Effect of Oxytocin on Plasma Concentrations of 13,14-Dihydro-15-Keto Prostaglandin F and the Oxytocin-Associated Neurophysin During the Estrous Cycle and Early Pregnancy in the Ewe

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Abstract
The effect of exogenous oxytocin on plasma concentrations of the prostaglandin (PG) F metabolite 13,14-dihydro-15-keto-PGF (PGFM) and the oxytocin-associated neurophysin (OT-N) during the estrous cycle and early pregnancy in the ewe was investigated. Ewes were given oxytocin (250 mU, i.v.) on Days 3 (n = 4), 8 (n = 5), 13 (n = 4) or 14 (n = 5) of the estrous cycle, and a further 6 ewes were injected on Days 13 (n = 2) and 14 (n = 4) of pregnancy. No significant rises in plasma concentrations of PGFM were observed on Days 3 and 8 of the estrous cycle and on Days 13 and 14 of pregnancy. A marked increase in plasma PGFM concentrations occurred on Day 14 of the estrous cycle with the PGFM levels rising from a mean basal value of 120 pg/ml to a mean maximum value of 415 pg/ml within 2-10 min of administering oxytocin (P < 0.001). No increases in plasma OT-N concentrations were found in early pregnancy and only 1 of 4 ewes at Day 14 of the cycle showed any significant increase in OT-N concentrations. There evidently is an increase in the responsivenes of the uterine-PGF secretory system to oxytocin during the latter stages of the estrous cycle. During early pregnancy this response was blocked by the presence of the embryo. The lack of a consistent rise in plasma OT-N concentrations in ewes during the time of elevated plasma PGFM concentrations does not support the view that oxytocin from the corpus luteum is stimulated by an increase in uterine PGF secretion.