HUMAN PLACENTAL MEMBRANES CONTAIN PREDOMINANTLY ET(B) RECEPTORS

The endothelin (ET) receptors on human placental membranes (HPMs), cou pled to fluomicrospheres, have been characterized by examining the bin ding of I-125-ET-1 in a scintillation proximity assay (SPA). Specific binding of I-125-ET-1 was potently inhibited by ET-1 (IC50: 80 pM), ET -3 (IC50: 170 pM), and the ET(B) receptor-selective agonists sarafotox in S6c (S6c; IC50: 210 pM) and alanine1,3,11,15-ET-1 (4-Ala-ET-1; IC50 :3.56 nM). In contrast, the ET(A) receptor-selective antagonist BQ123 (D-Val-Leu-D-Trp-D-Asp-Pro) only weakly (28% at 10 muM) inhibited I-12 5-ET-1 binding. In addition, the inhibition curves for ET-3 and 4-Ala- ET-I were shallow, with slopes less than unity, indicating binding-sit e heterogeneity. In keeping with this finding, the presence of a small population of ET(A) receptors was confirmed by the ability of BQ123 ( 1 muM) to reduce the maximum binding capacity of the HPMs for I-125-ET -1 by approximately 17%, without affecting the affinity for the radiol igand. In conclusion, these results suggest that the HPM-SPA system co ntains predominantly (approximately 80%) ET(B) receptors, with a small ET(A) receptor population. These findings should be taken into accoun t when this assay system is used to identify novel endothelin receptor ligands.