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 a nd intramammary pressure recordings were obtained from nine primiparou s pigs (190-220 kg). Data were acquired over 16 h of normal parturitio n, during which thirty-five piglets were born. 2. Oxytocin secretion d uring parturition, when measured by radioimmunoassay (RIA) in blood co llected 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 pea ks of concentration superimposed on this baseline, with a slow periodi city of 4-12 min. These substantial peaks in secretion mere not tempor ally related to fetal expulsion or visible abdominal contractions. 3. A small (13%) but significant increase in plasma oxytocin was also see n when assay data from the minutes coinciding with a birth were meaned and compared with the following minutes. This rise did not persist in to further minutes. 4. Intramammary pressure recordings revealed a hig hly repeatable and characteristic phenomenon in that fetal expulsion w as followed after 33.74 +/- 1.31 s (mean +/- S.E.M. time from emergenc e of fetus to peak pressure rise) in thirty-three of thirty-five insta nces by a distinctive and rapid bolus release of oxytocin. These 'post partum oxytocin pulses' could be closely mimicked by injections of exo genous oxytocin (0.03-1.0 ng kg(-1); lag time from jugular injection t o peak pressure rise, 20.44 +/- 0.99 s). The timing of this event coin cided with the small postpartum pulse measurable by radioimmunoassay. The larger but slower episodes of oxytocin release between births prod uced no response in the mammary gland. 5. These results show that endo genous oxytocin secretion during parturition is pulsatile, and that fe tal expulsion results in an additional but relatively small contributi on to circulating oxytocin, in the form of reflex postpartum pulses th at reach target after delivery is complete.