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

Abstract

The aim of the present study was to examine whether parameters of isolated mitochondria could account for the in vivo maximum oxygen uptake (V˙o 2 max) of human skeletal muscle.V˙o 2 max and work performance of the quadriceps muscle of six volunteers were measured in the knee extensor model (range 10–18 mmol O2 · min−1 · kg−1at work rates of 22–32 W/kg). Mitochondria were isolated from the same muscle at rest. Strong correlations were obtained betweenV˙o 2 max and a number of mitochondrial parameters (mitochondrial protein, cytochrome aa 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˙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 theV˙o 2 max. This might be due to systematic errors or to loose coupling of the mitochondrial respiration under intense exercise.