Responses of equine skeletal muscle characteristics to growth and training have been shown to differ between breeds. These differential responses may arise in part because muscle fiber type and mitochondrial density differ between breeds, even in untrained racing-bred horses. However, it is not known when these breed-specific differences manifest. To test the hypothesis that weanling Standardbreds (SB) and Thoroughbreds (TB) would have higher mitochondrial measures than Quarter Horses (QH), gluteus medius samples were collected from SB (mean ± SD; 6.2 ± 1.0 mo; n = 10), TB (6.1 ± 0.5 mo; n = 12), and QH (7.4 ± 0.6 mo; n = 10). Citrate synthase (CS) and cytochrome c oxidase (CCO) activities were assessed as markers of mitochondrial density and function, respectively. Mitochondrial oxidative (P) and electron transport system (E) capacities were assessed by high-resolution respirometry (HRR). Data for CCO and HRR are expressed as integrated (per mg protein and per mg tissue wet weight, respectively) and intrinsic (per unit CS). Data were analyzed using PROC MIXED in SAS v 9.4 with breed as a fixed effect. Mitochondrial density (CS) was higher for SB and TB than QH (P ≤ 0.0007). Mitochondrial function (integrated and intrinsic CCO) was higher in TB and QH than SB (P ≤ 0.01). Integrated CCO was also higher in TB than QH (P < 0.0001). However, SB had higher integrated maximum P (PCI+II) and E (ECI+II) than QH (P ≤ 0.02) and greater integrated and intrinsic complex II-supported E (ECII) than both QH and TB (P≤ 0.02), whereas TB exhibited higher integrated P with complex I substrates (PCI) than SB and QH (P ≤ 0.003) and higher integrated PCI+II and ECI+II than QH (P ≤ 0.02). In agreement, TB and QH had higher contribution of complex I (CI) to max E than SB (P ≤ 0.001), whereas SB had higher contribution of CII than QH and TB (P ≤ 0.002). Despite having higher mitochondrial density than QH and TB, SB showed lower CCO activity and differences in contribution of complexes to oxidative and electron transport system capacities. Breed differences in mitochondrial parameters are present early in life and should be considered when developing feeding, training, medication, and management practices.
Skeletal muscle fiber type composition and energetic profile of both mature trained and untrained racing-bred horses differ between breeds (Hodgson et al., 2014) and appear to diverge early in life (Bechtel and Kline, 1987; Kline and Bechtel, 1990). Stull and Albert (1980) proposed that these differences arise due to selection and adaptation for varying lengths and intensities of training and competition. Differences in adaptations to exercise have been demonstrated, and successfully raced Quarter Horses (Wood et al., 1988) and endurance horses (Rivero et al., 1993) have markedly distinctive muscle fiber type populations. Although all muscle fiber types are necessary for optimal performance, oxidative fibers and oxidative capacity are critical for longer bouts of exercise, such as Standardbred (Essèn-Gustavsson and Lindholm, 1985) and perhaps Thoroughbred (Lindholm et al., 1983) races. Conversely, fast twitch, nonoxidative fibers are extremely important for Quarter Horse racing (Wood et al., 1988), where explosive power is required, but exercise duration is shorter.
Previous studies established disparities in metabolic enzymes and muscle fiber type in mature horses of different breeds and disciplines, but there is no research comparing mitochondrial characteristics and oxidative capacity between racing breeds from a young age. Therefore, the aim of this study was to test the hypothesis that weanling Standardbreds (SB) and Thoroughbreds (TB) would have higher mitochondrial measures than Quarter Horses (QH).
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Via source Journal of Animal Science | Differential skeletal muscle mitochondrial characteristics of weanling racing-bred horses
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