Effects of inhibition of nitric oxide synthase on muscular arteries during exercise: nitric oxide does not contribute to vasodilation during exercise or in recovery

Background: Basal release of nitric oxide (NO) from the vascular endothelium regulates the tone of muscular arteries and resistance vasculature. Effects of NO on muscular arteries could be particularly important during exercise when shear stress may stimulate increased NO synthesis. Methods and Results: We investigated acute effects of NO synthase (NOS) inhibition on exercise haemodynamics using NG-monomethyl-L-arginine (L-NMMA), a non-selective NOS-inhibitor. Healthy volunteers (n=10, 5 female, 19-33 years) participated in a 2-phase randomised cross-over study, receiving L-NMMA (6mg/Kg, i.v. over 5 min) or placebo before bicycle exercise (25 to 150 W for 12 min). Blood pressure, cardiac output (measured by dilution of soluble and inert tracers) and femoral artery diameter were measured before, during and after exercise. At rest, L-NMMA reduced heart rate (by 16.2±4.3 bpm relative to placebo, P<0.01), increased peripheral vascular resistance (by 7.0 ±1.4 mmHg/L/min, P<0.001), mean arterial blood pressure (by 8.9±3.5 mmHg, P<0.05), and blunted an increase in femoral artery diameter that occurred immediately before exercise (change in diameter: 0.14±0.04 vs. 0.32±0.06 mm after L-NMMA and placebo, P<0.01). During/after exercise L-NMMA had no significant effect on peripheral resistance, cardiac output, or on femoral artery diameter. Conclusions: These results suggest that NO plays little role in modulating muscular artery function during exercise but that it may mediate changes in muscular artery tone immediately before exercise.