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The concentration of oxytocin receptors increased in the myometrium of pregnant women and reached maximum levels in early labor. Concentrations of oxytocin receptors were also high in the decidua and reached a maximum at parturition. In vitro, prostaglandin production by the decidua, but not by the myometrium, was increased by the addition of oxytocin. Oxytocin may therefore stimulate uterine contractions by acting both directly on the myometrium and indirectly on decidual prostaglandin production. Oxytocin receptors are probably crucial for the onset of human labor, and the stimulus for the increase in uterine prostaglandins may be oxytocin originating from the fetus.

Marked changes in the uterine binding of oxytocin (OT) occur in rats at the time of parturition or after treatment of ovariectomized rats with estrogen or progesterone. To ascertain that these binding sites represent the biological receptors for OT, we measured the uterine response to OT in various groups of rats in which specific OT binding was also determined. Intact pregnant rats and rats ovariectomized on day 20 of gestation and treated thereafter with oil, estradiol benzoate (5 micrograms/24 h), progesterone (5 mg/24 h), or estradiol and progesterone together had indwelling balloons inserted on day 20 for the recording of uterine response to either iv bolus injections or iv infusions of OT. The uterus was removed 24-48 h after balloon insertion, and OT binding to the particulate fraction as well as nuclear estrogen and cytosolic estrogen receptor concentrations were determined. An inverse correlation (r2 = 0.758) was found between the concentration of OT-binding sites and the threshold dose of OT, and a linear correlation was found between the concentration of binding sites and the uterine activity induced by OT infusion (r2 = 0.852). We conclude, therefore, that the high affinity (Kd, 1-2 nM) binding sites for OT represent the physiological receptors. The concentration of these sites increased progressively during estrogen treatment. Progesterone completely inhibited this estrogen-induced rise. After ovariectomy, there was a modest, but significant, increase in OT receptor concentration which also was prevented by progesterone. The increase in OT receptor concentration was correlated with the estrogen receptor concentration in intact pregnant and estrogen-treated ovariectomized animals, but not in the other groups of animals. The apparent affinity of the receptors for OT was not significantly affected by hormone treatment. We conclude that the concentration of receptors is a major factor controlling the uterine responsiveness to OT, and that the receptor concentrations are regulated by ovarian hormones in a manner related to estrogen receptor activation. In addition, estrogen appeared to enhance the coupling of OT receptor occupancy to the tissue response to OT.

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