Effect of intra-ovarian infusion of oxytocin on plasma progesterone concentrations in pregnant ewes.

The function of oxytocin receptors in the corpus luteum of pregnant ewes was investigated by infusing saline or oxytocin (100 ng/min) into the utero-ovarian artery of pregnant ewes (62 +/- 5 days, n = 12). During a 4-h infusion, plasma oxytocin (OT) concentration increased to 268 +/- 80 pg OT/ml in the OT-infused group and remained unchanged at 2.5 +/- 1.5 pg OT/ml in the saline-infused group. Progesterone concentration in jugular venous plasma (17 +/- 9 ng/ml) rapidly decreased during oxytocin infusion to 59 +/- 10% and 26 +/- 9% of control at 1.5 and 2 h, respectively; the utero-ovarian venous concentration of 64 +/- 38 ng/ml decreased by a similar magnitude during oxytocin infusion. Electron microscopy of corpora lutea, removed at the end of the experiments, showed no indication of luteolytic changes following oxytocin infusion. It was concluded that oxytocin markedly and rapidly reduces progesterone secretion in pregnant ewes.