Estrogen and progestogen binding in the myometrium of the ewe. II. Regulation by estradiol and progesterone.

Two experiments were conducted to study the role of estradiol and progesterone in regulating steroid binding sites in sheep uterine myometrium. Exogenous estradiol increased the concentration of estradiol binding sites in nuclei and subsequently increased the total cell content of estradiol binding sites. Estradiol increased the number of progestogen binding sites (promegestone binding) and may have enhanced the translocation of progestogen binding sites to nuclei. Progesterone decreased the number of progestogen binding sites and blocked estradiol-induced increase in cytoplasmic estradiol binding sites. Progesterone did not block the estradiol-induced increase in estradiol binding sites in nuclei. Estradiol increases myometrial RNA to DNA ratios with in 24 hr of a single injection, whereas progesterone did not. Progesterone blocked the estradiol-induced increase in RNA:DNA. Estradiol apparently promotes the synthesis of both types of binding site. Progesterone, on the other hand, appears to inhibit estrogen-induced synthesis of steroid binding sites by blocking estradiol action at the nucleus. Progesterone may also decrease the number of progestogen binding site through a mechanism other than its "anti-estrogenic" action.