Fetal and postnatal oxytocin secretion in the pig before and after a prostaglandin challenge

Maturation of oxytocin secretion was studied in anaesthetized male and female 80 (n=42) and 105 (n=35) days-old fetal pigs and in conscious unrestrained piglets 4 weeks post partum (n = 11). Blood samples were taken from the maternal jugular vein and simultaneously from the exteriorized umbilical artery of fetuses (n=4/sow) kept in utero under acute experimental conditions. Four weeks-old piglets were catheterized chronically. After initial observation of basal oxytocin secretion PGF(2alpha) (0.5 mg in fetal and 2.5 mg in postnatal pigs as an intramuscular injection) was used as a challenge for oxytocin release. In sham groups physiological saline was injected. After treatment blood was collected up to 25 min (fetuses 80 days), 35 min (fetuses 105 days) or 125 min (piglets). Sampling interval was 5 min (fetuses 80 d) or 10 min (other groups). Maternal oxytocin concentrations remained at a stable level throughout the experiment and were not altered by PGF(2alpha) application to the fetus. In the fetus, preinjection baseline oxytocin levels were obtained for all age groups with no sex difference. PGF(2alpha) resulted in a rise (P<0.001) of fetal and postnatal oxytocin release within 5 min. In the day 80 fetal group only 60% of the animals responded with an increase of oxytocin concentration while 100% of the animals responded in all other age groups. Oxytocin levels remained elevated for the rest of the observation period in all age groups. Again, no sex difference was detected. The data demonstrate that oxytocin secretion is present and could be stimulated in the fetal pig as early as day 80 of fetal life. Maturational changes expressed by an increase in the number of fetuses responding to a challenge of PGF(2alpha) with a release of oxytocin become apparent between day 80 and 105 of pregnancy.