Identification of a novel endothelin receptor in Xenopus laevis liver

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 (Kd) of 93.1 and 70.9 pM and maximum binding (Bmax) 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 (IC50 1 and 0.3 nM, respectively), whereas S6c BQ123, selective for ETB and ETA receptors, respectively, did not have any inhibitory effect up to 1 microM. 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 ETB receptors in its affinities for ET-1 and ET-3, we propose that this receptor be called the ETBX receptor.