Identification of a novel endothelin receptor in Xenopus laevis liver

Abstract

Membranes prepared from Xenopus liver displayed high density of high affinity endothelin (ET) binding sites. These sites have the same affinity for [ 125I] ET-1 and [ 125I] ET-3. Scatchard analysis of the specific binding from saturation binding experiments revealed an apparent dissociation constant (K d) of 93.1 and 70.9 pM and maximum binding (B max) of 602 and 651 fmol/mg protein for [ 125I] ET-1 and [ 125I] ET-3, respectively. Competition binding experiments using [ 125I] ET-1 and unlabelled ET-1, ET-3, S6c, and BQ123 indicated that ET-1 and ET-3 were the most potent in displacing [ 125I] ET-1 binding from these membranes (IC 501 and 0.3 nM, respectively), whereas S6c BQ123, selective for ET B and ET A receptors, respectively, did not have any inhibitory effect up to 1 μM. These data clearly indicate that the ET receptors present in Xenopus liver membranes belong to a new subtype of ET receptor. Because it resembled mammalian ET B receptors in its affinities for ET-1 and ET-3, we propose that this receptor be called the ET BX receptor.