Although hypoxia tolerance in heterothermic mammals is well established, it is unclear whether the adaptive significance stems from hypoxia or other cellular challenge associated with euthermy, hibernation, or arousal. In the present study, blood gases, hemoglobin O2 saturation (So2), and indexes of cellular and physiological stress were measured during hibernation and euthermy and after arousal thermogenesis. Results show that arterial O2 tension (PaO2) and So2 are severely diminished during arousal and that hypoxia-inducible factor (HIF)-1α accumulates in brain. Despite evidence of hypoxia, neither cellular nor oxidative stress, as indicated by inducible nitric oxide synthase (iNOS) levels and oxidative modification of biomolecules, was observed during late arousal from hibernation. Compared with rats, hibernating Arctic ground squirrels ( Spermophilus parryii) are well oxygenated with no evidence of cellular stress, inflammatory response, neuronal pathology, or oxidative modification following the period of high metabolic demand necessary for arousal. In contrast, euthermic Arctic ground squirrels experience mild, chronic hypoxia with low So2 and accumulation of HIF-1α and iNOS and demonstrate the greatest degree of cellular stress in brain. These results suggest that Arctic ground squirrels experience and tolerate endogenous hypoxia during euthermy and arousal.
Habitat selection by arctic ground squirrels (Spermophilus parryii)
