PRE‐ AND POSTSYNAPTIC α‐ADRENOCEPTOR BLOCKING ACTIVITY OF RAUBASINE IN THE RAT VAS DEFERENS

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Abstract
1 The actions of raubasine, yohimbine and corynanthine at pre- and postsynaptic α-adrenoceptors were studied in the rat vas deferens. 2 Low frequency electrical stimulation of the isolated vas deferens of the rat produced regular contractions that were inhibited by low concentrations of clonidine. This inhibition was presynaptic in origin and involved α-adrenoceptors. 3 Presynaptic α-adrenoceptor antagonist activity was assessed by studying the effect of increasing antagonist concentrations on cumulative clonidine dose-response curves on the stimulated vas deferens. 4 Postsynaptic α-adrenoceptor antagonist activity in the isolated vas deferens was assessed by comparing control cumulative noradrenaline dose-response curves in the absence and in the presence of increasing concentrations of antagonists. 5 The results indicate that raubasine and corynanthine preferentially block postsynaptic α-adrenoceptors. Yohimbine is more potent in blocking pre- than postsynaptic α-adrenoceptors. The ratio of the pre/postsynaptic potency declines in the order yohimbine > raubasine > corynanthine.