Prolonged progesterone treatment increases the concentration of uterine oxytocin receptors in ovariectomized ewes

Abstract

The effect of prolonged progesterone treatment on the concentrations of endometrial oxytocin receptors (OTr) was studied in chronically ovariectomized ewes. The ewes were primed with progesterone and oestradiol-17β (E 2-17β) to induce oestrus (day of oestrus=Day 0) and then injected with graduated doses of progesterone, using vegetable oil as a vehicle, for 14, 16, 18 or 20 days. The control ewes were injected with the vehicle. The concentrations of OTr on the day of cessation of treatment were measured by a receptor binding assay. During the period of treatment, plasma progesterone concentrations rose above 1 ng ml −1 by Day 4 and remained above 2 ng ml −1 thereafter. Treatment with progesterone for 14 days significantly ( P < 0.01) reduced the concentrations of OTr in the endometrium compared with the control ewes. However, the concentrations of OTr were increased after 16 or more days of progesterone treatment. These results indicate that in ovariectomized ewes progesterone can exert a suppressive effect on the concentrations of endometrial OTr; however, prolongation of progesterone treatment would lead to OTr eventually becoming refractory to the suppressive effect of progesterone and an increase in the concentrations of OTr.