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1 Department of Physiology, University of Glamorgan, Pontypridd, South Wales, United Kingdom
2 Department of Medicine, Queens University, Belfast, Ireland
3 Department of Medicine, University of California San Diego, San Diego, California, USA
* To whom correspondence should be addressed. E-mail: dbailey1{at}glam.ac.uk.
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 
-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) consistent with the
generation of secondary, lipid-derived oxygen (O2)-centered alkoxyl and carbon-centered alkyl radicals. Radical outflow appeared to be more intimately associated with predicted decreases in intracellular PO2 (iPO2) as opposed to measured increases in leg O2 uptake
(VO2), with greater outflow recorded between 25 to 70% WRMAX (P < 0.05 vs. 70 to 100% WRMAX). This bias was confirmed when radical venoarterial concentration differences were expressed relative to changes in the convective components of O2 extraction and flow (25 to 70% WRMAX P < 0.05 vs. 70 to 100% WRMAX, P > 0.05). Exercise also resulted in a net outflow of other potentially related redox-reactive parameters including hydrogen ions (H+), norepinephrine, myoglobin, lactate dehydrogenase and uric acid whereas exchange of
lipid/lipoproteins, ascorbic acid and selected lipid soluble antioxidants 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, H+ generation, norepinephrine auto-oxidation, peroxidation of damaged tissue and xanthine oxidase activation.
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