Identification of ET B receptor subtypes using linear and truncated analogs of ET

Abstract

Using canine spleen and lung membranes as model systems, we have shown the presence of subtypes of ET B receptors. This classification was done based on the binding profiles of various analogs of ET-1 that have been identified as ET B-selective. Saturation binding experiments performed with [ 125I] ET-3 and [ 125I] IRL-1260 (ET B-selective ligands) indicated that [ 125I] IRL-1620 labeled 80–90% of [ 125I]ET-3 binding sites in canine lung, whereas in canine spleen, the binding of [1251] IRL-1620 was 10–20% of [ 125I]ET-3 binding. In addition, competition binding experiments using ET B-selective agonists [Ala 1,3,11,15]-ET-1 (also known as 4-Ala ET-1), N-acetyl-[A1a 11,15] ET-1 (6-21) also known as BQ3020 and Suc-[Glu 9, Ala 11,15] ET-1 (8-21) also known as IRL-1620 indicated that all three ligands displaced [ 125I]ET-3 from canine lung membranes with ∼ 500−1000 fold greater affinity than canine spleen membranes. On the other hand, ET-1, ET-3, S6a, S6b, and S6d displayed very similar IC 50 values in both preparations, except S6c which was ∼20 fold less potent in canine spleen compared to lung. These data indicate that ET B receptors present in canine spleen are different from those present in lung and that ET B-selective linear as well as truncated analogs of ET-1 are good tools to identify these subtypes of ET B receptors.