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

The relationship between maximal oxygen consumption rate ( \(\dot V{\text{O}}_{{\text{2max}}}\) ) 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 andm. masseter, as well as samples of muscles collected in a whole-body sampling procedure, were analyzed by electron microscopy. \(\dot V{\text{O}}_{{\text{2max}}}\) per kilogram body mass was 2.7× greater in horses than steers. This higher \(\dot V{\text{O}}_{{\text{2max}}}\) 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 \(\dot V{\text{O}}_{{\text{2max}}}\) . The \(\dot V{\text{O}}_{{\text{2max}}}\) 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.