Nongenomic inhibition of oxytocin binding by progesterone in the ovine uterus

Progesterone (P-4) has been reported to inhibit oxytocin (OT) binding to its receptor in isolated murine endometrial membranes. The purpose of the present research was to 1) examine the in vivo and in vitro effect of P-4 on the binding of OT to its receptor in the ovine endometrium and 2) determine whether the endometrial plasma membranes have high-affinity binding sites for P-4. Ovariectomized ewes were pretreated with a sequence of estradiol-17beta (2 days) and P-4 (5 days) before being treated with estradiol-17beta plus either vehicle (corn oil), P-4. or P-4 + mifepristone (RU 486) for 3 consecutive days. Treatment of ewes with 10 mg P-4/day for 3 days suppressed binding of OT (P < 0.01) compared with that of controls, whereas concomitant treatment with the progestin antagonist RU 486 (10 mg/day) blocked the effect of P-4. Similarly, incubation of endometrial plasma membranes with P-4 (5 ng/ml) inhibited binding of OT (P < 0.05), whereas this effect of P-4 was blocked by the presence of RU 486 (10 ng/ml). By radioreceptor assay, the endometrial plasma membranes were found to contain a high-affinity binding site for P-4 and the progestin agonist promegestone (K-d 1.2 X 10(-9) and 1.74 X 10(-10)M, respectively). Incubation of endometrial plasma membranes with P-4 (5 ng/ml) significantly increased the concentration of progestin binding sites. Binding of labeled promegestone (R 5020) was competitively inhibited by excess unlabeled R 5020, P-4, RU 486, and OT but not by estradiol-17beta, cortisol, testosterone, and arginine vasopressin. These data suggest a direct suppressive action of P-4 on the binding of OT to OT receptors in the ovine endometrial plasma membrane.