Oxytocin Receptors and Parturition. II. Concentrations of Receptors for Oxytocin and Estrogen in the Gravid and Nongravid Uterus at Term*

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Abstract
Intrauterine volume apparently plays a role in the timing of parturition. To determine the effects of intrauterine volume on the concentrations of these myometrial receptors, receptor levels in gravid and nongravid uterine horns from unilaterally pregnant rats at or near the time of parturition were compared. Myometria from gravid horns contained substantially greater concentrations of DNA and protein per horn than did myometria from the contralateral nongravid horns. The amounts of receptors per horn for estrogen in the nuclear and cytosol fractions and for oxytocin were 7.5, 5.2 and 4.8 times greater, respectively, in myometria from gravid horns. The concentration of receptors per cell was the same in gravid and nongravid horns at or near parturition. Uterine stretch apparently causes hyperplasia and hypertrophy of the myometrium, indicative of more DNA and protein per gravid horn than nongravid horn. The sharp rise in concentration per cell of receptors for estrogen and oxytocin occurring about the time of labor appears to be induced by hormonal and not physical factors.