Mitochondrial Function in an In Vitro Model of Skeletal Muscle of Patients With Protracted Critical Illness and Intensive Care Unit–Acquired Weakness
Background: Functional mitochondria in skeletal muscle of patients with protracted critical illness and intensive care unit–acquired weakness are depleted, but remaining mitochondria have increased functional capacities of respiratory complexes II and III. This can be an adaptation to relative abundancy of fatty acid over glucose caused by insulin resistance. We hypothesized that the capacity of muscle mitochondria to oxidize fatty acid is increased in protracted critical illness. Methods: We assessed fatty acid oxidation (FAO) and mitochondrial functional indices in vitro by using extracellular flux analysis in cultured myotubes obtained by isolating and culturing satellite cells from vastus lateralis muscle biopsy samples from patients with ICU-acquired weakness (n = 6) and age-matched healthy controls (n = 7). Bioenergetic measurements were performed at baseline and after 6 days of exposure to free fatty acids (FFAs). Results: Mitochondrial density in myotubes from ICU patients was 69% of healthy controls (P = .051). After adjustment to mitochondrial content, there were no differences in adenosine triphosphate (ATP) synthesis or the capacity and coupling of the respiratory chain. FAO capacity in ICU patients was 157% of FAO capacity in controls (P = .015). In myotubes of ICU patients, unlike healthy controls, the exposure to FFA significantly (P = .009) increased maximum respiratory chain capacity. Conclusion: In an in vitro model of skeletal muscle of patients with protracted critical illness, we have shown signs of adaptation to increased FAO. Even in the presence of glucose and insulin, elevation of FFAs in the extracellular environment increased maximal capacity of the respiratory chain.