alpha-Adrenoceptor blocking properties of raubasine in pithed rats

1 Raubasine was compared with yohimbine and corynanthine in pithed rats. Antagonist activity at alpha 1-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 alpha 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 alpha 1-adrenoceptors and that yohimbine is more potent in blocking alpha 2-than alpha 1-adrenoceptors.