N-acetylcysteine attenuates the decline in muscle Na+K+-pump activity and delays fatigue during prolonged exercise in humans

摘要

Reactive oxygen species (ROS) have been linked with both depressed Na⁺,K⁺-pump activity and skeletal muscle fatigue. This study investigated N-acetylcysteine (NAC) effects on muscle Na⁺,K⁺-pump activity and potassium (K⁺) regulation during prolonged, submaximal endurance exercise. Eight well-trained subjects participated in a double-blind, randomised, crossover design, receiving either NAC or saline (CON) intravenous infusion at 125 mg kg⁻¹ h⁻¹ for 15 min, then 25 mg kg⁻¹ h⁻¹ for 20 min prior to and throughout exercise. Subjects cycled for 45 min at 71% V˙O2peak, then continued at 92% V˙O2peak until fatigue. Vastus lateralis muscle biopsies were taken before exercise, at 45 min and fatigue and analysed for maximal in vitro Na⁺,K⁺-pump activity (K⁺-stimulated 3-O-methyfluorescein phosphatase; 3-O-MFPase). Arterialized venous blood was sampled throughout exercise and analysed for plasma K⁺ and other electrolytes. Time to fatigue at 92% V˙O2peak was reproducible in preliminary trials (c.v. 5.6 ± 0.6%) and was prolonged with NAC by 23.8 ± 8.3% (NAC 6.3 ± 0.5 versus CON 5.2 ± 0.6 min, P < 0.05). Maximal 3-O-MFPase activity decreased from rest by 21.6 ± 2.8% at 45 min and by 23.9 ± 2.3% at fatigue (P < 0.05). NAC attenuated the percentage decline in maximal 3-O-MFPase activity (%Δactivity) at 45 min (P < 0.05) but not at fatigue. When expressed relative to work done, the %Δactivity-to-work ratio was attenuated by NAC at 45 min and fatigue (P < 0.005). The rise in plasma [K⁺] during exercise and the Δ[K⁺]-to-work ratio at fatigue were attenuated by NAC (P < 0.05). These results confirm that the antioxidant NAC attenuates muscle fatigue, in part via improved K⁺ regulation, and point to a role for ROS in muscle fatigue.