An empirical basis for the competition by dexamethasone to progesterone receptors as estimated with the synthetic progestin R5020.

Normal mammary glands of mice contain both progesterone and glucocorticoid receptors and the levels of both these receptors are modulated as a function of development. Measurement of progesterone receptors using the synthetic progestin, R5020, has led to conflicting data both with regard to the presence of progesterone receptor during certain developmental stages of the mammary gland and the ability of the glucocorticoids to compete for specific R5020 binding sites. In this report we have identified experimental conditions which allow for the separate measurements of the progesterone and glucocorticoid receptors in the same cytosol. If sulfhydryl reducing agents such as dithiothreitol are excluded from the assay buffer, R5020 binds to only a single class of high affinity sites in mammary cytosol of virgin mice and these binding sites exhibit a strict steroid specificity characteristic of progesterone receptors. In contrast, if dithiothreitol is included in the buffers, R5020 binds not only to the high affinity sites but also to certain saturable lower affinity sites; these lower affinity sites for R5020 also bind glucocorticoids such as dexamethasone. These findings should facilitate more accurate quantitation of both progesterone and glucocorticoid receptors in normal and neoplastic tissues and also be applicable to studies on the mechanism(s) of progesterone action.