Vasoconstriction mediated by postsynaptic alpha 2-adrenoceptor stimulation

The postsynaptic alpha-adrenoceptors involved in vasoconstriction brought about by B-HT 933 (2-amino-6-ethyl-4,5,7,8-tetrahydro-6H-oxazolo-[5,4-d]-azepin) administered i.v. to pithed, normotensive rats were characterized. The rate of onset of the hypertensive response to i.v. B-HT 933 is slower than that induced by (-)-phenylephrine, an agonist of alpha 1-adrenoceptors. The antagonism of the alpha-adrenoceptor blocking drugs rauwolscine, yohimbine and corynanthine was quantified towards B-HT 933-induced increases in diastolic pressure. Rauwolscine (pA2 = 7.06) and yohimbine (pA2 = 6.83) were effective antagonists, whereas corynanthine proved much less potent (pA2 = 5.03). On the basis of the reported selectivity of yohimbine and its two diastereoisomers rauwolscine and corynanthine for alpha 1- and alpha 2-adrenoceptors, it is concluded that the postsynaptic alpha-adrenoceptors triggered by B-HT 933 are of the alpha 2-type. B-HT 933 identifies a subclass of postsynaptic alpha 2-adrenoceptors in vascular smooth muscle distinct from postsynaptic alpha 1-adrenoceptors. Both types of alpha-adrenoceptors are likely to be involved in the mediation of vasoconstriction.