Nongenomic inhibition of oxytocin binding by progesterone in ovine uteri Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pv63g248f

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  • Effects of progesterone (P₄) and mifepristone (RU 486) on binding of oxytocin (OT) to its ovine uterine receptor (OTR) were studied in two experiments. Two additional experiments were conducted to evaluate specific binding sites for P4 in endometrial plasma membranes. In Exp. 1, ovariectomized ewes were injected with estradiol-17β (E₂) and P₄ to simulate an abbreviated estrous cycle (7 days) and then treated daily with 25 μg E₂ plus the following steroids for an additional 3 days: Grp. 1) controls (none), 2) 10 mg P₄; 3)10 mg P₄ + 10 mg RU 486. Endometrium removed on Day 11 was stored at -80°C until analyzed for OTR. For Exp. 2, endometrial plasma membranes (1 mg protein/mi) from control ewes were incubated for 1 h (25°C) with the following steroids: 1) controls (C), 2) P₄ (5 ng/mI) and 3) P₄ + RU 486 (10 ng/ml), and analyzed for OTR. Scatchard analysis revealed high affinity OT binding sites (Kd of 1.01 X 10⁻¹⁰M). in vivo and in vitro treatment with P4 suppressed the binding of OT to OTR (p<0.01) and treatment with RU 486 blocked the effect of P₄ (p>O.05). In Exp. 3, membranes recovered from control ewes as in Exp. 1 were analyzed for specificity of P₄ binding using [³H] P₄ and [³H] promegestone (R5020). Scatchard analysis revealed high affinity P4 and R5020 binding sites (Kd 1.2 x 10⁻⁹ and 1.74 x 10⁻¹⁰M, respectively). Binding of [³H] R5020 to plasma membranes was inhibited by competition with a 200-fold excess of unlabeled R5020, P₄, RU 486 and OT (37-49%) but not by E₂, cortisol, or testosterone. In Exp. 4, membranes were incubated in vitro with P₄ or P₄ + RU 486 and then analyzed for specific binding of [³H] R5020. Progestin binding was greater in membranes exposed to P4 compared to controls or those incubated with P₄ + RU 486 (p<0.05). These data suggest a direct interaction between P₄ and OTR at the level of the membrane that suppresses binding of OT to the OTR.
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