One-leg inactivity induces a reduction in mitochondrial oxidative capacity, intramyocellular lipid accumulation and reduced insulin signalling upon lipid infusion: a human study with unilateral limb suspension

Aims/hypothesis Physical inactivity, low mitochondrial function, increased intramyocellular lipid (IMCL) deposition and reduced insulin sensitivity are common denominators of chronic metabolic disorders, like obesity and type 2 diabetes. Yet, whether low mitochondrial function predisposes to insulin resistance in humans is still unknown. Methods Here we investigated, in an intervention study, whether muscle with low mitochondrial oxidative capacity, induced by one-legged physical inactivity, would feature stronger signs of lipid-induced insulin resistance. To this end, ten male participants (age 22.4 +/- 4.2 years, BMI 21.3 +/- 2.0 kg/m(2)) underwent a 12 day unilateral lower-limb suspension with the contralateral leg serving as an active internal control. Results In vivo, mitochondrial oxidative capacity, assessed by phosphocreatine (PCr)-recovery half-time, was lower in the inactive vs active leg. Ex vivo, palmitate oxidation to (CO2)-C-14 was lower in the suspended leg vs the active leg; however, this did not result in significantly higher [C-14]palmitate incorporation into triacylglycerol. The reduced mitochondrial function in the suspended leg was, however, paralleled by augmented IMCL content in both musculus tibialis anterior and musculus vastus lateralis, and by increased membrane bound protein kinase C (PKC) theta. Finally, upon lipid infusion, insulin signalling was lower in the suspended vs active leg. Conclusions/interpretation Together, these results demonstrate, in a unique human in vivo model, that a low mitochondrial oxidative capacity due to physical inactivity directly impacts IMCL accumulation and PKC theta translocation, resulting in impaired insulin signalling upon lipid infusion. This demonstrates the importance of mitochondrial oxidative capacity and muscle fat accumulation in the development of insulin resistance in humans.