N-acetylcysteine fails to modulate the in vitro function of sarcoplasmic reticulum of diaphragm in the final phase of fatigue

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Abstract
In the present study, we tested the hypothesis whether N-acetylcysteine (NAC), a non-specific antioxidant, might influence fatigue by modulating Ca2+-handling capacity by the sarcoplasmic reticulum (SR). In the presence (10 mm) or absence of NAC, bundles of rat diaphragm were stimulated with tetanic trains (350 ms, 30-40 Hz) at 1 train every 2 s for 300 s. SR functions, as assessed by SR Ca2+-uptake and release rates and SR Ca2+-ATPase activity, were measured in vitro on muscle homogenates. Following the 300-s stimulation, the force developed by NAC-treated muscles is approximately 1.8-fold higher (P < 0.05) than that of muscles without NAC treatment. Stimulation elicited an 18-30% depression in SR function (P < 0.05). Despite the differing degrees of fatigue between NAC-treated and non-treated muscles, SR functions in these muscles were reduced to similar extents. These results suggest that modulation of SR function measured in vitro may not be a major contributor to inhibition of diaphragmic fatigue with antioxidant, at least, in the final phase of fatigue where force output is remarkably reduced.