Reactive oxygen species formation in the transition to hypoxia in skeletal muscle

Many tissues produce reactive oxygen species (ROS) during reoxygenation after hypoxia or ischemia; however, whether ROS are formed during hypoxia is controversial. We tested the hypothesis that ROS are generated in skeletal muscle during exposure to acute hypoxia before reoxygenation. Isolated rat diaphragm strips were loaded with dihydrofluorescein-DA (Hfluor-A), a probe that is oxidized to fluorescein ( Fluor) by intracellular ROS. Changes in fluorescence due to Fluor, NADH, and FAD were measured using a tissue fluorometer. The system had a detection limit of 1 mu M H2O2 applied to the muscle superfusate. When the superfusion buffer was changed rapidly from 95% O-2 to 0%, 5%, 21%, or 40% O-2, transient elevations in Fluor were observed that were proportional to the rise in NADH fluorescence and inversely proportional to the level of O-2 exposure. This signal could be inhibited completely with 40 mu M ebselen, a glutathione peroxidase mimic. After brief hypoxia exposure ( 10 min) or exposure to brief periods of H2O2, the fluorescence signal returned to baseline. Furthermore, tissues loaded with the oxidized form of the probe (Fluor-DA) showed a similar pattern of response that could be inhibited with ebselen. These results suggest that Fluor exists in a partially reversible redox state within the tissue. When Hfluor-loaded tissues were contracted with low-frequency twitches, Fluor emission and NADH emission were significantly elevated in a way that resembled the hypoxia-nduced signal. We conclude that in the transition to low intracellular PO2, a burst of intracellular ROS is formed that may have functional implications regarding skeletal muscle O-2-sensing systems and responses to acute metabolic stress.