Correction: 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.Methodology/Principal Findings: 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 Ns 21 Nmg 21 wet weight) with combined Complex I and II (CI+II) substrate supply (malate+glutamate+succinate) increased from 77618 in overweight horses to 103618, 122615, and 129612 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 0.8560.10 (N = 32) of electron transfer capacity, independent of fitness level. In 15 trained horses, OXPHOS capacity increased from 119612 to 134637 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/Significance: 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.

In Vitro Assays for the Assessment of Impaired Mitochondrial Bioenergetics in Equine Atypical Myopathy

“…Regarding our ratios, CI P and CI + CII P are relative to OXPHOS capacity whereas CI + CII E and CII E are relative to ET capacity. Also, it is assumed that S-pathway linked respiration is not influenced by uncoupling [13,21] and therefore CII E = CII P . The ratios obtained for SUIT1 and SUIT2 in MCPA-treated cells were compared to ratios calculated in MCPA affected horses.…”

“…Oxygen concentration (µM) and oxygen flux per million cells (pmol O 2 •s −1 •10 6 cells −1 ) were recorded online using DatLab software (Oroboros Instruments, Austria). Experiments were conducted using specific SUIT protocols and oxygen levels were maintained between 200 and 500 µM O 2 to avoid any oxygen-related limitations of respiration and to align with previous studies [13,21,27]. Oxygen flux was expressed as respiration per million cells (pmol O 2 •s −1 •10 6 cells −1 ), or as control ratios, namely flux control ratios (FCR).…”

“…Previous studies performed on skeletal muscle show that structural alterations are associated with mitochondrial functional consequences [13]. Using high-resolution respirometry (HRR), a severe depression of mitochondrial oxidative phosphorylation (OXPHOS) and electron transfer system capacities (ET capacity) in AM affected horses was found [13] using standardized substrate-uncoupler-inhibitor titration (SUIT) protocols validated for respirometric assessments of equine muscle cells [21].…”