Role of oxytocin in the initiation of term and preterm labor in rats: Changes in oxytocin receptor density and plasma oxytocin concentration and the effect of an oxytocin antagonist, L-366,509

OBJECTIVE: Our purpose was to compare the functional roles of oxytocin in t erm and preterm labor in rats by both biochemical and pharmacologic means. STUDY DESIGN: We determined the myometrial oxytocin receptor density and th e maternal plasma concentrations of oxytocin and progesterone on gestationa l days 18, 20, and 22 (morning) and at the onset of delivery (day 22 aftern oon) in rats with labor at term and at the onset of delivery (day 20 aftern oon) in rats in preterm labor induced by the combined use of bilateral ovar iectomy and estradiol injection. We also evaluated the effects of an oxytoc in antagonist, L-366,509, on the initiation of both term and preterm labor. RESULTS: The number of tritiated oxytocin binding sites in myometrial membr anes rapidly increased on gestational day 22 (morning) in rats with term la bor. Plasma progesterone level decreased in an inverse fashion. A rapid inc rease in circulating oxytocin concentration was observed at the onset of de livery in rats in labor at term. Both the plasma oxytocin concentration and the receptor density had the same Values in rats with preterm labor as in rats with term labor. L-366,509 delayed the initiation of labor in rats wit h term and preterm labor in a dose-dependent manner. CONCLUSION: It is confirmed biochemically and pharmacologically that oxytoc in plays an important role in the initiation of both term and preterm labor in rats. The oxytocin antagonist examined was able to delay term and prete rm labor, so it might prove useful in clinical practice for the treatment o f preterm labor.