Progesterone binding to mineralocorticoid receptors: in vitro and in vivo studies

In previous studies using expressed recombinant human mineralocorticoid receptors (MR), progesterone was reported to have widely divergent affinity, from approximately 10 nM to < 10 pM. In the present studies, cytosol preparations of colon or hippocampus were incubated with [3H]aldosterone or [3H]progesterone, alone or with excess RU-486, and the ability of each steroid to compete for MR was determined. In guinea pigs, progesterone has equivalent affinity to aldosterone for MR in vitro, and in rats three times of that aldosterone, with no differences between tissues. In vivo, in both epithelial (kidney, colon) and nonepithelial tissues (heart, hippocampus), progesterone was 10- to 100-fold less potent a competitor than aldosterone for MR, both in the absence of transcortin (8-day-old rats) and in adult mice. Bolus injection of [3H]progesterone was not specifically bound in any of the four tissues. Whether progesterone at steady state may bid for MR occupancy under conditions of high circulating free levels (in utero, luteal phase, pregnancy), presumably to act as an antagonist to cortisol/corticosterone in unprotected nonepithelial receptors, thus remains to be determined.