Postjunctional alpha-adrenoceptors. Alpha 1 and alpha 2 subtypes in rat vasculature in vitro and in vivo.

The postsynaptic alpha-adrenoceptors in rat aorta and in pithed rat were investigated according to their sensitivity to nine alpha-adrenergic agonists and to the selective antagonists yohimbine (alpha 2) and prazosin (alpha 1) and the nonselective one, phentolamine. In addition, in radioligand binding studies, the affinity and selectivity of the drugs were determined on rat cerebral cortex using [3H] yohimbine and [3H] prazosin. On rat aorta, prazosin is 1,000 times more potent than yohimbine against each alpha-adrenoceptor agonist, whether alpha 1- or alpha 2-selective. Rat aorta probably contains only alpha 1-adrenoceptors. Pressor effects in pithed rats are mediated by post-junctional alpha 1- and alpha 2-adrenoceptors. The dose-response curve for alpha-methylnorepinephrine in the presence of prazosin, using Hofstee's plots, revealed alpha 1- and alpha 2-adrenoceptors, respective proportions being 80.5 and 19.5%.