Effects of aging on mitochondrial function in skeletal muscle of Quarter Horses

Skeletal muscle function, aerobic capacity and mitochondrial (Mt) function have been found to decline with age in humans and rodents. However, not much is known about age-related changes in Mt function in equine skeletal muscle. Here, we compared fiber type composition and Mt function in Gluteus medius and Triceps brachii muscle between young (age 1.8±0.1 years, n=24) and aged (age 17-25 years, n=10) Quarter Horses. The percentage of myosin heavy chain (MyHC)-IIX was lower in aged compared to young muscles (Gluteus, P=0.092; Triceps, P=0.012), while the percentages of MyHC-I (Gluteus; P<0.001) and MyHC-IIA (Triceps; P=0.023) were increased. Mass-specific Mt density, indicated by citrate synthase activity, was unaffected by age in Gluteus, but decreased in aged Triceps (P=0.023). Cytochrome c oxidase (COX) activity per mg tissue and per Mt unit decreased with age in Gluteus (P<0.001 for both) and Triceps (P<0.001; P=0.003, respectively). Activity of 3-Hydroxyacyl-CoA dehydrogenase per mg tissue was unaffected by age, but increased per Mt unit in aged Gluteus and Triceps (P=0.023, P<0.001, respectively). Mt respiration of permeabilized muscle fibers per mg tissue was unaffected by age in both muscles. Main effects of age appeared when respiration was normalized to Mt content, with increases in LEAK, OXPHOS capacity and electron transport system capacity (P=0.038; P=0.045; P=0.007, respectively), independent of muscle. In conclusion, equine skeletal muscle aging was accompanied by a shift in fiber type composition, decrease in Mt density and COX activity, but preserved Mt respiratory function.

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