Circulatory mechanisms underlying adaptive increases in thermogenic capacity in high-altitude deer mice

We examined the circulatory mechanisms underlying adaptive increases in thermogenic capacity in deer mice (Peromyscus maniculatus) native to the cold hypoxic environment at high altitudes. Deer mice from high- and low-altitude populations were born and raised in captivity to adulthood, and then acclimated to normoxia or hypobaric hypoxia (simulating hypoxia at ∼4300 m). Thermogenic capacity [maximal O₂ consumption (V̇_O), during cold exposure] was measured in hypoxia, along with arterial O₂ saturation (Sa_O ) and heart rate (f_H). Hypoxia acclimation increased V̇_O by a greater magnitude in highlanders than in lowlanders. Highlanders also had higher Sa_O  and extracted more O₂ from the blood per heartbeat (O₂ pulse=V̇_O/f_H). Hypoxia acclimation increased f_H, O₂ pulse and capillary density in the left ventricle of the heart. Our results suggest that adaptive increases in thermogenic capacity involve integrated functional changes across the O₂ cascade that augment O₂ circulation and extraction from the blood.