Total muscle mitochondrial volume in relation to aerobic capacity of horses and steers

The relationship between maximal oxygen consumption rate ( {Mathematical expression}) and mitochondrial content of skeletal muscles was examined in horses and steers (n=3 each). Samples of the heart left ventricle, diaphragm, m. vastus medialis, m. semitendinosus, m. cutaneous thoracicus and m. masseter, as well as samples of muscles collected in a whole-body sampling procedure, were analyzed by electron microscopy. {Mathematical expression} per kilogram body mass was 2.7× greater in horses than steers. This higher {Mathematical expression} was in proportion to the higher total volume of mitochondria in horse versus steer muscle when analyzed from the whole-body samples and from the locomotor muscle samples. In non-locomotor muscles, total mitochondrial volume was greater in horses than steers, but not in proportion to their differences in {Mathematical expression}. The {Mathematical expression} of the mitochondria was estimated to be close to 4.5 ml O₂·ml^-1 mitochondria in both species. It is concluded that in a comparison of a highly aerobic to a less aerobic mammalian species of similar body size, a higher oxidative potential may be found in all muscles of the more aerobic species. This greater oxidative potential is achieved by a greater total volume of skeletal muscle mitochondria.