Mitochondrial respiration is enhanced in TWEAK-KO mice. Respiratory activity of mitochondria isolated from wild-type and TWEAK-KO mice subjected to an acute bout of treadmill running: (A) Extracellular flux assay of isolated mitochondria: mitochondrial activity was measured in the absence of substrate (state 1 respiration) or in the presence of pyruvate (5 mM), malate (2.5 mM) and ADP (1 mM) (state 3 respiration). State 4 respiration was estimated by measuring the oxygen consumption rate (OCR) after the addition of oligomycin (1 μg/ml). FCCP (4 μM) was added to assess maximal activity of mitochondria respiring on substrates providing electrons for complex I (that is, pyruvate and malate) or complex II (that is, succinate in the presence of rotenone; 10 mM and 1 μM, respectively). (B) Group data derived from panel A. N = 3 mice per group. *P <0.05 versus wild-type. FCCP, carbonyl cyanide p-trifluoromethoxyphenylhydrazone; KO, knockout; TWEAK, TNF-like weak inducer of apoptosis.