Oxytocin and the initiation of human parturition. I. Prostaglandin release during induction of labor by oxytocin.

Concentrations of plasma prostaglandins E and F and the 15-keto-13,14 dihydrometabolite of PGF2 alpha (PGFM) were determined by radioimmunoassay in 15 women who underwent induction of labor with oxytocin. Plasma PGFM rose significantly during the oxytocin infusion in nine women who went on to deliver vaginally but did not change in six women in whom induction of labor failed. Plasma PGE and PGF levels also rose during the infusion in the nine women with successful induction of labor but the changes were not statistically significant. In comparison to the six women in whom induction failed, however, plasma PGE in the nine women with successful induction reached significantly higher levels. Oxytocin infusions elicited uterine contractions of similar frequency in both groups of women, but the cervix failed to dilate in the six women in whom induction failed. The oxytocin-induced rise in plasma PGFM is, therefore, not simply a consequence of uterine contractions. We suggest that oxytocin stimulates PGF production in the pregnant uterus when it is appropriately sensitized to oxytocin, causing a potentiation of the oxytocin-induced contractions which is necessary for the contractions to become efficient in dilating the cervix. We further suggest that the stimulation of PGF production by oxytocin is mediated by oxytocin receptors, probably in the decidua.