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. 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.
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. 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). 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. TRPM8 expression on the skin partly mediated the micturition responses. 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, 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.