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

Low frequency (0.1 Hz) electrical stimulation of the rat isolated vas deferens produced regular contractions that were inhibited by low concentrations of clonidine. The inhibition of the vas deferens produced by clonidine was presynaptic in origin and involved α‐adrenoceptors. 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. 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. 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. The findings support the view that presynaptic and postsynaptic α‐adrenoceptors differ in their sensitivity to α‐adrenoceptor antagonists.