UNIQUE CONTRACTILE ACTION OF ENDOTHELINS ON PORCINE ISOLATED URETER AND CHARACTERIZATION OF THE ENDOTHELIN-BINDING SITES

Endothelin produced rhythmic contractions in porcine isolated ureter, which was sensitive to nitrendipine and EDTA. Dose response studies indicated that the rhythmic contractile response of the ureter became apparent at 1 nM and its frequency increased markedly with increasing concentrations of endothelin-1. Endothelin-2 and endothelin-3 were also equally effective. To characterize the endothelin-binding sites on the ureter, we carried out affinity labeling and competitive binding assays. Affinity labeling with I-125-endothelin-1 yielded one major (60 K) and two minor (36 K and 31 K) bands; when I-125-endothelin-3 was used, only 60 K and 31 K bands were labeled. Competitive binding assay using radiolabeled endothelin-1 and endothelin-3 revealed that endothelin-1 was much more effective than endothelin-3 in displacing I-125-endothelin-1; similarly, endothelin-3 was more potent than endothelin-1 in displacing I-125-endothelin-3, indicating that there are at least two distinct endothelin receptors in the ureter: one which is specific for endothelin-1 and the other specific for endothelin-3.