HORMONAL-CONTROL OF CONCENTRATIONS OF ENDOMETRIAL OXYTOCIN RECEPTORS IN THE EWE

Uterine oxytocin receptors have been shown to play a major role in the regulation of uterine prostaglandin F2alpha release during the oestrous cycle and early pregnancy in sheep. The concentration of endometrial oxytocin receptors increases sharply from around Day 13 of the oestrous cycle to reach a maximum between Days 15 and 16. The high concentration of endometrial oxytocin receptors at this time coincides with the release of endogenous uterine prostaglandin F2alpha during luteal regression and the maximum uterine prostaglandin F2alpha response to an oxytocin stimulus. The concentration of uterine oxytocin receptors appears to be regulated by both progesterone and oestradiol. Studies in ovariectomized ewes have shown that initially progesterone lowers the concentration of endometrial oxytocin receptors, but after prolonged treatment with progesterone the concentration of oxytocin receptors increases; this suggests that the uterine-PGF2alpha response to oxytocin has become refractory to the inhibitory effects of progesterone. The concentration of endometrial oxytocin receptors is also lowered by short-term oestradiol treatment. However, oestrogen treatment of ewes after long-term treatment with progesterone does not result in an increase in the concentration of oxytocin receptors following the cessation of progesterone treatment. On the basis of these and other data it is proposed that in the normal oestrous cycle the concentration of endometrial oxytocin receptors is initially depressed by both oestradiol and progesterone but that the marked increase in the concentration of oxytocin receptors over Days 13-16 of the cycle is due primarily to the withdrawal of the inhibitory influence of progesterone alone. During early pregnancy the release of uterine prostaglandin F is suppressed. The concentration of uterine oxytocin receptors during early pregnancy is low compared with that of non-pregnant ewes; this may be due to ovine trophoblast protein (oTP-1), which is secreted by the embryo between Days 8 and 25 of pregnancy. It is possible that oTP-1 may prevent the withdrawal of the inhibitory effect of progesterone on uterine oxytocin receptors. This, however, remains speculative.