IDENTIFICATION OF A NOVEL ENDOTHELIN RECEPTOR IN XENOPUS-LAEVIS LIVER

Membranes prepared from Xenopus liver displayed high density of high a ffinity endothelin (ET) binding sites. These sites have the same affin ity for [I-125] ET-1 and [I-125] ET-3. Scatchard analysis of the speci fic binding from saturation binding experiments revealed an apparent d issociation constant (K-d) of 93.1 and 70.9 pM and maximum binding (B- max) of 602 and 651 fmol/mg protein for [I-125] ET-1 and [I-125] ET-3, respectively. Competition binding experiments using [I-125] ET-1 and unlabelled ET-1, ET-3, S6c, and BQ123 indicated that ET-1 and ET-3 wer e the most potent in displacing [I-125] ET-1 binding from these membra nes (IC(50)1 and 0.3 nM, respectively), whereas S6c BQ123, selective f or ET(B) and ET(A) receptors, respectively, did not have any inhibitor y effect up to 1 mu M. These data clearly indicate that the ET recepto rs present in Xenopus liver membranes belong to a new subtype of ET re ceptor. Because it resembled mammalian ET(B) receptors in its affiniti es for ET-1 and ET-3, we propose that this receptor be called the ET(B X) receptor.