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

¹Department of Exercise and Sport Science, Manchester Metropolitan University, Alsager ST7 2HL; and ²Department of Anaesthesia, Wythenshawe Hospital, Manchester M23 9LT, United Kingdom

Submitted 16 April 2003 ; accepted in final form 17 November 2003

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 (O₂) by nitric oxide and result in a speeding of phase II pulmonary O₂ 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 O₂ 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 O₂ 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 O₂, the duration of phase I, or the amplitude of the primary O₂ response. However, the time constant of the O₂ 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 O₂ 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 O₂ kinetics and a significant increase in the amplitude of the O₂ slow component in the transition to heavy-intensity cycle exercise in men. The speeding of the primary component O₂ 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 O₂ by nitric oxide. The cause of the larger O₂ 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.

respiratory; gas exchange

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