Hormonal regulation of oxytocin-induced prostaglandin F-2 alpha secretion by the bovine and ovine uterus in vivo

In long-term ovariectomized ewes and cows, endometrial oxytocin receptors r est at relatively high levels but oxytocin is unable to induce prostaglandi n F-2 alpha release. A series of studies were carried out to investigate th e roles of physiological levels of progesterone and estradiol in "activatin g" these receptors in terms of permitting oxytocin-induced prostaglandin F- 2 alpha release. In long-term ovariectomized cows, treatment with progester one, but not estradiol, resulted in the induction of responsiveness to oxyt ocin. This responsiveness appeared within 2 d of progesterone treatment, re ached a maximum by 6 d and was maintained to Day 18. In ovariectomized ewes , while estradiol treatment did induce temporary responsiveness to oxytocin after 3 d of treatment, treatment with progesterone was required to induce sustained responsiveness that appeared by Day 9 of treatment and was maint ained to Day 12. Measurement of endometrial receptors for oxytocin revealed a significant decline in oxytocin receptors by Day 6 of progesterone treat ment when responsiveness to oxytocin was maximal, demonstrating that recept or concentrations were not a limiting factor. The most likely mechanism by which progesterone treatment induces responsiveness to oxytocin may be thro ugh the up regulation of post receptor signaling pathways and/or enzymes in volved in prostaglandin synthesis. (C) 2001 Elsevier Science Inc. All right s reserved.