CHARACTERIZATION OF OXYTOCIN RECEPTORS IN RABBIT AMNION INVOLVED IN THE PRODUCTION OF PROSTAGLANDIN-E2

We characterized oxytocin (OT) receptors in purified plasma membranes from amnion, decidua, and myometrium of late pregnant rabbits using an iodinated OT antagonist (OTA). Saturation studies showed similar K(d) values for specific binding sites in the 100- to 250-pM range in all three tissues. OT receptor concentrations in decidua and myometrium did not change until the day of labor (day 31), when they rose about 2.5- and 18-fold, respectively. Increases in amnion receptors were first apparent on day 28 and continued to maximal levels on day 31. There was an increase of about 230-fold from day 26 to labor, reaching 9.5 pmol/mg protein. Competition studies using analogs showed that ligand specificities of amnion and decidual membranes were indistinguishable. Those of myometrial membranes were somewhat different, possibly owing to the presence of both AVP receptors and OT receptors in the myometrium. Binding of OTA corresponded to the OT-induced release of prostaglandin E2 (PGE2) by amnion cells in culture. The effects of OT were dose dependent, agonist specific, and selectively inhibited by OTA. Amnion cells from days 22 and 28 did not respond significantly to either OT or phorbol 12-myristate 13-acetate (PMA), but cells from day 30 pregnant rabbits responded strongly to both. In contrast, calcium ionophore stimulated comparable amounts of PGE2 release from cells cultured on day 22, 28, or 30. These studies show that specific, high affinity OT receptors are associated with the release of PGE2 from rabbit amnion cells. Increases in amnion OT receptor and protein kinase-C activity precede by several days the increases in receptor concentrations in decidua and myometrium, suggesting important roles for the amnion and OT in the initiation of labor in rabbits.