Effect of Oxytocin and Estradiol on Uterine Prostaglandin Release in Nonpregnant and Early-Pregnant Ewes1

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Abstract
The effects of exogenous oxytocin (OT) and estradiol-17.beta. (E2) on plasma concentration of prostaglandin (PG) E2 and 13, 14-dihydro-15-keto-PGF2.alpha. (PGFM) were investigated on Day 14-15 (NP) of the estrous cycle and Days 14-16 (PI) and 21-25 (EP) of pregnancy in the ewe. Basal concentrations of PGFM were significantly elevated in utero-ovaran venous (UOV) plasma on Day 14 of pregnancy (4.05 .+-. 0.81 nM, mean .+-. SEM) compared to that observed on Day 14 of the cycle or Days 21-25 of pregnancy (2.29 .+-. 1.3 nM and 1.06 .+-. 0.56 nM, respectively). PGFM release increased significantly following intera-arterial bolus injections of 50, 500, and 5000 mU OT at 2-h intervals in all experiment groups. There was no significant difference in area and peak height of the PGFM response between the 3 groups studied. The time to peak PGFM responses was, however, significantly longer in the PI group. No significant changes in concentration of PGFM were observed in ay experimental group following 1-h infusions of E2 at 5, 50, and 500 pmol/min. Long-term (15-18 h) infusion of E2 at 83 pmol/min increased the peak height of the OT-induced PGFM response at both stages of gestation studied. PGF2 concentrations in UOV plasma were < 0.05 nM in all samples studied. These results demonstrate that PG release can be induced in response to OT during the period in which ovine trophoblastic protein-1 (oTP-1) is released by the conceptus. During pregnancy, oTP-1 does not appear to inhibit the E2 induction of uterine OT receptors.