Incubation with sodium nitrite attenuates fatigue development in intact single mouse fibres at physiological P O 2 .

Dietary nitrate (NO3 - ) supplementation, which increases plasma nitrite (NO2 - ) concentration, has been reported to attenuate skeletal muscle fatigue development. Sarcoplasmic reticulum (SR) calcium (Ca2+ ) release is enhanced in isolated single skeletal muscle fibres following NO3 - supplementation or NO2 - incubation at a supra-physiological P O 2 but it is unclear whether NO2 - incubation can alter Ca2+ handling and fatigue development at a near-physiological P O 2 . We hypothesised that NO2 - treatment would improve Ca2+ handling and delay fatigue at a physiological P O 2 in intact single mouse skeletal muscle fibres. Each muscle fibre was perfused with Tyrode solution pre-equilibrated with either 20% ( P O 2 ∼150 Torr) or 2% O2 ( P O 2  = 15.6 Torr) in the absence and presence of 100 µM NaNO2 . At supra-physiological P O 2 (i.e. 20% O2 ), time to fatigue was lowered by 34% with NaNO2 (control: 257 ± 94 vs. NaNO2 : 159 ± 46 s, Cohen's d = 1.63, P < 0.05), but extended by 21% with NaNO2 at 2% O2 (control: 308 ± 217 vs. NaNO2 : 368 ± 242 s, d = 1.14, P < 0.01). During the fatiguing contraction protocol completed with NaNO2 at 2% O2 , peak cytosolic Ca2+ concentration ([Ca2+ ]c ) was not different (P > 0.05) but [Ca2+ ]c accumulation between contractions was lower, concomitant with a greater SR Ca2+ pumping rate (P < 0.05) compared to the control condition. These results demonstrate that increased exposure to NO2 - blunts fatigue development at near-physiological, but not at supra-physiological, P O 2 through enhancing SR Ca2+ pumping rate in single skeletal muscle fibres. These findings extend our understanding of the mechanisms by which increased NO2 - exposure can mitigate skeletal muscle fatigue development.