Influence of N-acetylcysteine administration on pulmonary O₂ uptake kinetics and exercise tolerance in humans.

We investigated the influence of the antioxidant N-acetylcysteine (NAC) on plasma nitrite concentration ([NO₂⁻]), pulmonary oxygen uptake (V(O₂)) kinetics and exercise tolerance. Eight males completed 'step' moderate- and severe-intensity cycle exercise tests following infusion of either NAC (125 mg kg⁻¹ h⁻¹ for 15 min followed by 25 mg kg⁻¹ h⁻¹ until the termination of exercise) or Placebo (PLA; saline). Following the initial loading phase, NAC infusion elevated plasma free sulfhydryl groups compared to placebo (PLA: 4 ± 2 vs. NAC: 13 ± 3 μ M g⁻¹; P < 0.05) and this elevation was preserved throughout the protocol. The administration of NAC did not significantly influence plasma [NO₂⁻] or V(O₂) kinetics during either moderate- or severe-intensity exercise. Although NAC did not significantly alter severe-intensity exercise tolerance at the group mean level (PLA: 776 ± 181 vs. NAC: 878 ± 284 s; P > 0.05), there was appreciable inter-subject variability in the response: four subjects had small reductions in exercise tolerance with NAC compared to PLA (-4%, -8%, -11%, and -14%) while the other four showed substantial improvements (+24%, +24%, +40%, and +69%). The results suggest that exercise-induced redox perturbations may contribute to fatigue development in recreationally-active adults.