Arginine flux and nitric oxide production during human pregnancy and postpartum

OBJECTIVE: To compare second-trimester, third-trimester, and postpartum arginine flux and nitric oxide production using infusions of the stable isotope L-[¹⁵N₂]-arginine in normal human gestation. METHODS: Kinetic measurements were made in pregnant volunteers with uncomplicated singleton gestations in mid gestation, late gestation, and more than 8 weeks postpartum. A bolus of 4.95 μmol/kg of labeled arginine was administered, followed by an infusion at 4.95 μmol/kg per hour for 6 hours. The isotopic enrichment of plasma arginine and nitrite or nitrate (NO_x) was determined by gas chromatography, mass spectrometry, or both. We used the Kolmogorov-Smirnov test for normality, repeated-measures analysis of variance, and Newman-Keuls test. P < .05 denoted statistical significance. RESULTS: The rate of turnover of the intravascular NO_x pool was significantly higher in mid gestation compared with late gestation and almost reached statistical significance when compared with postpartum values (6.2 ± 0.9 versus 4.3 ± 0.8 [P < .02] versus 3.7 ± 2.1% pool/hour; P = .08). Arginine flux was significantly higher in early compared with late gestation and postpartum (107.8 ± 13.9 versus 72.5 ± 16.1 versus 82 ± 8.8 μmol/kg per hour, respectively; P < .01). CONCLUSION: Arginine and nitric oxide production is higher in mid gestation. This suggests a role for nitric oxide in early cardiovascular adaptation in human gestations.