The Whole‐Body and Muscle Specific Metabolic Costs of Repair Following Volumetric Muscle Loss Injury: The Role of Limited Physical Activity and Adjunctive Oxidative Modulation

Abstract

Skeletal muscle has a significant metabolic demand, and the repair and regeneration of muscle is metabolically taxing. Following traumatic injuries in which there is an abundant loss of muscle resulting in chronic functional deficits, such as volumetric muscle loss (VML), the pathologic comorbidities of injury are known to further impair whole‐body and muscle‐specific metabolism. This drives an imbalance in the metabolic demands and supply to the muscle. This study examined the impact of restricting activity following injury on whole‐body metabolism and muscle function. And evaluated if adjunctive oxidative modulation with a β2‐receptor agonist (formoterol) would improve metabolism and contractile function following VML; as has been evident in other severe injury models. Adult male C57Bl/6J mice (n=36) underwent unilateral VML to the posterior muscles of the hindlimb and were randomly assigned to 8 weeks of unrestricted (28x18x12.5 cm) or restricted (12.5x8.5x6.3 cm) activity, with or without formoterol (0.3 mg/kg/day) treatment; a subset of mice were injury naïve, age‐matched controls. Longitudinal 24‐hr ambulation, whole‐body metabolic rate, and respiratory exchange ratio (RER) were evaluated 1 week pre‐ and 2 and 6 weeks post‐VML. Glucose tolerance was evaluated ~7 weeks post‐VML, and at 8 weeks post‐VML maximal in vivo torque was evaluated prior to muscle harvest for biochemical examination. There was no longitudinal difference in body mass (p=0.839), however treatment with formoterol resulted in a greater gastrocnemius mass (p=0.019) and maximal torque of the posterior compartment (p=0.049), independent of activity. Notably, mice within restricted housing ambulated ~41% less, coinciding with ~10% decreased whole‐body metabolic rate (p<0.001). The primary hypothesis was that restricting activity would worsen metabolic impairments following VML, which could be remedied by formoterol treatment. Six weeks post‐VML, RER was lowest for mice untreated and unrestricted, and highest for those untreated with restricted activity (~3.6% difference; p=0.034) due largely to ~21% decreased lipid oxidation when activity was restricted. Formoterol treatment did not impact RER regardless of activity, suggesting a hypermetabolic state when activity is limited post‐VML that is attenuated by formoterol. Although there were no carbohydrate oxidation differences, formoterol treatment lowered blood glucose indicating enhanced glucose uptake (p<0.001). In the muscle remaining, restricting activity decreased pyruvate dehydrogenase activity (p=0.002); however, formoterol moderated this alteration despite reduced mitochondrial abundance as indicated by lower citrate synthase activity (p=0.019). Overall, adjunctive oxidative modulation improves contractile function and attenuates impairments in whole‐body RER and muscle specific metabolic activity resulting from VML or activity restriction. Ongoing work is evaluating specific metabolites and lipids in the muscle remaining after VML to elucidate mechanisms of impaired metabolism after VML.