STUDIES ON OXYTOCIN IN THE BABOON DURING PREGNANCY AND DELIVERY

Oxytocin was determined by radioimmunoassay in pregnant baboons throughout gestation, in the fetus at caesarean section and after oxytocin infusion into the mother and fetus. Serial maternal plasma oxytocin in 6 baboons during pregnancy showed a significant correlation between the gestational age and maternal plasma oxytocin concentrations with a correlation coefficient [r] of 0.3185 and P < 0.005. Seventy-one out of 75 plasma samples (94.7%) during pregnancy had detectable levels of oxytocin. Uterine vein plasma had higher oxytocin concentrations than maternal plasma and amniotic fluid (18.6 .+-. 4.6 pg/ml; mean .+-. SE) but lower concentrations than umbilical, jugular vein and cardiac blood from the fetus. Fetal pituitary gland contained 5.4-26.1 .mu.g oxytocin/g. Regular uterine contractions were established with i.v. oxytocin of 4-20 mU[units]/min and the plasma oxytocin measured showed a significant correlation with the uterine activity achieved (r = 0.64, P < 0.001). The disappearance of plasma oxytocin at 179 days gestation gave an apparent half-life [T 1/2] of 1.1 and 1.7 min in 2 baboons with a late T 1/2 of 9.9 and 17.3 min, respectively. In 1 baboon at 171 days gestation, the apparent T 1/2 of oxytocin was 9.9 min. The metabolic clearance rates were calculated to be 3.1, 3.2 and 11.7 ml/kg per min, respectively. The production rates were 97, 74, 390 pg/kg per min, respectively. Oxytocin injected into the umbilical vessel near term showed an increase in oxytocin concentration in maternal and uterine vein plasma and amniotic fluid, suggesting that oxytocin can cross the placenta from the fetal to the maternal side. In the baboon, oxytocin is present throughout pregancy, uterine activity can be correlated with plasma oxytocin during oxytocin infusion, fetal circulation has higher oxytocin concentration than maternal blood and oxytocin probably can cross the placenta from fetus to the mother.