Mechanical unloading during microgravity causes skeletal muscle atrophy and impairs mitochondrial energetics. The elevated production of reactive oxygen species (ROS) by mitochondria and Nox2, coupled with impairment of stress protection (e.g., SIRT1, antioxidant enzymes), contribute to atrophy. We tested the hypothesis that the SIRT1 activator, SRT2104 would rescue unloading-induced mitochondrial dysfunction. Mitochondrial function in rat gastrocnemius and soleus muscles were evaluated under three conditions (10 days): ambulatory control (CON), hindlimb unloaded (HU), and hindlimb-unloaded-treated with SRT2104 (SIRT). Oxidative phosphorylation, electron transfer capacities, H₂O₂ production, and oxidative and antioxidant enzymes were quantified using high-resolution respirometry and colorimetry. In the gastrocnemius, (1) integrative (per mg tissue) proton LEAK was lesser in SIRT than in HU or CON; (2) intrinsic (relative to citrate synthase) maximal noncoupled electron transfer capacity (E_CI+II) was lesser, while complex I-supported oxidative phosphorylation to E_CI+II was greater in HU than CON; (3) the contribution of LEAK to E_CI+II was greatest, but cytochrome c oxidase activity was lowest in HU. In both muscles, H₂O₂ production and concentration was greatest in SIRT, as was gastrocnemius superoxide dismutase activity. In the soleus, H₂O₂ concentration was greater in HU compared to CON. These results indicate that SRT2104 preserves mitochondrial function in unloaded skeletal muscle, suggesting its potential to support healthy muscle cells in microgravity by promoting necessary energy production in mitochondria.

Many factors support or restrict equine performance. Energy supply is required for cellular function and thus is an important factor contributing to performance in all disciplines. Energy metabolism can be divided into two primary processes: aerobic and anaerobic metabolism. Aerobic metabolism occurs in the mitochondria, colloquially known as the powerhouses of the cell, and is a primary and efficient source of skeletal muscle energy. Mitochondrial aerobic metabolism begins with the breakdown of carbohydrates, lipids, and proteins within and outside the mitochondria to produce reducing equivalents. In the presence of oxygen, mitochondria reduce these equivalents during the process of oxidative phosphorylation (OXPHOS), establishing a protonmotive force that drives the production of the primary source of cellular energy, adenosine triphosphate (ATP). Conversely, in the absence of oxygen, sugar is converted into energy by anaerobic metabolism, a less efficient form of energy production compared with aerobic metabolism. While each equine sport has specific demands requiring different levels of support from each metabolic system, mitochondrial function is necessary at some level for all horses.

Coenzyme Q10 (CoQ10) is an essential component of the mitochondrial electron transfer system and a potent antioxidant. The impact of CoQ10 supplementation on mitochondrial capacities and the muscle proteome is largely unknown. This study determined the effect of CoQ10 supplementation on muscle CoQ10 concentrations, antioxidant balance, the proteome, and mitochondrial respiratory capacities. In a randomized cross-over design, six Thoroughbred horses received 1600 mg/d CoQ10 or no supplement (control) for 30-d periods separated by a 60-d washout. Muscle samples were taken at the end of each period. Muscle CoQ10 and glutathione (GSH) concentrations were determined using mass spectrometry, antioxidant activities by fluorometry, mitochondrial enzyme activities and oxidative stress by colorimetry, and mitochondrial respiratory capacities by high-resolution respirometry. Data were analyzed using mixed linear models with period, supplementation, and period × supplementation as fixed effects and horse as a repeated effect. Proteomics was performed by tandem mass tag 11-plex analysis and permutation testing with FDR < 0.05. Concentrations of muscle CoQ10 (p = 0.07), GSH (p = 0.75), and malondialdehyde (p = 0.47), as well as activities of superoxide dismutase (p = 0.16) and catalase (p = 0.66), did not differ, whereas glutathione peroxidase activity (p = 0.003) was lower when horses received CoQ10 compared to no supplement. Intrinsic (relative to citrate synthase activity) electron transfer capacity with complex II (E_CII) was greater, and the contribution of complex I to maximal electron transfer capacity (FCR_PCI and FCR_PCIG) was lower when horses received CoQ10 with no impact of CoQ10 on mitochondrial volume density. Decreased expression of subunits in complexes I, III, and IV, as well as tricarboxylic acid cycle (TCA) enzymes, was noted in proteomics when horses received CoQ10. We conclude that with CoQ10 supplementation, decreased expression of TCA cycle enzymes that produce NADH and complex I subunits, which utilize NADH together with enhanced electron transfer capacity via complex II, supports an enhanced reliance on substrates supplying complex II during mitochondrial respiration.

Recurrent Exertional Rhabdomyolysis (RER) is a skeletal muscle disease that can intermittently impede careers of equine athletes. Manifesting as episodes of muscular stiffness and degeneration, RER causes lost training and competition days. RER is associated with abnormal intracellular calcium (Ca ²⁺ ) regulation. Calcium released from the sarcoplasmic reticulum to initiate contraction may also be transported into mitochondria, which impacts energy production. RER-susceptible horses exhibit down-regulated mitochondrial proteins but upregulated mitochondrial genes. Thus, mitochondrial function may be impacted by Ca ²⁺ irregularities in RER horses. Fourteen race-fit female Thoroughbred horses were used to test the hypothesis that horses which had previously experienced at least 2 ER episodes (n = 8; mean ± SD 3.1 ± 1.1 yr) would have impaired skeletal muscle mitochondrial capacities compared to control racehorses (n = 6; 3.5 ± 1.4 yr). Gluteus medius samples were analyzed for oxidative phosphorylation ( P) and electron transfer ( E) capacities via high-resolution respirometry. Contrary to our hypothesis, integrative (per mg) NADH-linked P ( P _N ), maximal P ( P _NS ), maximal E ( E _NS ) and succinate-linked E ( E _S ) tended to be greater ( P ≤ 0.1) while intrinsic (relative to citrate synthase activity) P _N and E _S tended to be greater ( P ≤ 0.1) and P _NS and E _NS were greater ( P ≤ 0.03) in RER-susceptible compared to control horses. Horses susceptible to RER appeared to have greater skeletal muscle mitochondrial capacities, which might be consequent to aberrant intramuscular Ca ²⁺ handling in vivo. The clinical significance of this finding remains unclear but further investigation could lead to mitochondrially-targeted treatment and management strategies.

Energy-dense feeds commonly provided to equine athletes may be high in starch, which alter gastrointestinal microbiota and could promote systemic inflammation. To test the hypothesis that exercise-induced inflammation would be greater in horses receiving a starch- versus fiber-based concentrate. Quarter Horses (mean±SD 16±1mo; 337±30kg) received either a fiber-based control (CON; 7 fillies, 8 geldings) or an isocaloric, isonitrogenous starch concentrate (STARCH; 8 fillies, 7 geldings) for 24d. Fecal metagenomics were evaluated on d0 and 21. Blood inflammatory mediators were quantified on d0, d21, and surrounding a 2-h submaximal exercise test (SET) on d22. On d21, CON horses had greater Lactobacillaceae (∼5.7% vs. ∼2.4% in STARCH), while STARCH had greater Lachnospiraceae (∼38% vs. ∼32% in CON) but diet alone did not impact inflammatory markers. On d22, CRP increased at 24h post-SET in all horses (P<0.0001). By 48h, CRP returned to pre-SET in STARCH but remained elevated in CON (P=0.0005), resulting in greater CRP in CON than STARCH at 48h (P=0.02). TNFα increased from pre-SET to 1h in STARCH horses (P=0.02), then returned to pre by 6h. In CON horses, TNFα increased at 24h (P=0.04) and remained elevated at 48h (P=0.0005). Throughout the SET, CON had greater IL-10 than STARCH horses (P=0.005). SAA, IL-4, IL-8, and vascular endothelial growth factor (VEGF)-A were differentially impacted by the SET but were unaffected by diet. Contrary to our hypothesis, fiber-fed horses appeared to elicit a more robust acute inflammatory response to exercise than starch-fed horses despite an altered gastrointestinal microbiome.