Binding of 5α-dihydroprogesterone and other progestins to female rat anterior pituitary nuclear extracts

Abstract

The specific binding of 5α-dihydroprogesterone (5α-DHP), progesterone and R5020 to anterior pituitary nuclear extracts was studied using ovariectomized rats treated with estradiol benzoate and progesterone. The binding equilibrium association constant for 5α-dihydroprogesterone with different preparations of nuclear extract ranged from 4.0 ± 0.54 μM −1 to 59 ± 10 μM −1. The association constants for progesterone and R5020 were 0.39 ± 0.081 nM −1 and 1.5 ± 0.15 nM −1, respectively. The binding of 5α-DHP was specific in that it could be competed only by R5020, progesterone and 5α-DHP and not by other progesterone metabolites and other hormonal steroids tested. With [ 3H]-progesterone and [ 3H]R5020 as ligands the most efficient competitors also were R5020, progesterone and 5α-DHP. Estrogen priming of ovariectomized rats consistently and significantly increased the number of binding sites for all three progestins and subsequent progesterone treatment enabled their detection at higher levels in the nuclei.