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 rates in six Drosophila 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 affected 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 capacities were 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 compensatory oxidation of alternative substrates occurred at temperatures that tended to correlate 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.

中文翻译：

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临界高温下线粒体底物使用的戏剧性变化：使用果蝇的比较研究

等温热容忍度对物种分布至关重要，但目前对热容忍度的生理基础仍然知之甚少。线虫的功能在包括昆虫在内的某些等温线中处于临界高温，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。为了检验该假设，我们测量了六个果蝇中的线粒体耗氧率高分辨率呼​​吸测定法测定具有不同耐热性的物种。使用底物-解偶联剂-抑制剂滴定方案，我们检查了电子传输系统的特定步骤，以研究低于，包围和高于生物热极限的温度如何影响线粒体功能和底物氧化。在良性温度（19和30°C）下，复杂的I辅助呼吸（CI-OXPHOS）是最大OXPHOS的最重要因素。在较高的温度（34、38、42和46°C）下，CI-OXPHOS显着下降，最终在42和46°C降至非常低的水平。在所有温度下，配合物I的酶催化能力均保持完整，因此CI-OXPHOS的降低不太可能直接由配合物I的高温变性/失活直接引起。尽管CI-OXPHOS有所降低，通过替代性底物的氧化，在所有物种中都保持了最大的OXPHOS容量；脯氨酸，琥珀酸酯，尤其是3-磷酸甘油，表明在超过机体热量极限的温度下重要的线粒体柔韧性。有趣的是，替代底物的这种补偿性氧化发生在倾向于与物种耐热性相关的温度下，使得耐热物种在比敏感物种更高的温度下可以捍卫“正常”的线粒体功能。未来的研究应调查为什么会干扰CI-OXPHOS，以及这如何影响ATP的生产率。表明在超过机体热量极限的温度下重要的线粒体柔韧性。有趣的是，替代底物的这种补偿性氧化发生在倾向于与物种耐热性相关的温度下，使得耐热物种在比敏感物种更高的温度下可以捍卫“正常”的线粒体功能。未来的研究应调查为什么会干扰CI-OXPHOS，以及这如何影响ATP的生产率。表明在超过机体热量极限的温度下重要的线粒体柔韧性。有趣的是，替代底物的这种补偿性氧化发生在倾向于与物种耐热性相关的温度下，使得耐热物种在比敏感物种更高的温度下可以捍卫“正常”的线粒体功能。未来的研究应调查为什么会干扰CI-OXPHOS，以及这如何影响ATP的生产率。