ELEVATION OF NITRATE LEVELS IN PREGNANT EWES AND THEIR FETUSES

OBJECTIVE: Nitric oxide is a potent vasodilator released by endothelia l cells that may play an important role in modulating maternal and fet al vascular tone in normal pregnancy. The current study was designed t o evaluate whether plasma or urine nitrite and nitrate (the metabolite s of nitric oxide) concentrations are elevated in pregnant compared wi th those of nonpregnant sheep and whether the nitrate concentrations i n the fetal circulation were increased in comparison with the maternal circulation. STUDY DESIGN: Eleven pregnant sheep and seven nonpregnan t oophorectomized sheep were instrumented with catheters in the matern al and fetal femoral arteries and veins, uterine and umbilical veins, and amniotic cavity. Blood, urine, and amniotic fluid samples were col lected for nitrate determination at least 5 days after surgery. After extraction nitrate was reduced to nitrite and quantitated with the Gri ess reagent. RESULTS: Arterial plasma nitrate concentrations in the pr egnant sheep were significantly elevated compared with those of nonpre gnant sheep (5.0 +/- 0.9 vs 2.5 +/- 0.6 mu mol/L, p < 0.05). The urina ry nitrate concentrations were also significantly increased in the pre gnant sheep compared with those of nonpregnant sheep (89.9 +/- 16.3 vs 23.1 +/- 4.5 nmol/mg creatinine, p < 0.01). Fetal plasma nitrate conc entrations were ninefold higher than the maternal nitrate concentratio ns (43.9 +/- 7 vs 5.0 +/- 0.9 mu mol/L, p < 0.01), whereas amniotic fl uid concentrations were extremely high (133.8 +/- 13.8 mu mol/L, n = 3 ). No venous-arterial differences were measurable across either the ma ternal or fetal sides of the placenta. CONCLUSION: Nitrate concentrati ons in pregnant sheep and their fetuses are increased. The increased n itrate concentrations in the maternal and fetal circulations may refle ct the increased nitric oxide synthesis, which may in part mediate the cardiovascular adaptations to normal pregnancy and the low systemic a nd umbilical vascular resistance in the fetus.