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

Abstract

The alpha-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 alpha-adrenoceptor agonists and antagonists under condition where beta-adrenoceptors and neuronal and extraneuronal uptakes were inhibited. Noradrenaline (non-selective), phenylephrine (alpha 1-selective) and clonidine (alpha 2-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 (alpha 1-selective), phentolamine (non-selective) and yohimbine (alpha 2-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 alpha-adrenoceptors is present. The actual pA2 values for these antagonists are comparable to those reported previously in tissues said to contain mainly alpha 1-adrenoceptors. Thus, we concluded that the postsynaptic alpha-adrenoceptors of the rabbit trigone, urethra and prostate mediating contraction belong to the alpha 1-subtype.