Evidence for the presence of vascular dopamine receptors in the rabbit

In vitro experiments, using dopamine-induced relaxations of several pre-contracted rabbit blood vessels, confirmed the presence of renal dopamine receptors and suggested the existence of similar receptors in the portal vein and aorta. With jejunal muscle, however, dopamine's (spontaneous) relaxant effect was mediated by beta-adrenoceptors. The relaxant responses in the blood vessels were small, and quantitative information proved difficult to obtain. Therefore, a method of measuring alpha-adrenoceptor mediated contractile responses to dopamine was developed. Comparison of the respective antagonism of these responses produced by an alpha-adrenoceptor antagonist, phentolamine, and a dopamine receptor antagonist, haloperidol, showed differences between vascular and intestinal smooth muscle. The results suggest that this method is able to detect the presence of dopamine receptors in isolated tissues.