α-Adrenoceptor antagonism by apoyohimbine and some observations on the pharmacology of α-adrenoceptors in the rat anococcygeus and vas deferens

Abstract

1 α-Adrenoceptor antagonism of several test drugs was assessed against adrenergic contractile responses to field stimulation in rat vas deferens and anococcygeus, the prejunctional inhibitory effect of xylazine in vas deferens and the contractile effects of α-adrenoceptor agonists in anococcygeus. 2 Against the adrenergic nerve-induced contraction in vas deferens, the potency series was WB 4101> prazosin > apoyohimbine > corynanthine > yohimbine > rauwolscine. 3 Against the inhibitory effect of xylazine in vas deferens the potency series was apoyohimbine > rauwolscine = yohimbine > WB 4101 > prazosin and corynanthine. 4 In anococcygeus, against the contractile responses to adrenergic nerve stimulation or to the agonists amidephrine, noradrenaline and xylazine, the potency series was apoyohimbine > corynanthine > rauwolscine. 5 These results show that apoyohimbine is more potent than the yohimbine sterioisomers as an antagonist at α₁₋ and α₂-adrenoceptors but is no more selective. 6 The assay methods employed confirm the current classification of ‘α’-receptors and drugs.