PULSATILE SECRETION OF OXYTOCIN DURING PARTURITION IN THE PIG - TEMPORAL RELATIONSHIP WITH FETAL EXPULSION

1. To assess changes in oxytocin release as they occur in relation to the rapid progress of events at fetal expulsion, continuous automated blood withdrawals (3 ml min(-1)) from an indwelling jugular catheter and intramammary pressure recordings were obtained from nine primiparous pigs (190-220 kg). Data were acquired over 16 h of normal parturition, during which thirty-five piglets were born. 2. Oxytocin secretion during parturition, when measured by radioimmunoassay (RIA) in blood collected and pooled every minute, showed a baseline secretion (19.8-88.37 pg ml(-1)) that was raised relative to preterm values. Analysis of individual secretion profiles revealed significant fluctuations or peaks of concentration superimposed on this baseline, with a slow periodicity of 4-12 min. These substantial peaks in secretion mere not temporally related to fetal expulsion or visible abdominal contractions. 3. A small (13%) but significant increase in plasma oxytocin was also seen when assay data from the minutes coinciding with a birth were meaned and compared with the following minutes. This rise did not persist into further minutes. 4. Intramammary pressure recordings revealed a highly repeatable and characteristic phenomenon in that fetal expulsion was followed after 33.74 +/- 1.31 s (mean +/- S.E.M. time from emergence of fetus to peak pressure rise) in thirty-three of thirty-five instances by a distinctive and rapid bolus release of oxytocin. These 'postpartum oxytocin pulses' could be closely mimicked by injections of exogenous oxytocin (0.03-1.0 ng kg(-1); lag time from jugular injection to peak pressure rise, 20.44 +/- 0.99 s). The timing of this event coincided with the small postpartum pulse measurable by radioimmunoassay. The larger but slower episodes of oxytocin release between births produced no response in the mammary gland. 5. These results show that endogenous oxytocin secretion during parturition is pulsatile, and that fetal expulsion results in an additional but relatively small contribution to circulating oxytocin, in the form of reflex postpartum pulses that reach target after delivery is complete.