Regulation of free radical outflow from an isolated muscle bed in exercising humans.

Incremental knee extensor (KE) exercise performed at 25, 70, and 100% of single-leg maximal work rate (WRMAX) was combined with ex vivo electron paramagnetic resonance (EPR) spectroscopic detection of α-phenyl- tert -butylnitrone (PBN) adducts, lipid hydroperoxides (LH), and associated parameters in five males. Blood samples were taken from the femoral arterial and venous circulation that, when combined with measured changes in femoral venous blood flow, permitted a direct examination of oxidant exchange across a functionally isolated contracting muscle bed. KE exercise progressively increased the net outflow of LH and PBN adducts (100% > 70% > 25% WRMAX, P 0.05). Exercise also resulted in a net outflow of other potentially related redox-reactive parameters, including hydrogen ions, norepinephrine, myoglobin, lactate dehydrogenase, and uric acid, whereas exchange of lipid/lipoproteins, ascorbic acid, and selected lipid-soluble anti-oxidants was unremarkable. These findings provide direct evidence for an exercise intensity-dependent increase in free radical outflow across an active muscle bed that was associated with an increase in sarcolemmal membrane permeability. In addition to increased mitochondrial electron flux subsequent to an increase in O2 extraction and flow, exercise-induced free radical generation may also be regulated by changes in iPo2, hydrogen ion generation, norepinephrine autoxidation, peroxidation of damaged tissue, and xanthine oxidase activation.