Effect of fetal intravascular 4-aminoantipyrine infusions on myometrial activity (contractures) at 125 to 143 days' gestation in the pregnant sheep.

Myometrial activity of low amplitude and long duration--contractures--is present throughout gestation in the pregnant ewe and other species. This activity differs from the contractions of labor and delivery. Between 125 and 143 days' gestation, 4-aminoantipyrine infused into the fetus at rates that produced maternal uterine vein plasma concentrations of 4-aminoantipyrine of 5.95 +/- 1.23 (mean +/- SEM, n = 5) mg X 100 ml-1 markedly depressed both total myometrial electromyographic activity to 28.6% and frequency of contractures to 30.5% of preinfusion values. By 60 minutes of infusion, maternal uterine vein plasma 13,14-dihydro-15-keto prostaglandin F2 alpha concentration was reduced to 14% of the resting level. Two conclusions were drawn from this study. First, prostaglandins are involved in the regulation of contractures. Second, when infusions of 4-aminoantipyrine are used to quantify uterine or umbilical blood flow, the possibility exists that changes will occur in physiologic systems that are modified by prostaglandins.