Effects of intravenous N-acetylcysteine infusion on time to fatigue and potassium regulation during prolonged cycling exercise.

The production of reactive oxygen species in skeletal muscle is linked with muscle fatigue. This study investigated whether the antioxidant compound N-acetylcysteine (NAC) augments time to fatigue during prolonged, submaximal cycling exercise. Seven men completed a double-blind, crossover study, receiving NAC or placebo before and during cycling exercise, comprising 45 min at 70% of peak oxygen consumption (Vo2 peak) and then to fatigue at 90% Vo2 peak. NAC was intravenously infused at 125 mg.kg-1.h-1 for 15 min and then 25 mg.kg-1.h-1 for 20 min before and throughout exercise, which was continued until fatigue. Arterialized venous blood was analyzed for NAC concentration, hematology, and plasma electrolytes. NAC induced no serious adverse reactions and did not affect hematology, acid-base status, or plasma electrolytes. Time to fatigue was reproducible in preliminary trials (coefficient of variation 7.4 +/- 1.2%) and was not augmented by NAC (NAC 14.6 +/- 4.5 min; control 12.8 +/- 5.4 min). However, time to fatigue during NAC trials was correlated with Vo2 peak (r = 0.78; P < 0.05), suggesting that NAC effects on performance may be dependent on training status. The rise in plasma K+ concentration at fatigue was attenuated by NAC (P < 0.05). The ratio of rise in K+ concentration to work and the percentage change in time to fatigue tended to be inversely related (r = -0.71; P < 0.07). Further research is required to clarify a possible training status-dependent effect of NAC on muscle performance and K+ regulation.