, pretreatment with antisense oligonucleotide of mitogen-activated protein kinase phosphatase-1 (MKP-1) inhibited the proapoptotic effect mediated by the AT2R. Furthermore, in cultured rat vascular smooth muscle cells (VSMC), Ang II was shown to stimulate mRNA expression and protein synthesis of a PTPase with selective activity for MAP kinase (Duff et al., 1993). A similar increase in MKP-1 mRNA levels, following AT2R activation, has also been reported in adult rat ventricular myocytes (Fischer et al., 1998). Consistent with these findings, AT2R overexpression in VSMC (Nakajima et al., 1995) demonstrated an inhibition of AT1R-mediated MAP kinase activity (extracellular-regulated kinases (ERK) 1 and 2), which presumably involves activation of a particular MKP-1. Collectively, these results strongly suggest that MKP-1 is one of the phosphatases involved in AT2R signal transduction.

SHP-1 is a soluble PTPase that has been implicated in the termination of signalling by cytokines and growth factor receptors, and there is now evidence to suggest that it may also serve as an early transducer in AT2R signalling (Bedecs et al., 1997; Lehtonen et al., 1999; Cui et al., 2001; Shibasaki et al., 2001). In PC12W cells (Lehtonen et al., 1999) and rat foetal VSMC (Cui et al., 2001), a functional link has been established between AT2R, SHP-1 tyrosine phosphatase activation and apoptosis, whereby transfection of an inactive SHP-1 mutant into cells not only prevented SHP-1 activation, but also inhibited AT2R-mediated apoptosis (Lehtonen et al., 1999; Cui et al., 2001). Dephosphorylation of phosphothreonine by serine/threonine phosphatase (PP2A) can also inactivate MAP kinase (Gallinat et al., 2000). Indeed, studies in neurons cultured from neonatal rat hypothalamus and brain stem indicate that AT2R stimulation activates PP2A (Huang et al., 1995), thereby inhibiting AT1R-mediated MAP kinase activation (Huang et al., 1996) and inducing apoptosis (Shenoy et al., 1999). However, in some circumstances, AT2R stimulation transiently increased ERK phosphorylation prior to inhibition of MAP kinases within the process of cell differentiation (Stroth et al., 2000).

It is worth noting that, in addition to modulating the more extensively studied ERK pathway (Nakajima et al., 1995; Huang et al., 1996; Yamada et al., 1996, 1998; Bedecs et al., 1997; Horiuchi et al., 1997), AT2Rs are capable of inducing the dephosphorylation of other protein kinsases. Janus kinases and signal transducers and activators of transcription (STAT) represent important signalling pathways through which Ang II (via the AT1R), and other growth factors, stimulate VSMC proliferation (Marrero et al., 1995; de Gasparo et al., 2000). In AT2R cDNA-transfected rat VSMC, R3T3 fibroblasts and mouse foetal VSMC (which express AT2R naturally), stimulation of AT2 receptors has been shown to reduce AT1 receptor-mediated tyrosine phosphorylation of STAT1, STAT2 and STAT3 (Horiuchi et al., 1999b), and also inhibits effects of the growth factors, epidermal growth factor (EGF) and platelet-derived growth factor (PDGF), on STAT1 activity (Horiuchi et al., 1999b). Thus, it appears that Ang II can impair the activity of various growth factor signalling pathways through the activation of AT2R.

Collectively, the studies performed to date demonstrate that AT2R stimulation can activate tyrosine and serine/threonine phosphatases, depending on the cell line in question. These phosphatases serve to reverse, or at least counter-regulate, the cell proliferative- and growth-promoting effects mediated by the various protein kinases in response to AT1R activation.

While AT2R-mediated ERK inactivation has been clearly established in cell culture, the question remains as to whether or not this modulatory influence extends to a physiological setting. Interestingly, Masaki et al (1998) reported that transgenic mice overexpressing cardiac AT2R exhibit reduced cardiac ERK activity, relative to their wild-type litter mates. Conversely, foetal VSMC from AT2R-null mice demonstrate a generally enhanced growth phenotype, as well as increased basal- and serum-induced ERK phosphorylation levels (Akishita et al., 1999). On the basis of these genetic manipulation studies, at least, it appears that ERK inactivation by the AT2R may play a physiological role in vivo, in relation to cardiac and vascular growth.

NO/cyclic GMP pathway

A number of studies have demonstrated a link between AT2R activation and alterations in cellular cyclic GMP levels. Ang II elicits an increase in cyclic GMP levels, in cultured bovine aortic endothelial cells (Wiemer et al., 1993; Saito et al., 1996), via an AT2R-mediated, NO (and presumably soluble guanylyl cyclase)-dependent pathway (Wiemer et al., 1993). A stimulatory effect of Ang II on cellular NO and/or cyclic GMP levels has also been reported in other aortic endothelial cell preparations (Pueyo et al., 1998) and cultured N1E-115 neuroblastoma cells (Zarahn et al., 1992; Chaki & Inagami, 1993). Interestingly, Ang II-induced activation of the cellular NO-cyclic GMP pathway may be mediated partially (Zarahn et al., 1992), or in some cases, exclusively (Caputo et al., 1995; Saito et al., 1996; Pueyo et al., 1998) by AT1R.

Ang II is also reported to stimulate NO release directly in isolated blood vessels (Seyedi et al., 1995; Thorup et al., 1998; Thorup et al., 1999); a component of which is mediated by AT2R. In dog coronary microvessels and large coronary arteries, the Ang II-induced increase in nitrite levels was abolished by both AT1R and AT2R antagonists (losartan and PD 123319, respectively), as well as by the nonselective Ang II receptor antagonist, saralasin (Seyedi et al., 1995). In isolated perfused rat renal arteries, losartan significantly reduced Ang II-stimulated NO release, as measured by a NO-sensitive microelectrode, without abolishing the response altogether (Seyedi et al., 1995; Thorup et al., 1998, 1999), and it was concluded that the residual, losartan-insensitive increase in NO production may be mediated by an AT2R mechanism. Collectively, the results of studies performed to date in both cell culture and isolated vascular preparations, suggest that AT1R and AT2R may not always act in direct opposition to each other, at least at the level of cyclic GMP production.

Role of bradykinin

A study performed by Siragy et al. (1996) in conscious, uninephrectomised dogs initiated the concept that AT2R may stimulate the release of endogenous bradykinin, in addition to NO in vivo. In that study, a non-AT1R was identified as mediating renal bradykinin and cyclic GMP production in response to endogenous RAS activation (Siragy et al., 1996). With the use of the same renal microdialysis technique, bradykinin has indeed been shown to stimulate NO release via the activation of B2 receptors (Siragy et al., 1997). Subsequent studies have revealed that both endogenous Ang II (following dietary Na+ restriction) and exogenously infused Ang II stimulate an increase in cyclic GMP content in renal interstitial fluid of conscious rats; an effect that is abolished by AT2R blockade (Siragy & Carey, 1996) or AT2R antisense oligonucleotide (Moore et al., 2001), as well as NOS inhibition (Siragy & Carey, 1997, 1999; Siragy et al., 2000) and bradykinin B2 receptor blockade (Siragy & Carey, 1999; Siragy et al., 2000; 2001). In a renal wrap model of hypertension, Ang II infusion elicited an AT2R-mediated increase in bradykinin levels (Siragy & Carey, 1999), thereby confirming a direct link between AT2R activation and subsequent bradykinin synthesis/release.

Genetic studies involving either the targeted deletion of the AT2R gene (Siragy et al., 1999a) or AT2 receptor over expression in VSMC of transgenic mice (Tsutsumi et al., 1999) have provided further support for a link between AT2R-mediated vasodepression and associated bradykinin and/or NO production. Specifically, AT2R-null mice exhibit markedly reduced basal- and Ang II-induced cyclic GMP and bradykinin levels in renal interstitial fluid, and are hypersensitive to the pressor and antidiuretic effects of Ang II, relative to their wild-type litter mates (Siragy et al., 1999a). It has been suggested that the exaggerated vascular reactivity to Ang II in AT2R-null mice is at least partially due to an increase in vascular AT1R expression (Tanaka et al., 1999); however, it is unlikely that this would account for the observed deficiency of the bradykinin–NO–cyclic GMP vasodilator cascade. On the other hand, AT2R overexpression in VSMC in mice unmasked an Ang II-induced increase in aortic cyclic GMP content (which was reversed by cotreatment with either AT2 or B2 receptor antagonists, or NOS inhibition), and was associated with complete abolishment of the pressor response to Ang II in vivo; an effect that was also reversed by these same inhibitors (Tsutsumi et al., 1999). In addition, in stroke-prone spontaneously hypertensive rats, acute Ang II infusion produced a significant increase in the cyclic GMP content of aortic explants via a mechanism that involves AT2R and endothelial-derived bradykinin and NO (Gohlke et al., 1998). While the precise nature of the AT2R/bradykinin interaction is not fully understood, it has been proposed that Ang II, in reducing intracellular pH levels in endothelial cells, may in turn activate acid-optimum kininogenases to cleave bradykinin from intracellularly stored kininogens (Wiemer et al., 1993; Tsutsumi et al., 1999). However, there are also recent data indicating that AT2R stimulation increases cyclic GMP independently of bradykinin B2 receptors since cyclic GMP levels were in fact enhanced in PC12W cells in the presence of B2 receptor blockade (Zhao et al., 2003); consistent with findings we have also observed in rat uterine arteries (Hannan et al., 2003b).

Thus, the weight of evidence presented to date indicates that the AT2Rs are located within the cardiovascular system (heart, kidney, vasculature–albeit at lower levels of expression than AT1Rs) with appropriate signal transduction pathways for potentially important functional effects (e.g. direct vasodilator pathways and indirect anti-AT1R trophic effects).

AT2R-mediated relaxation/vasodilatation

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

There is an increasing amount of literature that has demonstrated AT2R-mediated relaxation directly in a range of isolated arteries including rabbit renal arterioles (Arima et al., 1997; Endo et al., 1997, 1998), rabbit cerebral arteries (Haberl, 1994), and rat mesenteric arteries (Matrougui et al., 1999, 2000; Dimitropoulou et al., 2001; Katada & Majima, 2002; Widdop et al., 2002). In the presence of an AT1R antagonist, Ang II caused an approximate 30% increase in the diameter of preconstricted, microperfused rabbit afferent (Arima et al., 1997; Endo et al., 1998) and efferent (Endo et al., 1997) arterioles in a PD123319-sensitive manner. An AT2 vasodilator effect was confirmed using the selective AT2R agonist, CGP42112, in the absence of AT1R blockade (Arima et al., 1997). Moreover, it has been suggested that, in the rabbit afferent arteriole, AT2R-mediated, endothelium-dependent relaxation occurs via a cytochrome P-450-dependent, NO-independent pathway, which may involve the production of epoxyeicosatrienoic acid and subsequent opening of large conductance, Ca2+-activated K+ channels (Arima et al., 1997). Interestingly, impaired renal AT2R vasodilator function is associated with exaggerated Ang II vasoconstrictor responses in the afferent arterioles of prehypertensive SHR rats (Endo et al., 1998). Collectively, these studies demonstrate a potentially important role of AT2R in the regulation of glomerular haemodynamics. Moreover, recent studies from our laboratories indicate the complex nature of the renal effects of AT2R, at least in the rabbit, since AT2R activation counteracted both AT1R-mediated vasoconstriction in the cortex and, unexpectedly, also AT1R-mediated vasodilation in the medulla (Duke et al., 2003).

Ang II has also been shown to stimulate flow-induced dilatation of perfused rat mesenteric arteries in situ (Matrougui et al., 1999, 2000), whereby AT2R blockade produced a decrease in diameter of arterial branches submitted to pressure and flow; an effect that was prevented by NOS inhibition and endothelial disruption (Matrougui et al., 1999). This suggests that endogenous Ang II may activate endothelial AT2R-mediated NO release, thereby contributing to flow-induced dilatation. Indeed, Matrougui et al. (1999) reported that AT2Rs mediate between 20 and 39% of dilatation in response to shear stress. Flow-mediated dilatation of perfused rat mesenteric arteries in the presence of AT1R antagonism is also inhibited significantly by a bradykinin B2 receptor antagonist, as well as by PD123319 (Katada & Majima, 2002). Interestingly, in that study, arteries isolated from kininogen-deficient Brown Norway Katholiek rats displayed markedly impaired AT2R-mediated vasodilator responses, relative to their wild-type counterparts (Katada & Majima, 2002), suggesting an important role of endogenous bradykinin synthesis in the mechanisms underlying acute AT2R vasodilatation. However, Dimitropoulou et al. (2001) reported, on the basis of functional, whole-cell and single-channel patch-clamp studies, that Ang II relaxes rat mesenteric microvessels via stimulation of AT2R, with the subsequent opening of large-conductance, calcium- and voltage-activated K+ (BKCa) channels, leading to membrane repolarisation and vasodilation. It was proposed that arachidonic acid metabolites may serve as intermediate messengers in this novel, endothelium-independent AT2-BKCa channel pathway.

Thus, acute AT2R-mediated vasodilator responses may be endothelium-dependent (Arima et al., 1997; Matrougui et al., 1999) or -independent (Dimitropoulou et al., 2001), according to the techniques employed, and appear to involve a range of signalling pathways, including NO (Matrougui et al., 1999) and bradykinin production (Katada & Majima, 2002), activation of cytochrome P-450 epoxygenase pathways (Arima et al., 1997) and modulation of K+ channel activity (Arima et al., 1997; Dimitropoulou et al., 2001).

In other circumstances, direct AT2R-mediated relaxation was not found (Zwart et al., 1998), but instead the AT2R antagonist PD123319 enhanced AT1R-mediated contraction in uterine arteries (Zwart et al., 1998; McMullen et al., 1999; St-Louis et al., 2001; Hannan et al., 2003a), which implied a vasodilator action of AT2R, most likely involving NO and bradykinin (Hannan et al., 2003a). The sensitivity of the experimental preparation is an important consideration since conventional wire myographs are less likely to detect AT2R-mediated relaxation (Zwart et al., 1998) compared with cannulated blood vessel preparations (Matrougui et al., 1999, 2000; Dimitropoulou et al., 2001; Katada & Majima, 2002; Widdop et al., 2002). In one study, when complete concentration–response curves were constructed for AT2R activation in isolated mesenteric arteries, the relaxation produced was concentration-dependent but relatively small (∼25%) compared with maximum acetylcholine-mediated relaxation (Widdop et al., 2002). However, others reported full AT2R-mediated relaxation using the same preparation (Dimitropoulou et al., 2001). While the discrepancy between the two studies may relate to the use of different precontraction agents, even submaximal changes in resistance-like vessels could potentially result in marked haemodynamic effects. Moreover, this PD123319-sensitive AT2R-mediated vasodilatation, in contrast to AT1R-mediated vasoconstriction, was a highly reproducible phenomenon; maintained even in the presence of chronic AT1R blockade, when circulating Ang II levels are elevated (Widdop et al., 2002). This preservation of AT2R function is critical when considering its potential physiological role in the antihypertensive effects of AT1R antagonists (see later).

In vivo evidence for AT2R-mediated vasodilatation is less exhaustive and has largely come from blood pressure (BP) measurements using two separate approaches: either indirectly based on enhanced Ang II-mediated vasoconstriction in the presence of AT2R blockade or infusing Ang II in the presence of AT1R blockade in order to stimulate AT2R (Scheuer & Perrone, 1993; Munzenmaier & Greene, 1996; Gohlke et al., 1998). While these latter studies provided functional evidence of AT2R activation, there were no uniform BP-lowering effects, most likely because any potential BP reductions may have been masked by direct vasoconstriction caused by infusion of a large dose of Ang II alone (Gohlke et al., 1998). Instead, Barber et al. (1999) selectively stimulated AT2R using the agonist CGP42112 and found that it caused a depressor response in conscious SHR in the presence of AT1 receptor blockade; an effect which was subsequently confirmed by Carey et al. (2001a).

More recently, direct haemodynamic measurements have confirmed the inferences made from BP measurements. In particular, AT2R stimulation (Ang II infused in the presence of AT1R blockade) caused coronary vasodilatation, but not renal vasodilatation, in anaesthetised rats previously given a myocardial infarction (Schuijt et al., 2001), although no vasodilatation was evident in noninfarcted anaesthetised rats (Schuijt et al., 1999). In addition, Lambers et al. (2000) reported that AT1R blockade unmasked Ang II-mediated increases in uterine blood flow, which were reversed by PD123319 and NOS inhibition. Similarly, Ang II-induced renal vasodilatation was unmasked by AT1R blockade and this effect was enhanced with renal failure (Bautista et al., 2001). More recently, the AT2R depressor response previously observed in SHR (Barber et al., 1999) has been examined in haemodynamically instrumented conscious rats. Against a background of AT1R blockade, the AT2R agonist, CGP42112, caused generalised vasodilatation in the renal, mesenteric and hindquarters circulations of SHR but not WKY rats (Li & Widdop, 2003). Consistent with a vasodilator role for the AT2R, it was also established in the same study that the AT2R antagonist PD123319 itself exerted haemodynamic effects consisting of modest pressor activity and renal and mesenteric vasoconstriction. Thus, these data suggest that the AT2R tonically modulated vascular tone in the renal and mesenteric circulations, at least in conscious SHR (Li & Widdop, 2003), which agrees with a recent study in which antisense oligodeoxynucleotide directed against the AT2R mRNA infused into the renal interstitium in rats increased blood pressure (Moore et al., 2001). In this context, a functional role of AT2R may also be apparent in human forearm vasculature. In healthy male volunteers, Ang II evoked dose-dependent vasoconstriction in the forearm circulation (Phoon & Howes, 2001), whereas this effect was converted into dose-dependent vasodilatation in elderly female patients who were on 3-week candesartan therapy (Phoon & Howes, 2002). In the latter study, PD123319 elevated baseline forearm vascular resistance suggesting a tonic AT2R vasodilator influence.

In terms of the chronic pharmacodynamic effects of selective AT2R stimulation, the combination of Ang II infusion and AT1R blockade failed to decrease BP (Li et al., 1998; Cao et al., 1999; Diep et al., 1999; Tea et al., 2000), most likely because of large pressor doses of Ang II invariably being employed. However, in mice overexpressing AT2R in vasculature, an Ang II infusion over 14 days, in the presence of an AT1R antagonist, caused a prolonged depressor effect in these transgenic mice (Tsutsumi et al., 1999). Moreover, Carey and colleagues demonstrated that either CGP42112 or Ang II combined with valsartan progressively decreased systolic BP (SBP) in normotensive rats over 8–9 days to a greater extent than AT1R blockade alone. Furthermore, a 4-day infusion of CGP42112 alone actually lowered SBP (Carey et al., 2001b), which is consistent with a persistent vasodilator action of AT2R without desensitisation, even in the presence of AT1R blockade (Widdop et al., 2002).

Therefore, it may be speculated that AT2R-mediated haemodynamic effects (Tsutsumi et al., 1999; Bautista et al., 2001; Schuijt et al., 2001; Li & Widdop, 2003) were more apparent in pathological states in which there is often an upregulation of AT2R. For example, AT2R-mediated vasodilatation occurred in SHR but not in WKY rats (Barber et al., 1999; Li & Widdop, 2003), although this was not the case using the isolated mesenteric artery (Matrougui et al., 2000). However, the conscious animal data are consistent with analogous in vivo protocols in which AT2R-mediated increases in vascular cGMP production occurred in SHRSP (Gohlke et al., 1998) but not in WKY rats (Pees et al., 2003).

Structural effects mediated by AT2R

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

In vitro

Distinct from the plethora of acute pharmacodynamic effects of AT2R already described, there is an increasing body of evidence that AT2R may tonically modulate cardiovascular structure, although the reported changes are, in some case, contradictory.

Given the signalling profiles of AT2R, much research has focused on its role in growth and remodelling. A variety of in vitro/in situ studies have found that AT2R activation exerts antigrowth effects, largely based on potentiated growth in the presence of the AT2R antagonist PD123319. Using cultured rat coronary endothelial cells, Stoll et al. (1995) reported that Ang II did not induce proliferation unless AT2R were blocked, whereas Ang II alone caused AT1R-mediated growth of VSMC. This study nicely illustrated the fact that net growth depends on the expression of AT2R on appropriate cellular targets, since only the endothelial cells expressed both AT1R and AT2R. Consistent with those data, in VSMC transected with AT2R, PD123319 unmasked Ang II-mediated proliferation (Nakajima et al., 1995). Experiments performed using cardiomyocytes (Booz & Baker, 1996; van Kesteren et al., 1997), cardiac fibroblasts (Ohkubo et al., 1997; Ozawa et al., 1996; Tsuzuki et al., 1996b; van Kesteren et al., 1997) and isolated perfused hypertrophic hearts (Bartunek et al., 1999), all reported an increase in AngII-induced growth with AT2R blockade.

Cellular differentiation is also closely linked to the antiproliferative and regenerative effects of AT2R stimulation. Early studies indicated that AT2R activation inhibited proliferation and promoted differentiation in PC12W and NG108-15 cells (Laflamme et al., 1996; Meffert et al., 1996), as well as axonal regeneration in vitro and in vivo (Lucius et al., 1998). Apoptosis is also considered to play an important role in normal development as well as in response to pathological changes, such that the process of cardiovascular remodelling is determined by the balance between cell growth and proliferation versus apoptosis. AT2R evokes proapoptotic effects in a number of cell types in vitro including PC12W, fibroblasts and VSMC (Yamada et al., 1996; 1998; Tsuzuki et al., 1996a; Dimmeler et al., 1997), while AT2R-mediated growth inhibition in endothelial cells involves remodelling of the extracellular matrix components (Fischer et al., 2001).

In vivo

In the in vivo setting, there are a number of different experimental models that mirror the in vitro, growth-potentiating effect of PD123319. These include PD123319-induced enhanced angiogenesis (Munzenmaier & Greene, 1996), increased media thickness (Ceiler et al., 1998; Diep et al., 1999), increased neointima (Akishita et al., 2000a), increased cardiac and renal fibrosis (Ohkubo et al., 1997; Morrissey & Klahr, 1999) as well as enhanced atherosclerotic development (Daugherty et al., 2001). However, in other studies, PD123319 did not alter LV or vascular hypertrophy (Makino et al., 1997; Ohkubo et al., 1997; Li et al., 1998; Tea et al., 2000; Varagic et al., 2001).

Furthermore, there are also reports that oppose the conventional view that AT2R is linked with antigrowth since PD123319 actually blocked vascular hypertrophy in SHR or Ang II-mediated vascular hypertrophy and fibrosis (Levy et al., 1996; Sabri et al., 1997; Otsuka et al., 1998; Cao et al., 1999). Moreover, Cao et al. (2002) recently suggested that PD 123319 was reno-protective in subtotally nephrectomised rats, since it reduced proteinuria and inflammatory markers of renal injury, albeit in a similar fashion to AT1R blockade. Unusually, AT2R stimulation also promoted cellular proliferation in normal kidney, but was associated with apoptosis (Cao et al., 2000). Similarly, Ang II acting at both AT1R and AT2R has been shown to stimulate NFκB, which is a known proinflammatory mediator (Ruiz-Ortega et al., 2000; 2001; Wolf et al., 2002).

On the other hand, selective AT2R stimulation using CGP42112 (Janiak et al., 1992) or vascular AT2R over expression (Nakajima et al., 1995) inhibited neointimal growth. These data, together with previous in vitro studies using CGP42112 (Stoll et al., 1995; Ozawa et al., 1996; Dimmeler et al., 1997), further support an antigrowth role for AT2R.

Mice

Discrepancies noted between some of the fore-mentioned rat studies may relate to the variety of experimental models that have been used, notwithstanding differences in drug doses, length of treatment, etc. Conceivably, targeted deletion of the AT2R would help resolve these issues. Table 1 lists the basal effects of the AT2R in cardiac and vascular tissue, as deduced from either targeted deletion or cardiac overexpression of AT2R. In keeping with the antigrowth role of AT2R, a number of studies have found that vascular pathologies have been exacerbated in the AT2R knock out animals with aortic banding or femoral cuffs, although there was no effect on cardiac hypertrophy (Table 1). These changes included enhanced perivascular fibrosis of coronary arteries, vascular thickening, neointimal growth and apoptosis. By contrast, Inagami and colleagues, using a different AT2R knockout strain, reported that AT2R stimulation was actually responsible for pressure-overload cardiac hypertrophy and fibrosis caused by aortic banding or Ang II infusion (i.e. absent in AT2R knockout mice) (Senbonmatsu et al., 2000; Ichihara et al., 2001). These controversial data (Schneider & Lorell, 2001) are consistent with a number of previously mentioned studies in rats suggesting prohypertrophic/proliferative actions of AT2R (Levy et al., 1996; Sabri et al., 1997; Cao et al., 1999; 2000; 2002), although recent studies using this same AT2 knockout strain found no differences from wild-type controls with respect to basal cardiac structure and function (Xu et al., 2002).

Table 1. Cardiovascular structural effects in mice that have been attributed to the AT2Ra
StudybExperimental interventionStrainCardiac hypertrophyPerivascular fibrosisInterstitial fibrosisVascular remodelling
  • a

    Changes in structural indices refer to effects of AT2R on the particular experimental intervention, and not the effect of experimental intervention per se.

  • b

    The majority of studies have used AT2R knockout mice, except those using mice with cardiac AT2R overexpression# [RIGHTWARDS ARROW], no effect; −ve, inhibitory effect; +ve, excitatory effect; −, not determined.

Akishita et al. (2000b)Aortic bandingFVB/N[RIGHTWARDS ARROW]−ve−ve
Wu et al. (2002)Aortic bandingFVB/N[RIGHTWARDS ARROW]−ve−ve
Senbonmatsu et al. (2000)Aortic bandingC57BL/6+ve+ve
Ichihara et al. (2001)Ang II infusionC57BL/6+ve+ve+ve
Sugino et al. (2001)#Ang II infusionC57BL/6[RIGHTWARDS ARROW]
Kurisu et al. (2003)#Ang II infusionC57BL/6[RIGHTWARDS ARROW]−ve−ve
Brede et al. (2001)untreatedFVB/N−ve
Yang et al. (2002)#MI (4 weeks)C57BL/6[RIGHTWARDS ARROW] ([UPWARDS ARROW] ED wall thickness)
Xu et al. (2002)MI (24 weeks)C57BL/6[RIGHTWARDS ARROW][RIGHTWARDS ARROW]
Ichihara et al. (2002)MI (1 week)C57BL/6+ve+ve
Oishi et al. (2003)MI (2 weeks)FVB/N−ve[RIGHTWARDS ARROW] (subthreshold stimuli)[RIGHTWARDS ARROW] (subthreshold stimuli)
(Ma et al. (1998)Uretal ligationC57BL/6−ve (renal)−ve (renal)
Akishita et al. (2000a)Femoral artery cuffFVB/N−ve
Suzuki et al., (2002)Femoral artery cuff −ve
Wu et al. (2001)Femoral artery cuffFVB/N−ve

Clearly, there are discrepancies between studies that may relate to the development of the AT2R knock outs independently by two groups (Hein et al., 1995; Ichiki et al., 1995). As pointed out, these strains differ slightly with respect to basal BP, pressor sensitivity to exogenous Ang II and genetic background (Hein et al., 1995; Ichiki et al., 1995; Schneider & Lorell, 2001), and even with regard to the structural indices measured (Inagami & Senbonmatsu, 2001). One also has to be cognisant of the potential for compensatory changes that contribute to phenotype. Indeed, the relatively small increase in basal BP in the AT2R knock out, despite reductions in vasodilator (bradykinin, cGMP) signalling pathways (Siragy et al., 1999a), was explained on the basis that there was an upregulation of AT1R-mediated vasodilator prostanoids that offset any substantial hypertension in the AT2R knockout model (Siragy et al., 1999b).

Thus, there are reported differences between the AT2R knockout strains with respect to cardiac hypertrophy when applying conventional loads using either Ang II or aortic banding (Table 1). AT2R exerted either no change or caused cardiac hypertrophy in response to these stimuli, although the magnitude of cardiac hypertrophy induced by aortic banding in the corresponding wild types differed substantially between studies (Akishita et al., 2000b; Senbonmatsu et al., 2000; Schneider & Lorell, 2001). Nevertheless, the lack of evidence for AT2R antigrowth in the heart per se (Opie & Sack, 2001) is consistent with recent studies in which AT2R was overexpressed in cardiomyocytes but did not alter cardiac mass (Masaki et al., 1998; Sugino et al., 2001; Kurisu et al., 2003), although Yang et al. (2002) reported greater end-diastolic wall thickness and higher ejection fraction at baseline in these transgenic mice than in wild-type controls.

In addition, the divergent effects of the AT2R knockout strains on cardiac fibrosis may relate to the measurement of different fibrotic indices between studies (Table 1) (Inagami & Senbonmatsu, 2001). However, one would have to reconcile contrasting AT2R effects on cardiac interstitial versus coronary perivascular fibrosis in those studies, whereas this is not the case in mice overexpressing cardiac AT2R. In this model, there was a significant reduction in the degree of Ang II-induced cardiac interstitial and perivascular fibrosis observed (Kurisu et al., 2003), implying that AT2R negatively regulates cardiac fibrosis, in line with the majority of data obtained from rats. Moreover, this AT2R antifibrotic effect was mediated via a kinin/NO-dependent mechanism (Kurisu et al., 2003). Curiously, in the AT2R knockout strain which exhibited no cardiac fibrosis (i.e. profibrotic AT2R phenotype: Senbonmatsu et al., 2000; Ichihara et al., 2001), Ma et al. (1998) reported enhanced renal fibrosis (i.e. antifibrotic AT2R phenotype), implicating tissue-specific bidirectional fibrotic changes.

Myocardial infarction (MI) has also been produced in mice, but again with conflicting results that may relate to the different times examined after MI and/or strains (Table 1). In the AT2R knockout strain which could not evoke a prohypertrophic/fibrotic response (Senbonmatsu et al., 2000; Ichihara et al., 2001), there was increased rupture immediately following MI although survival rate was not different from controls 6 weeks after MI (Ichihara et al., 2002). By contrast, in the AT2R knockout strain which exhibited enhanced perivascular fibrosis (Akishita et al., 2000b), the survival rate was lower than controls 2 weeks after MI but without any difference in the incidence of rupture (Oishi et al., 2003). Moreover, the MI-induced left ventricular enlargement and fibrosis seen in wild types was attenuated in one study (Ichihara et al., 2002) but enhanced in another (Oishi et al., 2003), in line with the contrasting pre-existing phenotypes. However, others have reported no differences in MI remodelling after 24 weeks (Xu et al., 2002). In addition, in mice with cardiac AT2R overexpression, left ventricular function was enhanced compared with wild types, as assessed by magnetic resonance imaging techniques (Yang et al., 2002). Moreover, this left ventricular remodelling was preserved when measured invasively and noninvasively 28 days after myocardial infarction (Yang et al., 2002).

Collectively, in the context of growth modulatory effects of AT2R, there is good evidence for AT2R to inhibit basal growth with respect to neointimal/peripheral vessel injury, albeit in a limited number of studies. There is also reasonable consensus that AT2R does not alter cardiac hypertrophy appreciably, but does regress cardiac fibrosis in both rats and mice (Table 1). However, the contradictory findings derived from seemingly similar AT2R knockout strains emphasise the need for pharmacological studies in wild-type mice to clarify the role of AT2R in this species.

The ability of AT2R to inhibit perivascular/interstitial fibrosis and neointimal growth, while not altering cardiac mass, probably relates to the level of AT2R expression in these tissues. AT2R are constitutively expressed on cultured fibroblasts (Dudley et al., 1991; Dudley & Summerfelt, 1993) and are present at perivascular and vascular sites (Nora et al., 1998; Akishita et al., 2000b; Suzuki et al., 2002). AT2R are less abundant in cardiac myocytes compared with cardiac fibroblasts, and are further upregulated in fibroblasts under conditions of cardiac load/pathology (Ohkubo et al., 1997; Tsutsumi et al., 1998; Wharton et al., 1998). Moreover, AT2R activation at these sites has been linked with reduced collagen synthesis and inhibition of growth of cardiac fibroblasts and mitogen signals (Ohkubo et al., 1997; Tsutsumi et al., 1998). On the other hand, Mifune et al. (2000) reported that AT2R activation caused collagen production in cultured vascular smooth muscle cells transfected with AT2R, and this fact has been used as an argument to support a profibrotic role for AT2R (Inagami & Senbonmatsu, 2001). However, there were heterogeneous effects reported from the same study, since AT2R activation in fact inhibited collagen production in cultured fibroblasts (Mifune et al., 2000), which is the effector cell more likely to be involved in the fibrotic process.

Role of AT2R in cardiovascular action of AT1R blockade

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

Evidence has been presented here and elsewhere (Matsubara, 1998; Horiuchi et al., 1999a) describing opposing actions of AT1R (predominantly excitatory) and AT2R (predominantly inhibitory). AT1R blockade has emerged as an effective treatment for hypertension and heart failure, in much the same manner as ACE inhibitors. However, these RAS inhibitors have divergent effects with respect to plasma levels of Ang II. Unlike ACE inhibitors, AT1R blockade increases circulating levels of Ang II that could in theory act on the unopposed AT2R. In this way, the ability of Ang II to stimulate AT2R in the presence of blockade of (excitatory) AT1R could provide additional complementary therapeutic benefit. Indeed, this point is increasingly being used as a marketing ploy to distinguish AT1R blockers from ACE inhibitors; therefore, it is timely to review this hypothesis.

An early study performed using anaesthetised SHR suggested that AT2R may be involved in the acute natriuretic and diuretic effects of losartan because PD123319 and Hoe 140 attenuated these effects although they did not affect the losartan-induced fall in BP (Munoz-Garcia et al., 1995). More conclusive evidence for an AT2R contribution to the effect of an AT1R antagonist was provided in the seminal study by Liu et al. (1997), who used the conventional rat coronary artery ligation model of heart failure. These authors found that chronic treatment with the AT1R antagonist, L-158809, improved left ventricular ejection fraction and caused regression of cardiac hypertrophy and fibrosis. These effects were partly attributed to unopposed AT2R activation since the AT2R antagonist PD123319 reversed the L-158809-induced changes in ejection fraction, LV volumes and myocyte cross-sectional area. In addition, the L-158809-induced reduction in cardiac fibrosis tended to be reversed, although the latter effect was not significant. Moreover, the involvement of AT2R activation during AT1R blockade also involved subsequent bradykinin production (Liu et al., 1997), which is consistent with the vasodilator signalling pathways already described.

Thereafter, a number of acute and short-term studies have examined the AT2R contribution to AT1R blockade by examining the potential reversal, by PD123319, of the effects of ‘sartan’ compounds. Jalowy et al. (1998) established that the size of the myocardial infarct in pigs caused by ischaemia/reperfusion over several hours was reduced by the AT1R antagonist candesartan. Remarkably, a 30-min pretreatment with either AT2R or bradykinin B2 receptor antagonists (PD123319 or Hoe 140) reversed the beneficial effect of AT1R blockade, which was consistent with earlier data from anaesthetised SHR (Munoz-Garcia et al., 1995). In addition, acute studies from the laboratories of Siragy and Carey (1996), (1997) have documented that, in conscious rats, Ang II infusions or sodium depletion caused an increase in renal interstitial fluid levels of cGMP that was blocked by PD123319. Furthermore, valsartan reduced SBP in sodium-depleted rats and renal-dependent hypertensive rats, and this effect of valsartan was blocked by a 30-min coinfusion of the AT2R antagonist PD123319 (Siragy & Carey, 1999; Siragy et al., 2000), although this ‘PD123319 reversal’ has not always been seen in studies by the same group (Siragy et al., 2002).

In some respects, it is quite remarkable that PD could reverse the effects of AT1R blockade in the fore-mentioned acute studies because it was shown some years ago that PD123319 actually displaced the metabolite of losartan from protein binding sites in a nonspecific manner. This served to increase the degree of AT1R blockade, which was assessed as inhibition of Ang II-mediated vasoconstriction (Widdop et al., 1992). These findings were subsequently confirmed by others (Wong et al., 1992). However, this interaction was only examined acutely over 24 h, and so the extent of this potentially confounding issue during chronic treatment conditions is not known.

Several studies of a more chronic nature in rats have also assessed changes in cardiac and vascular structure as well as BP (Table 2 ). In addition to the reported AT2R involvement in the effects of chronic AT1R blockade in heart failure (Liu et al., 1997), it was claimed that PD 123319, given as a daily, 70-min infusion for 1 week, reversed the losartan-induced reduction in SBP in sodium-depleted rats, as measured noninvasively by tail cuff method (Gigante et al., 1998). However, we could not confirm these findings in analogous experiments in which direct arterial BP measurements were made (Jones et al., 1999). In other studies, AT2R blockade exerted negligible effects on the antihypertensive effect of AT1R blockade in SHR, but PD123319 did in fact reverse valsartan-induced reductions in vascular smooth muscle growth and vascular mass. Moreover, this vascular remodelling involved AT2R-mediated smooth muscle cell apoptosis (Tea et al., 2000). By contrast, Varagic et al. (2001) found that PD123319 treatment caused a small reversal of losartan-induced antihypertensive effect in SHR. In addition, despite a lack of effect on left ventricular mass, AT2R blockade fully reversed candesartan-induced reductions in cardiac fibrosis, assessed by biochemical assay (Varagic et al., 2001). Thus, partial (Liu et al., 1997) or full (Varagic et al., 2001) reversal by PD123319, of reduced cardiac fibrosis caused by AT1R blockade, occurred after 2–3 months treatment with this AT2R antagonist. In this context, chronic treatment with PD 123319 alone to myopathic hamsters enhanced cardiac interstitial fibrosis after 44, but not 20, weeks, implicating growth-modulatory effects only after prolonged treatment (Ohkubo et al., 1997). This finding may help explain the partial effect of PD123319 on reversing the antifibrotic effect of AT1R blockade in the heart failure setting (Liu et al., 1997), notwithstanding the complex heterogeneous changes in AT2R expression that may be model and/or tissue specific (see earlier).

Table 2. Contribution of the AT2R to effects of chronic AT1R blockadea
StudyStrainExperimental designBlood pressureCardiac hypertrophyCardiac fibrosisVascular remodelling
  • a

    Assessed by the ability of PD123319 to reverse the effects of AT1R blockade in rats (‘PD reversal’), or an attenuated effect of AT1R antagonist in AT2R knockout mice.

  • a

    Yes, AT2R does contribute; no, AT2R does not contribute; —, not determined.

Liu et al. (1997)Lewis ratMI, ‘PD reversal’NoYesYes (trend only)
Gigante et al. (1998)Wistar ratLow salt diet, ‘PD reversal’Yes
Jones et al. (1999)WKY ratLow salt diet, ‘PD reversal’NoNo
Tea et al. (2000)SHR‘PD reversal’NoYes
Varagic et al. (2001)SHR‘PD reversal’Yes (small)NoYes
Collister et al. (2002)Sprague–Dawley rat‘PD reversal’No (further [DOWNWARDS ARROW] BP)
Xu et al. (2002)AT2R KO mouse (C57BL/6)MI, valsartanNoYesYes
Wu et al. (2002)AT2R KO mouse (FVB/N)Aortic banding, valsartanNo (but sub-depressor valsartan)NoYesYes
Wu et al. (2001)AT2R KO mouse (FVB/N)Femoral artery cuff, valsartanNoYes

Interestingly, in all the fore-mentioned studies examining an AT1R/AT2R interaction, SBP was measured using the noninvasive, but relatively stressful, tail cuff method. When BP was measured by radiotelemetry, combined AT1R and AT2R blockade actually further reduced BP relative to AT1R blockade alone, albeit in normotensive rats measured only over a 10-day period (Collister et al., 2002), which could imply a nonspecific interaction (Widdop et al., 1992).

Thus, there are very few studies that have assessed the ability of PD 123319 to reverse the antihypertensive and remodelling effects of AT1R antagonists under chronic treatment conditions, partly because of a lack of availability of the AT2R antagonist. While there is some evidence for structural changes (Table 2), there appears to be, at best, only a minor role of AT2R in the BP-lowering effects of AT1R antagonists when given chronically (e.g. Varagic et al., 2001). Nevertheless, the lack of reversal, by PD123319, of chronic ‘sartan’-induced haemodynamic changes, can still be reconciled with the well-described direct AT2R-mediated vasodilator effect in vitro (see earlier) or in vivo (Barber et al., 1999; Carey et al., 2001a; Li & Widdop, 2003). AT2R-mediated vasodilatation relies on directly stimulating vascular AT2R (with Ang II or CGP42112). On the other hand, the determination of whether or not PD123319 can reverse ‘sartan’-induced hypotension is often used as an indication of potential AT2R vasodilator involvement in AT1R blockade. However, any observed response will be the net effect of (opposing) interactions between pharmacodynamic (genuine ‘PD reversal’) and pharmacokinetic (nonspecific, enhanced AT1R blockade (Widdop et al., 1992) events.

In mice, three studies published to date have examined the effects of valsartan in AT2R knockout mice under three different pathological states (Table 2). In each case, the ability of valsartan to regress neointimal formation induced by femoral artery cuff (Wu et al., 2001), inhibit perivascular fibrosis and coronary artery thickening induced by aortic banding (Wu et al., 2002) or improve cardiac haemodynamics after MI (Xu et al., 2002) was attenuated in the mice lacking AT2R compared with wild types. Interestingly, these studies were conducted using both the antigrowth (Wu et al., 2001, 2002) and prohypertrophic/fibrotic (Xu et al., 2002) AT2R knockout phenotypes. Clearly, additional rat and mice studies are required to elucidate fully the role of AT2R in the setting of chronic AT1R blockade.

Thus, on close inspection, there are remarkably few chronic treatment studies on which to judge the somewhat seductive hypothesis, based mainly on acute studies, that AT2R stimulation contributes to the cardiovascular effects of AT1R antagonists. The striking AT2R effects on BP, inferred from experiments in which the acute administration of PD 123319 reversed the acute antihypertensive effects of AT1R antagonists, have not been seen in the limited number of chronic studies published using rats. On the other hand, there was evidence for regression of cardiac fibrosis and vascular remodelling evoked by AT2R activation, which are of greater physiological importance during long-term antihypertensive therapy, although the AT2R effects on cardiac hypertrophy were more equivocal (Table 2). Limited studies in mice also would point towards a role of AT2R in the effects of AT1R antagonists.

In the clinical setting, the most recent meta-analysis comparing AT1R blockade with either placebo or ACE inhibition did not report a clear-cut superiority of AT1R blockade in reducing all-cause mortality or hospitalisation rate in patients with heart failure (Jong et al., 2002) despite an earlier smaller analysis reporting a survival benefit with AT1R blockade (Sharma et al., 2000). As such, these clinical data may somewhat dampen the interest generated from experimental studies on the possible involvement of AT2R stimulation in the therapeutic effects of AT1R antagonists. However, several large trials, which will more than double the current patient population surveyed, are yet to conclude (Dickstein & Kjekshus, 1999; Pfeffer et al., 2000). Therefore, before we reject the hypothesis that AT2R counter-regulates AT1R function in the setting of heart failure (Opie & Sack, 2001), it is possible that there may be refinement of current evidence that does not readily distinguish AT1R antagonists from ACE inhibitors, at least in the heart failure population.

In addition, there is evidence that indicates AT1R antagonists are not all the same (Siragy, 2002). In this context, there is the intriguing possibility that AT1R antagonists may differentially stimulate AT2R-mediated effects in vivo. Recently Siragy et al. (2002) found that valsartan, but not losartan, caused prolonged elevations in cGMP levels in renal interstitial fluid of sodium-depleted rats. The fact that these effects were blocked by PD123319 would imply that the degree of AT2R activation caused by different AT1R antagonists may differ.

Of course, there is great difficulty in addressing any AT1R/AT2R interaction in the clinical situation. Nevertheless, this issue has been indirectly examined in recent preliminary studies using two different patient populations (Phoon & Howes, 2001, 2002). In one study in elderly female patients, Ang II caused dose-dependent forearm vasodilatation when tested during 3-week candesartan treatment; interestingly, a short-term infusion of PD 123319 in these patients elevated baseline forearm vascular resistance, suggesting that tonic AT2R-mediated vasodilatation contributes to the haemodynamic profile of AT1R blockade (Phoon & Howes, 2002). However, the AT2R antagonist did not appear to block Ang II-induced vasodilatation, indicating perhaps non-AT2R vasodilator mechanisms in response to exogenous Ang II were also involved (Phoon & Howes, 2002). Moreover, it was unclear if the potential AT2R involvement was due to the antihypertensive therapy and/or a unique patient population since, in an earlier study in healthy male volunteers without AT1R blockade, Ang II caused forearm vasoconstriction and PD123319 did not affect baseline forearm vascular resistance (Phoon & Howes, 2001).

Conclusions

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

AT2R function is likely to be context-specific, as recently suggested (Schneider & Lorell, 2001). This likelihood is well exemplified by studies at a number of levels: (i) Growth modulatory effects of Ang II in vitro depend on the type of AT receptor on a given cell. AT2R are natively expressed on cultured endothelial cells but not on cultured VSMC, such that antiproliferative actions of AT2R offset AT1R-mediated growth-promoting effects in endothelial cells but not VSMC (Nakajima et al., 1995; Stoll et al., 1995), (ii) Bidirectional changes in effector response can be elicited by AT2R depending on the cell type. AT2R evoked increased collagen production in VSMC and mesangial cells, but decreased collagen production in fibroblasts (Mifune et al., 2000). (iii) AT2R are upregulated in cardiac hypertrophy and heart failure (Lopez et al., 1994; Tsutsumi et al., 1998; Wharton et al., 1998), which impacts on whether or not an AT2R involvement is noted with respect to hypertrophy/growth and fibrosis (Liu et al., 1997; Ohkubo et al., 1997; Tsutsumi et al., 1998; Bartunek et al., 1999; Varagic et al., 2001). (iv) AT2R exerts vasodilatation per se in hypertensive and failing states compared with appropriate controls (Barber et al., 1999; Schuijt et al., 2001; Li & Widdop, 2003). Likewise, an involvement of AT2R in the acute (Siragy & Carey, 1999; Siragy et al., 2000) or chronic (Liu et al., 1997) therapeutic effects of AT1R blockade has usually been observed in experimental models in which there is increased RAS activity and/or pathological states where there is more likely to be an upregulation of AT2R.

Thus, it is apparent that there is marked tissue heterogeneity which is likely to reflect the balance of AT1/AT2 receptor expression in various tissues, which may be partly determined by the choice of experimental model. Indeed, the fact that there appears to be a greater diversity of AT2R effects on cardiac hypertrophy (stimulatory, inhibitory or no effect) than on cardiac fibrosis (predominantly inhibitory), most likely reflects the greater AT2R expression on cardiac fibroblasts (Ohkubo et al., 1997; Tsutsumi et al., 1998; Wharton et al., 1998). An alternative view that AT2R causes stimulatory effects, while increasingly being reported (Senbonmatsu et al., 2000; Ichihara et al., 2001), requires further consideration in the context of pharmacological studies to match genetic manipulations.

In any case, the elucidation of the actions of AT2R has gained prominence partly because of a postulated role in the therapeutic effects of AT1R antagonists. Current experimental data, although still incomplete, supports a role for the AT2R in contributing to the regression of structure caused by AT1R blockade in a context-specific manner. However, while the most recent meta-analysis of human clinical trials comparing AT1R antagonists with either placebo or ACE inhibition did not report a clear-cut superiority of AT1R blockade in reducing all-cause mortality or hospitalisation rate in patients with heart failure (Jong et al., 2002), more subtle aspects of cardiovascular remodelling between these two classes of antihypertensives are yet to be investigated, as is any potential AT2R involvement.

In addition, other factors need to be borne in mind when treating with an AT1R antagonist. For example, AT1R blockade and/or Ang II can themselves increase AT2R expression in vasculature and endothelial cells in some (Gigante et al., 1997; De Paolis et al., 1999; Bonnet et al., 2001), but not all (Otsuka et al., 1998) studies. In this context, recent data also showed that an over expression of AT2R in VSMC downregulates AT1R expression as well as basal DNA synthesis and proliferation of VSMC from WKY rats (Jin et al., 2002) but not from SHR (Su et al., 2002). Therefore, therapeutic outcome may be influenced, not only by pathological status of AT2R expression, but also by the autocrine/paracrine regulation of the cellular milieu in the target organs.

Future directions

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

Many of the actions attributed to AT2R pathophysiology have been inferred from changes in function due to AT2R blockade (either pharmacological using virtually one compound, or gene deletion methods) in either the presence or absence of AT1R blockade. Thus, the development of other novel AT2R agonists and antagonists is required in order to limit our reliance on too few available AT2R ligands.

Further studies examining the cardiovascular effects of chronic selective AT2R stimulation per se, as well in combination with AT1R antagonists, are imperative in order to elucidate further the pathophysiological role of the AT2R in cardiovascular disease. The fact that there was a sustained, enhanced effect of combined AT2R stimulation plus AT1R blockade that was greater than AT1R blockade alone (Carey et al., 2001a), together with the absence of functional AT2R desensitisation (Widdop et al., 2002), suggests that directly targeting the AT2R in cardiovascular disease will be a fruitful avenue for future research (Barber et al., 1999; Carey et al., 2001b; Siragy & Carey, 2001).

Acknowledgments

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References

The studies from our own laboratories have been supported by the National Health & Medical Research Council of Australia, the National Heart Foundation of Australia, Monash University, the Clive & Vera Ramaciotti Foundations, and the ANZ Trustees.

References

  1. Top of page
  2. Abstract
  3. Distribution of AT2 receptors
  4. AT2R signalling
  5. AT2R-mediated relaxation/vasodilatation
  6. Structural effects mediated by AT2R
  7. Role of AT2R in cardiovascular action of AT1R blockade
  8. Conclusions
  9. Future directions
  10. Acknowledgments
  11. References
  • AKISHITA, M., HORIUCHI, M., YAMADA, H., ZHANG, L., SHIRAKAMI, G., TAMURA, K., OUCHI, Y. & DZAU, V.J. (2000a). Inflammation influences vascular remodeling through AT2 receptor expression and signaling. Physiol. Genomics, 2, 1320.
  • AKISHITA, M., ITO, M., LEHTONEN, J.Y., DAVIET, L., DZAU, V.J. & HORIUCHI, M. (1999). Expression of the AT2 receptor developmentally programs extracellular signal-regulated kinase activity and influences fetal vascular growth. J. Clin. Invest., 103, 6371.
  • AKISHITA, M., IWAI, M., WU, L., ZHANG, L., OUCHI, Y., DZAU, V.J. & HORIUCHI, M. (2000b). Inhibitory effect of angiotensin II type 2 receptor on coronary arterial remodeling after aortic banding in mice. Circulation, 102, 16841689.
  • ARIMA, S., ENDO, Y., YAOITA, H., OMATA, K., OGAWA, S., TSUNODA, K., ABE, M., TAKEUCHI, K., ABE, K. & ITO, S. (1997). Possible role of P-450 metabolite of arachidonic acid in vasodilator mechanism of angiotensin II type 2 receptor in the isolated microperfused rabbit afferent arteriole. J. Clin. Invest., 100, 28162823.
  • ASANO, K., DUTCHER, D.L., PORT, J.D., MINOBE, W.A., TREMMEL, K.D., RODEN, R.L., BOHLMEYER, T.J., BUSH, E.W., JENKIN, M.J., ABRAHAM, W.T., RAYNOLDS, M.V., ZISMAN, L.S., PERRYMAN, M.B. & BRISTOW, M.R. (1997). Selective downregulation of the angiotensin II AT1-receptor subtype in fatting human ventricular myocardium. Circulation, 95, 11931200.
  • BAKER, K.M., BOOZ, G.W. & DOSTAL, D.E. (1992). Cardiac actions of angiotensin II: role of an intracardiac renin-angiotensin system. Ann. Rev. Physiol., 54, 227241.
  • BARBER, M.N., SAMPEY, D.B. & WIDDOP, R.E. (1999). AT(2) receptor stimulation enhances antihypertensive effect of AT(1) receptor antagonist in hypertensive rats. Hypertension, 34, 11121116.
  • BARTUNEK, J., WEINBERG, E.O., TAJIMA, M., ROHRBACH, S. & LORELL, B.H. (1999). Angiotensin II type 2 receptor blockade amplifies the early signals of cardiac growth response to angiotensin II in hypertrophied hearts. Circulation, 99, 2225.
  • BAUTISTA, R., SANCHEZ, A., HERNANDEZ, J., OYEKAN, A. & ESCALANTE, B. (2001). Angiotensin II type AT(2) receptor mRNA expression and renal vasodilatation are increased in renal failure. Hypertension, 38, 669673.
  • BEDECS, K., ELBAZ, N., SUTREN, M., MASSON, M., SUSINI, C., STROSBERG, A.D. & NAHMIAS, C. (1997). Angiotensin II type 2 receptors mediate inhibition of mitogen-activated protein kinase cascade and functional activation of SHP-1 tyrosine phosphatase. Biochem J., 325, 449454.
  • BONNET, F., COOPER, M.E., CAREY, R.M., CASLEY, D. & CAO, Z. (2001). Vascular expression of angiotensin type 2 receptor in the adult rat: influence of angiotensin II infusion. J Hypertens., 19, 10751081.
  • BOOZ, G.W. & BAKER, K.M. (1996). Role of type 1 and type 2 angiotensin receptors in angiotensin II-induced cardiomyocyte hypertrophy. Hypertension, 28, 635640.
  • BOTTARI, S.P., KING, I.N., REICHLIN, S., DAHLSTROEM, I., LYDON, N. & DE GASPARO, M. (1992). The angiotensin AT2 receptor stimulates protein tyrosine phosphatase activity and mediates inhibition of particulate guanylate cyclase. Biochem. Biophys. Res., Commun., 183, 206211.
  • BRECHLER, V., REICHLIN, S., DE GASPARO, M. & BOTTARI, S.P. (1994). Angiotensin II stimulates protein tyrosine phosphatase activity through a G-protein independent mechanism. Receptors Channels, 2, 8998.
  • BREDE, M., HADAMEK, K., MEINEL, L., WIESMANN, F., PETERS, J., ENGELHARDT, S., SIMM, A., HAASE, A., LOHSE, M.J. & HEIN, L. (2001). Vascular hypertrophy and increased P70S6 kinase in mice lacking the angiotensin II AT(2) receptor. Circulation, 104, 26022607.
  • BRINK, M., ERNE, P., DE GASPARO, M., ROGG, H., SCHMID, A., STULZ, P. & BULLOCK, G. (1996). Localization of the angiotensin II receptor subtypes in the human atrium. J. Mol. Cell Cardiol., 28, 17891799.
  • BUISSON, B., LAFLAMME, L., BOTTARI, S.P., DE GASPARO, M., GALLO-PAYET, N. & PAYET, M.D. (1995). A G protein is involved in the angiotensin AT2 receptor inhibition of the T-type calcium current in non-differentiated NG108-15 cells. J. Biol. Chem., 270, 16701674.
  • BURRELL, J.H. & LUMBERS, E.R. (1997). Angiotensin receptor subtypes in the uterine artery during ovine pregnancy. Eur. J. Pharmacol., 330, 257267.
  • BUSCHE, S., GALLINAT, S., BOHLE, R.M., REINECKE, A., SEEBECK, J., FRANKE, F., FINK, L., ZHU, M., SUMNERS, C. & UNGER, T. (2000). Expression of angiotensin AT(1) and AT(2) receptors in adult rat cardiomyocytes after myocardial infarction. A single-cell reverse transcriptase–polymerase chain reaction study. Am. J. Pathol., 157, 605611.
  • CAO, Z., BONNET, F., CANDIDO, R., NESTEROFF, S.P., BURNS, W.C., KAWACHI, H., SHIMIZU, F., CAREY, R.M., DE GASPARO, M. & COOPER, M.E. (2002). Angiotensin type 2 receptor antagonism confers renal protection in a rat model of progressive renal injury. J. Am. Soc. Nephrol., 13, 17731787.
  • CAO, Z., DEAN, R., WU, L., CASLEY, D. & COOPER, M.E. (1999). Role of angiotensin receptor subtypes in mesenteric vascular proliferation and hypertrophy. Hypertension, 34, 408414.
  • CAO, Z., KELLY, D.J., COX, A., CASLEY, D., FORBES, J.M., MARTINELLO, P., DEAN, R., GILBERT, R.E. & COOPER, M.E. (2000). Angiotensin type 2 receptor is expressed in the adult rat kidney and promotes cellular proliferation and apoptosis. Kidney Int., 58, 24372451.
  • CAPUTO, L., BENESSIANO, J., BOULANGER, C.M. & LEVY, B.I. (1995). Angiotensin II increases cGMP content via endothelial angiotensin II AT1 subtype receptors in the rat carotid artery. Arterioscler. Thromb. Vasc. Biol., 15, 16461651.
  • CAREY, R.M., HOWELL, N.L., JIN, X.H. & SIRAGY, H.M. (2001a). Angiotensin type 2 receptor-mediated hypotension in angiotensin type-1 receptor-blocked rats. Hypertension, 38, 12721277.
  • CAREY, R.M., JIN, X.H. & SIRAGY, H.M. (2001b). Role of the angiotensin AT2 receptor in blood pressure regulation and therapeutic implications. Am. J. Hypertens., 14, 98S102S.
  • CEILER, D.L., NELISSEN-VRANCKEN, H.J., DEMEY, J.G. & SMITS, J.F. (1998). Effect of chronic blockade of angiotensin II-receptor subtypes on aortic compliance in rats with myocardial infarction. J. Cardiovasc. Pharmacol., 31, 630637.
  • CHAKI, S. & INAGAMI, T. (1993). New signaling mechanism of angiotensin II in neuroblastoma neuro-2A cells: activation of soluble guanylyl cyclase via nitric oxide synthesis. Mol. Pharmacol., 43, 603608.
  • CHANG, R.S. & LOTTI, V.J. (1991). Angiotensin receptor subtypes in rat, rabbit and monkey tissues: relative distribution and species dependency. Life Sci., 49, 14851490.
  • CIUFFO, G.M., VISWANATHAN, M., SELTZER, A.M., TSUTSUMI, K. & SAAVEDRA, J.M. (1993). Glomerular angiotensin II receptor subtypes during development of rat kidney. Am. J. Physiol., 265, F264F271.
  • COLLISTER, J.P., SOUCHERAY, S.L. & OSBORN, J.W. (2002). Chronic hypotensive effects of losartan are not dependent on the actions of angiotensin II at AT2 receptors. J. Cardiovasc. Pharmacol., 39, 107116.
  • COX, D.A. & COHEN, M.L. (1996). Effects of oxidized low-density lipoprotein on vascular contraction and relaxation: clinical and pharmacological implications in atherosclerosis. Pharmacol. Rev., 48, 319.
  • CUI, T., NAKAGAMI, H., IWAI, M., TAKEDA, Y., SHIUCHI, T., DAVIET, L., NAHMIAS, C. & HORIUCHI, M. (2001). Pivotal role of tyrosine phosphatase SHP-1 in AT2 receptor-mediated apoptosis in rat fetal vascular smooth muscle cell. Cardiovasc. Res., 49, 863871.
  • DAUGHERTY, A., MANNING, M.W. & CASSIS, L.A. (2001). Antagonism of AT2 receptors augments angiotensin II-induced abdominal aortic aneurysms and atherosclerosis. Br. J. Pharmacol., 134, 865870.
  • DE GASPARO, M., CATT, K.J., INAGAMI I WRIGHT, J.W. & UNGER, T.H. (2000). International Union of Pharmacology. XXIII. The Angiotensin II Receptors. Pharmacol. Rev., 52, 415472.
  • DE GASPARO, M. & SIRAGY, H.M. (1999). The AT2 receptor: fact, fancy and fantasy. Regul. Peptides, 81, 1124.
  • DE PAOLIS, P., PORCELLINI, A., GIGANTE, B., GILIBERTI, R., LOMBARDI, A., SAVOIA, C., RUBATTU, S. & VOLPE, M. (1999). Modulation of the AT2 subtype receptor gene activation and expression by the AT1 receptor in endothelial cells. J. Hypertens., 17, 18731877.
  • DICKSTEIN, K. & KJEKSHUS, J. (1999). Comparison of the effects of losartan and captopril on mortality in patients after acute myocardial infarction: the OPTIMAAL trial design. Optimal Therapy in Myocardial Infarction with the Angiotensin II Antagonist Losartan. Am. J. Cardiol., 83, 477481.
  • DIEP, Q.N., LI, J.S. & SCHIFFRIN, E.L. (1999). In vivo study of AT(1) and AT(2) angiotensin receptors in apoptosis in rat blood vessels. Hypertension, 34, 617624.
  • DIMITROPOULOU, C., WHITE, R.E., FUCHS, L., ZHANG, H., CATRAVAS, J.D. & CARRIER, G.O. (2001). Angiotensin II relaxes microvessels via the AT(2) receptor and Ca(2+)-activated K(+) (BK(Ca)) channels. Hypertension., 37, 301307.
  • DIMMELER, S., RIPPMANN, V., WEILAND, U., HAENDELER, J. & ZEIHER, A.M. (1997). Angiotensin II induces apoptosis of human endothelial cells* Protective effect of nitric oxide. Circ. Res., 81, 970976.
  • DUDLEY, D.T., HUBBELL, S.E. & SUMMERFELT, R.M. (1991). Characterization of angiotensin II (AT2) binding sites in R3T3 cells. Mol. Pharmacol., 40, 360367.
  • DUDLEY, D.T. & SUMMERFELT, R.M. (1993). Regulated expression of angiotensin II (AT2) binding sites in R3T3 cells. Regul. Peptides, 44, 199206.
  • DUFF, J.L., MARRERO, M.B., PAXTON, W.G., CHARLES, C.H., LAU, L.F., BERNSTEIN, K.E. & BERK, B.C. (1993). Angiotensin II induces 3CH134, a protein–tyrosin phosphatase, in vascular smooth muscle cells. J. Biol. Chem., 268, 2603726040.
  • DUKE, L.M., EPPEL, G.A., WIDDOP, R.E. & EVANS, R.G. (2003). Disparate roles of AT2 receptors in the renal cortical and medullary circulations of anesthetized rabbits. Hypertension, 42, 200205.
  • ENDO, Y., ARIMA, S., YAOITA, H., OMATA, K., TSUNODA, K., TAKEUCHI, K., ABE, K. & ITO, S. (1997). Function of angiotensin II type 2 receptor in the postglomerular efferent arteriole. Kidney Int. Suppl., 63, S205S207.
  • ENDO, Y., ARIMA, S., YAOITA, H., TSUNODA, K., OMATA, K. & ITO, S. (1998). Vasodilation mediated by angiotensin II type 2 receptor is impaired in afferent arterioles of young spontaneously hypertensive rats. J. Vasc. Res., 35, 421427.
  • FISCHER, J.W., STOLL M HAHN, A.W. & UNGER, T. (2001). Differential regulation of thrombospondin-1 and fibronectin by angiotensin II receptor subtypes in cultured endothelial cells. Cardiovasc. Res., 51, 784791.
  • FISCHER, T.A., SINGH, K., O'HARA, D.S., KAYE, DM. & KELLY, R.A. (1998). Role of AT1 and AT2 receptors in regulation of MAPKs and MKP-1 by ANG II in adult cardiac myocytes. Am. J. Physiol., 275, H906H916.
  • GALLINAT, S., BUSCHE, S., RAIZADA, M.K. & SUMNERS, C. (2000). The angiotensin II type 2 receptor: an enigma with multiple variations. Am. J. Physiol. Endocrinol. Metab., 278, E357E374.
  • GIGANTE, B., PIRAS, O., DE PAOLIS, P., PORCELLINI, A., NATALE, A. & VOLPE, M. (1998). Role of the angiotensin II AT2-subtype receptors in the blood pressure-lowering effect of losartan in salt-restricted rats. J. Hypertens., 16, 20392043.
  • GIGANTE, B., RUBATTU, S., RUSSO, R., PORCELLINI, A., ENEA, I., DE PAOLIS, P., SAVOIA, C., NATALE, A., PIRAS, O. & VOLPE, M. (1997). Opposite feedback control of renin and aldosterone biosynthesis in the adrenal cortex by angiotensin II AT1-subtype receptors. Hypertension, 30, 563568.
  • GOETTE, A., ARNDT, M., ROCKEN, C., SPIESS, A., STAACK, T., GELLER, J.C., HUTH, C., ANSORGE, S., KLEIN, H.U. & LENDECKEL, U. (2000). Regulation of angiotensin II receptor subtypes during atrial fibrillation in humans. Circulation, 101, 26782681.
  • GOHLKE, P., PEES, C. & UNGER, T. (1998). AT2 receptor stimulation increases aortic cyclic GMP in SHRSP by a kinin-dependent mechanism. Hypertension, 31, 349355.
  • GOLDFARB, D.A., DIZ, D.I., TUBBS, R.R., FERRARIO, C.M. & NOVICK, A.C. (1994). Angiotensin II receptor subtypes in the human renal cortex and renal cell carcinoma. J. Urol., 151, 208213.
  • GOTO, M., MUKOYAMA, M., SUGAWARA, A., SUGANAMI, T., KASAHARA, M., YAHATA, K., MAKINO, H., SUGA, S., TANAKA, I. & NAKAO, K. (2002). Expression and role of angiotensin II type 2 receptor in the kidney and mesangial cells of spontaneously hypertensive rats. Hypertens. Res., 25, 125133.
  • GRONE, H.J., SIMON, M. & FUCHS, E. (1992). Autoradiographic characterization of angiotensin receptor subtypes in fetal and adult human kidney. Am. J. Physiol., 262, F326F331.
  • GROSS, V., SCHUNCK, W.H., HONECK, H., MILIA, A.F., KARGEL, E., WALTHER, T., BADER, M., INAGAMI, T., SCHNEIDER, W. & LUFT, F.C. (2000). Inhibition of pressure natriuresis in mice lacking the AT2 receptor. Kidney Int., 57, 191202.
  • HABERL, R.L. (1994). Role of angiotensin receptor subtypes in the response of rabbit brain arterioles to angiotensin. Stroke, 25, 14761479 discussion 1480.
  • HANNAN, R.E., DAVIS, E.A. & WIDDOP, R.E. (2003a). Functional role of angiotensin II AT2 receptor in modulation of AT1 receptor-mediated contraction in rat uterine artery: involvement of bradykinin and nitric oxide. Br. J. Pharmacol. in press.
  • HANNAN, R.E., GASPARI, T.A., DAVIS, E.A. & WIDDOP, R.E. (2003b). Differential regulation by AT1 and AT2 receptors of angiotensin II-stimulated cyclic GMP production in rat uterine artery and aorta. Hypertension. submitted.
  • HAYWOOD, G.A., GULLESTAD, L., KATSUYA, T., HUTCHINSON, H.G., PRATT, R.E., HORIUCHI, M. & FOWLER, M.B. (1997). AT1 and AT2 angiotensin receptor gene expression in human heart failure. Circulation, 95, 12011206.
  • HEIN, L., BARSH, G.S., PRATT, R.E., DZAU, V.J. & KOBILKA, B.K. (1995). Behavioural and cardiovascular effects of disrupting the angiotensin II type-2 receptor in mice [published erratum appears in Nature 1996; 380(6572):366]. Nature, 377, 744747.
  • HENRION, D., KUBIS, N. & LEVY, B.I. (2001). Physiological and pathophysiological functions of the AT(2) subtype receptor of angiotensin II: from large arteries to the microcirculation. Hypertension., 38, 11501157.
  • HORIUCHI, M., AKISHITA, M. & DZAU, V.J. (1999a). Recent progress in angiotensin II type 2 receptor research in the cardiovascular system. Hypertension, 33, 613621.
  • HORIUCHI, M., HAYASHIDA, W., AKISHITA, M., TAMURA, K., DAVIET, L., LEHTONEN, J.Y. & DZAU, V.J. (1999b). Stimulation of different subtypes of angiotensin II receptors, AT1 and AT2 receptors, regulates STAT activation by negative crosstalk. Circ. Res., 84, 876882.
  • HORIUCHI, M., HAYASHIDA, W., KAMBE, T., YAMADA, T. & DZAU, V.J. (1997). Angiotensin type 2 receptor dephosphorylates Bcl-2 by activating mitogen-activated protein kinase phosphatase-1 and induces apoptosis. J. Biol. Chem., 272, 1902219026.
  • HUANG, X.C., RICHARDS, EM. & SUMNERS, C. (1995). Angiotensin II type 2 receptor-mediated stimulation of protein phosphatase 2A in rat hypothalamic/brainstem neuronal cocultures. J. Neurochem., 65, 21312137.
  • HUANG, X.C., RICHARDS, E.M. & SUMNERS, C (1996). Mitogen-activated protein kinases in rat brain neuronal cultures are activated by angiotensin II type 1 receptors and inhibited by angiotensin II type 2 receptors. J. Biol. Chem., 271, 1563515641.
  • ICHIHARA, S., SENBONMATSU, T., PRICE Jr, E., ICHIKI, T., GAFFNEY, F.A. & INAGAMI, T. (2001). Angiotensin II type 2 receptor is essential for left ventricular hypertrophy and cardiac fibrosis in chronic angiotensin II-induced hypertension. Circulation, 104, 346351.
  • ICHIHARA, S., SENBONMATSU, T., PRICE Jr, E., ICHIKI, T., GAFFNEY, F.A. & INAGAMI, T. (2002). Targeted deletion of angiotensin II type 2 receptor caused cardiac rupture after acute myocardial infarction. Circulation, 106, 22442249.
  • ICHIKI, T., LABOSKY, P.A., SHIOTA, C., OKUYAMA, S., IMAGAWA, Y., FOGO, A., NIIMURA, F., ICHIKAWA, I., HOGAN, B.L. & INAGAMI, T. (1995). Effects on blood pressure and exploratory behaviour of mice lacking angiotensin II type-2 receptor. Nature, 377, 748750.
  • INAGAMI, T. & SENBONMATSU, T. (2001). Dual effects of angiotensin II type 2 receptor on cardiovascular hypertrophy. Trends Cardiovasc. Med., 11, 324328.
  • JALOWY, A., SCHULZ, R., DORGE, H., BEHRENDS, M. & HEUSCH, G. (1998). Infarct size reduction by AT1-receptor blockade through a signal cascade of AT2-receptor activation, bradykinin and prostaglandins in pigs. J. Am. Coll. Cardiol., 32, 17871796.
  • JANIAK, P., PILLON, A., PROST, J.F. & VILAINE, J.P. (1992). Role of angiotensin subtype 2 receptor in neointima formation after vascular injury. Hypertension, 20, 737745.
  • JIN, X.Q., FUKUDA, N., SU, J.Z., LAI, Y.M., SUZUKI, R., TAHIRA, Y., TAKAGI, H., IKEDA, Y., KANMATSUSE, K. & MIYAZAKI, H. (2002). Angiotensin II type 2 receptor gene transfer downregulates angiotensin II type 1a receptor in vascular smooth muscle cells. Hypertension, 39, 10211027.
  • JONES, E.S., PAULL, J.R.A. & WIDDOP, R.E. (1999). Does stimulation of the angiotensin AT2 receptor contribute to the antihypertensive effect of AT1 receptor antagonists in salt-restricted rats Proc. Aust. Soc. Clin. Exp. Pharmacol. Toxicol., 6, 84.
  • JONG, P., DEMERS, C., MCKELVIE, R.S. & LIU, P.P. (2002). Angiotensin receptor blockers in heart failure: meta-analysis of randomized controlled trials. J. Am. Coll. Cardiol., 39, 463470.
  • KATADA, J. & MAJIMA, M. (2002). AT(2) receptor-dependent vasodilation is mediated by activation of vascular kinin generation under flow conditions, [comment]. Br. J. Pharmacol., 136, 484491.
  • KIMURA, B., SUMNERS, C. & PHILLIPS, M.I. (1992). Changes in skin angiotensin II receptors in rats during wound healing. Biochem. Biophys. Res. Commun., 187, 10831090.
  • KURISU, S., OZONO, R., OSHIMA, T., KAMBE, M., ISHIDA, T., SUGINO, H., MATSUURA, H., CHAYAMA, K., TERANISHI, Y., IBA, O., AMANO, K. & MATSUBARA, H. (2003). Cardiac angiotensin II type 2 receptor activates the kinin/NO system and inhibits fibrosis. Hypertens., 41, 99107.
  • LAFLAMME, L., DE GASPARO, M., GALLO, J.M., PAYET, M.D. & GALLO-PAYET, N. (1996). Angiotensin II induction of neurite outgrowth by AT2 receptors in NG108-15 cells. Effect counteracted by the AT1 receptors. J. Biol. Chem., 271, 2272922935.
  • LAMBERS, D.S., GREENBERG, S.G. & CLARK, K.E. (2002). Functional role of angiotensin II type 1 and 2 receptors in regulation of uterine blood flow in nonpregnant sheep. Am. J. Physiol., 278, H353H359.
  • LEHTONEN, J.Y., DAVIET, L., NAHMIAS, C., HORIUCHI, M. & DZAU, V.J. (1999). Analysis of functional domains of angiotensin II type 2 receptor involved in apoptosis. Mol. Endocrinol., 13, 10511060.
  • LEVY, B.I., BENESSIANO, J., HENRION, D., CAPUTO, L., HEYMES, C., DURIEZ, M., POITEVIN, P. & SAMUEL, J.L. (1996). Chronic blockade of AT2-subtype receptors prevents the effect of angiotensin II on the rat vascular structure. J. Clin. Invest., 98, 418425.
  • LI, J.S., TOUYZ, R.M. & SCHIFFRIN, E.L. (1998). Effects of AT1 and AT2 angiotensin receptor antagonists in angiotensin II-infused rats. Hypertension, 31, 487492.
  • LI, X.C. & WIDDOP, R.E. (2003). AT2 receptor-mediated vasodilatation is unmasked by AT1 receptor blockade in conscious SHR. Br. J. Pharmacol. submitted.
  • LIU, K.L., LO, M., GROUZMANN, E., MUTTER, M. & SASSARD, J. (1999). The subtype 2 of angiotensin II receptors and pressure-natriuresis in adult rat kidneys. Br. J. Pharmacol., 126, 826832.
  • LIU, Y.H., YANG, X.P., SHAROV, V.G., NASS, O., SABBAH, H.N., PETERSON, E. & CARRETERO, O.A. (1997). Effects of angiotensin-converting enzyme inhibitors and angiotensin II type 1 receptor antagonists in rats with heart failure* Role of kinins and angiotensin II type 2 receptors. J. Clin. Invest., 99, 19261935.
  • LO, M., LIU, K.L., LANTELME, P. & SASSARD, J. (1995). Subtype 2 of angiotensin II receptors controls pressure-natriuresis in rats. J. Clin. Invest., 95, 13941397.
  • LOPEZ, J.J., LORELL, B.H., INGELFINGER, J.R., WEINBERG, E.O., SCHUNKERT, H., DIAMANT, D. & TANG, S.S. (1994). Distribution and function of cardiac angiotensin AT1- and AT2-receptor subtypes in hypertrophied rat hearts. Am. J. Physiol., 267, H844H852.
  • LUCIUS, R., GALLINAT, S., ROSENSTIEL, P., HERDEGEN, T., SIEVERS, J. & UNGER, T. (1998). The angiotensin II type 2 (AT2) receptor promotes axonal regeneration in the optic nerve of adult rats. J. Exp. Med., 188, 661670.
  • MA, J., NISHIMURA, H., FOGO, A., KON, V., INAGAMI, T. & ICHIKAWA, I. (1998). Accelerated fibrosis and collagen deposition develop in the renal interstitium of angiotensin type 2 receptor null mutant mice during ureteral obstruction. Kidney Int., 53, 937944.
  • MAKINO, N., SUGANO, M., OHTSUKA, S., SAWADA, S. & HATA, T. (1999). Chronic antisense therapy for angiotensinogen on cardiac hypertrophy in spontaneously hypertensive rats. Cardiovasc. Res., 44, 543548.
  • MAKINO, N., SUGANO, M., OTSUKA, S. & HATA, T. (1997). Molecular mechanism of angiotensin II type I and type II receptors in cardiac hypertrophy of spontaneously hypertensive rats. Hypertension, 30, 796802.
  • MARRERO, M.B., SCHIEFFER, B., PAXTON, W.G., HEERDT, L., BERK, B.C., DELAFONTAINE, P. & BERNSTEIN, K.E. (1995). Direct stimulation of Jak/STAT pathway by the angiotensin II AT1 receptor. Nature, 375, 247250.
  • MASAKI, H., KURIHARA, T., YAMAKI, A., INOMATA, N., NOZAWA, Y., MORI, Y., MURASAWA, S., KIZIMA, K., MARUYAMA, K., HORIUCHI, M., DZAU, V.J., TAKAHASHI, H., IWASAKA, T., INADA, M. & MATSUBARA, H. (1998). Cardiac-specific overexpression of angiotensin II AT2 receptor causes attenuated response to AT1 receptor-mediated pressor and chronotropic effects. J. Clin. Invest., 101, 527535.
  • MATROUGUI, K., LEVY, B.I. & HENRION, D. (2000). Tissue angiotensin II and endothelin-1 modulate differently the response to flow in mesenteric resistance arteries of normotensive and spontaneously hypertensive rats. Br. J. Pharmacol., 130, 521526.
  • MATROUGUI, K., LOUFRANI, L., HEYMES, C., LEVY, BI. & HENRION, D. (1999). Activation of AT(2) receptors by endogenous angiotensin II is involved inflow-induced dilation in rat resistance arteries. Hypertension, 34, 659665.
  • MATSUBARA, H. (1998). Pathophysiological role of angiotensin II type 2 receptor in cardiovascular and renal diseases. Circ. Res., 83, 11821191.
  • MATSUMOTO, T., OZONO, R., OSHIMA, T., MATSUURA, H., SUEDA, T., KAJIYAMA, G. & KAMBE, M. (2000). Type 2 angiotensin II receptor is downregulated in cardiomyocytes of patients with heart failure. Cardiovasc. Res., 46, 7381.
  • MCMULLEN, J.R., GIBSON, K.J., LUMBERS, E.R. & BURRELL, J.H. (2001). Selective down-regulation of AT2 receptors in uterine arteries from pregnant ewes given 24-h intravenous infusions of angiotensin II. Regul. Peptides, 99, 119129.
  • MCMULLEN, J.R., GIBSON, K.J., LUMBERS, E.R., BURRELL, J.H. & WU, J. (1999). Interactions between AT1 and AT2 receptors in uterine arteries from pregnant ewes. Eur. J. Pharmacol., 378, 195202.
  • MEFFERT, S., STOLL, M., STECKELINGS, U.M., BOTTARI, S.P. & UNGER, T. (1996). The angiotensin II AT2 receptor inhibits proliferation and promotes differentiation in PC12W cells. Mol. Cell Endocrinol., 122, 5967.
  • MIFUNE, M., SASAMURA, H., SHIMIZU-HIROTA, R., MIYAZAKI, H. & SARUTA, T. (2000). Angiotensin II type 2 receptors stimulate collagen synthesis in cultured vascular smooth muscle cells. Hypertension, 36, 845850.
  • MIYATA, TV., PARK, F., LI, X.F. & COWLEY Jr, A.W. (1999). Distribution of angiotensin AT1 and AT2 receptor subtypes in the rat kidney. Am. J. Physiol., 277, F437F446.
  • MOORE, A.F., HEIDERSTADT, N.T., HUANG, E., HOWELL, N.L., WANG, Z.Q., SIRAGY, H.M. & CAREY, R.M. (2001). Selective inhibition of the renal angiotensin type 2 receptor increases blood pressure in conscious rats. Hypertension, 37, 12851291.
  • MORRISSEY, J.J. & KLAHR, S. (1999). Effect of AT2 receptor blockade on the pathogenesis of renal fibrosis. Am. J. Physiol., 276, F39F45.
  • MUNOZ-GARCIA, R., MAESO, R., RODRIGO, E., NAVARRO, J., RUILOPE, L.M., CASAL, M.C., CACHOFEIRO, V. & LAHERA, V. (1995). Acute renal excretory actions of losartan in spontaneously hypertensive rats: role of AT2 receptors, prostaglandins, kinins and nitric oxide. J. Hypertens., 13, 17791784.
  • MUNZENMAIER, D.H. & GREENE, A.S. (1996). Opposing actions of angiotensin II on microvascular growth and arterial blood pressure. Hypertension, 27, 760765.
  • NAHMIAS, C., CAZAUBON, S.M., BRIEND-SUTREN, M.M., LAZARD, D., VILLAGEOIS, P. & STROSBERG, A.D. (1995). Angiotensin II AT2 receptors are functionally coupled to protein tyrosine dephosphorylation in N1E-115 neuroblastoma cells. Biochem. J., 306, 8792.
  • NAKAJIMA, M., HUTCHINSON, H.G., FUJINAGA, M., HAYASHIDA, W., MORISHITA, R., ZHANG, L., HORIUCHI, M., PRATT, R.E. & DZAU, V.J. (1995). The angiotensin II type 2 (AT2) receptor antagonizes the growth effects of the AT1 receptor: gain-of-function study using gene transfer. Proc. Nat. Acad. Sci. U.S.A., 92, 1066310667.
  • NIO, Y., MATSUBARA, H., MURASAWA, S., KANASAKI, M. & INADA, M. (1995). Regulation of gene transcription of angiotensin II receptor subtypes in myocardial infarction. J. Clin. Invest., 95, 4654.
  • NORA, E.H., MUNZENMAIER, D.H., HANSEN-SMITH, F.M., LOMBARD, J.H. & GREENE, A.S. (1998). Localization of the ANG II type 2 receptor in the microcirculation of skeletal muscle. Am. J. Physiol., 275, H1395H1403.
  • OHKUBO, N., MATSUBARA, H., NOZAWA, Y., MORI, Y., MURASAWA, S., KIJIMA, K., MARUYAMA, K., MASAKI, H., TSUTUMI, Y., SHIBAZAKI, Y., IWASAKA, T. & INADA, M. (1997). Angiotensin type 2 receptors are reexpressed by cardiac fibroblasts from failing myopathic hamster hearts and inhibit cell growth and fibrillar collagen metabolism. Circulation, 96, 39543962.
  • OISHI, Y., OZONO, R., YANO, Y., TERANISHI, Y., AKISHITA, M., HORIUCHI, M., OSHIMA, T. & KAMBE, M. (2003). Cardioprotective role of AT2 receptor in postinfarction left ventricular remodeling. Hypertension, 41, 814818.
  • OPIE, L.H. & SACK, M.N. (2001). Enhanced angiotensin II activity in heart failure: revaluation of the counterregulatory hypothesis of receptor subtypes. Circ. Res., 88, 654658.
  • OTSUKA, S., SUGANO, M., MAKINO, N., SAWADA, S., HATA, T. & NIHO, Y. (1998). Interaction of mRNAs for angiotensin II type 1 and type 2 receptors to vascular remodeling in spontaneously hypertensive rats. Hypertension, 32, 467472.
  • OZAWA, Y., SUZUKI, Y., MURAKAMI, K. & MIYAZAKI, H. (1996). The angiotensin II type 2 receptor primarily inhibits cell growth via pertussis toxin-sensitive G proteins. Biochem. Biophys. Res. Commun., 228, 328333.
  • OZONO, R., WANG, Z.Q., MOORE, A.F., INAGAMI, T., SIRAGY, H.M. & CAREY, R.M. (1997). Expression of the subtype 2 angiotensin (AT2) receptor protein in rat kidney. Hypertension, 30, 12381246.
  • PEES, C., UNGER, T. & GOHLKE, P. (2003). Effect of angiotensin AT(2) receptor stimulation on vascular cyclic GMP production in normotensive Wistar–Kyoto rats. Int. J. Biochem. Cell Biol., 35, 963972.
  • PFEFFER, M.A., MCMURRAY, J., LEIZOROVICZ, A., MAGGIONI, A.P., ROULEAU, J.L., VAN DE WERF, F., HENIS, M., NEUHART, E., GALLO, P., EDWARDS, S., SELLERS, M.A., VELAZQUEZ, E. & CALIFF, R. (2000). Valsartan in acute myocardial infarction trial (VALIANT): rationale and design. Am. Heart J., 140, 727750.
  • PHOON, S. & HOWES, L.G. (2002). Forearm vasodilator response to angiotensin II in elderly women receiving candesartan: role of AT(2)- receptors. J. Renin Angiotensin Aldosterone Syst., 3, 3639.
  • PHOON, S. & HOWES, L.G. (2001). Role of angiotensin type 2 receptors in human forearm vascular responses of normal volunteers. Clin. Exp. Pharmacol. Physiol., 28, 734736.
  • PUEYO, M.E., ARNAL, J.F., RAMI, J. & MICHEL, J.B. (1998). Angiotensin II stimulates the production of NO and peroxynitrite in endothelial cells. Am. J. Physiol., 274, C214C220.
  • REGITZ-ZAGROSEK, V., FRIEDEL, N., HEYMANN, A., BAUER, P., NEUSS, M., ROLFS, A., STEFFEN, C., HILDEBRANDT, A., HETZER, R. & FLECK, E. (1995). Regulation, chamber localization, and subtype distribution of angiotensin II receptors in human hearts. Circulation, 91, 14611471.
  • ROGG, H., DE GASPARO, M., GRAEDEL, E., STULZ, P., BURKART, F., EBERHARD, M. & ERNE, P. (1996). Angiotensin II-receptor subtypes in human atria and evidence for alterations in patients with cardiac dysfunction. Eur. Heart J., 17, 11121120.
  • ROULSTON, C.L., LAWRENCE, A.J., JARROTT, B. & WIDDOP, R.E. (2003). Localization of AT(2) receptors in the nucleus of the solitary tract of spontaneously hypertensive and Wistar Kyoto rats using [1251] CGP42112: upregulation of a non-angiotensin II binding site following unilateral nodose ganglionectomy. Brain Res., 968, 139155.
  • RUIZ-ORTEGA, M., LORENZO, O., RUPEREZ, M., ESTEBAN, V., SUZUKI, Y., MEZZANO, S., PLAZA, J.J. & EGIDO, J. (2001). Role of the renin-angiotensin system in vascular diseases: expanding the field. Hypertension, 38, 13821387.
  • RUIZ-ORTEGA, M., LORENZO, O., RUPEREZ, M., KONIG, S., WITTIG, B. & EGIDO, J. (2000). Angiotensin II activates nuclear transcription factor kappaB through AT(1) and AT(2) in vascular smooth muscle cells: molecular mechanisms. Circ. Res., 86, 12661272.
  • SABRI, A., LEVY B.I POITEVIN, P., CAPUTO, L., FAGGIN, E., MAROTTE, F., RAPPAPORT, L. & SAMUEL, J.L. (1997). Differential roles of AT1 and AT2 receptor subtypes in vascular trophic and phenotypic changes in response to stimulation with angiotensin II. Arterioscler. Thromb. Vasc. Biol., 17, 257264.
  • SAITO, S., HIRATA, Y., EMORI, T., IMAI, T. & MARUMO, F. (1996). Angiotensin II activates endothelial constitutive nitric oxide synthase via AT1 receptors. Hypertens. Res. Clin. Exp., 19, 201206.
  • SCHEUER, D.A. & PERRONE, M.H. (1993). Angiotensin type 2 receptors mediate depressor phase of biphasic pressure response to angiotensin. Am. J. Physiol., 264, R917R923.
  • SCHNEIDER, M.D. & LORELL, B.H. (2001). AT(2), judgment day: which angiotensin receptor is the culprit in cardiac hypertrophy Circulation, 104, 247248.
  • SCHUIJT, M.P., BASDEW, M., VAN VEGHEL, R., DE VRIES, R., SAXENA, P.R., SCHOEMAKER, R.G. & DANSER, A.H. (2001). AT(2) receptor-mediated vasodilation in the heart: effect of myocardial infarction. Am. J. Physiol. — Heart Circ. Physiol., 281, H2590H2596.
  • SCHUIJT, M.P., DE VRIES, R., SAXENA, P.R. & DANSER, A.M. (1999). No vasoactive role of the angiotensin II type 2 receptor in normotensive Wistar rats. J. Hypertens., 17, 18791884.
  • SECHI, L.A., GRADY, E.F., GRIFFIN, C.A., KALINYAK, J.E. & SCHAMBELAN, M. (1992a). Distribution of angiotensin II receptor subtypes in rat and human kidney. Am. J. Physiol., 262, F236F240.
  • SECHI, L.A., GRIFFIN, C.A., GRADY, E.F., KALINYAK, J.E. & SCHAMBELAN, M. (1992b). Characterization of angiotensin II receptor subtypes in rat heart. Circ. Res., 71, 14821489.
  • SENBONMATSU, T., ICHIHARA, S., PRICE JR, E., GAFFNEY, F.A. & INAGAMI, T. (2000). Evidence for angiotensin II type 2 receptor-mediated cardiac myocyte enlargement during in vivo pressure overload. J. Clin. Invest., 106, R25R29.
  • SEYEDI, N., XU, X., NASJLETTI, A. & HINTZE, T.H. (1995). Coronary kinin generation mediates nitric oxide release after angiotensin receptor stimulation. Hypertension, 26, 164170.
  • SHANMUGAM, S., LLORENS-CORTES, C., CLAUSER, E., CORVOL, P. & GASC, J.M. (1995). Expression of angiotensin II AT2 receptor mRNA during development of rat kidney and adrenal gland. Am. J. Physiol., 268, F922F930.
  • SHARMA, D., BUYSE M PITT, B. & RUCINSKA, E.J. (2000). Meta-analysis of observed mortality data from all-controlled, double-blind, multiple-dose studies of losartan in heart failure. Losartan Heart Failure Mortality Meta-analysis Study Group. Am. J. Cardiol., 85, 187192.
  • SHENOY, U.V., RICHARDS, E.M., HUANG, X.C. & SUMNERS, C. (1999). Angiotensin II type 2 receptor-mediated apoptosis of cultured neurons from newborn rat brain. Endocrinology, 140, 500509.
  • SHIBASAKI, Y., MATSUBARA, H., NOZAWA, Y., MORI, Y., MASAKI, H., KOSAKI, A., TSUTSUMI, Y., UCHIYAMA, Y., FUJIYAMA, S., NOSE, A., IBA, O., TATEISHI, E., HASEGAWA, T., HORIUCHI, M., NAHMIAS, C. & IWASAKA, T. (2001). Angiotensin II type 2 receptor inhibits epidermal growth factor receptor transactivation by increasing association of SHP-1 tyrosine phosphatase. Hypertension, 38, 367372.
  • SIRAGY, H.M. (2002). Angiotensin receptor blockers: how important is selectivity Am. J. Hypertens., 15, 10061014.
  • SIRAGY, H.M. & CAREY, R.M. (1996). The subtype-2 (AT2) angiotensin receptor regulates renal cyclic guanosine 3′, 5′-monophosphate and AT1 receptor-mediated prostaglandin E2 production in conscious rats [see comments]. J. Clin. Invest., 97, 19781982.
  • SIRAGY, H.M. & CAREY, R.M. (1997). The subtype 2 (AT2) angiotensin receptor mediates renal production of nitric oxide in conscious rats. J. Clin. Invest., 100, 264269.
  • SIRAGY, H.M. & CAREY, R.M. (1999). Protective role of the angiotensin AT2 receptor in a renal wrap hypertension model. Hypertension, 33, 12371242.
  • SIRAGY, H.M. & CAREY, R.M. (2001). Angiotensin type 2 receptors: potential importance in the regulation of blood pressure. Curr. Opin. Nephrol. Hypertens., 10, 99103.
  • SIRAGY, H.M., DE GASPARO, M. & CAREY, R.M. (2000). Angiotensin type 2 receptor mediates valsartan-induced hypotension in conscious rats. Hypertension, 35, 10741077.
  • SIRAGY, H.M., DE GASPARO, M., EL-KERSH, M. & CAREY, R.M. (2001). Angiotensin-converting enzyme inhibition potentiates angiotensin II type 1 receptor effects on renal bradykinin and cGMP. Hypertension, 38, 183186.
  • SIRAGY, H.M., EL-KERSH, M.A., DE GASPARO, M., WEBB, R.L. & CAREY, R.M. (2002). Differences in AT2-receptor stimulation between AT1-receptor blockers valsartan and losartan quantified by renal interstitial fluid cGMP. J. Hypertens., 20, 11571163.
  • SIRAGY, H.M., INAGAMI, T., ICHIKI, T. & CAREY, R.M. (1999a). Sustained hypersensitivtiy to angiotensin II and its mechanism in mice lacking the subtype-2 (AT2) angiotensin receptor. Proc. Nat. Acad. Sci. U.S.A., 96, 65066510.
  • SIRAGY, H.M., JAFFA, A.A. & MARGOLIUS, H.S. (1997). Bradykinin B2 receptor modulates renal prostaglandin E2 and nitric oxide. Hypertension, 29, 757762.
  • SIRAGY, H.M., JAFFA, A.A., MARGOLIUS, H.S. & CAREY, R.M. (1996). Renin-angiotensin system modulates renal bradykinin production. Am. J. Physiol., 271, R1090R1095.
  • SIRAGY, H.M., SENBONMATSU, T., ICHIKI, T., INAGAMI, T. & CAREY, R.M. (1999b). Increased renal vasodilator prostanoids prevent hypertension in mice lacking the angiotensin subtype-2 receptor. J. Clin. Invest., 104, 181188.
  • SR-LOUIS, J., SICOTTE, B., BEDARD, S. & BROCHU, M. (2001). Blockade of angiotensin receptor subtypes in arcuate uterine artery of pregnant and postpartum rats. Hypertension., 38, 10171023.
  • STOLL, M., STECKELINGS, U.M., PAUL, M., BOTTARI, S.P., METZGER, R. & UNGER, T. (1995). The angiotensin AT2-receptor mediates inhibition of cell proliferation in coronary endothelial cells. J. Clin. Invest., 95, 651657.
  • STROTH, U., BLUME, A., MIELKE, K. & UNGER, T. (2000). Angiotensin AT(2) receptor stimulates ERK1 and ERK2 in quiescent but inhibits ERK in NGF-stimulated PC12W cells. Mol. Brain Res., 78, 175180.
  • SU, J.Z., FUKUDA, N., JIN, X.Q., LAI, Y.M., Suzuki, R., Tahira, Y., TAKAGI, H., IKEDA, Y., KANMATSUSE, K. & MIYAZAKI, H. (2002). Effect of AT2 receptor on expression of AT1 and TGF-beta receptors in VSMCs from SHR. Hypertension, 40, 853858.
  • SUGINO, H., OZONO, R., KURISU, S., MATSUURA, H., ISHIDA, M., OSHIMA, T., KAMBE, M., Teranishi, Y., Masaki, H. & Matsubara, H. (2001). Apoptosis is not increased in myocardium overexpressing type 2 angiotensin II receptor in transgenic mice. Hypertension, 37, 13941398.
  • SUZUKI, J., IWAI, M., NAKAGAMI, H., WU, L., CHEN, R., SUGAYA, T., HAMADA, M., HIWADA, K. & HORIUCHI, M. (2002). Role of angiotensin II-regulated apoptosis through distinct AT1 and AT2 receptors in neointimal formation. Circulation, 106, 847853.
  • SUZUKI, J., MATSUBARA, H., URAKAMI, M. & INADA, M. (1993). Rat angiotensin II (type 1A) receptor mRNA regulation and subtype expression in myocardial growth and hypertrophy. Circ. Res., 73, 439447.
  • TANAKA, M., TSUCHIDA, S., IMAI, T., FUJII, N., MIYAZAKI, H., ICHIKI, T., NARUSE, M. & INAGAMI, T. (1999). Vascular response to angiotensin II is exaggerated through an upregulation of AT1 receptor in AT2 knockout mice. Biochem. Biophys. Res. Commun., 258, 194198.
  • TEA, B.S., DER SARKISSIAN, S., TOUYZ, R.M., HAMET, P. & DEBLOIS, D. (2000). Proapoptotic and growth-inhibitory role of angiotensin II type 2 receptor in vascular smooth muscle cells of spontaneously hypertensive rats in vivo. Hypertension, 35, 10691073.
  • THORUP, C., KORNFELD, M., GOLIGORSKY, M.S. & MOORE, L.C. (1999). ATI receptor inhibition blunts angiotensin II-stimulated nitric oxide release in renal arteries. J. Am. Soc. Nephrol., 10, S220S224.
  • THORUP, C., KORNFELD, M., WINAVER, J.M., GOLIGORSKY, M.S. & MOORE, L.C. (1998). Angiotensin-II stimulates nitric oxide release in isolated perfused renal resistance arteries. Pflugers Archiv.–Eur. J. Physiol., 435, 432434.
  • TOUYZ, R.M., ENDEMANN, D., HE, G., LI, J.S. & SCHIFFRIN, E.L. (1999). Role of AT2 receptors in angiotensin II-stimulated contraction of small mesenteric arteries in young SHR. Hypertension, 33, 366372.
  • TSUTSUMI, Y., MATSUBARA, H., MASAKI, H., KURIHARA, H., MURASAWA, S., TAKAI, S., MIYAZAKI, M., NOZAWA, Y., OZONO, R., NAKAGAWA, K., MIWA, T., KAWADA, N., MORI, Y., SHIBASAKI, Y., TANAKA, Y., FUJIYAMA, S., KOYAMA, Y., FUJIYAMA, A., TAKAHASHI, H. & IWASAKA, T. (1999). Angiotensin II type 2 receptor overexpression activates the vascular kinin system and causes vasodilation. J. Clin. Invest., 104, 925935.
  • TSUTSUMI, Y., MATSUBARA, H., OHKUBO, N., MORI, Y., NOZAWA, Y., MURASAWA, S., KIJIMA, K., MARUYAMA, K., MASAKI, H., MORIGUCHI, Y., SHIBASAKI, Y., KAMIHATA, H., INADA, M. & IWASAKA, T. (1998). Angiotensin II type 2 receptor is upregulated in human heart with interstitial fibrosis, and cardiac fibroblasts are the major cell type for its expression. Circ. Res., 83, 10351046.
  • TSUZUKI, S., EGUCHI, S. & INAGAMI, T. (1996a). Inhibition of cell proliferation and activation of protein tyrosine phosphatase mediated by angiotensin II type 2 (AT2) receptor in R3T3 cells. Biochem. Biophys. Res. Commun., 228, 825830.
  • TSUZUKI, S., MATOBA, T., EGUCHI, S. & INAGAMI, T. (1996b). Angiotensin II type 2 receptor inhibits cell proliferation and activates tyrosine phosphatase. Hypertension, 28, 916918.
  • UNGER, T. (1999). The angiotensin type 2 receptor: variations on an enigmatic theme. J. Hypertens., 17, 17751786.
  • VAN KESTEREN, C.A., VAN HEUGTEN, H.A., LAMERS, J.M., SAXENA, P.R., SCHALEKAMP, M.A. & DANSER, A.H. (1997). Angiotensin II-mediated growth and antigrowth effects in cultured neonatal rat cardiac myocytes and fibroblasts. J. Mol. Cell. Cardiol., 29, 21472157.
  • VARAGIC, J., SUSIC, D. & FROHLICH, E.D. (2001). Coronary hemodynamic and ventricular responses to angiotensin type 1 receptor inhibition in SHR: interaction with angiotensin type 2 receptors. Hypertension, 37, 13991403.
  • VISWANATHAN, M. & SAAVEDRA, J.M. (1992). Expression of angiotensin II AT2 receptors in the rat skin during experimental wound healing. Peptides, 13, 783786.
  • VISWANATHAN, M., TSUTSUMI, K., CORREA, F.M. & SAAVEDRA, J.M. (1991). Changes in expression of angiotensin receptor subtypes in the rat aorta during development. Biochem. Biophys. Res. Communi., 179, 13611367.
  • WANG, Z.Q., MILLATT, L.J., HEIDERSTADT, N.T., SIRAGY, H.M., JOHNS, R.A. & CAREY, R.M. (1999). Differential regulation of renal angiotensin subtype AT1A and AT2 receptor protein in rats with angiotensin-dependent hypertension. Hypertension, 33, 96101.
  • WANG, Z.Q., MOORE, A.F., OZONO, R., SIRAGY, H.M. & CAREY, R.M. (1998). Immunolocalization of subtype 2 angiotensin II (AT2) receptor protein in rat heart. Hypertension, 32, 7883.
  • WHARTON, J., MORGAN, K., RUTHERFORD, R.A., CATRAVAS, J.D., CHESTER, A., WHITEHEAD, B.F., DELEVAL, M.R., YACOUB, M.H. & POLAK, J.M. (1998). Differential distribution of angiotensin AT2 receptors in the normal and failing human heart. J. Pharmacol. Exp. Therapeutics, 284, 323336.
  • WIDDOP, R.E., GARDINER, S.M., KEMP, P.A. & BENNETT, T. (1992). Inhibition of the haemodynamic effects of angiotensin II in conscious rats by AT2-receptor antagonists given after the AT1-receptor antagonist, EXP 3174. Bri. J. Pharmacol., 107, 873880.
  • WIDDOP, R.E., MATROUGUI, K., LEVY, B.I. & HENRION, D. (2002). AT2 receptor-mediated relaxation is preserved after long-term AT1 receptor blockade. Hypertension, 40, 516520.
  • WIEMER, G., SCHOLKENS, B.A., BUSSE, R., WAGNER, A., HEITSCH, H. & LINZ, W. (1993). The functional role of angiotensin II-subtype AT2-receptors in endothelial cells and isolated ischemic rat hearts. Pharm. Pharmacol. Lett., 3, 2427.
  • WOLF, G., WENZEL, U., BURNS, K.D., HARRIS, R.C., STAHL, R.A. & THAISS, F. (2002). Angiotensin II activates nuclear transcription factor-kappaB through AT1 and AT2 receptors. Kidney Int., 61, 19861995.
  • WONG, P.C., CHRIST, D.D. & TIMMERMANS, P.B. (1992). Enhancement of losartan (DuP 753)-induced angiotensin II receptor antagonism by PD123177 in rats. Eur. J. Pharmacol., 220, 267270.
  • WU, L., IWAI, M., NAKAGAMI, H., CHEN, R., SUZUKI, J., AKISHITA, M., DE GASPARO, M. & HORIUCHI, M. (2002). Effect of angiotensin II type 1 receptor blockade on cardiac remodeling in angiotensin II type 2 receptor null mice. Arteriosclerosis, Thrombosis Vasc. Biol., 22, 4954.
  • WU, L., IWAI, M., NAKAGAMI, H., LI, Z., CHEN, R., SUZUKI, J., AKISHITA, M., DE GASPARO, M. & HORIUCHI, M. (2001). Roles of angiotensin II type 2 receptor stimulation associated with selective angiotensin II type 1 receptor blockade with valsartan in the improvement of inflammation-induced vascular injury. Circulation., 104, 27162721.
  • XU, J., CARRETERO, O.A., LIU, Y.H., SHESELY, E.G., YANG, F., KAPKE, A. & YANG, X.P. (2002). Role of AT2 receptors in the cardioprotective effect of AT1 antagonists in mice. Hypertension, 40, 244250.
  • XU, Y., CLANACHAN, A.S. & JUGDUTT, B.I. (2000). Enhanced expression of angiotensin II type 2 receptor, inositol 1,4, 5-trisphosphate receptor, and protein kinase cepsilon during cardioprotection induced by angiotensin II type 2 receptor blockade. Hypertension, 36, 506510.
  • YAMADA, T., AKISHITA, M., POLLMAN, M.J., GIBBONS, G.H., DZAU, V.J. & HORIUCHI, M. (1998). Angiotensin II type 2 receptor mediates vascular smooth muscle cell apoptosis and antagonizes angiotensin II type 1 receptor action: an in vitro gene transfer study. Life Sci., 63, L2894295.
  • YAMADA, T., HORIUCHI, M. & DZAU, V.J. (1996). Angiotensin II type 2 receptor mediates programmed cell death. Proc. Natl. Acad. Sci. U.S.A., 93, 156160.
  • YANG, Z., BOVE, C.M., FRENCH, B.A., EPSTEIN, F.H., BERR, S.S., DIMARIA, J.M., GIBSON, J.J., CAREY, R.M. & KRAMER, C.M. (2002). Angiotensin II type 2 receptor overexpression preserves left ventricular function after myocardial infarction. Circulation, 106, 106111.
  • ZARAHN, E.D., YE, X., ADES, A.M., REAGAN, L.P. & FLUHARTY, S.J. (1992). Angiotensin-induced cyclic GMP production is mediated by multiple receptor subtypes and nitric oxide in N1E-115 neuroblastoma cells. J. Neurochem., 58, 19601963.
  • ZHAO, Y., BIERMANN, T., LUTHER, C., UNGER, T., CULMAN, J. & GOHLKE, P. (2003). Contribution of bradykinin and nitric oxide to AT2 receptor-mediated differentiation in PC12 W cells. J. Neurochem., 85, 759767.
  • ZHUO, J., ALLEN, A.M., ALCORN, D., ALDRED, G.P., MACGREGOR, D.P. & MENDELSOHN, F.A. (1995). The distribution of angiotensin II receptors. In: Hypertension: Pathophysiology, Diagnosis and Management. ed. Laragh, J.H. & Brenner, B.M. pp. 17391762. New York: Raven Press, Ltd..
  • ZHUO, J., DEAN, R., MACGREGOR, D., ALCORN, D. & MENDELSOHN, F.A. (1996). Presence of angiotensin II AT2 receptor binding sites in the adventitia of human kidney vasculature. Clin. Exp. Pharmacol. Physiol.–Suppl., 3, S1478154.
  • ZWART, A.S., DAVIS, E.A. & WIDDOP, R.E. (1998). Modulation of AT1 receptor-mediated contraction of rat uterine artery by AT2 receptors. Br. J. Pharmacol., 125, 14291436.