High‐Resolution Respirometry May Identify Early Indicators for Exertional Rhabdomyolysis in Standardbred Racehorses

Abstract

IntroductionDuring racing, mitochondrial oxidative phosphorylation (OXPHOS) is a major source of energy for muscle contraction. Assessment of muscle OXPHOS capacity by high‐resolution respirometry (HRR) might identify animals with altered energy metabolism and consequently increased risk of exertional rhabdomyolysis (ER).MethodsProspective cohort study. Ten healthy French Standardbreds in active training, aged 4–7 years, recruited at the start of a competitive racing season, underwent a standardised field exercise test with determination of speed at onset of lactate accumulation (VLa4) and measurement of serum creatine kinase activity (CK) two hours post‐exercise. Triceps brachii muscle microbiopsies were obtained for determination of OXPHOS and electron transfer system (ETS) capacities with HRR. Associations between HRR variables and occurrence of ER over the following four months of racing were analysed using univariate logistic regression.ResultsTwo horses developed ER during the study period: one within one month of recruitment, another presented multiple minor episodes over the season. For these ER horses OXPHOS/ETS ratio was lower, and Complex I/Complex I + II ratio and Complex I /Complex II ratio were higher than non‐affected horses in the study (perfect prediction of occurrence of ER with ratio of ≤0.74, ≥0.60 and ≥0.715 respectively). VLa4 for ER horses was within the range of non‐affected horses. Post‐exercise CK was within normal limits for all horses in the study.ConclusionsOXPHOS and ETS capacities and their derived ratios indicate dysfunctional muscle energy metabolism whereas VLa4 and CK were unremarkable in horses subsequently affected by ER.Ethical Animal ResearchThe study was approved by Liège University's Animal Ethics Committee (agreement N° 07‐629); owner informed consent was obtained. Sources of funding: Part of the study protocol was funded by Les Haras Nationaux (France). Competing interests: The Oxygraph‐2k HRR apparatus was provided on loan by OROBOROS Instruments (Innsbruck, Austria); there are no patents, products in development or marketed products to declare.