Nitric oxide biomarkers increase during exercise-induced vasodilation in the forearm

The purpose of the study was to determine if exercise-induced vasodilation was associated with an increase in forearm plasma levels of nitric oxide (NO) biomarkers (NO₂^- + NO₃^- and L-citrulline). Twelve healthy subjects (27 ± 6 yrs) performed incremental rhythmic forearm exercise with the nondominant hand for 6 min each at 15, 30 and 45% of maximal voluntary contraction (MVC). Forearm blood flow (FBF) was determined in the exercise arm using venous occlusion plethysmography. Blood samples were obtained from the antecubital vein of the exercise and nonexercise arms for the measurement of NO biomarkers. In the exercise arm, FBF increased by a mean of 150%, 335% and 585% above baseline at 15, 30 and 45% of MVC, respectively. (ANOVA, P = 0.0001). Venous plasma NO₂^- + NO₃^- levels increased from 24 ± 4 μmol/L at baseline, to 29 ± 5, 32 ± 4 and 35 ± 4 μmol/L (ANOVA, P = 0.0001). Venous plasma L-citrulline levels increased from 31 ± 5 μmol/L at baseline to 58 ± 10, 87 ± 7 and 141 ± 15 μmol/L (ANOVA, P = 0.0001). There was a linear relationship between FBF and venous plasma NO₂^- + NO₃^- (slope = 0.38 ± 0.10, P = 0.0007) and between L-citrulline, (slope = 5.1 ± 1.3, P = 0.0004). Venous plasma levels of NO₂^- + NO₃^- and L-citrulline in the nonexercise arm were unchanged. These results demonstrate that exercise-induced vasodilation in the forearm is associated with forearm plasma levels of NO₂^- + NO₃^- and L-citrulline, in vivo markers of NO production.