Alpha 1- and alpha 2-adrenoceptors in the smooth muscle of isolated rabbit urinary bladder and urethra.

The present experiment was undertaken to determine the existence of alpha 1- and alpha 2-adrenoceptors in the smooth muscle of the rabbit bladder dome, trigone and proximal urethra. In the dome pretreated with propranolol (10(-6) M), phenylephrine (10(-5) M-10(-3) M) and norepinephrine (10(-7) M-10(-5) M) caused only a small contraction but norepinephrine, only at high concentrations (10(-4) M-10(-3) M), produced a small relaxation. Clonidine, however, had no effect on the dome. In both trigone and urethra, phenylephrine, clonidine and norepinephrine caused dose-dependent contractions. The contractile response to phenylephrine or norepinephrine was significantly greater than that to clonidine in the trigone but no such difference was observed in the urethra. Prazosin (10(-8) M-10(-6) M, alpha 1-adrenoceptor antagonist) produced a rightward shift of the phenylephrine and clonidine dose-response curve in both the trigone and urethra. Yohimbine (10(-8) M-10(-6) M, alpha 2-adrenoceptor antagonist) inhibited the response to clonidine without significantly affecting the responses to phenylephrine. These studies indicated that alpha 1- and alpha 2-adrenoceptors are present in both the trigone and proximal urethra. In the dome, only alpha 1-adrenoceptors are sparsely distributed.