Simultaneous assessment of endothelial function, nitric oxide synthase activity, nitric oxide-mediated signaling, and oxidative stress in individuals with and without hypercholesterolemia

BACKGROUND: Endothelial function is impaired in hypercholesterolemia and atherosclerosis. Based on mostly indirect evidence, this impairment is attributed to reduced synthesis or impaired biological activity of endothelium-derived nitric oxide (NO). It was the aim of this study to directly estimate and compare whole-body NO production in normo- and hypercholesterolemia by applying a nonradioactive stable isotope dilution technique in vivo. METHODS: We enrolled 12 normocholesterolemic and 24 hypercholesterolemic volunteers who were all clinically healthy. To assess whole-body NO synthesis, we intravenously administered L-[guanidino-(N-15(2))]arginine and determined the urinary excretion of N-15-labeled nitrate, the specific end product of NO oxidation in humans, by use of gas chromatographymass spectrometry. In addition, we measured flow-mediated vasodilation (FMD) of the brachial artery expression of endothelial NOS (eNOS) in platelets: plasma concentration of the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA), and urinary excretion of 8-isoprostaglandin F-2 alpha (8-iso-PGF(2 alpha)). RESULTS: After infusion of L-[guanidino-(N-15(2))-arginine, cumulative excretion of N-15-labeled-nitrate during 48 h was 40% [95% CI 15%-66%] lower in hypercholesterolemic than normocholesterolemic volunteers [mean 9.2 (SE 0.8) mu mol vs 15.4 (2.3) mu mol/l, P = 0.003]. FMD was on average 36% [4%-67%] lower in hypercholesterolemic than normocholesterolemic volunteers [6.3 (4.0)% vs 9.4 (4.6)%, P = 0.027]. Normalized expression of NOS protein in platelets was also significantly lower in hypercholesterolemic volunteers, whereas there were no significant differences in plasma ADMA concentration or urinary excretion of 8-iso-PGF(2 alpha) between the 2 groups. CONCLUSIONS: This study provides direct evidence for a decreased whole body NO synthesis rate in healthy people with hypercholesterolemia. (C) 2007 American Association for Clinical Chemistry.