Oxytocin Receptors and Parturition. III. Increases in Estrogen Receptor and Oxytocin Receptor Concentrations in the Rat Myometrium during Prostaglandin F2α-Induced Abortion*

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Abstract
Marked elevations in the concentrations of receptors for estrogen (E) and oxytocin (OT) in the rat myometrium occur during spontaneous labor. These increases appear to be correlated with a decrease in plasma progesterone (P). In this study we sought to determine if these same changes in plasma steroid and myometrial receptor concentrations can be made to occur prematurely when pregnancy is terminated with prostaglandin Fα, (PGF2α). The administration of PGF2α to rats on day 18 of pregnancy resulted in premature delivery on day 20. PGF2α-induced labor was associated with a rise in the concentration of myometrial receptors for E and OT. The administration of P to these rats inhibited the increases in concentration of receptors for E and OT on day 20 and prevented premature delivery. In contrast, PGF2α administration on day 15, when plasma P is near maximal, resulted in marginal changes in myometrial receptor concentrations and in uninterrupted pregnancy. PGF2α treatment on day 10 increased the concentrations of E and OT receptors by day 11 and resorption of the fetuses by day 12. These effects of PGF2α were reversed by exogenous P. The effects of PGF2α appear to be mediated by its luteolytic activity for several reasons. Exogenous P blocked PGF2α-induced changes in myometrial receptor concentrations. Azastene, which inhibits progesterone synthesis, gave the same results as PGF2α on day 10 of pregnancy. Finally, the ratio of plasma P to E concentrations was inversely proportional to the concentration of myometrial OT receptors in all of the rats treated with PGF2α or azastene. We postulate that the biochemical events that precede PGF2α- induced abortion are the same as those in spontaneous labor. Namely, after P withdrawal, there is an increase in the concentration of myometrial E receptors. The increase in E receptors and their occupancy by E causes the appearance of more OT receptors, which may then trigger labor by interacting with circulating OT. (Endocrinology106: 739, 1980)