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1 School of Applied Sciences, University of Glamorgan, Pontypridd, South Wales CF37 1DL; 2 Department of Medicine, Queen's University, Belfast BT12 6BJ; 3 Department of Medicine, University of Liverpool, Liverpool L69 3GA, United Kingdom; and 4 Divison of Physics and 5 Department of Medicine, University of California San Diego, La Jolla, California 92093
There is no direct evidence to
support the contention that contracting skeletal muscle and/or
associated vasculature generates free radicals in exercising humans.
The unique combination of isolated quadriceps exercise and the
measurement of femoral arterial and venous free radical concentrations
with the use of electron paramagnetic resonance (EPR) spectroscopy
enabled this assumption to be tested in seven healthy men. Application
of ex vivo spin trapping using 
-phenyl-tert-butylnitrone
(PBN) resulted in the detection of oxygen- or carbon-centered free
radicals (aN = 1.38 ± 0.01 mT and
a
-hydrogen coupling constants, respectively) with
consistently higher EPR signal intensities of the PBN spin adduct
observed in the venous compared with the arterial circulation
(P < 0.05). Incremental exercise further increased the
venoarterial intensity difference [85 ± 58 arbitrary units (AU)
at 24 ± 6% maximal work rate (WRmax) vs. 387 ± 214 AU at 69 ± 7% WRmax; P < 0.05]. When combined with measured changes in femoral venous blood
flow (
), this resulted in a net adduct outflow of 130 ± 118 and 1,146 ± 582 AU/min (P < 0.05), which was
positively associated with leg oxygen uptake (r2 = 0.47, P < 0.05) and
(r2 = 0.47, P < 0.05). These results provide the first evidence for oxygen- or
carbon-centered free radical outflow from an active muscle bed in humans.
spin trapping; blood flow; oxygen uptake
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