The effects of physical inactivity on skeletal muscle metabolic function

Abstract

Bed rest leads to multiple adverse physiological consequences, comparable to the effects of physical inactivity. The role of cellular metabolism in the regulation of these physiological changes during bed rest has not been studied in detail. The aim of this study was to test if prolonged bed rest decreases mitochondrial function, muscle mass and impacts upon mitochondrial dynamics in healthy young men. We also evaluated the impact a micronutrient cocktail supplement had in mitigating these changes. 60 days bed rest resulted a number of anthropometric changes including reduction in lean muscle mass (p<0.05) and an increase in fat mass (p<0.05). These changes were accompanied by an increase in RQ indicative of a switch from fat to carbohydrate oxidation at rest (p<0.05). Mitochondrial respiration, expressed per mg of wet muscle weight, was lower during ETS (CI), ETS (CII) and CII respiration following 60 days of bed rest (p<0.05), a change which was not evident when normalised to mitochondrial content. We concluded that the adaptations in the regulators of mitochondrial density/amount may explain these changes. In particular, we observed a significant decrease in mitochondrial fusion marker, OPA1 and fission marker, Drp1 (p<0.05). Our intervention mitigated these changes, increasing OPA1:Drp1 ratio (p<0.05) suggesting our intervention increased mitochondrial elongation. A comparison between our 60-day study and a previously conducted 21-day bed rest study emphasised the important role OPA1 may be having throughout bed rest epitomised by a shift in mitochondrial dynamics over time from a fragmented phenotype (21-day BR) to a rebalancing of fusion-fission (60-day BR). Further in-vitro analysis uncovered the novel role SIRT4 has on these processes with knockdown increasing OPA1-mediated mitochondrial fusion while increasing ADP-stimulated OXPHOS (p<0.05) indicative of SIRT4s regulation of free fatty acid oxidation. Bed rest leads to a series of adverse physiological and cellular adaptations which may be limited by micronutrient supplementation. Although further clarification is needed, SIRT4 may have a role in this regard with mechanisms linking mitochondrial dynamics and function