Stimulation of ovine placental lactogen secretion by arachidonic acid

To determine whether arachidonic acid stimulates the secretion of ovine placental lactogen (oPL), arachidonic acid was infused as an intravenous bolus into pregnant ewes and fetuses. Plasma oPL concentrations were determined in mothers and fetuses before and for 5 h after infusion. The administration of 12.5 mg arachidonic acid (0.15-0.2 mg/kg, n = 11 experiments) to the pregnant ewes caused an increase in maternal plasma oPL concentrations of 73.9 +/- 15.6% (S.E.M.) and 60.8 +/- 18.1% above the pretreatment concentrations at 4 and 5 h respectively (P less than 0.01 in each instance). The infusion of 25 mg arachidonic acid (n = 8) caused increases of 96.0 +/- 19.1% and 100.3 +/- 26.4% (P less than 0.005), and the stimulation was not inhibited by the cyclo-oxygenase inhibitors indomethacin and ibuprofen. In contrast to arachidonic acid, vehicle alone or palmitic acid had no effects on plasma oPL concentrations. Despite the increase in maternal plasma oPL concentrations, plasma oPL concentrations in the fetus remained unchanged after the maternal infusions. The infusion of arachidonic acid (0.5-1.5 mg/kg) directly into six fetuses had no effects on either fetal or maternal oPL concentrations. These studies indicate that arachidonic acid stimulates maternal plasma oPL concentrations but has no effect on fetal oPL concentrations and the stimulation of oPL secretion is not due to the conversion of arachidonic acid to prostaglandins or other cyclo-oxygenase products.