Li Zuo, Benjamin K Pannell, Anthony T Re, Thomas M Best, Peter D Wagner

PO2 cycling, often referred to as intermittent hypoxia, involves exposing tissues to brief cycles of low oxygen environments immediately followed by hyperoxic conditions. After experiencing long-term hypoxia, muscle can be damaged during the subsequent reintroduction of oxygen, which leads to muscle dysfunction via reperfusion injury. The protective effect and mechanism behind PO2 cycling in skeletal muscle during reoxygenation have yet to be fully elucidated. We hypothesize that PO2 cycling effectively increases muscle fatigue resistance through reactive oxygen species (ROS), protein kinase B (Akt), extracellular signal-regulated kinase (ERK), and certain mitochondrial channels during reoxygenation. Using a dihydrofluorescein fluorescent probe, we detected the production of ROS in mouse diaphragmatic skeletal muscle in real time under confocal microscopy. Muscles treated with PO2 cycling displayed significantly attenuated ROS levels (n = 5; p < 0.001) as well as enhanced force generation when compared to controls during reperfusion (n = 7; p < 0.05). We also used inhibitors for signaling molecules or membrane channels such as ROS, protein kinase B (Akt), extracellular signal-regulated kinase (ERK), as well as chemical stimulators to close mitochondrial ATP-sensitive potassium channel (KATP) or open mitochondrial permeability transition pore (mPTP). All these blockers or stimulators abolished improved muscle function with PO2 cycling treatment. This current investigation has discovered a correlation between KATP and mPTP and the PO2 cycling pathway in diaphragmatic skeletal muscle. Thus, we have identified a unique signaling pathway which may involve ROS, Akt, ERK and mitochondrial channels responsible for PO2 cycling protection during reoxygenation conditions in the diaphragm.