WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• L-Arginine is a semiessential amino acid that is converted to nitric oxide (NO) by NO synthase (NOS).• NO improves endothelial function by elevating cyclic guanosine monophosphate.• However, oral L-arginine treatment in humans is hampered by extensive metabolism. WHAT THIS STUDY ADDS• Oral L-citrulline supplementation raises plasma L-arginine concentration and augments NO-dependent signalling in a dose-dependent manner.• L-Citrulline may thus be an alternative to L-arginine in patients with impaired NOS activity. AIMSOral L-arginine supplementation has been used in several studies to improve endothelium-dependent, nitric oxide (NO)-mediated vasodilation. L-Arginine treatment is hampered by extensive presystemic elimination due to intestinal arginase activity. In contrast, L-citrulline is readily absorbed and at least in part converted to L-arginine. The aim of our study was to assess this metabolic conversion and its subsequent pharmacodynamic effects. METHODSIn a double-blind, randomized, placebo-controlled cross-over study, 20 healthy volunteers received six different dosing regimes of placebo, citrulline, and arginine. Pharmacokinetic parameters (Cmax, Tmax, Cmin, AUC) were calculated after 1 week of oral supplementation. The ratio of plasma L-arginine over asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase (arginine/ADMA ratio), urinary cyclic guanosine monophosphate (cGMP) and nitrate excretion rates, and flow-mediated vasodilation (FMD) was measured to assess pharmacodynamic effects. RESULTSL-Citrulline dose-dependently increased AUC and Cmax of plasma L-arginine concentration more effectively than L-arginine (P < 0.01). The highest dose of citrulline (3 g bid) increased the Cmin of plasma L-arginine and improved the L-arginine/ADMA ratio from 186 Ϯ 8 (baseline) to 278 Ϯ 14 [P < 0.01, 95% confidence interval (CI) 66, 121]. Moreover, urinary nitrate and cGMP were increased from 92 Ϯ 10 to 125 Ϯ 15 mmol mmol -1 creatinine (P = 0.01, 95% CI 8, 58) and from 38 Ϯ 3.3 to 50 Ϯ 6.7 nmol mmol -1 creatinine (P = 0.04, 95% CI 0.4, 24), respectively. No treatment improved FMD over baseline. However, pooled analysis of all FMD data revealed a correlation between the increase of arginine/ADMA ratio and improvement of FMD. CONCLUSIONOur data show for the first time that oral L-citrulline supplementation raises plasma L-arginine concentration and augments NO-dependent signalling in a dose-dependent manner.

Background-Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, induces endothelial dysfunction. Although elevated ADMA has been associated with an increased risk of cardiovascular disease events and death in referral samples, the prognostic significance of ADMA in the community has not been adequately evaluated. Methods and Results-We related plasma ADMA, L-arginine (Arg), and the ratio of Arg to ADMA to the incidence of cardiovascular disease (fatal or nonfatal myocardial infarction, coronary insufficiency, angina pectoris, stroke or transient ischemic attack, intermittent claudication, or heart failure) and death in 3320 Framingham Offspring Study participants (1769 women; mean age, 59 years). Over a follow-up period of 10.9 years, 281 individuals of 2956 free of cardiovascular disease at baseline developed incident cardiovascular disease, and 285 participants died. In multivariable models adjusting for established risk factors and other biomarkers (B-type natriuretic peptide, renin, homocysteine, urine albumin excretion, and C-reactive protein), ADMA and the ratio of Arg to ADMA were significantly associated with all-cause mortality (adjusted-hazard ratio [HR] per 1-SD increment, 1.21; 95% CI, 1.07 to 1.37; and HR per 1-SD increment, 0.80; 95% CI, 0.69 to 0.93, respectively), whereas Arg was not (HR per 1-SD increment, 0.89; 95% CI, 0.77 to 1.02). We noted effect modification by diabetes status; ADMA was associated with death in individuals without diabetes (adjusted HR per 1-SD increment, 1.30; 95% CI, 1.13 to 1.50) but not in individuals with diabetes (adjusted HR per 1-SD increment, 0.85; 95% CI, 0.62 to 1.16). ADMA, Arg, and the ratio of Arg to ADMA were not associated with cardiovascular disease incidence (PϾ0.10). Conclusions-In our large community-based sample, ADMA was significantly associated with all-cause mortality, particularly in nondiabetic individuals. (Circulation. 2009;119:1592-1600.)

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Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of nitric oxide synthase (NOS). Elevated ADMA plasma levels have been reported in connection with diseases associated with an impaired endothelial L-arginine-NO pathway and endothelial dysfunction, such as atherosclerosis, hypercholesterolemia, chronic heart failure, diabetes mellitus, and hypertension. NO production by NOS is decreased due to elevated ADMA levels. In fact, there is increasing interest in determination of ADMA levels in samples of various origins. The aim of this work was to develop a precise and easy immunoassay in contrast to the existing methods, such as HPLC, liquid chromatography-mass spectrometry (LC-MS) and gas chromatography (GC)-MS. We determined cross-reactivity in our immunoassay of 1.2% for symmetric dimethylarginine and <0.02% for L-arginine. The limit of quantitation was 0.05 micromol/l. We found good correlation of the values measured when we compared our assay with LC-tandem MS (n = 29; r = 0.984; p < 0.0001). We determined ADMA levels in human serum and plasma, mouse and rat plasma, and cell culture supernatant. For human plasma we found a mean of 0.65 micromol/l in healthy subjects. In the plasma of mice and rats we found mean concentrations of 1.05 and 1.09 micromol/l, respectively.

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