Effects of in vivo and in vitro L-arginine supplementation on healthy human vessels.

We studied the influence of dietary L-arginine (L-ARG) supplementation on forearm resistance arteries in vivo and the effect of exogenous addition of L-ARG to subcutaneous arteries isolated from gluteal biopsies. Twenty-six healthy males were recruited, and 16 were randomly allocated in a double-blind protocol to receive either oral L-ARG 20 g/day or placebo for 28 days. We examined responses to acetylcholine (ACh), sodium nitroprusside (SNP) and NG-monomethyl-L-arginine (L-NMMA) on forearm resistance arteries using venous occlusion plethysmography performed before and after supplementation of L-ARG (or placebo). L-ARG 20 g/day had no effect on plasma L-ARG levels (% mol based on total amino acid pool; before vs. after L-ARG 3.43 +/- 0.31 vs. 3.76 +/- 0.05), weekly blood pressure (BP) measurements, or plasma biochemical analysis of liver function enzymes, urea, or electrolyte levels. On the other hand, analysis of the major amino acids in plasma showed a significant difference in profile after L-ARG, but not placebo supplementation (Mann Whitney U test, p < 0.05), indicating a domino effect of chronic oral L-ARG supplementation on other amino acids. This may result from a change in appetite and thus protein intake after L-ARG supplementation. At the dose given, neither L-ARG nor placebo had any effect on forearm blood flow (FBF) responses to ACh (area under the dose-response curve, before vs. after L-ARG 1,763 +/- 260.1 vs. 1,862.8 +/- 163.6 U, Student's paired t test; p > 0.05), SNP, or L-NMMA. Gluteal skin biopsies were performed on 10 different untreated subjects. Subcutaneous arteries were isolated and mounted as ring preparations in isometric small vessel myographs. Full concentration-response curves to norepinephrine (NE), ACh, substance P, and a single response to SNP (10 microM) were obtained with and without addition of either L- or D-ARG 10 microM. Both L-ARG [-log EC50 (M) before vs. after arginine 7.12 +/- 0.15 vs. 6.66 +/- 0.16, Student's paired t test, p < 0.005] and D-ARG [-log EC50 (M) before vs. after arginine 7.36 +/- 0.17 vs. 6.85 +/- 0.18; Student's paired t test, p < 0.05] significantly antagonized responses to NE in subcutaneous arteries isolated from healthy humans. With the exception of a subset of vessels in which some endothelial dysfunction was observed, neither of the isomers of arginine had any effect on the responses to ACh, substance P, or SNP. In the subset vessels already described (n = 5), in which responses to ACh were < 90% maximal dilatation, L- but not D-ARG significantly increased the potency to ACh [-log EC50 (M) before vs. after L-ARG 7.42 +/- 0.20 vs. 8.27 +/- 0.28. Student's paired t test, p < 0.05]. We conclude that oral supplementation with L-ARG 20 g/day for 28 days does not affect endothelial function in normal healthy adults, possibly because the dose given in the current study was inadequate or because chronic oral administration leads to dissipation of arginine to other pathways, as evidenced by the change in total amino acid profile but not L-ARG plasma concentration, or because L-ARG cannot improve normal endothelium-mediated vasodilatation. These concepts are supported by our findings that responses to ACh and substance P were not altered by L-ARG in subcutaneous arteries isolated from healthy subjects.