Role of progesterone in the control of endometrial oxytocin receptors at luteolysis in sheep

Abstract

Merino ewes were given a prostaglandin synthetase inhibitor, Finadyne (50 mg flunixin meglumine ml-1), on days 14-16 of the oestrous cycle (day of oestrus = day 0), Finadyne on days 14-16 plus PGF(2α) on days 15-16, or progesterone on days 14-17 plus PGF(2α) on days 15-16. Blood samples were taken once a day on days 10-14 and three times a day on days 15-16 for progesterone measurement. The concentrations of oxytocin receptors were measured in the endometrial (pooled caruncular and intercaruncular) tissues collected on day 17. Treatment of ewes with Finadyne resulted in the maintenance of high plasma concentrations of progesterone and a small, but nonsignificant, reduction in the concentrations of endometrial oxytocin receptors. Co-administration of PGF(2α) reversed this effect of Finadyne. Treatment with both progesterone and PGF(2α) increased the concentrations of progesterone in plasma and significantly reduced the concentrations of endometrial oxytocin receptors compared with those in the control ewes. These data indicate that withdrawal of progesterone from the circulation as a result of spontaneous luteolysis or by a PGF(2α)-induced luteolysis caused an increase in the concentrations of oxytocin receptors. However, maintenance of plasma progesterone concentrations over the period of normal luteolysis only partially inhibited the concentrations of endometrial oxytocin receptors. These results suggest that the increase in the concentrations of oxytocin receptors at luteolysis in the naturally cycling ewes may be due to the loss of the inhibitory effects of progesterone on uterine oxytocin receptors.