PHARMACOLOGY OF [H-3]PROSTAGLANDIN E-1 [H-3]PROSTAGLANDIN E-2 AND [H-3]PROSTAGLANDIN F2-ALPHA BINDING TO EP3 AND FP PROSTAGLANDIN RECEPTOR-BINDING SITES IN BOVINE CORPUS-LUTEUM - CHARACTERIZATION AND CORRELATION WITH FUNCTIONAL DATA/

Specific binding of [H-3]prostaglandin (PG) E-1, [H-3]PGE(2) and [H-3] PGF(2 alpha) to washed total particulate homogenates of bovine corpus luteum comprised 60 to 82% of total binding. Scatchard analysis of com petition data revealed the presence of an apparent single population o f binding sites for [H-3]PGE(1) and [H-3]PGE(2) with dissociation cons tants (K(d)s) of 2.76 to 3.39 nM and apparent receptor density (B-max) of 1.5 to 1.56 pmol/g wet weight (n = 3-4). However, [H-3]PGF(2a) app eared to interact with two classes/states of binding sites (K-d1 = 6.5 1 +/- 0.65 nM, B-max1 = 2.33 +/- 0.26 pmol/g wet weight; K-d2 = 986 +/ - 269 nM; B-max2 = 44.8 +/- 11.3 pmol/g wet weight, n = 11). Specific [H-3]PGE(1) and [H-3]PGE(2) binding was most potently (nanomolar affin ity) inhibited by PGs with high selectivity for the EP3 receptor subty pe (e.g., GR63799, sulprostone, enprostil) but was weakly (K(i)s > 1 m u M) influenced by EP1-selective (SC-19220), FP-selective (fluprosteno l, PHXA85), DP-selective (BWA868C; ZK118182), IP-selective (iloprost) and TP-selective (U46619) PGs. Specific [H-3]PGF(2 alpha) binding was potently displaced by FP-selective agents such as fluprostenol, PHXA85 and cloprostenol with nanomolar affinities (n = 3-25), but weakly (K( i)s > 1 mu M) by other PGs showing high selectivity for other PG recep tor subtypes mentioned above. The relative specificities and potencies of EP3- and FP-selective PGs tested in the binding assays were confir med using various functional assays. These studies have provided stron g pharmacological evidence for the similarity of [H-3]PGE, and [H-3]PG E(2) binding to EP3 receptors and for [H-3]PGF(2 alpha) binding to FP receptors in washed bovine corpus luteum homogenates.