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

Abstract

We examined the circulatory mechanisms underlying adaptive increases in thermogenic capacity in deer mice (<i>Peromyscus maniculatus</i>) 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 (<i>V̇</i>_O₂,max), during cold exposure] was measured in hypoxia, along with arterial O₂ saturation (<i>S</i>a_O₂ ) and heart rate (<i>f</i>_H). Hypoxia acclimation increased <i>V̇</i>_O₂,max by a greater magnitude in highlanders than in lowlanders. Highlanders also had higher <i>S</i>a_O₂ and extracted more O₂ from the blood per heartbeat (O₂ pulse=<i>V̇</i>_O₂,max/<i>f</i>_H). Hypoxia acclimation increased <i>f</i>_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.