Zhang Yangfan, So Bog E, Farrell Anthony P
Department of Zoology, Faculty of Land and Food System, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
Metabolites. 2021 Jul 8;11(7):447. doi: 10.3390/metabo11070447.
The utility of measuring whole-animal performance to frame the metabolic response to environmental hypoxia is well established. Progressively reducing ambient oxygen (O) will initially limit maximum metabolic rate as a result of a hypoxemic state and ultimately lead to a time-limited, tolerance state supported by substrate-level phosphorylation when the O supply can no longer meet basic needs (standard metabolic rate, SMR). The metabolic consequences of declining ambient O were conceptually framed for fishes initially by Fry's hypoxic performance curve, which characterizes the hypoxemic state and its consequences to absolute aerobic scope (AAS), and Hochachka's concept of scope for hypoxic survival, which characterizes time-limited life when SMR cannot be supported by O supply. Yet, despite these two conceptual frameworks, the toolbox to assess whole-animal metabolic performance remains rather limited. Here, we briefly review the ongoing debate concerning the need to standardize the most commonly used assessments of respiratory performance in hypoxic fishes, namely critical O (the ambient O level below which maintenance metabolism cannot be sustained) and the incipient lethal O (the ambient O level at which a fish loses the ability to maintain upright equilibrium), and then we advance the idea that the most useful addition to the toolbox will be the limiting-O concentration (LOC) performance curve. Using Fry & Hart's (1948) hypoxia performance curve concept, an LOC curve was subsequently developed as an eco-physiological framework by Neil et al. and derived for a group of fish during a progressive hypoxia trial by Claireaux and Lagardère (1999). In the present review, we show how only minor modifications to available respirometry tools and techniques are needed to generate an LOC curve for individual fish. This individual approach to the LOC curve determination then increases its statistical robustness and importantly opens up the possibility of examining individual variability. Moreover, if peak aerobic performance at a given ambient O level of each individual is expressed as a percentage of its AAS, the water dissolved O that supports 50% of the individual's AAS (DO) can be interpolated much like the P for an O hemoglobin dissociation curve (when hemoglobin is 50% saturated with O). Thus, critical O, incipient lethal O, DO and P and can be directly compared within and across species. While an LOC curve for individual fish represents a start to an ongoing need to seamlessly integrate aerobic to anaerobic capacity assessments in a single, multiplexed respirometry trial, we close with a comparative exploration of some of the known whole-organism anaerobic and aerobic capacity traits to examine for correlations among them and guide the next steps.
通过测量动物整体性能来构建对环境低氧的代谢反应，其效用已得到充分证实。逐步降低环境中的氧气（O），最初会由于低氧状态而限制最大代谢率，最终当氧气供应无法满足基本需求（标准代谢率，SMR）时，会导致由底物水平磷酸化支持的限时耐受状态。环境氧气下降的代谢后果最初由弗莱的低氧性能曲线在概念上为鱼类构建，该曲线描述了低氧状态及其对绝对有氧范围（AAS）的影响，以及霍查克的低氧存活范围概念，该概念描述了在氧气供应无法支持SMR时的限时生命。然而，尽管有这两个概念框架，但评估动物整体代谢性能的工具仍然相当有限。在这里，我们简要回顾了关于标准化低氧鱼类最常用呼吸性能评估方法的持续争论，即临界氧（低于该环境氧水平维持代谢就无法持续）和初始致死氧（鱼类失去维持直立平衡能力时的环境氧水平），然后我们提出这样一个观点，即工具中最有用的补充将是极限氧浓度（LOC）性能曲线。利用弗莱和哈特（1948年）的低氧性能曲线概念，尼尔等人随后将LOC曲线发展为一个生态生理框架，并由克莱罗和拉加尔德（1999年）在一组鱼类的渐进性低氧试验中推导得出。在本综述中，我们展示了只需对现有的呼吸测量工具和技术进行微小修改，就能为个体鱼类生成LOC曲线。这种确定LOC曲线的个体方法随后提高了其统计稳健性，并且重要的是开启了检查个体变异性的可能性。此外，如果将每个个体在给定环境氧水平下的最大有氧性能表示为其AAS的百分比，那么支持个体AAS的50%的溶解氧（DO）就可以像氧血红蛋白解离曲线中的P50（当血红蛋白被氧饱和50%时）一样进行内插。因此，临界氧、初始致死氧、DO和P50可以在物种内和物种间直接进行比较。虽然个体鱼类的LOC曲线代表了在单一的多重呼吸测量试验中无缝整合有氧到无氧能力评估这一持续需求的开端，但我们最后对一些已知的全生物体无氧和有氧能力特征进行了比较探索，以研究它们之间的相关性并指导下一步研究。
