Characterization of receptors for angiotensin in the rat vas deferens

Abstract

Experiments were performed in the rat vas deferens to characterize the receptor for angiotensin mediating the potentiation of the sympathetic nerve stimulation by this peptide. For this purpose we measured the order of potency of various angiotensins, the affinity of two specific and competitive antagonists, and we compared the effects of several angiotensins in tissues desensitized by angiotensin II.The potency of natural angiotensins follows the order: AT_II > AT_I > AT_III the relative potency of a few analogues which resist degradation by proteolytic enzymes as well as the potency of three L-Ala analogues of AT_II show similar changes as those observed in other smooth muscle preparations (e.g. the rabbit aorta).Affinity of antagonists was evaluated by measuring pA₂ and is higher for [Leu⁸]-AT_II than for [des-Asp¹, Leu⁸]-AT_II. Both antagonists appear to be competitive since they displace the dose-response curves of AT_II and AT_III to the right without changing the slope of the curves. Desensitization with AT_II renders the tissues insensitive to AT_III and to other angiotensins without changing the response of the tissues to substance P.AT_II does not modify the action of exogenous NA on nonstimulated tissues. AT_I has no direct effect since its action is completely blocked in the presence of an inhibitor of the converting enzyme (SQ. 14225), while the responses of the vas deferens to AT_II and substance P are unaltered.It is concluded that the receptor for AT_II in the rat vas deferens is of the same type as the receptor mediating contraction of the rabbit aorta.