Some aspects on the mechanism of human labor and delivery

The balance between fetal oxytocin and placental leucine aminopeptidase (P-LAP) controls human uterine contraction around labor onset

A fetal pituitary hormone, oxytocin which causes uterine contractions, increases throughout gestation, and its increase reaches 10-fold from week 32 afterward. Oxytocin is, on the other hand, degraded by placental leucine aminopeptidase (P-LAP) which exists in both terminal villi and maternal blood. Maternal blood P-LAP increases with advancing gestation under the control of non-genomic effects of progesterone, which is also produced from the placenta. Progesterone is converted to estrogen by CYP17A1 localized in the fetal adrenal gland and placenta at term. The higher oxytocin concentrations in the fetus than in the mother demonstrate not only fetal oxytocin production but also its degradation and/or inhibition of leakage from fetus to mother by P-LAP. Until labor onset, the pregnant uterus is quiescent possibly due to the balance between increasing fetal oxytocin and P-LAP under control of progesterone. A close correlation exists between the feto-placental and maternal units in the placental circulation, although the blood in the two circulations does not necessarily mix. Fetal maturation results in progesterone withdrawal via the CYP17A1 activation accompanied with fetal oxytocin increase. Contribution of fetal oxytocin to labor onset has been acknowledged through the recognition that the effect of fetal oxytocin in the maternal blood is strictly regulated by its degradation by P-LAP under the control of non-genomic effects of progesterone. In all senses, the fetus necessarily takes the initiative in labor onset.

Effect of pregnancy on the metabolic clearance rate and the volume of distribution of oxytocin in the baboon

Pharmacokinetic parameters of oxytocin (OT) metabolism were determined during the last third of pregnancy and again 4-8 wk after delivery in the baboon. Animals were placed on a tether system with venous and arterial access and a continuous monitoring of uterine contractions during gestation. Two methods of determining OT pharmacokinetics were utilized (bolus injection vs. continuous infusion). The metabolic clearance rate of OT as determined during the bolus trials (n = 7) was 22.2 +/- 1.5 ml.min-1.kg-1 in pregnancy and 16.3 +/- 1.4 ml.min-1.kg-1 postpartum (P < 0.05), respectively, and 23.7 +/- 2.8 vs. 16.9 +/- 3.7 ml.min-1.kg-1 (P < 0.05), respectively, as determined during the 1-h infusion trials (n = 4). The initial dilution volume and the volume of distribution at steady state of OT after administration did not differ between pregnant and postpartum animals (P > 0.05). The mean residence time (MRT) of OT was shorter during pregnancy, 7.7 +/- 0.8 vs. 10.8 +/- 1.2 min postpartum (P < 0.05). In summary, OT metabolism during pregnancy in the baboon is characterized by 1) increased clearance rate (1.4-fold), 2) accelerated turnover due to the shorter MRT, and 3) unaltered distribution.