SELECTIVITY OF BLOCKING AGENTS FOR PRE‐ AND POSTSYNAPTIC α‐ADRENOCEPTORS

Abstract

1 Low frequency (0.1 Hz) electrical stimulation of the rat isolated vas deferens produced regular contractions that were inhibited by low concentrations of clonidine. 2 The inhibition of the vas deferens produced by clonidine was presynaptic in origin and involved α-adrenoceptors. 3 Presynaptic α-adrenoceptor antagonist activity was assessed by studying the effects of increasing concentrations of the antagonists on cumulative clonidine dose-response curves on the stimulated vas deferens. 4 Postsynaptic α-adrenoceptor antagonist activity was assessed by comparison of control cumulative noradrenaline dose-response curves with those in the presence of increasing concentrations of antagonists in the rat anococcygeus muscle. 5 The results indicate that yohimbine and phentolamine are more potent in blocking presynaptic than postsynaptic α-adrenoceptors. Phenoxybenzamine and prazosin block postsynaptic α-adrenoceptors preferentially. 6 The findings support the view that presynaptic and postsynaptic α-adrenoceptors differ in their sensitivity to α-adrenoceptor antagonists.