Mitochondrial diagnostics reveal diffuse impairments in skeletal muscle energetics in aging mice

Aging is associated with progressive declines in skeletal muscle mass, strength, and endurance, often linked to mitochondrial dysfunction. However, a complete understanding of mitochondrial impairments during aging is lacking. Herein, we examined how biological sex and aging affect muscle function and mitochondrial energy transduction. Male and female C57BL/6 mice at 16 and 26 mo of age (n = 48) were assessed for physical function, muscle contractility, histology, and mitochondrial bioenergetics. Using isolated limb muscle mitochondria, we used a diagnostic approach to evaluate respiration, redox potential, and membrane polarization under physiologically relevant energy demands. Aged mice had significantly lower grip strength (P = 2.7E-09), walking speed (P = 0.024), and endurance capacity (P = 1.24E-08). Muscle mass and contractile function were also significantly lower in 26-mo-old mice regardless of sex. Mitochondrial diagnostics revealed a significant reduction (30%-50%) in oxygen consumption rates across a range of energy demands and substrate conditions in both male and female 26-mo-old mice. Redox and membrane potentials were also reduced (P < 0.05) in aged mice, resulting in a lower respiratory efficiency when compared with 16-mo-old mice. Notably, aged males exhibited greater mitochondrial deficits with carbohydrate substrates, whereas aged females showed larger declines with fatty acid substrates. Aging induces diffuse impairments in mitochondrial energy transduction in skeletal muscle of mice of both sexes. The application of the mitochondrial diagnostics platform offers new insights into the changes in muscle mitochondria with aging and could enhance the identification of interventions for preserving mitochondrial health in aging.NEW & NOTEWORTHY This study uses a mitochondrial diagnostic platform to understand how aging and biological sex impact mitochondrial energy transduction. Findings from this diagnostic approach revealed diffuse, but severe, deficits in energy transduction that occur across a broad range of substrate conditions. Given the lack of published data on female aging mice, the work helps fill a gap in the literature regarding sex-dependent and -independent alterations in muscle aging.