Effects of prazosin and phentolamine on responses to field stimulation and to noradrenaline in vas deferens and anococcygeus muscle of the rat.

The effects of phentolamine (10(-6)M) and prazosin (10(-7)M) on contractile responses, in the presence of cocaine or nortriptyline, were investigated in vas deferens and anococcygeus muscle of rat. Cocaine (10(-5)M) and nortriptyline (10(-6)M) had no effect on maximum responses to field stimulation, (--)-noradrenaline, methoxamine, and acetylcholine in anococcygeus muscle. However, both cocaine (3.3 x 10(-5)M) and nortripthline (10(-6)M) potentiated maximum responses to (--)-noradrenaline without altering the maximum response to field stimulation in vas deferens. In vas deferens, phentolamine potentiated or inhibited the responses to field stimulation depending on the frequency whereas prazosin inhibited all responses to field stimulation. In anococcygeus muscle, both phentolamine and prazosin inhibited all responses to field stimulation. In vas deferens and anococcygeus muscle, prazosin was a more potent antagonist of responses to (--)-noradrenaline than phentolamine. These results show that prazosin is a more potent postsynaptic alpha-adrenoceptor antagonist than phentolamine and suggest that prazosin has little or no affinity for presynaptic alpha-adrenoceptors.