Oxytocin release and uterine activation during parturition iin rabbits.

Plasma oxytocin was measured by a specific RIA in blood of pregnant and parturient rabbits obtained through an indwelling cardiac catheter. Uterine activity was continuously recorded by means of indwelling intrauterine balloons. Plasma oxytocin concentrations were low throughout gestation (mean +/- SE, 16.1 +/- 2.0 pg/ml), with no rise toward term. During parturition, a significant increase in plasma oxytocin was observed in all but 3 of 21 rabbits studied. Plasma oxytocin rose rapidly and reached high levels within minutes of the beginning of labor contractions. The mean concentration was 193 +/- 55 pg/ml (SE) 30--60 sec before the expulsive phase. The highest oxytocin concentrations were usually observed at the delivery of the first fetus (mean, 258 +/- 89 pg/ml); this was followed by a rapid decline. Baseline levels were reached in 20--60 min. Uterine activity during parturition was well correlated with plasma oxytocin; an abrupt increase was observed a few minutes (5.1 +/- 1.2 min) before the expulsion of the young, followed by a gradual disappearance over 30--70 min. Little or no circulating oxytocin was detected during delivery in 3 rabbits. In these, the course of parturition was abnormal and protracted, resulting in a high percentage of stillborn young. The series of strong contractions associated with normal delivery was absent. By contrast, abortion in 2 rabbits was associated with elevated plasma oxytocin levels and increased uterine activity. These findings indicate that a substantial amount of oxytocin is released into the circulation during delivery and suggest that the normal activation of the uterus at parturition depends on oxytocin. The stimulus eliciting the release of oxytocin is not known. Dilatation of the birth canal by the passage of a fetus was not consistently followed by detectable oxytocin release, and during delivery, a second release of oxytocin was often unrelated to any apparent stimulus. Intact spinal cord caudal to T5 does not appear to be essential for the release of oxytocin at parturition, since spinal transsection in 2 rabbits was associated with normal oxytocin release at delivery. Injections of synthetic oxytocin caused a dose-dependent increase in plasma oxytocin and uterine activity. Disappearance of oxytocin from the circulation followed a double exponential curve; the mean half-life of the initial rapid phase was 1.82 min after single injections and the half-life of the low component was 26.5 min. The initial volume of distribution was close to the volume of the vascular compartment, and the total apparent volume of distribution was about twice the size of the extracellular compartment. After constant infusions, a half-life of 3.6 min was obtained for the uncorrected values during the initial phase.