Working underground: Respiratory adaptations in the blind mole rat

Mole rats (Spalax ehrenbergi superspecies) perform the heavy work of digging their subterranean burrows in Israel under highly hypoxic/hypercapnic conditions. Unlike must other mammals, they can achieve high levels of metabolic rate under these conditions, while their metabolic rate at low work rates is depressed. We explored, by comparing mole rats with white rats, whether and how this is related to adaptations in the design of the respiratory system, which determines the transfer of O₂ from the lung to muscle mitochondria. At the same body mass, mole rats were found to have a significantly smaller total skeletal muscle mass than ordinary white rats (- 22%). In contrast, the fractional volume of muscle mitochondria was larger by 46%. As a consequence, both species had the same total amount of mitochondria and achieved, under normoxia, the same V̇O₂(max). Whereas the O₂ transport capacity of the blood was not different, we found a larger capillary density (+31%) in the mole rat muscle, resulting in a reduced diffusion distance to mitochondria. The structural pulmonary diffusing capacity for O₂ was greater in the mole rat (+44%), thus facilitating O₂ uptake in hypoxia. We conclude that structural adaptations in lung and muscle tissue improve O₂ diffusion conditions and serve to maintain high metabolic rates in hypoxia but have no consequences for achieving V̇O₂(max) under normoxic conditions.