Postjunctional alpha-adrenoceptors in the isolated saphenous vein of the rabbit. Characterization and influence of angiotensin.
Experiments on the isolated saphenous vein of the rabbit have been performed in order to determine whether a coexistence of postjunctional alpha 1- and alpha 2-adrenoceptor subtypes can be demonstrated also under in vitro conditions. Prazosin, selective for alpha 1-receptors, and rauwolscine, selective for alpha 2-receptors, were used to antagonize the contractile response of the agonists phenylephrine (alpha 1), B-HT 920 (alpha 2) and noradrenaline (alpha 1/alpha 2). Each of the antagonists was equipotent against all three agonists; the effect of neither antagonist fulfilled the criteria for a competitive antagonism. The preferential alpha 1-receptor antagonist phenoxybenzamine, applied for irreversible blockade of alpha-receptors, reduced the effect of phenylephrine and B-HT 920 to the same degree. Thus, the results obtained with alpha-receptor antagonists cannot be reconciled with the coexistence of alpha 1- and alpha 2-receptors or the existence of only one of them. Contractile responses induced by alpha 1- and alpha 2-receptor agonists, respectively, could be differentiated by the calcium entry blocker nitrendipine. The effect of B-HT 920 was decreased whereas that of phenylephrine was hardly affected. Furthermore, we investigated the influence of angiotensin II on the effects of the alpha-adrenoceptor agonists as well as the antagonists. We observed firstly, that angiotensin, acting postsynaptically, potentiates the contractile response of certain alpha 2-receptor agonists and secondly, that in the presence of angiotensin the characteristics of the receptors as revealed by B-HT 920 are converted to typical alpha 2-adrenoceptors. It is concluded that the postjunctional alpha 2-receptors of the saphenous vein require the blood borne substance angiotensin for their expression.