title: Physical fitness and mitochondrial respiratory capacity in horse skeletal muscle.
abstract: BACKGROUND
  Within the animal kingdom, horses are among the most powerful aerobic athletic mammals. Determination of muscle respiratory capacity and control improves our knowledge of mitochondrial physiology in horses and high aerobic performance in general.
  
  
  RESULTS
  We applied high-resolution respirometry and multiple substrate-uncoupler-inhibitor titration protocols to study mitochondrial physiology in small (1.0-2.5 mg) permeabilized muscle fibres sampled from triceps brachii of healthy horses. Oxidative phosphorylation (OXPHOS) capacity (pmol O(2) • s(-1) • mg(-1) wet weight) with combined Complex I and II (CI+II) substrate supply (malate+glutamate+succinate) increased from 77 ± 18 in overweight horses to 103 ± 18, 122 ± 15, and 129 ± 12 in untrained, trained and competitive horses (N = 3, 8, 16, and 5, respectively). Similar to human muscle mitochondria, equine OXPHOS capacity was limited by the phosphorylation system to 0.85 ± 0.10 (N = 32) of electron transfer capacity, independent of fitness level. In 15 trained horses, OXPHOS capacity increased from 119 ± 12 to 134 ± 37 when pyruvate was included in the CI+II substrate cocktail. Relative to this maximum OXPHOS capacity, Complex I (CI)-linked OXPHOS capacities were only 50% with glutamate+malate, 64% with pyruvate+malate, and 68% with pyruvate+malate+glutamate, and ~78% with CII-linked succinate+rotenone. OXPHOS capacity with glutamate+malate increased with fitness relative to CI+II-supported ETS capacity from a flux control ratio of 0.38 to 0.40, 0.41 and 0.46 in overweight to competitive horses, whereas the CII/CI+II substrate control ratio remained constant at 0.70. Therefore, the apparent deficit of the CI- over CII-linked pathway capacity was reduced with physical fitness.
  
  
  CONCLUSIONS
  The scope of mitochondrial density-dependent OXPHOS capacity and the density-independent (qualitative) increase of CI-linked respiratory capacity with increased fitness open up new perspectives of integrative and comparative mitochondrial respiratory physiology.
authors.affiliation: Equine European Centre of Mont-le-Soie, University of Liege, Vielsalm, Belgium. dominique.votion@ulg.ac.be
authors.forename: Dominique-Marie
authors.initials: DM
authors.lastname: Votion
authors.forename: Erich
authors.initials: E
authors.lastname: Gnaiger
authors.forename: Hélène
authors.initials: H
authors.lastname: Lemieux
authors.forename: Ange
authors.initials: A
authors.lastname: Mouithys-Mickalad
authors.forename: Didier
authors.initials: D
authors.lastname: Serteyn
mesh_terms: D000818:Animals; D004579:Electron Transport; D042967:Electron Transport Complex I / Q000378:metabolism; D005260:Female; D006736:Horses / Q000502:physiology*; D008931:Mitochondria, Muscle / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000378:metabolism*; D058451:Oxidative Coupling; D010085:Oxidative Phosphorylation
chemical_list: D042967:Electron Transport Complex I