• P W Nathanielsz, M B Honnebier, C Mecenas, S L Jenkins, M L Holland, and K Demarest.
• College of Veterinary Medicine, Department of Physiology, Cornell University, Ithaca, New York 14853-6401, USA. pwn1@cornell.edu

AbstractThe oxytocin antagonist, atosiban (1-deamino-2-D-tyr(OET)-4-thr-8-orn-vasotocin/oxytocin), was infused i.v. to chronically instrumented pregnant baboons in the last third of pregnancy. Atosiban (6 microg/kg per min) inhibited myometrial electromyographic activity associated with spontaneous myometrial contractions that occurred around the onset of darkness between 134 and 162 days gestation (term 180 days gestation). The effect of atosiban on maternal heart rate was minimal. Maternal blood pressure remained unaltered during atosiban infusion. Fetal carotid arterial PO2 was unchanged during a 2-h infusion of atosiban. Transplacental passage of atosiban from mother to fetus was assessed at cesarean section under halothane anesthesia in four baboons and in two chronically instrumented fetuses in the absence of anesthesia. The maternal:fetal concentration gradient ranged from 9.2 to 22.8. Maternal atosiban clearance rates were 9.2-16.9 ml/kg per min. In conclusion, atosiban was very effective at inhibiting spontaneously occurring nocturnal myometrial contractions during the last third of gestation in the pregnant baboon. Although atosiban crosses the placenta relatively freely, there was no effect on fetal oxygenation.

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