Male lower urinary tract symptoms and α1D-adrenoceptors

Authors


Correspondence: Osamu Ishizuka M.D., Department of Urology, Shinshu University School of Medicine, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan. Email: ishizuk@shinshu-u.ac.jp

Abstract

Historically, α1-adrenoceptors have been classified into three subtypes (α1A, α1B and α1D) that are widely distributed in various organs. Research on the α1D-adrenoceptors in the bladder, urethra and prostate has focused on the relationship between expression levels and symptoms of bladder outlet obstruction, and the implications and functional roles of α1D-adrenoceptors subtypes in these organs. The α1D-adrenoceptor messenger ribonucleic acid and protein seem to be increased in obstructed bladders or small capacity bladders. In contrast, α1D-adrenoceptor subtype knock-out mice have been found to have a prolonged voiding interval. Interestingly, an α1D-adrenoceptor antagonist was found to inhibit the facilitation of afferent nerve activity for the micturition reflex induced by intravesical infusion of acetic acid. Clinically, patients who felt urgency at low filling volumes and had a small bladder capacity were found to have more α1D-adrenoceptor messenger ribonucleic acid in their bladder mucosa than patients who felt urgency at high filling volumes and had a large bladder capacity. An α1D-adrenoceptor antagonist was found to increase the first desired volume and the maximum desired volume while decreasing detrusor overactivity in pressure flow studies. Thus, α1D-adrenoceptors in the lower urinary tract might play an important role in the pathophysiology of lower urinary tract disorders.

Abbreviations & Acronyms
AR

adrenoceptor

ATP

adenosine 5'-triphosphate

BOO

bladder outlet obstruction

BPO

benign prostatic obstruction

FDV

first desired volume

FVC

frequency volume chart

IPSS

international prostate symptom score

KO

knock-out

LUTS

lower urinary tract symptom

mRNA

messenger ribonucleic acid

OVX

ovariectomy

PFS

pressure flow study

RT–PCR

reverse transcriptase polymerase chain reaction

SDV

strong desired volume

SHR

spontaneous hypertensive rats

TRPM8

transient receptor potential channel melastatin member 8

WT

wild-type

Basic studies

α1-Adrenoceptor subtypes

Drugs targeting AR producing activation and/or inhibition are some of the most widely used therapeutic agents in clinical medicine. AR bind and are activated by the endogenous catecholamines, adrenaline and noradrenaline. α1-AR subtype cDNA encoding three α1-AR subtypes (α1A, α1B and α1D) have been cloned and characterized pharmacologically.[1-6] The α1D-AR have been shown to have 10 to 100-fold higher affinity for the endogenous neurotransmitters, norepinephrine and epinephrine, compared with the α1A- or α1B-AR subtypes.[6] This finding provides a potentially important mechanistic rationale for targeting α1D-ARs when treating LUTS.

Expression of α1-AR subtypes in animal models and afferent nerve activity modulation through α1D-AR in voiding function

In some animal models associated with BOO, hypertension and aging have been found to complicate urinary bladder dysfunction.[7-10] In each of these models, an elevated expression level of α1D-AR mRNA or α1D-AR protein has been shown (Fig. 1). To counteract these pathological changes, some of which are, at least partly, caused by α1D-AR stimulation, it is reasonable to assume that an α1D-AR antagonist could be useful.

Figure 1.

The expression level of α1D-AR mRNA or α1D-AR protein increases by bladder outlet obstruction, hypertension and/or aging. This might be related to the male lower urinary symptoms, and α1D-AR might become the target of the therapy.

Expression of α1-AR subtype in animal models

In animal studies, Hampel et al. found that BOO produced a sixfold increase in bladder weight versus sham operation, and significantly increased voiding frequency.[11] Although the bladder α1-AR density did not increase overall, striking changes in α1-AR subtype expression were shown. In control animals, 70% of α1-AR mRNA was the α1A subtype, 5% was α1B and 25% was α1D, whereas in obstructed animal bladders, α1-AR expression changed to 23% α1A, 2% α1B and 75% α1D. Changes in α1-AR mRNA expression were of similar magnitude throughout the bladder dome, mid body and base. Parallel changes were also evident at the protein level, with 100% α1A-AR expression in control animals changing to 36% (mean value) in animals with a fivefold or greater increase in bladder weight. The data of Hampel et al. are suggestive, but whether a change in α1D density could have functional consequences was not investigated. However, it has been shown pharmacologically that elevated urinary frequency in obstructed rats was decreased with tamsulosin (α1A/1D antagonist), but not with 5-Me-urapidil (α1A antagonist).[12] Furthermore, Barendrecht et al. reported that relaxation responses to the endogenous agonist noradrenaline through β-AR are turned into α1-AR-mediated contraction responses in BOO, possibly as a result of upregulation of α1D-AR.[10] These findings support the hypothesis that the α1D-ARs are mechanistically involved in the development of storage symptoms, and they are plausible targets for therapeutic interventions to achieve a stable bladder condition and control storage symptoms. Recently, changes in the distribution of the α1-AR subtype in the urinary bladder have been shown in a rat BOO model.[13] The study was carried out with a sham group and a BOO group to evaluate the impact of BOO on α1-AR expression. Although there was no major difference in weight between the sham and BOO groups, urinary bladder weight was higher in the BOO group (0.76 g) than in the sham group (0.11 g). The expression of α1D-AR subtype was higher in the urothelium in the BOO group than in the sham group. The expression of α1A-AR mRNA was markedly reduced in the BOO group (0.57) compared with the sham group (2.43). In contrast, the expression of α1D-AR mRNA was notably higher in the BOO group (1.79) than in the sham group (0.71). An actual difference in the α1-AR subtype between the smooth muscle layers and the urothelium could not be detected.

Persson et al. showed that bladder function in SHR differed from that in control rats, and was characterized by a decrease in bladder capacity and micturition volume, as well as by an increase in non-voiding bladder contractions.[7] The results have furthermore suggested differences in smooth muscle and neuronal responsiveness to norepinephrine between SHR and control rats. The distribution of α1-AR subtype in the urinary bladder of SHR has been reported; rats were assigned to two groups, and the reference group was fed a normal chow diet, whereas another group was fed experimental chow that contained 8% NaCl.[14] Blood pressure increased slightly depending on the NaCl diet. While the expression of α1D-AR subtype was increased in the NaCl diet group, the expression of α1A-AR subtype was dramatically decreased in both bladder urothelium and smooth muscle layers. The expression level of α1D-AR mRNA was similarly increased, and that of α1A was decreased. These findings suggest that the dynamics of α1-AR expression could partly contribute to bladder function.

Using rat bladder, Dmitrieva et al. investigated whether aging affects expression of α1D-AR and whether α1D-AR mediates contraction.[9] Immunofluorescent staining for α1D-AR was detected in sections of the urothelium. Furthermore, western blotting confirmed more α1D-AR in the bladder in aged rats than in young rats. Moreover, phenylephrine increased intravesical pressure in a concentration-dependent manner in both young and aged rats, and the effect of phenylephrine was significantly greater in aged than in young rats.

It was also found that prazosin and BMY7378, an α1D-AR antagonist, decreased the contractile response induced by phenylephrine in a concentration-dependent manner in aged and young rats. BMY7378 produced significantly greater inhibition in aged rats, whereas prazosin did not. These findings suggest that α1D-AR was overexpressed in aged rat bladder, resulting in enhancement of the contractile response.

Voiding function and afferent nerve activity modulation through α1D-AR

To examine whether a molecule is really important in some physiological responses, animal genetic models might have advantages. Two groups recently reported that the α1D-AR subtype plays a unique role in voiding in α1D-AR KO mice. Chen et al. showed that the α1D-AR subtype has an important role in regulating bladder function.[15] Mean daily voiding frequency was significantly lower in α1D-AR KO mice (9 times) than in WT mice (16 times). Mean volume per void was significantly larger in α1D-AR KO mice than in WT mice. Similarly, cystometric analysis showed larger bladder capacity (140%) and voided volume (146%) in α1D-AR KO mice than in WT mice. Furthermore, Wang et al. suggested that locally-released noradrenaline activates urothelial α1D-AR and affects urinary bladder function.[16] Cystometry using WT mice showed that intravesical infusion of noradrenaline into the urinary bladder shortened the intercontraction interval in a dose-dependent manner, without changing the maximum voiding pressure. In contrast, α1D-AR KO mice showed no change in the intercontraction interval in response to intravesical infusion of noradrenaline. These findings indicate a predominant involvement of α1D-AR in the facilitation of the micturition reflex by noradrenaline.

Ishihama et al. showed that α1D-ARs were expressed in the urothelium of the rat bladder with western blotting and immunohistochemistry, and that inhibition of these receptors affects reflex voiding through an afferent nerve decrease.[17] The rather selective α1D-AR antagonist, naftopidil, prolonged the intercontraction interval during continuous infusion cystometrograms in conscious rats (143% of the control value) and suppressed the excitatory effect of intravesical infusion of 0.1% acetic acid on the intercontraction interval (220%). Naftopidil inhibited the bladder afferent nerve activity induced by bladder distension (32%) and acetic acid infusion (30%), and decreased ATP levels in the bladder perfusate during bladder distention (37%). Sugaya et al. also reported the effects of naftopidil on the urinary ATP level and bladder activity after bladder stimulation in rats using continuous cystometry with 0.1% acetic acid.[18] The shortened interval between bladder contractions with the acetic acid solution was recovered with naftopidil treatment. The urinary ATP level increase caused by the infusion of acetic acid was less with naftopidil. Therefore, naftopidil's inhibitory effect on bladder activity might be partly a result of blocking ATP release from the bladder epithelium.

A relationship between sensory nerve activation and an increased voiding reflex response is well known. Cold stress was found to significantly decrease the voiding interval, micturition volume and bladder capacity in conscious rats.[19] TRPM8 expression on the skin partly mediated the micturition responses.[20] But how does α1D-AR affect the aforementioned relationship? A novel and interesting finding associated with cold stress stimulation was recently reported using OVX rats,[21] which have been shown to have a decreased sensory afferent nerve threshold. Cold stress stimulates the skin TRPM8. The TRPM8-positive area in the skin was significantly higher (twofold) in OVX rats than in sham rats. In the OVX rats, the voiding interval was shortened from 3.0 min to 2.0 min, and the bladder capacity was smaller, from 0.73 mL to 0.39 mL under the low temperature condition as compared with the sham rats. Furthermore, naftopidil, an α1D-AR antagonist, blocked the OVX-induced effects (voiding interval, 4.7 min; bladder capacity, 0.83 mL). Therefore, the increased TRPM8 in OVX rats might result in α1D-AR mediated detrusor overactivity induced by cold stress under afferent nerve activation.

Clinical studies

α1-AR subtypes in the human lower urinary tract

Historically, Malloy et al. found species heterogeneity in α1-AR subtype expression (human vs rat), with α1D predominating in the human detrusor.[22] Total α1-AR expression in the human bladder was 6 fmol/mg total protein. Although significantly less than the α1-AR density in the human prostate, bladder α1-AR expression is threefold higher than the density in the human coronary artery.[23] At a subtype level, α1D-AR are twice as abundant as α1A-AR in the human detrusor at both the mRNA and protein levels; no α1B-AR were found in the human detrusor. Overall α1D-AR expression in human tissue is more limited than that of other α1-AR subtypes.[24]

Functional responses of α1-AR in obstructed bladder

Nomiya et al. found that α1-AR were expressed at low levels in human bladder, and α1D-AR mRNA was increased 1.8-fold in obstructed patients compared with the control group, but the difference was not significant.[25] Their functional study showed that phenylephrine at concentrations up to 10−4 mol/L produced no contractile response in obstructed or control bladders. However, Chapple et al. showed that an α1 agonist produced responses in six of 11 patients with overactive detrusor bladder specimens.[26] Furthermore, Bouchelouche et al. showed significant contractile responses to phenylephrine in preparations from BOO bladders.[27] In contrast, α1 agonist responses were slight in all non-obstructed bladders, although potassium-induced contractions in these tissue strips were similar to those in BOO specimens. Another interesting finding was that phenylephrine induced contractile oscillations and tonic contractions in BOO preparations, whereas each type of contraction was dose-dependently inhibited by tamsulosin. Such inhibition was also obtained with the α1D-AR antagonist, BMY7378, by shifting the phenylephrine-induced dose–response curve. These results and previous animal investigations[11] support the hypothesis that α1D-AR might be important in storage symptoms associated with male LUTS.

The correlation between bladder expression of α1-AR and sensation in patients has been unclear. A recent investigation showed expression of α1-AR mRNA in the bladder mucosa of men with LUTS and BPO, and the association of α1-AR mRNA with urodynamic parameters during storage of experimentally-infused contrast medium.[28] Expressions of α1A- and α1B-AR mRNA in bladder mucosa from two groups (group 1: FDV ≤200 mL and/or SDV ≤300 mL; group 2: FDV ≥201 mL and/or SDV ≥301 mL) showed no significant differences between the groups with respect to α1A- and α1B-AR mRNA levels. However, mucosa from the first group of patients had significantly more α1D-AR mRNA than did that from the latter group of patients. There seem to be many molecular causes of storage symptoms,[29-32] not all necessarily involving urothelial α1-AR. Nevertheless, the finding of a relationship between urodynamic sensory parameters and the expression levels of urothelial α1-AR mRNA suggests that α1D-AR might play an important role in storage symptoms in male LUTS patients.

What is the role of the α1D-AR in the human bladder?

As the non-selective α1-AR antagonist, terazosin, relieves nocturia, as assessed by FVC,[33] one (or more) α1-AR subtypes must be responsible for this effect. Both tamsulosin, which is an α1A/1D-AR selective antagonist (rather α1A-AR selective), and naftopidil, which is an α1D/1A-AR selective antagonist (rather α1D-AR selective), reduce the nocturnal frequency in FVC by decreasing nocturnal urine volume.[34, 35] By contrast, silodosin, which is an α1A-AR highly-selective antagonist, does not decrease the number of night-time voidings in FVC.[36] Then, is there evidence for α1D-AR-related LUTS that was obtained by comparing the effects of a rather α1A-AR selective antagonist and a rather α1D-AR selective antagonist on bladder storage function? Nishino et al. reported interesting evidence from a PFS by comparing the two drugs in a randomized, cross-over design.[37] This study was carried out with an average prostate size of 20 mL in patients with severe symptoms (total IPSS ≥20). Tamsulosin and naftopidil caused no significant difference in voiding symptoms or total IPSS, but relief of storage symptoms, especially nocturia, was significantly greater with naftopidil. In PFS, moreover, the increases in the maximum desired volume and the first desired volume were higher with naftopidil than with tamsulosin. In seven subjects who showed disappearance of involuntary contractions, disappearances were found in five subjects in each cross-over period. In one subject, however, involuntary contractions disappeared during the first naftopidil period, but they returned after the switch to tamsulosin. In another subject, the cross-over to naftopidil resulted in the disappearance of involuntary contractions, although the contractions continued with tamsulosin. In addition, both drugs decreased BOO grade. Kakizaki et al. have also investigated detrusor overactivity using a filling cystometry procedure in BPH patients with total IPSS storage symptom scores (frequency, urgency and nocturia) ≥7.[38] In nine patients who had detrusor overactivity before naftopidil treatment, filling cystometry was repeated after treatment. Bladder volume at first desire to void increased significantly from 174 ± 92 mL to 259 ± 109 mL. These investigations support the suggestion that α1D-AR might play an important role, at least in part, in sensory afferent nerve activity.

α1A- or α1D-AR subtype dominant in patients with BPH

Initial studies examining the α1-AR subtype in human prostate using RNase protection assays and in situ hybridization approaches showed that α1A-AR predominates at the RNA level.[39] However, recent evidence showed that, in addition to α1A-AR, the α1D-AR subtype was also present to a significant extent in the human prostate using the real time RT–PCR procedure.[40] This study showed that the median expression levels (interquartile range) of α1A-AR and α1D-AR were 1.25 (0.66–2.45) and 1.18 (0.71–2.27) × 1000 copies/β-actin, respectively, with no significant difference. Furthermore, the ratio of the mean expression level of each subtype to total α1-AR was 41.2% and 49.1% for α1A- and α1D-AR mRNA, respectively. The correlation between the expression of α1-AR subtype mRNA in the prostate and the clinical efficacy of subtype-selective α1-AR antagonists was also examined.[41] Patients who did not have malignant tumors based on prostate biopsy results were divided into two groups and given either tamsulosin or naftopidil. The efficacy of tamsulosin and naftopidil differed depending on the dominant α1-AR subtype in the prostate. Tamsulosin was more effective in patients with dominant expression of the α1A-AR subtype, whereas naftopidil was more effective in those with dominant expression of the α1D-AR subtype. Although this theory might not be clinically applicable to all BPH patients because of the inconvenience of biopsy, the α1-AR subtype mRNA expression level in the prostate could be a predictor of the efficacy of subtype selective α1-AR antagonists. Genetic differences might be responsible for the diverse responses to these drugs.

Conclusion

There is evidence that targeting only α1A-AR might not provide comprehensive therapy for LUTS associated with BPH/BPO. The presence of α1D-AR in the lower urinary tract suggests that this subtype might play an important role in the pathophysiology of male LUTS.

Conflict of interest

None declared.

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