Department of Biology and Centre for Forest Interdisciplinary Research, University of Winnipeg, Winnipeg, Manitoba, Canada.
Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada.
J Physiol. 2019 Sep;597(18):4809-4829. doi: 10.1113/JP277183. Epub 2019 Aug 13.
For small mammals living in a cold, hypoxic environment, supplying enough O to sustain thermogenesis can be challenging. While heterothermic mammals are generally more tolerant of cold and hypoxia than homeothermic mammals, how they regulate O supply and demand during progressive cooling in hypoxia is largely unknown. We show that as ambient temperature is reduced in hypoxia, body temperature falls in both homeotherms and heterotherms. In the homeothermic rat, a decrease in O consumption rate and lung O extraction accompany this fall in body temperature, despite a relative hyperventilation. Paradoxically, in heterothermic mice, hamsters and ground squirrels, body temperature decreases more than in the homeothermic rat, even though they maintain ventilation, increase lung O extraction and maintain or elevate their O consumption rates. Variation in cold and hypoxia tolerance among homeotherms and heterotherms reflects divergent strategies in how O supply and demand are regulated under thermal and hypoxic challenges.
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 O supply versus O 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% O ) or environmental hypoxia (7% O ), while reducing ambient temperature from 38 to 5°C. We found that when ambient temperature was reduced in normoxia, all species increased their O consumption rate and ventilation in parallel, maintaining a constant ventilatory equivalent and level of lung O 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 O consumption rate and lung O 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 O extraction and maintained or elevated their O 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 O supply and O demand, and that enhanced cold and hypoxia tolerance in heterotherms may stem from an improved ability to extract O from the inspired air.
对于生活在寒冷、缺氧环境中的小型哺乳动物来说，提供足够的氧气来维持体温可能是一项挑战。虽然异温动物通常比恒温动物更能耐受寒冷和缺氧，但它们在缺氧时如何调节氧气的供应和需求以适应体温逐渐下降的情况在很大程度上是未知的。我们发现，在缺氧环境中，当环境温度降低时，恒温动物和异温动物的体温都会下降。在恒温动物大鼠中，尽管相对过度通气，但随着体温下降，耗氧量和肺氧气摄取率都会下降。矛盾的是，在异温动物小鼠、仓鼠和地松鼠中，体温下降的幅度比恒温动物大鼠更大，尽管它们维持通气，增加肺氧气摄取，并维持或提高氧气消耗率。恒温动物和异温动物之间对寒冷和缺氧的耐受性差异反映了在面临寒冷和缺氧挑战时，它们在调节氧气供应和需求方面的策略存在差异。
与恒温动物相比，异温动物被报道对寒冷和缺氧具有异常的耐受性。我们假设这种耐受性的差异源于恒温动物和异温动物在暴露于缺氧时调节氧气供应与需求的策略不同，这种差异是由于它们对低温的适应程度不同。为了验证这一假设，我们将不同程度表现出异温特性的成年啮齿动物（恒温动物：大鼠；兼性异温动物：小鼠和仓鼠；和强制异温动物：地松鼠）暴露于常氧（21%氧气）或环境缺氧（7%氧气）中，同时将环境温度从 38°C 降低至 5°C。我们发现，当环境温度在常氧下降低时，所有物种都平行地增加了氧气消耗率和通气量，保持恒定的通气当量和肺氧气摄取水平。令人惊讶的是，所有物种的体温都下降了，异温动物的体温下降更为显著。然而，当环境温度在缺氧下降低时，恒温动物大鼠尽管通气当量增加，但耗氧量和肺氧气摄取却下降，表明相对过度通气。另一方面，异温动物（小鼠、仓鼠和地松鼠）降低了通气当量，但增加了肺氧气摄取，并维持或提高了氧气消耗率，但它们的体温下降幅度甚至比大鼠还要大。这些结果与这样一种观点一致，即恒温动物和异温动物在匹配氧气供应和需求的策略上存在差异，而异温动物对寒冷和缺氧的耐受性增强可能源于从吸入空气中提取氧气的能力提高。
