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

Serum estradiol levels increased throughout late gestation, with a significant relationship to uterine oxytocin mRNA and receptors.

• Estradiol concentrations rose during late gestation, correlating with increased uterine oxytocin mRNA and estrogen receptors.
• Serum progesterone levels decreased after day 19, while uterine progesterone receptors remained stable.
• Uterine prostaglandin E2 levels increased progressively, peaking the evening before delivery.
• Uterine oxytocin receptors did not increase until the morning of delivery, with oxytocin peptide levels rising only during parturition.
• Tamoxifen treatment delayed parturition by 24 hours and inhibited the increase in estradiol, estrogen receptors, and oxytocin mRNA and peptide.
• The findings support the idea that estradiol is crucial for normal parturition in rats, suggesting their potential relevance as a model for human parturition.

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