Oxytocin Receptors and Parturition in the Guinea Pig1

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Abstract
We have previously reported that parturition in the rat was preceded by an abrupt rise in the concentration of myometrial receptors for oxytocin (OT) and estradiol-17β (E). These increases corresponded to a decrease in plasma progesterone (P) concentrations. Because there is no prepartum decline in plasma P concentrations in the guinea pig, we sought to determine whether the concentrations of myometrial receptors for OT and E are elevated before term in this species. The concentration of OT receptors per myometrial cell in the guinea pig uterus rose during gestation and was maximal from Day 60 to about 12 h after parturition (about Day 69). OT receptor levels then fell sharply by the second postpartum day. These changes in OT receptor concentration correspond to previously published changes in the sensitivity of the guinea pig myometrium to OT. The concentration of E receptors per mg DNA in the nuclear fraction of the myometrium was unchanged during the first 50 days of gestation and, like OT receptor concentrations, rose to a maximum from Day 60 to about 12 h after parturition. The concentration of nuclear E receptors in individual animals was directly proportional to the concentration of OT receptors from Day 60 to 12 h postpartum. In contrast, there was no apparent relationship between the concentration of E receptors in the cytosol and OT receptors. Although plasma P levels remained high throughout gestation, the ratios of plasma E to P, as well as plasma E concentrations were maximal during pregnancy on Days 50 to 60, before the maxima in nuclear E and OT receptor concentrations were reached. Because the ratio of E to P was influenced primarily by plasma E concentrations, it is possible that increases in E levels stimulate the increases in receptor concentrations. Despite the absence of a fall in plasma P levels prior to parturition, the guinea pig resembles the rat in that there were increases in the concentration of OT receptors and nuclear E receptors in the myometrium. The increases in OT receptor concentration in both species correspond to changes in the myometrial sensitivity to OT. The two species differ, however, in that the sudden increase in OT receptor concentration in the rat myometrium appears to be the veritable trigger for parturition, whereas in the guinea pig the elevation of OT receptors to maximal levels about 9 days before parturition appears to have a permissive role in oxytocin-induced labor.