Mitochondrial Adaptation in Insulin Resistant Human Skeletal Muscle

Introduction: The integrity, morphology, structure and functionality of skeletal muscle mitochondria in patients with type 2 diabetes (T2D) is heavily disputed. Thus, the purpose of this investigation was to characterize mitochondrial phenotypes across the insulin resistance spectrum.

Methods: 30 sedentary adults aged 18-55 were grouped as lean (BMI 21±0.6, N=13), obese (BMI 32±0.9, N=12), or T2D (BMI 33±2.5, N=5) based on oral glucose tolerance tests and body mass index (BMI). All subjects completed a 3-day, inpatient stay consisting of cardio-metabolic testing, body composition analysis and a hyperinsulinemic-euglycemic clamp study. On the day of the clamp, muscle specimens were obtained basally and under insulin-stimulated conditions. Mitochondrial membrane potential (δψ_m) was measured by confocal microscopy using the cationic fluorophore TMRM. Oxidative phosphorylation (OXPHOS) was assessed in permeabilized fibers via high resolution respirometry (Oroboros Oxygraph-2k). Muscle mitochondrial ultrastructure and content were determined by transmission electron microscopy (TEM) of fixed muscle sections.

Results: There was a significant, step-wise decrease in insulin-stimulated glucose uptake across the metabolic spectrum (P<0.001). No differences in resting δψm were observed, however, insulin stimulation depolarized δψm in all groups (P<0.05). Those with T2D exhibited increased basal complex III (P<0.05) and IV (P<0.05) activity vs. lean and obese controls. Furthermore, obese controls and T2D displayed augmented complex IV responsiveness to insulin stimulation (P<0.01). Threshold analysis of muscle sections revealed a progressive loss of mitochondrial content, as well as accumulation of lipid droplets across the metabolic spectrum (P<0.05).

Conclusion: Collectively, these findings provide direct evidence of mitochondrial adaptation in insulin resistant and diabetic skeletal muscle.