REGULATION OF THE ESTROGEN-RECEPTOR IN THE DECIDUA BASALIS OF THE PREGNANT RAT

This study examines the distribution of the estrogen receptor (ER) in the mesometrial decidua basalis (DB) and the chorioallantoic placenta between Days 8 and 21 of pregnancy (Day 1 = presence of vaginal sperm) and its regulation by estradiol and progesterone. Immunocytochemistry revealed that ER was localized within nuclei of cells of the DB but not in trophoblastic cells (Day 10) or in cells of the junctional zone (JZ) or labyrinth zone (LZ). Western blot analysis and estradiol binding assays of DB, JZ, and LZ also revealed that only DB expressed ER. Native ER (66 kDa) was most abundant on Days 8 and 9, and declined 67% on Day 11 (p < 0.01), becoming barely detectable by Day 17. A truncated ER moiety (49 kDa) gradually increased, becoming the dominant form on Day 13. The effects of estradiol and progesterone on ER were studied during periods of growth and decline of DB (i.e., Days 8-10 and 12-14, respectively), Bats were ovariectomized on Day 8 or 12 and treated with estradiol daily (0.2, 0.75, or 2 mu g, s.c.), with a progesterone pellet, or with both, for 48 h. Progesterone, but not estradiol, stimulated the 66-kDa ER moiety and ER binding activity (p < 0.01). Estradiol administered with progesterone antagonized progesterone action, at least in part, by enhancing expression of the 49-kDa ER at the expense of the native form (p < 0.01). Thus, progesterone up-regulated ER whereas estradiol down-regulated ER in rat DB.