Effect of skeletal muscle mitochondrial phenotype on H₂O₂ emission

Reactive oxygen species (ROS) are a key output of the skeletal muscle mitochondrial information processing system both at rest and during exercise. In skeletal muscle, mitochondrial ROS release depends on multiple factors; however, fiber-type specific differences remain ambiguous in part owing to the use of mitochondria from mammalian muscle that consist of mixed fibers. To elucidate fiber-type specific differences, we used mitochondria isolated from rainbow trout (Oncorhynchus mykiss) red and white skeletal muscles that consist of spatially distinct essentially pure red and white fibers. We first characterized the assay conditions for measuring ROS production (as H₂O₂) in isolated fish red and white skeletal muscle mitochondria (RMM and WMM) and thereafter compared the rates of emission during oxidation of different substrates and the responses to mitochondrial electron transport system (ETS) pharmacological modulators. Our results showed that H₂O₂ emission rates by RMM and WMM can be quantified using the same protein concentration and composition of the Amplex UltraRed-horseradish peroxidase (AUR-HRP) detection system. For both RMM and WMM, protein normalized H₂O₂ emission rates were highest at the lowest protein concentration tested and decreased exponentially thereafter. However, the absolute values of H₂O₂ emission rates depended on the calibration curves used to convert fluorescent signals to H₂O₂ while the trends depended on the normalization strategy. We found substantial qualitative and quantitative differences between RMM and WMM in the H₂O₂ emission rates depending on the substrates being oxidized and their concentrations. Similarly, pharmacological modulators of the ETS altered the magnitudes and trends of the H₂O₂ emission differently in RMM and WMM. While comparable concentrations of substrates elicited maximal albeit quantitively different emission rates in RMM and WMM, different concentrations of pharmacological ETS modulators may be required for maximal H₂O₂ emission rates depending on muscle fiber-type. Taken together, our study suggests that biochemical differences exist in RMM compared with WMM that alter substrate oxidation and responses to ETS modulators resulting in fiber-type specific mitochondrial H₂O₂ emission rates.