Serum nitrite sensitively reflects endothelial NO formation in human forearm vasculature: evidence for biochemical assessment of the endothelial L-arginine-NO pathway

Objective: A reduced bioactivity of endothelial nitric oxide (NO) has been implicated in the pathogenesis of atherosclerosis. In humans, the endotheli al L-arginine-NO pathway has been indirectly assessed via the flow response to endothelium-dependent vasodilators locally administered into the corona ry, pulmonary or forearm circulation. However, biochemical quantification o f endothelial NO formation in these organ circulations has been hampered so far because of the rapid metabolism of NO. Therefore, we aimed to work out a reliable biochemical index to assess endothelial NO formation in human c irculation. Methods: In 33 healthy volunteers, forearm blood flow (FBF) was measured by standard techniques of venous occlusion plethysmography at res t, after local application of the endothelium-dependent vasodilator acetylc holine (ACH), the endothelium-independent vasodilator papaverine (PAP), the stereospecific inhibitor of endothelial NO synthase (eNOS) L-NMMA, and L-a rginine (ARG), the natural substrate of eNOS. In parallel, nitrite and nitr ate concentrations in blood samples taken from the antecubital vein were me asured by HPLC using anion-exchange chromatography in combination with elec trochemical and ultraviolet detection following a specific sample preparati on method. Results: ACH dose-dependently increased resting FBF (from 3.0+/- 0.3 to 10.4+/-0.9 ml/min per 100 mi tissue) and serum nitrite concentration (from 402+/-59 to 977+/-82 nmol/l, both p<0.05, n=12). A significant corre lation was observed between the changes in FBF and the serum nitrite concen tration (r=0.61, p<0.0001). L-NMMA reduced resting FBF and endothelium-depe ndent vasodilation by 30% and this was paralleled by a significant reductio n in serum nitrite concentration at the highest dose of ACH (n=9, p<0.001). PAP increased FBF more than fourfold, but did not affect serum nitrite con centration (n=11), whereas ARG significantly increased both FBF and nitrite . Basal serum nitrate amounted to 25+/-4 mu mol/l and remained constant dur ing the application of AGH, PAP and L-NMMA. Conclusions: The concentration of serum nitrite sensitively reflects changes in endothelial NO formation i n human forearm circulation. This biochemical measure may help to character ize the L-arginine-NO pathway in disease states associated with endothelial dysfunction and to further elucidate its pathophysiological significance f or the development of atherosclerosis in humans. (C) 1999 Elsevier Science B.V. All rights reserved.