Progesterone Receptors in Mammary Gland of the Pregnant Rat*

Abstract

The characteristics of pregnant rat mammary gland progesterone receptors have been studied by the use of two synthetic probes, [³H]R5020 (³H-labeled 17α,21-dimethyl- 19-nor-pregn-4,9-diene-3,20-dione) and [³H]ORG 2058 (³H-labeled 16a-ethyl-21-hydroxy-19-nor-pregn-4-ene-3,20-dione). [³H] ORG 2058 bound with high affinity (K_d at 4 C, ∼0.5 nM) to an apparent single class of displaceable sites, with a hierarchy of competition as follows: ORG 2058 > R5020 ≥ progesterone >>> dexamethasone. In contrast, [³H]R5020 showed binding in mammary gland cytosols which was nonlinear with protein concentration, of intermediate affinity (K_d at 4 C, ∼5 nM), and poorly displaced by nonradioactive R5020; the hierarchy of competitors was R5020 » progesterone » dexamethasone = ORG 2058. Parallel studies in rat uterine cytosols showed both [³H]0RG 2058 and [³H]R5020 to bind with high affinity to an apparently identical class of sites; the hierarchy of competition for both was ORG 2058 ≥ R5020 > progesterone » dexamethasone. We conclude that in pregnant rat mammary gland, as opposed to rat uterus and other tissues, R5020 is unsuitable as a progesterone receptor probe; however, the affinity, capacity, and specificity characteristics of progesterone receptors in this tissue can be described by the use of ORG 2058.