The effects of alpha-adrenoceptor agonists and antagonists on responses of transmurally stimulated prostatic and epididymal portions of the isolated vas deferens of the rat.

1 The effects of alpha-adrenoceptor agonists and antagonists on contractile responses of transmurally stimulated prostatic and epididymal portions of the rat isolated vas deferens were examined. 2 Responses to single stimuli consisted of two phases, the first predominant in the prostatic and the second in the epididymal portion. The first phase was resistant to alpha-adrenoceptor antagonists but the second was reduced in a dose-related manner in the order of potency prazosin greater than azapetine greater than phentolamine greater than labetalol greater than yohimbine. 3 Both phases of the response to a single stimulus were reduced by clonidine but only the first could be reliably restored by yohimbine. 4 Trains of transmural stimuli produced biphasic responses, an early rapid component predominant in the prostatic and a slow secondary component predominant in the epididymal portion. The effects of alpha-adrenoceptor antagonists on these responses were complex. Prazosin produced the most straightforward inhibition of responses with relative resistance of the early rapid component. Only yohimbine and phentolamine produced increases in responses which could be pre-junctional in origin. 5 The alpha-adrenoceptor agonists, oxymetazoline and clonidine, reduced while phenylephrine increased responses to trains of stimuli. 6 These results are discussed in relation to the nature of the innervation of rat vas deferens and the usefulness of the preparation in pharmacological tests for activity at alpha-adrenoceptors.