Oxidation/reduction state of cytochrome oxidase during repetitive contractions

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Abstract
There is disagreement regarding whether inadequate O2 determines maximal O2 uptake (.ovrhdot.VO2max) and lactic acid output (.ovrhdot.L) during muscular activity activity. Direct assessment of mitochondrial cytochrome oxidase (cytochrome a-a3) oxidation/reduction (O/R) state should provide an unequivocal answer for this issue. A new near-infrared spectrophotometric method was used to measure the O/R state of cytochrome a-a3 of dog gastrocnemius-plantaris muscle in situ during repetitive isotonic twitch and tetanic contractions. Three contraction frequencies were used for each contraction type in alternating sequence to provide a wide range of .ovrhdot.VO2 up to .ovrhdot.VO2max. .ovrhdot.VO2 and L were measured after 3 and 9 min of a 10-min contraction period, and 15 min were allowed for recovery between contraction periods. .ovrhdot.VO2 increased with contraction frequency. L was variably increased with contraction frequency at 3 min and uptake usually occurred at 9 min, except at the highest tetanic frequency. The O/R span of cytochrome a-a3 was determined by respiring the animals with 100% N2 to determine the most reduced state. This was followed by respiration with 100% O2, which gave the most oxidized state transiently during recovery. Within this span in muscles at rest, cytochrome a-a3 was 50-80% oxidized. During contractions of both types at all frequencies, cytochrome a-a3 always became more oxidized by an additional 10-20%. These findings should put to rest any arguments that inadequate O2 is a determinant of .ovrhdot.VO2max or .ovrhdot.L under the conditions of these experiments: repetitive contractions with free flow in self-perfused muscles and normoxia.