Occurrence of common binding sites for progestin and glucocorticoid in the lactating mammary gland of the rat.

Previous reports have shown that in the cytosol from rat mammary gland, the high affinity binding of glucocorticoid is strongly inhibited by unlabeled progestin and that of progestin is strongly inhibited by glucocorticoid. Experiments were carried out with [3H]dexamethasone and [3H]R5020 to determine whether the binding sites for glucocorticoid and progestin are the same. Thermal inactivation experiments showed that the molecular properties of the [3H]-dexamethasone-binding sites closely resemble those of the [3H]R5020-binding sites. The dissociation constant of the [3H]R5020-receptor complex was almost exactly the same as the inhibition constant of unlabeled R5020 for [3H]dexamethasone-binding sites. The same correlation was observed between the dissociation constant and the inhibition constant of dexamethasone. In addition, no [3H]R5020-binding was observed after saturating the [3H]-dexamethasone-binding sites. These results led us to conclude that common binding sites for [3H]dexamethasone and [3H]R5020 exist in the cytosol from the rat mammary gland. The results of further experiments, including Scatchard analyses and dEAE-cellulose chromatography, strongly suggest that there are at least two classes of [3H]dexamethasone-binding sites, and that one of them binds both glucocorticoid and progestin stably.