PO(2) Cycling Reduces Diaphragm Fatigue by Attenuating ROS Formation

Prolonged muscle exposure to low PO(2) conditions may cause oxidative stress resulting in severe muscular injuries. We hypothesize that PO(2) cycling preconditioning, which involves brief cycles of diaphragmatic muscle exposure to a low oxygen level (40 Torr) followed by a high oxygen level (550 Torr), can reduce intracellular reactive oxygen species (ROS) as well as attenuate muscle fatigue in mouse diaphragm under low PO(2). Accordingly, dihydrofluorescein (a fluorescent probe) was used to monitor muscular ROS production in real time with confocal microscopy during a lower PO(2) condition. In the control group with no PO(2) cycling, intracellular ROS formation did not appear during the first 15 min of the low PO(2) period. However, after 20 min of low PO(2), ROS levels increased significantly by ∼30% compared to baseline, and this increase continued until the end of the 30 min low PO(2) condition. Conversely, muscles treated with PO(2) cycling showed a complete absence of enhanced fluorescence emission throughout the entire low PO(2) period. Furthermore, PO(2) cycling-treated diaphragm exhibited increased fatigue resistance during prolonged low PO(2) period compared to control. Thus, our data suggest that PO(2) cycling mitigates diaphragm fatigue during prolonged low PO(2). Although the exact mechanism for this protection remains to be elucidated, it is likely that through limiting excessive ROS levels, PO(2) cycling initiates ROS-related antioxidant defenses.