Inhibition of spontaneous uterine contractions during the last trimester in pregnant baboons by an oxytocin antagonist.

Abstract

The effect of a potent oxytocin antagonist, produced in our laboratories, on spontaneous uterine contractions in the pregnant baboon was examined. Three types of uterine contractions were studied: immediately after operation, during the nocturnal period, and near or at labor. Bolus intravenous injections of oxytocin antagonist were given and uterine activity was examined +/- 1 hour after the injection. The oxytocin antagonist caused a precipitate decrease (approximately 70%) in contractile force (mean amplitude x frequency) in the first 15 minutes after injection (p less than 0.05); this force diminished to approximately 90% at the end of 1 hour for both nocturnal and labor contractions. In contrast, uterine contractions immediately after operation were diminished by only 60% within 60 minutes after the oxytocin antagonist. These results indicate that the oxytocin antagonist is a potent inhibitor and suggest that oxytocin is a primary regulator of spontaneous nocturnal and labor uterine contractions in the pregnant baboon.