Malcolm J. Brown

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

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 (%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.

Infusion with the antioxidant N-acetylcysteine attenuates early adaptive responses to exercise in human skeletal muscle

Aim:  Production of reactive oxygen species (ROS) in skeletal muscle is markedly increased during exercise and may be essential for exercise adaptation. We, therefore, investigated the effects of infusion with the antioxidant N‐acetylcysteine (NAC) on exercise‐induced activation of signalling pathways and genes involved in exercise adaptation in human skeletal muscle.

Alkalosis increases muscle K+ release, but lowers plasma [K+] and delays fatigue during dynamic forearm exercise

Alkalosis enhances human exercise performance, and reduces K+ loss in contracting rat muscle. We investigated alkalosis effects on K+ regulation, ionic regulation and fatigue during intense exercise in nine untrained volunteers. Concentric finger flexions were conducted at 75% peak work rate (∼3 W) until fatigue, under alkalosis (Alk, NaHCO3, 0.3 g kg−1) and control (Con, CaCO3) conditions, 1 month apart in a randomised, double‐blind, crossover design. Deep antecubital venous (v) and radial arterial (a) blood was drawn at rest, during exercise and recovery, to determine arterio‐venous differences for electrolytes, fluid shifts, acid–base and gas exchange. Finger flexion exercise barely perturbed arterial plasma ions and acid–base status, but induced marked arterio‐venous changes. Alk elevated [HCO3−] and P  CO 2, and lowered [H+] (P < 0.05). Time to fatigue increased substantially during Alk (25 ± 8%, P < 0.05), whilst both [K+]a and [K+]v were reduced (P < 0.01) and [K+]a‐v during exercise tended to be greater (P= 0.056, n= 8). Muscle K+ efflux at fatigue was greater in Alk (21.2 ± 7.6 µmol min−1, 32 ± 7%, P < 0.05, n= 6), but peak K+ uptake rate was elevated during recovery (15 ± 7%, P < 0.05) suggesting increased muscle Na+,K+‐ATPase activity. Alk induced greater [Na+]a, [Cl−]v, muscle Cl− influx and muscle lactate concentration ([Lac−]) efflux during exercise and recovery (P < 0.05). The lower circulating [K+] and greater muscle K+ uptake, Na+ delivery and Cl− uptake with Alk, are all consistent with preservation of membrane excitability during exercise. This suggests that lesser exercise‐induced membrane depolarization may be an important mechanism underlying enhanced exercise performance with Alk. Thus Alk was associated with improved regulation of K+, Na+, Cl− and Lac−.

Antioxidant treatment with N‐acetylcysteine regulates mammalian skeletal muscle Na+–K+‐ATPase α gene expression during repeated contractions

Exercise increases Na+–K+ pump isoform gene expression and elevates muscle reactive oxygen species (ROS). We investigated whether enhanced ROS scavenging induced with the antioxidant N‐acetylcysteine (NAC) blunted the increase in Na+–K+ pump mRNA during repeated contractions in human and rat muscle. In experiment 1, well‐trained subjects received saline or NAC intravenously prior to and during 45 min cycling. Vastus lateralis muscle biopsies were taken pre‐infusion and following exercise. In experiment 2, isolated rat extensor digitorum longus muscles were pre‐incubated without or with 10 mm NAC and then rested or stimulated electrically at 60 Hz for 90 s. After 3 h recovery, muscles were frozen. In both experiments, the muscles were analysed for Na+–K+ pump α1, α2, α3, β1, β2 and β3 mRNA. In experiment 1, exercise increased α2 mRNA by 1.0‐fold (P= 0.03), but α2 mRNA was reduced by 0.40‐fold with NAC (P= 0.03). Exercise increased α3, β1 and β2 mRNA by 2.0‐ to 3.4‐fold (P < 0.05), but these were not affected by NAC (P > 0.32). Neither exercise nor NAC altered α1 or β3 mRNA (P > 0.31). In experiment 2, electrical stimulation increased α1, α2 and α3 mRNA by 2.3‐ to 17.4‐fold (P < 0.05), but these changes were abolished by NAC (P > 0.07). Electrical stimulation almost completely reduced β1 mRNA but only in the presence of NAC (P < 0.01). Neither electrical stimulation nor NAC altered β2 or β3 mRNA (P > 0.09). In conclusion, NAC attenuated the increase in Na+–K+ pump α2 mRNA with exercise in human muscle and all α isoforms with electrical stimulation in rat muscle. This indicates a regulatory role for ROS in Na+–K+ pump α isoform mRNA in mammalian muscle during repeated contractions.