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Determination of nitric oxide (NO) synthesis in vivo is essential to understand the pathophysiologic role and therapeutic implications of the L-arginine/NO pathway in pediatric diseases. The aim of this study was to establish a noninvasive, sensitive, specific, and reliable approach to determine whole-body NO synthesis in healthy children. Seventeen healthy children (eight boys/nine girls, 4-16 y) were studied twice, and six of them on three occasions. Fasting children received a single oral dose of nonradioactive L-[15N]2-guanidino arginine (5 mg/kg body weight). Complete 24-h urine collections were subsequently performed on an ambulatory basis. Total urinary nitrate excretion and [15N]nitrate enrichments were determined using high-pressure liquid chromatography and gas chromatography-isotope ratio mass spectrometric techniques. The mean urinary [15N]nitrate enrichments on the 0-12-h/12-24-h collection periods of three study visits were 0.9309%/0.5910%, 0.9056%/0.6214%, and 0.9087%/0.6059%. The levels of 24-h urinary [15N]nitrate excretion [mean (95% confidence interval)] for three study visits were 11.70 (8.85-14.54), 12.21 (9.61-14.82), and 11.37 (7.96-14.77) [mu]g [15N]nitrogen-nitrate/mmol creatinine, respectively. Within-subject coefficient of variation for 24-h urinary [15N]nitrate excretion was 11.87%. Agreement among results was assessed by intraclass correlation coefficient (0.93) and coefficient of repeatability (4.08). The percentage of L-[15N]2-guanidino arginine dose directed to nitric oxide synthesis was 0.221% [0.181-0.261]. Multiple regression analysis showed age as the predictor variable of whole-body NO synthesis. These results show for the first time that a single oral administration of L-[15N]2-guanidino arginine can be used to reliably and specifically determine whole-body NO synthesis in children.

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