30 Intrinsic skeletal muscle mitochondrial function decreases while mitochondrial volume density increases from 6 months to 3-years-old in Thoroughbreds

Abstract

As the primary source of cellular energy, mitochondria are critical for athletic performance, and the intricacies of mitochondrial electron transfer may be related to Thoroughbred race performance. However, little is known about specific mitochondrial adaptations to growth in the equine athlete. To test the hypothesis that mitochondrial metrics would increase with age, gluteus medius samples were collected from the same Thoroughbred horses as weanlings (4 colts, 9 fillies; mean ± SD, 6 ± 1mo), yearlings (4 colts, 10 fillies; 20 ± 1mo), 2-year-olds (2yo; 3 geldings, 10 mares; 32 ± 1mo), and 3yo (2 geldings, 3 mares; 44 ± 1mo) from the same Kentucky farm over 4 years. Thoroughbreds were broke to ride in August of their yearling year then began race training. Mitochondrial oxidative phosphorylation (P) and electron transfer (E) capacities were determined via high-resolution respirometry. Citrate synthase (CS) and cytochrome c oxidase (CCO) activities were quantified as measures of mitochondrial volume density and function, respectively. Data were analyzed using PROC MIXED in SASv9.4 with age and sex as fixed effects, sire and dam as random effects, and age as a repeated effect with horse as the subject. Intrinsic (relative to CS activity) CCO activity, P supported by complex I (PCI), maximal P (PCI+II), and maximal E (ECI+II) weregreatest in weanlings compared with all other ages (P ≤ 0.003) and greater in yearlings than 2yo (P ≤ 0.05). Intrinsic E supported by complex II only (ECII) was greater in weanlings than 2 or 3yo (P ≤ 0.01) and greater in yearlings than 2yo (P ≤ 0.04). Weanlings also had a greater contribution of PCI+II to ECI+II than yearlings, 2yo, or 3yo (P ≤ 0.05). However, CS activity was greatest in 3yo (P ≤ 0.03) and lowest in weanlings (P ≤ 0.0001) with yearlings and 2yo intermediate, indicating expansion of the mitochondrial network with age and training. Additionally, 3yo had the greatest integrative (per mg tissue) PCI of all ages (P ≤ 0.005). Two and 3yo had greater integrative PCI+II, ECI+II, and ECII capacities than weanlings (P ≤ 0.05). Overall, weanlings had the greatest intrinsic mitochondrial capacities and function but 2 and 3yo Thoroughbreds had greater mitochondrial volume density resulting in greater integrative capacities. Skeletal muscle mitochondria of young Thoroughbreds appear to adapt to meet energetic demands of growth and race training by increasing volume density rather than intrinsic capacity, but further research is warranted to characterize the impacts of various training programs on mitochondrial metrics.