Antioxidant treatments do not improve force recovery after fatiguing stimulation of mouse skeletal muscle fibres

The contractile performance of skeletal muscle declines during intense activities, i.e. fatigue develops. Fatigued muscle can enter a state of prolonged low-frequency force depression (PLFFD). PLFFD can be due to decreased tetanic free cytosolic [Ca2+] ([Ca2+](i)) and/or decreased myofibrillar Ca2+ sensitivity. Increases in reactive oxygen and nitrogen species (ROS/RNS) may contribute to fatigue-induced force reductions. We studied whether pharmacological ROS/RNS inhibition delays fatigue and/or counteracts the development of PLFFD. Mechanically isolated mouse fast-twitch fibres were fatigued by sixty 150ms, 70Hz tetani given every 1s. Experiments were performed in standard Tyrode solution (control) or in the presence of: NADPH oxidase (NOX) 2 inhibitor (gp91ds-tat); NOX4 inhibitor (GKT137831); mitochondria-targeted antioxidant (SS-31); nitric oxide synthase (NOS) inhibitor (l-NAME); the general antioxidant N-acetylcysteine (NAC); a cocktail of SS-31, l-NAME and NAC. Spatially and temporally averaged [Ca2+](i) and peak force were reduced by approximate to 20% and approximate to 70% at the end of fatiguing stimulation, respectively, with no marked differences between groups. PLFFD was similar in all groups, with 30Hz force being decreased by approximate to 60% at 30min of recovery. PLFFD was mostly due to decreased tetanic [Ca2+](i) in control fibres and in the presence of NOX2 or NOX4 inhibitors. Conversely, in fibres exposed to SS-31 or the anti ROS/RNS cocktail, tetanic [Ca2+](i) was not decreased during recovery so PLFFD was only caused by decreased myofibrillar Ca2+ sensitivity. The cocktail also increased resting [Ca2+](i) and ultimately caused cell death. In conclusion, ROS/RNS-neutralizing compounds did not counteract the force decline during or after induction of fatigue.