EFFECT OF THE OXYTOCIN ANTAGONIST ATOSIBAN (1-DEAMINO-2-D-TYR(OET)-4-THR-8-ORN-VASOTOCIN OXYTOCIN) ON NOCTURNAL MYOMETRIAL CONTRACTIONS, MATERNAL CARDIOVASCULAR FUNCTION, TRANSPLACENTAL PASSAGE, AND FETAL OXYGENATION IN THE PREGNANT BABOON DURING THE LAST 3RD OF GESTATION/

The oxytocin antagonist, atosiban amino-2-D-tyr(OET)4-thr-8-orn-vasoto cin/oxytocin), was infused i.v. to chronically instrumented pregnant b aboons in the last third of pregnancy. Atosiban (6 mu g/kg per min) in hibited myometrial electromyographic activity associated with spontane ous myometrial contractions that occurred around the onset of darkness between 134 and 162 days gestation (term 180 days gestation). The eff ect of atosiban on maternal heart rate was minimal. Maternal blood pre ssure remained unaltered during atosiban infusion. Fetal carotid arter ial PO2 was unchanged during a 2-h infusion of atosiban. Transplacenta l passage of atosiban from mother to fetus was assessed at cesarean se ction under halothane anesthesia in four baboons and in two chronicall y instrumented fetuses in the absence of anesthesia. The maternal:feta l concentration gradient ranged from 9.2 to 22.8. Maternal atosiban cl earance rates were 9.2-16.9 ml/kg per min. In conclusion, atosiban was very effective at inhibiting spontaneously occurring nocturnal myomet rial contractions during the last third of gestation in the pregnant b aboon. Although atosiban crosses the placenta relatively freely, there was no effect on fetal oxygenation.