Influence of L-NAME on pulmonary O2 uptake kinetics during heavy-intensity cycle exercise

We hypothesized that inhibition of nitric oxide synthase (NOS) by N(G)-nitro-L-arginine methyl ester (L-NAME) would alleviate the inhibition of mitochondrial oxygen uptake (Vo(2)) by nitric oxide and result in a speeding of phase II pulmonary Vo(2) kinetics at the onset of heavy-intensity exercise. Seven men performed square-wave transitions from unloaded cycling to a work rate requiring 40% of the difference between the gas exchange threshold and peak Vo(2) with and without prior intravenous infusion of L-NAME (4 mg/kg in 50 ml saline over 60 min). Pulmonary gas exchange was measured breath by breath, and Vo(2) kinetics were determined from the averaged response to two exercise bouts performed in each condition. There were no significant differences between the control (C) and L-NAME conditions (L) for baseline Vo(2), the duration of phase I, or the amplitude of the primary Vo(2) response. However, the time constant of the Vo(2) response in phase II was significantly smaller (mean +/- SE: C: 25.1 +/- 3.0 s; L: 21.8 +/- 3.3 s; P < 0.05), and the amplitude of the Vo(2) slow component was significantly greater (C: 240 +/- 47 ml/min; L: 363 +/- 24 ml/min; P < 0.05) after L-NAME infusion. These data indicate that inhibition of NOS by L-NAME results in a significant (13%) speeding of Vo(2) kinetics and a significant increase in the amplitude of the Vo(2) slow component in the transition to heavy-intensity cycle exercise in men. The speeding of the primary component Vo(2) kinetics after L-NAME infusion indicates that at least part of the intrinsic inertia to oxidative metabolism at the onset of heavy-intensity exercise may result from inhibition of mitochondrial Vo(2) by nitric oxide. The cause of the larger Vo(2) slow-component amplitude with L-NAME requires further investigation but may be related to differences in muscle blood flow early in the rest-to-exercise transition.