Hypoxia alters the thermogenic response to cold in adult homeothermic and heterothermic rodents.

Compared to homeothermic mammals, heterothermic mammals are reported to be exceptionally tolerant of cold and hypoxia. We hypothesised that this variation in tolerance stems from divergent strategies in how homeotherms and heterotherms regulate O2 supply versus O2 demand when exposed to hypoxia during progressive cooling. To test this hypothesis, we exposed adult rodents ranging in their degree of heterothermic expression (homeotherm: rats, facultative heterotherms: mice and hamsters, and obligate heterotherm: ground squirrels) to either normoxia (21% O2 ) or environmental hypoxia (7% O2 ), while reducing ambient temperature from 38 to 5°C. We found that when ambient temperature was reduced in normoxia, all species increased their O2 consumption rate and ventilation in parallel, maintaining a constant ventilatory equivalent and level of lung O2 extraction. Surprisingly, body temperature fell in all species, significantly so in the heterotherms. When ambient temperature was reduced in hypoxia, however, the homeothermic rat decreased their O2 consumption rate and lung O2 extraction despite an increase in their ventilatory equivalent, indicative of a relative hyperventilation. Heterotherms (mice, hamsters and ground squirrels), on the other hand, decreased their ventilatory equivalent, but increased lung O2 extraction and maintained or elevated their O2 consumption rates, yet their body temperature fell even more than in the rat. These results are consistent with the idea that homeotherms and heterotherms diverge in the strategies they use to match O2 supply and O2 demand, and that enhanced cold and hypoxia tolerance in heterotherms may stem from an improved ability to extract O2 from the inspired air.