1 The actions of raubasine, yohimbine and corynanthine at pre-and postsynaptic oa-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 a-adrenoceptors. 3 Presynaptic x-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 a-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 aadrenoceptors. Yohimbine is more potent in blocking pre-than postsynaptic a-adrenoceptors. The ratio of the pre/postsynaptic potency declines in the order yohimbine > raubasine > corynanthine.
1 Raubasine was compared with yohimbine and corynanthine in pithed rats. Antagonist activity at a1-adrenoceptors was evaluated on the pressor response to electrical stimulation of the spinal sympathetic outflow and to phenylephrine administration, both being reduced by raubasine in the dose range 1 to 4 mg/kg. Corynanthine was quantitatively similar, but yohimbine was not only less potent but also in doses of 0.125 to 0.5 mg/kg enhanced the effects of electrical stimulation. 2 Antagonist activity at 0(2-adrenoceptors was determined against the inhibitory effects of clonidine on tachycardia induced by electrical stimulation of cardiac sympathetic nerves and against the pressor responses to B-HT-933 injection. Raubasine up to 4 mg/kg, like corynanthine, did not affect the pressor responses to B-HT-933 nor did it reduce the inhibitory effect of clonidine. By contrast yohimbine reduced the response to BHT-933 and antagonized clonidine as well as enhancing the tachycardia caused by electrical stimulation. 3 The results indicate that, in vivo, raubasine, like corynanthine, is a selective antagonist at o1-adrenoceptors and that yohimbine is more potent in blocking M2-than ac-adrenoceptors.
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