Mitochondrial respiration in hummingbird flight muscles

Respiration rates of muscle mitochondria in flying hummingbirds range from 7 to 10 ml of O2 per cm3 of mitochondria per min, which is about 2 times higher than the range obtained in the locomotory muscles of mammals running at their maximum aerobic capacities (VO2max). Capillary volume density is higher in hummingbird flight muscles than in mammalian skeletal muscles. Mitochondria occupy approximately 35% of fiber volume in hummingbird flight muscles and cluster beneath the sarcolemmal membrane adjacent to capillaries to a greater extent than in mammalian muscles. Measurements of protein content, citrate synthase activity, and respiratory rates in vitro per unit mitochondrial volume reveal no significant differences between hummingbird and mammalian skeletal muscle mitochondria. However, inner membrane surface areas per unit mitochondrial volume [Sv(im,m)] are higher than those in mammalian muscle. We propose that both mitochondrial volume densities and Sv(im,m) are near their maximum theoretical limits in hummingbirds and that higher rates of mitochondrial respiration than those observed in mammals are achieved in vivo as a result of higher capacities for O2 delivery and substrate catabolism.