ETA AND ETB SPECIFIC LIGANDS SYNERGISTICALLY ANTAGONIZE ENDOTHELIN-1 BINDING TO AN ATYPICAL ENDOTHELIN RECEPTOR IN PRIMARY RAT ASTROCYTES

Using a whole-cell binding procedure with long incubations at low temp erature and subsequent acid stripping, we have characterized an atypic al endothelin (ET) receptor in primary rat cortical astrocyte cultures . We found the following: (a) no competition for I-125-ET-1 binding by the ETA antagonists BQ-123 and LU 135252 or the ETB agonist IRL 1620; (b) weak competition by the ETB antagonist BQ-788 and by the predomin ant ETB ligand ET-3; (c) potent synergistic competition of ETA and ETB ligands in combination for I-125-ET-1 binding; (d) potent competition of ET-1 with any of the radioligands used, I-125-ET-1, I-125-IRL 1620 , and [H-3]BQ-123; (e) lack of competition of IRL 1620 and BQ-123 with the respective other radioligand; (f) shifting of the amount of acid- strippable I-125-ET-1 binding from 20 to 80% by ETB ligands and to 4% by ETA ligands; and (g) as a control, typical ETA and ETB binding char acteristics of the RAT-I fibroblast and the U373MG astrocytoma cell li ne, respectively, under our assay conditions. The unusual binding prop erties of astrocytic ET receptors described in this study appear to be the result of several binding sites in the receptor for different ET ligands or ligand epitopes.