Pulsatile secretion of oxytocin during parturition in the pig: temporal relationship with fetal expulsion.

Abstract

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 min1) 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-) 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 were 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 organs after delivery is complete.The precise effects of the high circulating oxytocin concentrations that are seen during second-stage labour remain unclear, with much of the uncertainty being due to a lack of information on release patterns during the rapid

The influence of estradiol and progesterone on the concentrations of uterine oxytocin receptors and plasma PGFM in response to oxytocin in ovariectomized gilts

“…The magnitude of the PGFM response above the baseline increased only in seven of the twenty-five studied gilts. The ineffectiveness of OT treatment was not due to the dose of the peptide used since the injection of 20 IU OT resulted in its plasma level being comparable to the values measured in sows during parturition [16], mating [22] or suckling [34]. Moreover, in our earlier study (Franczak, data unpublished) 20 IU OT were administered to cyclic gilts on days 15 to 18 of the oestrous cycle and resulted in a significant increase of plasma PGFM concentration.…”

“…However, in pigs, luteolysis is not preceded by a temporal increase in endometrial OTR [29,33]. In sows, the concentration of plasma PGF 2a metabolite (PGFM) significantly increased in response to the OT administered around luteolysis (days [14][15][16], but there is no response during the early luteal phase (days 4-6) and a weak response is found in the mid-luteal phase (days 9-12) of the oestrous cycle [6,20]. These data indirectly suggest that cyclic changes of estrogens (E 2 ) and the P 4 level during the oestrous cycle may influence uterine OTR concentration and PGF 2a response to OT.…”

Indomethacin blocks pre-partum nest building behaviour in the pig (Sus scrofa): effects on plasma prostaglandin F metabolite, oxytocin, cortisol and progesterone

“…Oxytocin Extraction from plasma using C 18 SepPak Cartridges (Waters Chromatography, Milford, MA, USA) and the RIA were carried out as described (Thornton et al 1986, Gilbert et al 1994, Boulton et al 1997b) using an antiserum previously described (Sheldrick & Flint 1981). The extraction recovery was 87%.…”

“…Food was withheld for 24 h before induction of anaesthesia (Gilbert et al 1994) and surgical placement of catheters (Burne et al 1999). Catheter patency and sterility were maintained as described (Burne et al 1999).…”

The effect of opioid antagonism and environmental restriction on plasma oxytocin and vasopressin concentrations in parturient gilts

“…One of the major roles of oxytocin in the progress of parturition is believed to be in the co-ordination of uterine contractions (rat: Summerlee 1981, Antonijevic et al 1995, rabbit: O'Byrne et al 1986, pig: Taverne et al 1979 and foetal expulsion (Gilbert et al 1994), with oxytocin receptors in the uterus increasing dramatically prior to parturition (Soloff et al 1979, Alexandrova & Soloff 1980.…”