Repeated hyperbaric oxygen exposure accelerates fatigue and impairs SR-calcium release in mice

Breathing hyperoxic gas is common in diving and accelerates fatigue after prolonged and repeated exposure. The mechanism(s) remain unknown but may be related to increased oxidants that interfere with skeletal muscle calcium trafficking or impair aerobic ATP production. To determine these possibilities, C57BL/6J mice were exposed to hyperbaric oxygen (HBO₂) for 4-h on three consecutive days or remained in room air. Post-final exposure, fatigue was determined by grip strength and run to exhaustion tests. Other measurements included indices of oxidant stress and antioxidant defenses, mitochondrial bioenergetics, caffeine-induced sarcoplasmic reticulum-calcium release, and S-nitrosylation of ryanodine receptor 1 (RyR1). Despite grip strength being unaffected by repeated HBO₂ exposure, mean running time was reduced by 50%. In skeletal muscle from HBO₂ exposed mice, superoxide production was significantly increased, resulting in elevated lipid and DNA (nuclear and mitochondrial) oxidation. Accompanying increased oxidant stress was a reduction in glutathione content and increased Sod1 and Hmox1 gene expression; Ucp3 mRNA was reduced. Mitochondrial respiration, membrane potential, and NAD⁺/NADH were not influenced by HBO₂. In contrast, caffeine induced SR-calcium release was reduced by 66% and S-nitrosylation of RyR1 was increased by 45%. Exposing mice to repeated HBO₂ increases oxidant stress that activates some antioxidant responses. Mitochondrial function is not altered and could be related to decreased production of UCP3 that serves to maintain the electrochemical proton gradient. S-nitrosylation of RyR1 may promote SR-calcium leak and reduce content, a potential mechanism for repeated HBO₂-induced fatigue.