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

Abstract

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% , then continued at 92% 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% 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 (%Deltaactivity) at 45 min (P < 0.05) but not at fatigue. When expressed relative to work done, the %Deltaactivity-to-work ratio was attenuated by NAC at 45 min and fatigue (P < 0.005). The rise in plasma [K(+)] during exercise and the Delta[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.

Figure 1. Skeletal muscle maximal in vitro K⁺ stimulated 3-O-methylfluorescein phosphatase activity (3-O-MFPase, Na⁺,K⁺-pump activity), measured preinfusion at rest, after 45 min cycling at 71% V˙_O2peak, and then after cycling at 92% V˙_O2peak continued to fatigue

Subjects were well trained individuals and were infused intravenously with either saline (CON, filled bar), or N-acetylcysteine (NAC, open bar). The muscle maximal 3-O-MFPase activity is expressed as nmol min⁻¹ (g wt weight)⁻¹ *less than preinfusion (time main effect; P < 0.001). Values are means ± s.e.m.; n = 8.

Figure 2. Percentage (%) change from resting preinfusion in muscle maximal in vitro 3-O-MFPase activity during prolonged submaximal exercise in well trained individuals

A, percentage change in maximal in vitro 3-O-MFPase activity; B, percentage change in maximal in vitro 3-O-MFPase activity-to-work ratio, at 45 min and at fatigue. Filled bars, CON; open bars, NAC. †NAC < CON (P < 0.05). Values are means ± s.e.m.; n = 8.

Figure 3. Effect of NAC (•) and CON (▵) infusion on plasma potassium concentration ([K⁺]) during and after prolonged submaximal exercise

Shaded bar denotes exercise, comprising 45 min at 71% V˙_O2peak, then continued to fatigue (F) at 92% V˙_O2peak. *Different from preinfusion (time main effect, P < 0.005), **greater than 45 min (P < 0.05). Values are means ± s.e.m.;n = 8.

Figure 4. Effect of NAC and CON infusion on the rise in plasma K⁺ above preinfusion (Δ[K⁺]) during prolonged submaximal exercise

Filled bars, CON; open bars, NAC. *different from 15 min (time main effect; P < 0.05); †NAC < CON (treatment main effect; P < 0.05). Values are means ± s.e.m.; n = 8.