TWEAK regulates mitochondrial oxidative capacity and glycolytic flux. Extracellular flux analysis of primary differentiated myotubes treated with 0, 10, or 100 ng/ml TWEAK for 72 hours. (A) Mitochondrial function assay: after three baseline measurements, inhibitors or activators of electron transport were added sequentially to intact myotubes. Oxygen consumption rate (OCR) measurements were recorded after each exposure. O, oligomycin; F, FCCP; and A, antimycin A/rotenone. The OCR was normalized to total protein in each well. (B) Indices of mitochondrial function calculated from assay results in panel A. (C) Extracellular acidification rates (ECAR) of cells treated without or with TWEAK; ECAR is a surrogate measure of lactate and therefore is used as a measure of glycolytic flux. N = 3 per group; *P <0.05 versus control. FCCP, carbonyl cyanide p-trifluoromethoxyphenylhydrazone; TWEAK, TNF-like weak inducer of apoptosis.