Endothelin Receptor Heterogeneity; Structure Activity, Autoradiographic and Functional Studies

Abstract

Radioligand binding studies were used to define the binding constants for inhibition of [¹²⁵I]endothelin-1 binding by endothelin-1 (ET-1), endothelin-3 (ET-3) and a synthetic analogue of ET-1, [Ala^1,3,11,15]ET-1 in rat aorta and cerebellum. The inhibition curves for the three substances were similar in the cerebellum (Ki, ET-1 = ET-3 = [Ala^1,3,11,15]ET-1) but differed in the aorta (Ki, ET-1 <<< [Ala^1,3,11,15]ET-1) with the ET-3 inhibition curves being biphasic, indicating binding to two sites. In functional studies in the aorta, in contrast to findings in the cerebellum, [Ala^1,3,11,15]ET-1 was inactive and ET-3 was a partial agonist. These findings support the idea of heterogeneity of endothelin receptors. Autoradiographic studies were performed to identify the cellular localisation of endothelin-1 receptors in the two tissues. In the aorta binding was to all elements of the vasculature including the endothelium, smooth muscle, periadventitial connective tissue and nerves. In the cerebellum binding sites were located predominantly over the neuronal elements of the granular cell layer. The cellular localisation of the high and low affinity sites for ET-3, and the functional significance of the high affinity binding site for ET-3, remain to be determined.