Oxytocin receptors and parturition. I. Control of oxytocin receptor concentration in the rat myometrium at term.

Rat myometrium exhibited a marked rise in the concentration of oxytocin (OT) receptors during parturition. The elevation began several hours before labor, was maximal during labor, and declined several hours later. In the perinatal period, the change in OT receptor concentration was proportional to the ratio of plasma estradiol to progesterone levels. Several hours before the increase in OT receptor concentration, there was a proportional increase in estrogen receptor concentration in both the cytosol and nuclear fractions of the myometrium. In view of the known action of estrogens in increasing the concentration of OT receptors in rat uterus, we propose that the following sequence of events occurs in the initiation of labor in the rat. The decline in serum progesterone permits estradiol to stimulate the synthesis of estrogen receptors in the myometrium. This increased concentration of estrogen receptors and their occupancy by estradiol stimulates the appearance of more OT receptors, which then trigger labor by interacting with circulating OT.