title: Oxidation/reduction state of cytochrome oxidase during repetitive contractions.
abstract: There is disagreement regarding whether inadequate O2 determines maximal O2 uptake (VO2max) and lactic acid output (L) during muscular 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 VO2 up to VO2max. 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. 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 VO2max or L under the conditions of these experiments: repetitive contractions with free flow in self-perfused muscles and normoxia.
authors.affiliation: Department of Physiology, College of Medicine, University of Florida, Gainesville 32610.
authors.forename: W N
authors.initials: WN
authors.lastname: Stainsby
authors.forename: W F
authors.initials: WF
authors.lastname: Brechue
authors.forename: D M
authors.initials: DM
authors.lastname: O'Drobinak
authors.forename: J K
authors.initials: JK
authors.lastname: Barclay
mesh_terms: D000818:Animals; D017936:Cytochrome a Group; D003580:Cytochromes / Q000378:metabolism; D004285:Dogs; D003576:Electron Transport Complex IV / Q000378:metabolism*; D007551:Isotonic Contraction / Q000502:physiology*; D007773:Lactates / Q000378:metabolism; D008931:Mitochondria, Muscle / Q000201:enzymology; D009119:Muscle Contraction / Q000502:physiology*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D013053:Spectrophotometry
chemical_list: D017936:Cytochrome a Group; D003580:Cytochromes; D007773:Lactates; D003576:Electron Transport Complex IV; D010100:Oxygen