The quantitative structural composition of skeletal muscle tissue shows a wide range of variability among different species of animals and in any one species among muscles with a different function. Moreover, experimental manipulations such as exercise training or chronic electrical stimulation can dramatically change the ultrastructural appearance of the muscles involved. Both in endurance exercise and in chronic electrical stimulation the volume density of mitochondria can be increased greatly (by more than three-fold in the stimulation experiments). This happens without an apparent change of the internal architecture of the mitochondria, since the surface density of the inner mitochondrial membranes remains constant. In situations where both the mitochondrial volume and the maximal rate of oxygen consumption of the muscle tissue are known, these two variables are found to be linearly related. It can be calculated that the ‘maximal’ oxygen consumption of a unit volume of mitochondria in muscle is close to 5 ml O2 min-1 cm-3 under comparable conditions in man, mouse and a series of African mammals. It is hypothesized that there is a constant volume of oxygen metabolized per unit volume of mitochondria and unit time under limiting conditions in working skeletal muscle tissue. Given the efficiency of muscular energy conversion, this would allow an estimate of the potential for aerobic power production of a muscle from measurement of its volume density of mitochondria.

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