Human skeletal muscle mitochondrial capacity

Under aerobic work, the oxygen consumption and major ATP production occur i n the mitochondria and it is therefore a relevant question whether the in v ivo rates can be accounted for by mitochondrial capacities measured in vitr o. Mitochondria were isolated from human quadriceps muscle biopsies in yiel ds of approximate to 45%. The tissue content of total creatine, mitochondri al protein and different cytochromes was estimated. A number of activities were measured in functional assays of the mitochondria: pyruvate, ketogluta rate, glutamate and succinate dehydrogenases, palmitoyl-carnitine respirati on, cytochrome oxidase, the respiratory chain and the ATP synthesis. The ac tivities involved in carbohydrate oxidation could account for in vivo oxyge n uptakes of 15-16 mmol O-2 min(-1) kg(-1) or slightly above the value meas ured at maximal work rates in the knee-extensor model of Saltin and co-work ers, i.e. without limitation from the cardiac output. This probably indicat es that the maximal oxygen consumption of the muscle is limited by the mito chondrial capacities. The in vitro activities of fatty acid oxidation corre sponded to only 39% of those of carbohydrate oxidation. The maximal rate of free energy production from aerobic metabolism of glycogen was calculated from the mitochondrial activities and estimates of the Delta G or ATP hydro lysis and the efficiency of the actin-myosin reaction. The resultant value was 20 W kg(-1) or approximate to 70% of the maximal in vivo work rates of which 10-20% probably are sustained by the anaerobic ATP production. The la ck of aerobic in vitro ATP synthesis might reflect termination of some crit ical interplay between cytoplasm and mitochondria.