OXYTOCIN INDUCES PROSTAGLANDIN-F2-ALPHA RELEASE IN PREGNANT COWS - INFLUENCE OF GESTATIONAL-AGE AND OXYTOCIN RECEPTOR CONCENTRATIONS

Brahman cows with known breeding dates received i.v. injections of eit her 10 or 100 IU oxytocin (OT) on Days 50, 150, 250, or 280 of gestati on (n = 6 for each stage). Concentrations of the prostaglandin (PG) F- 2 alpha metabolite, 13,14-dihydro-15-keto-prostaglandin (PGFM), and OT were measured in samples of peripheral plasma collected at 15-min int ervals for 1 h before and 1 h after treatment and then at 30-min inter vals for 3 h. Plasma progesterone was measured daily for 14 days after OT injections on Days 50 and 250 of gestation. The increase in plasma OT after injection was dose-dependent (p = 0.001) but not affected by stage of gestation. Plasma PGFM increased after OT in a dose- and sta ge-dependent manner (p = 0.0001). At Day 280, the increase in plasma P GFM after 100 IU OT was sevenfold greater than at Day 50. Plasma proge sterone declined significantly during the 7th to 12th days postinjecti on and returned to normal pregnancy values by the 14th day (4.4 +/- 0. 3 ng/ml) except in two cows treated on Day 50 of gestation that later aborted. In these, plasma progesterone was significantly lower, 2.6 +/ - 0.1 ng/ml. In a second experiment, the concentration of OT receptors was determined in endometrium collected from purebred Angus or Herefo rd cows slaughtered on Days 50, 150, 250, and 280 of gestation (n = 3 or 4 at each stage). Endometrial concentrations of OT receptor changed as a function of gestational age, increasing sixfold from Day 50 to D ay 280, which was parallel to the increase by OT of plasma M. Thus, en dometrial OT receptors are functionally coupled to PGF(2 alpha) releas e during pregnancy, and their concentration determines the magnitude o f CT-induced PGF(2 alpha) release during gestation. Consequently, endo genous OT is a factor in the regulation of PGF(2 alpha) release from t he bovine uterus during pregnancy and parturition.