Glucocorticoid receptors: documentation in the rat uterus

The question of the presence of specific glucocorticoid receptors in the rat uterus was reassessed. A high speed supernatant obtained from uteri of ovariectomized or intact 45-day old rats was incubated 18 h at 4 c in the presence of [3H]triamcinolone acetonide ([3H]TA) with or without excess dexamethasone, followed by a 10 min exposure to dextran-coated charcoal. A specific uterine glucocorticoid receptor was detected with a Kd=5.1 nM and the number of binding sites equal to 1.1 pmol/uterus. [3H]dexamethasone was observed to underestimate the number of specific receptor sites, probably due to the instability of the dexamethasone-receptor complex during treatment with charcoal. In addition, unlabeled TA was unsatisfactory for determining nonspecific binding because of the cross-reactivity with the progesterone receptor also present in uterine cytosol. The uterine glucocorticoid receptor exhibited the appropriate steroid specificity. Sucrose density gradient analysis of uterine cytosol revealed peaks of dexamethasone-competable [3H]TA binding at 7.2S and 5.75 in low salt and at 4S in 0.4M KCL. Similar components were observed in cytosol and nuclear fractions, respectively, 20 min after in vivo injection of 25 mu Cl [3H]TA. The physical characteristics of the specific glucocorticoid receptor were consistent with those determined for other receptor proteins. The documentation of the presence of glucocorticoid receptors in the rat uterus is important physiologically because of numerous reports of glucocorticoid effects on this reproductive tissue. If receptors could not be demonstrated, extensive studies of the mechanism of these effects would be necessary for a full understanding of uterine endocrinology.