Relationships among sex steroids, oxytocin, and their receptors in the rat uterus during late gestation and at parturition

Sex steroids and oxytocin (OT) produced within intrauterine tissues have been implicated in the regulation of parturition. The purpose of these studies was 1) to determine the relationships among estradiol (E(2)), progesterone (P-4), OT, and their receptors in uterine tissues during late gestation and parturition in the rat; 2) to observe the effects of the estrogen antagonist tamoxifen (TAM) on these factors; and 3) to evaluate the rat as a potential model for events at human parturition. Concentrations of E(2), P-4, PGE(2), and OT were measured by RIA. E(2) receptor (ER) was measured by enzyme immunoassay, and P-4 receptor (PR) and OT receptor (OTR) were measured by binding assays. OT messenger RNA (mRNA) was measured by ribonuclease protection assay. Groups (n = 5) of pregnant rats (normal gestation = 22 days) were treated with TAM (200 mg/day) or vehicle and killed on gestation day 19, 21, 21.5, or 22 or after delivery of the first pup. Serum E(2) increased throughout late gestation accompanied by an increase in uterine OT mRNA and ER. Serum P-4 declined after day 19, and uterine PR did not change significantly. Uterine PGE(2) in creased progressively, reaching peak levels the evening before delivery. Uterine OTR did not increase until the morning of delivery, and uterine OT peptide concentrations increased only during parturition. Parturition was significantly delayed by 24 h in the TAM-treated group. TAM inhibited the increase in serum E(2), uterine ER, and OT mRNA and peptide, but had no effect on serum P-4 or uterine PR levels. With TAM, the responses of uterine OTR and PGE(2) were significantly delayed, but still underwent a significant increase before the delayed parturition. These results support the hypothesis that E(2) stimulates the synthesis of ER, OT, and OTR within the rat uterus and is essential for normal parturition. P-4 withdrawal may be more important to the increases in OTR and PGE(2), but these are delayed in the absence of estrogen. These data also suggest that the rat may be a relevant model for human parturition.