NITRIC-OXIDE EXCRETION IN NORMAL AND HYPERTENSIVE PREGNANCIES

Objective: To determine whether nitric oxide (NO) excretion, assessed by the measurement of NO breakdown products, was increased in normal pregnancy but reduced in preeclampsia (PE). Methods: The study was conducted in a university teaching hospital and in the antenatal wards of a private community cooperative hospital. Measurements of urinary NO breakdown products were made on 24-h urines collected from 31 nonpregnant age-matched women, 56 normal pregnant women in their third trimester, 18 women with mild preeclampsia (equivalent to ''gestational'' or ''transient'' hypertension in pregnancy in other classifications), 30 women with severe preeclampsia (87% with proteinuria), and 13 women with essential hypertension. All women were eating a free diet. Main Outcome Measure: Twenty-four-hour urinary nitrite/nitrate excretion. Results: Urinary nitrate/nitrite excretion was similar among groups-micromoles per day, median (interquartile range): nonpregnant, 982 (763, 1534); normal pregnant, 980 (673, 1274); mild preeclampsia, 912 (684, 1307); severe preeclampsia, 899 (689, 1204); essential hypertension, 885 (821, 1302). Results were also similar among groups when nitrate/nitrite excretions were measured per millimole creatinine excreted. Conclusions: This study demonstrates that urinary nitrate/nitrite excretion is similar in normal and hypertensive pregnancies, specifically preeclampsia. One explanation for this finding is that small differences in nitrate production do exist among groups, but that these were masked by variable nitrate intake. Alternatively, these findings may mean that systemic and/or renal NO production is unaltered in normal and hypertensive pregnancy, and that reduced NO production is not part of the pathogenesis of preeclampsia. It is not possible to standardize diets of severely preeclamptic women (as they are very often delivered soon after admission to hospital), and future studies in human pregnancy will need to seek better methods of assessing nitric oxide production.