Zoophysiology, Department of Biology, Aarhus University, 8000 Aarhus C, Denmark
Zoophysiology, Department of Biology, Aarhus University, 8000 Aarhus C, Denmark.
J Exp Biol. 2021 Mar 19;224(Pt 6):jeb240960. doi: 10.1242/jeb.240960.
Ectotherm thermal tolerance is critical to species distribution, but at present the physiological underpinnings of heat tolerance remain poorly understood. Mitochondrial function is perturbed at critically high temperatures in some ectotherms, including insects, suggesting that heat tolerance of these animals is linked to failure of oxidative phosphorylation (OXPHOS) and/or ATP production. To test this hypothesis, we measured mitochondrial oxygen consumption rate in six species with different heat tolerance using high-resolution respirometry. Using a substrate-uncoupler-inhibitor titration protocol, we examined specific steps of the electron transport system to study how temperatures below, bracketing and above organismal heat limits affect mitochondrial function and substrate oxidation. At benign temperatures (19 and 30°C), complex I-supported respiration (CI-OXPHOS) was the most significant contributor to maximal OXPHOS. At higher temperatures (34, 38, 42 and 46°C), CI-OXPHOS decreased considerably, ultimately to very low levels at 42 and 46°C. The enzymatic catalytic capacity of complex I was intact across all temperatures and accordingly the decreased CI-OXPHOS is unlikely to be caused directly by hyperthermic denaturation/inactivation of complex I. Despite the reduction in CI-OXPHOS, maximal OXPHOS capacity was maintained in all species, through oxidation of alternative substrates - proline, succinate and, particularly, glycerol-3-phosphate - suggesting important mitochondrial flexibility at temperatures exceeding the organismal heat limit. Interestingly, this failure of CI-OXPHOS and compensatory oxidation of alternative substrates occurred at temperatures that correlated with species heat tolerance, such that heat-tolerant species could defend 'normal' mitochondrial function at higher temperatures than sensitive species. Future studies should investigate why CI-OXPHOS is perturbed and how this potentially affects ATP production rates.
变温动物的热耐受性对物种分布至关重要，但目前对热耐受性的生理基础仍知之甚少。一些变温动物（包括昆虫）在临界高温下，线粒体功能受到干扰，这表明这些动物的耐热性与氧化磷酸化（OXPHOS）和/或 ATP 产生的失败有关。为了验证这一假设，我们使用高分辨率呼吸测定法测量了六种耐热性不同的物种的线粒体耗氧率。通过使用底物-解偶联剂-抑制剂滴定方案，我们研究了电子传递系统的特定步骤，以研究低于、接近和高于生物体热限的温度如何影响线粒体功能和底物氧化。在温和的温度（19 和 30°C）下，复合物 I 支持的呼吸（CI-OXPHOS）是最大 OXPHOS 的主要贡献者。在较高的温度（34、38、42 和 46°C）下，CI-OXPHOS 大大减少，最终在 42 和 46°C 时降至非常低的水平。复合物 I 的酶催化能力在所有温度下都是完整的，因此 CI-OXPHOS 的减少不太可能直接由复合物 I 的热变性/失活引起。尽管 CI-OXPHOS 减少，但所有物种的最大 OXPHOS 能力都通过替代底物（脯氨酸、琥珀酸和特别甘油-3-磷酸）的氧化得到维持，这表明在超过生物体热限的温度下，线粒体具有重要的灵活性。有趣的是，CI-OXPHOS 的失败和替代底物的代偿性氧化发生在与物种耐热性相关的温度下，以至于耐热物种能够在比敏感物种更高的温度下维持“正常”的线粒体功能。未来的研究应探讨为什么 CI-OXPHOS 受到干扰，以及这如何潜在地影响 ATP 产生速率。
