Aerobic metabolism of human quadriceps muscle: in vivo data parallel measurements on isolated mitochondria

The aim of the present study was to examine whether parameters of isolated mitochondria could account for the in vivo maximum oxygen uptake ((V) over dot O-2 max) of human skeletal muscle. (V) over dot O-2 max and work performance of the quadriceps muscle of six volunteers were measured in the knee extensor model (range 10-18 mmol O-2 . min(-1) . kg(-1) at work rates of 22- 32 W/kg). Mitochondria were isolated from the same muscle at rest. Strong correlations were obtained between (V) over dot O-2 max and a number of mitochondrial parameters (mitochondrial protein, cytochrome alpha alpha (3), citrate synthase, and respiratory activities). The activities of citrate synthase, succinate dehydrogenase, and pyruvate dehydrogenase, measured in isolated mitochondria, corresponded to, respectively, 15, 3, and 1.1 times the rates calculated from (V) over dot O-2 max. The respiratory chain activity also appeared sufficient. Fully coupled in vitro respiration, which is limited by the rate of ATP synthesis, could account for, at most, 60% of the (V) over dot O-2 max. This might be due to systematic errors or to loose coupling of the mitochondrial respiration under intense exercise.