ObjectiveTo establish the role played by the circulating nitric oxide synthase inhibitorsNG-monomethyl-L-arginine (L-NMMA), asymmetrical dimethyl arginine (ADMA) and symmetric dimethyl arginine (SDMA) and its association with hypertension of children and adolescents.DesignWe measured plasma concentrations ofL-NMMA, ADMA and SDMA in 38 hypertensives (median age 7.7 years) and in nine healthy normotensive controls (median age 8.2 years) using high-performance liquid chromatography. In addition, their plasma renin activity was determined. The subjects' glomerular filtration rates were calculated from plasma creatinine and height measurements. To determine the vasoactive potency of the arginine analogues, concentration–response curves were plotted for the responses in isolated endothelium-intact and endothelium-denuded mouse aortic rings that had been pre-contracted by administration of a threshold concentration of phenylephrine.ResultsPlasma ADMA and SDMA concentrations in members of the hypertensive group [0.23 ± 0.03 and 1.37 ± 0.06 μmol/l, respectively (means ± SEM)] were significantly higher than those in members of the control group (ADMA 0.10 ± 0.01 μmol/l and SDMA 1.18 ± 0.06 μmol/l). Plasma concentrations ofL-NMMA were similar in members of the hypertensive (0.21 ± 0.01 μmol/l) and control (0.18 ± 0.02 μmol/l) groups. The glomerular filtration rate of the hypertensive group was below normal [70.4 ± 5.4 ml/min per 1.73 m2(mean ± SEM)] and was significantly associated with elevated plasma concentrations of ADMA (r = −0.77,P< 0.001), SDMA (r = −0.38,P= 0.02) andL-NMMA (r = 0.35,P= 0.03). Higher plasma ADMA concentrations were associated with a lower plasma renin activity (r = −0.36,P= 0.04). The vasoactive potencies of ADMA (concentration for half-maximal effect with the endothelium intact 25.4 ± 7.1 μmol/l) andL-NMMA (concentration for half-maximal effect with the endothelium intact 8.2 ± 2.9 μmol/l) was significantly (P< 0.05) greater than that of SDMA. Both ADMA andL-NMMA (at 3 μmol/ l concentrations) initiated a significant vasocontractile response from baseline (P= 0.03 andP< 0.001, respectively). These effects were absent after the endothelium had been removed. SDMA had no effect.ConclusionsPlasma ADMA and SDMA levels are increased in hypertensive children. By inference from in-vitro data, ADMA appears to attain sufficient concentrations to produce a significant change in vascular tone and hence might play a role in the pathophysiology of childhood hypertension.