Whole‐Body and Remaining Muscle‐Specific Metabolic Function is Limited Following Volumetric Muscle Loss

Abstract

Volumetric muscle loss (VML) is the traumatic loss of muscle, resulting in long-term functional deficits and various pathologic comorbidities. The metabolic consequences, whole-body and muscle-specific, of VML are unclear. We hypothesized that VML would decrease physical activity, respiratory exchange ratio (RER), and metabolic rate. A subset of mice underwent VML injury to the plantarflexors, 24-hr whole-body physical and metabolic activity was measured longitudinally. Prior to VML, 24-hr ambulation was 1.3km/day, RER was 0.91, and metabolic rate was 19kcal/kg/hr; by 6 weeks post-VML, daily ambulation did not change but 24-hr RER and metabolic rate decreased (0.88 and 17kcal/kg/hr p≤0.036). Intriguingly, active to inactive-phase RER, a marker of metabolic flexibility (fat vs. carbohydrate oxidation), was less post- compared to pre-VML injury suggesting whole-body metabolic inflexibility post-VML. Additionally, in a cross-over design, a subset of mice were assigned to restricted (12.5x8.5x6.3cm) or standard cages for 1-wk, to model clinical conditions. When activity was restricted ambulation decreased ~50% and 24-hr metabolic rate decreased ~23% (p≤0.002). In contrast, 24-hr RER increased ~4% (p<0.001), suggesting greater carbohydrate utilization and supporting future use of this restrictive model in combination with VML. We next hypothesized a VML-related disruption in the underlying oxidative physiology of the muscle remaining after VML contributed to whole-body metabolic inflexibility. Additional mice were evaluated at 1-, 4- and 8-weeks following VML or sham procedure. Various oxidative, contractile, and physiology evaluations of the gastrocnemius muscle were completed. 1-week post-VML, permeabilized myofibers from injured limbs had a decreased conductance of electrons through the electron transport chain with fats as a fuel, but not carbohydrates; providing a potential molecular mechanism to whole-body metabolic inflexibility. Interestingly, contractile properties support a slower myofiber phenotype that is typically more ready to use fat substrates for fuel. There was a VML-induced slowing of torque production and elevated twitch:tetani ratio. Maximum torque generated in VML injured muscles was significantly reduced compared to uninjured (334±28 vs. 557±66mN·m). Although there was no impact of VML on mitochondria content following injury there was a flattening of the capillary distribution per fiber with an impact at both 4- and 8-weeks post-VML. The average cross-sectional area of myofibers shifted right, with an increasing proportion of MyHCslow expressing fibers, again supporting slowing of the muscle following VML. The most salient findings herein suggest metabolic activity and flexibility are negatively impacted following VML injury despite a shift toward a slower phenotype. Metabolic inflexibility in humans is known to be associated with greater risk of comorbidities and cardiovascular disease, findings herein support a potential increased risk for VML-afflicted patients to develop various chronic diseases, such as metabolic disorder.