?1-Adrenoceptor subtype mediating contraction of the smooth muscle in the lower urinary tract and prostate of rabbits

Abstract

Summary The α-adrenoceptor subtype mediating contraction of the smooth muscle in the urinary bladder base (trigone), proximal urethra and prostate isolated from male rabbits was investigated by comparing the responsiveness to α-adrenoceptor agonists and antagonists under condition where β-adrenoceptors and neuronal and extraneuronal uptakes were inhibited. Noradrenaline (non-selective), phenylephrine (α₁-selective) and clonidine (α₂-selective) caused a dose-dependent contraction in the trigone, urethra and prostate. Phenylephrine acted as a full agonist whereas clonidine was a partial agonist. YM-12617 and prazosin (α₁-selective), phentolamine (non-selective) and yohimbine (α₂-selective) produced dose-dependent shifts to the right of the dose-response curves for noradrenaline, phenylephrine and clonidine in the all three tissues. YM-12617 (pA₂=9.77, 9.67 and 9.73 for trigone, urethra and prostate, respectively), prazosin (pA₂=8.26, 8.20 and 8.08), phentolamine (pA₂=7.67, 7.62 and 7.45) and yohimbine (pA₂=6.30, 6.30 and 5.94) showed constant pA₂ values irrespective of the agonists and tissues used, suggesting that only a single subclass of α-adrenoceptors is present. The actual pA₂ values for these antagonists are comparable to those reported previously in tissues said to contain mainly α₁-adrenoceptors. Thus, we concluded that the postsynaptic α-adrenoceptors of the rabbit trigone, urethra and prostate mediating contraction belong to the α₁-subtype.