Body temperature regulation during acclimation to cold and hypoxia in rats

Extreme environmental conditions present challenges for thermoregulation in homoeothermic organisms such as mammals. Such challenges are exacerbated when two stressors are experienced simultaneously and each stimulus evokes opposing physiological responses. This is the case of cold, which induces an increase in thermogenesis, and hypoxia, which suppresses metabolism conserving oxygen and preventing hypoxaemia. As an initial approach to understanding the thermoregulatory responses to cold and hypoxia in a small mammal, we explored the effects of acclimation to these two stressors on the body temperature (T-b) and the daily and ultradian T-b variations of Sprague-Dawley rats. As T-b is influenced by sleep-wake cycles, these T-b variations reflect underlying adjustments in set-point and thermosensitivity. The T-b of rats decreased precipitously during initial hypoxic exposure which was more pronounced in cold (T-b=33.4 +/- 0.13) than in room temperature (T-b=35.74 +/- 0.17) conditions. This decline was followed by an increase in T-b stabilising at a new level similar to 0.5 degrees C and similar to 1.4 degrees C below normoxic values at room and cold temperatures, respectively. Daily T-b variations were blunted during hypoxia with a greater effect in the cold. Ultradian T-b variations exhibited daily rhythmicity that disappeared under hypoxia, independent of ambient temperature. The adjustments in T-b during hypoxia and/or cold are in agreement with the hypothesis that an initial decrease in the T-b set-point is followed by its partial re-establishment with chronic hypoxia. This rebound of the T-b set-point might reflect cellular adjustments that would allow animals to better deal with low oxygen conditions, diminishing the drive for a lower T-b set-point. Cold and hypoxia are characteristic of high altitude environments. Understanding how mammals cope with changes in oxygen and temperature will shed light into their ability to colonize new environments along altitudinal dines and increase our understanding of how T-b is regulated under stimuli that impose contrasting physiological constraints. (C) 2014 Elsevier Ltd. All rights reserved.