Radioimmunoassay of plasma and pituitary oxytocin in pregnant rats during various stages of pregnancy and parturition.

Radioimmunoassayable plasma and pituitary oxytocin (ROT) was measured during pregnancy and parturition. The highest mean plasma ROT level was seen in maternal rats on day 20 of pregnancy followed by a non significant decrease on day 21 reaching the basal level just before delivery. Plasma ROT level declined continuously up to the time of delivery of the first pup and then gradually increased until the completion of parturition. The highest maternal mean pituitary ROT level was found on day 21 pregnancy. Just before delivery of the first pup, the mean pituitary ROT level significantly declined by 22%. Pituitary ROT level declined steadily during the delivery of the first five pups. This pattern of reduction was reversed and ROT levels increased between the delivery of the fifth pup and the completion of labor. The increase of maternal pituitary and plasma ROT levels during delivery of the pups suggest that maternal ROT may play a role in normal parturition. Fetal hypothalamic-pituitary ROT does not seem to play any part in parturition.

Oxytocin in maternal and fetal blood.

Radioimmunoassayable plasma oxytocin (OT) has been measured in maternal and fetal blood. Simultaneous samples were obtained in maternal forearm venous blood and in umbilical venous and arterial blood in 29 patients at term delivery. In addition, maternal forearm venous blood samples were also obtained 10 minutes prior to delivery. Mean OT level in maternal plasma at delivery was 82 +/- 12 muU/ml, and at 10 minutes prior to delivery the mean OT level was 90 +/- 11 muU/ml. The umbilical arterial plasma OT showed 95 +/- 12 muU/ml and the umbilical vein plasma OT was 60 +/- 10 muU/ml. Oxytocin levels higher in maternal blood than in fetal blood were found with the following incidence: In 51% of samples there was more OT in maternal venous blood than in umbilical arterial blood, and in 84% of samples there was more OT in maternal blood than umbilical vein blood. During the postpartum period, the mean maternal plasma OT was 66 +/- 8 muU/ml for the first day, and 50 +/- 9 muU/ml and 54 +/- 9 muU/ml for the second and third days, respectively. This study indicates that both the fetus and the mother are active producers of oxytocin.