Abstract

Disturbances in skeletal muscle lipid oxidation might induce ectopic fat deposition and lipotoxicity. Nevertheless, cellular mechanisms that regulate skeletal muscle lipid oxidation been fully determined. We aimed to determine whether there was association between relative whole body lipid oxidation and mitochondrial size or mitochondria-sarcoplasmic reticulum interactions in the skeletal muscle. Twelve healthy men were included [mean (standard deviation), 24.7 (1.5) yr old, 24.4 (2.6) kg/m(2)]. The respiratory quotient (RQ) was used to estimate relative lipid oxidation at rest and during exercise (50% maximal oxygen consumption, 600 kcal expended). A skeletal muscle biopsy was obtained from the vastus lateralis at rest Transmission electron microscopy was used to determine mitochondrial size and mitochondria-sarcoplasmic reticulum interactions (<= 50 nm of distance between organelles). Protein levels of fusion/fission regulator were measured in skeletal muscle by Western blot. Resting RQ and exercise RQ associated inversely with intermyofibrillar mitochondrial size (r = -0.66 and r = -0.60, respectively. P< 0.05). Resting RQ also associated inversely with the percentage of intermyofibrillar mitochondria-sarcoplasmic reticulum interactions (r = -0.62, P = 0.03). Finally, intermyofibrillar mitochondrial size associated inversely with droplet density (r = -0.66. P = 0.01) but directly with mitochondria fusion- to-fission ratio (r = 0.61, P = 0.03). Our results show that whole body lipid oxidation is associated with skeletal muscle intennyolibrillar mitochondrial size. fusion phenotype. and mitochondria-sarcoplasmic-reticulum interactions in nondiabetic humans.