Department of Biology, University of Turku, Turku, Finland.
Institute of Integrative Biology, Department of Evolution, Ecology and Behaviour, University of Liverpool, Liverpool, UK.
Acta Physiol (Oxf). 2019 Oct;227(2):e13299. doi: 10.1111/apha.13299. Epub 2019 Jun 5.
Gas transport concepts in vertebrates have naturally been formulated based on human blood. However, the first vertebrates were aquatic, and fish and tetrapods diverged hundreds of millions years ago. Water-breathing vertebrates live in an environment with low and variable O levels, making environmental O an important evolutionary selection pressure in fishes, and various features of their gas transport differ from humans. Erythrocyte function in fish is of current interest, because current environmental changes affect gas transport, and because especially zebrafish is used as a model in biomedical studies, making it important to understand the differences in gas transport between fish and mammals to be able to carry out meaningful studies. Of the close to thirty thousand fish species, teleosts are the most species-numerous group. However, two additional radiations are discussed: agnathans and elasmobranchs. The gas transport by elasmobranchs may be closest to the ancestors of tetrapods. The major difference in their haemoglobin (Hb) function to humans is their high urea tolerance. Agnathans differ from other vertebrates by having Hbs, where cooperativity is achieved by monomer-oligomer equilibria. Their erythrocytes also lack the anion exchange pathway with profound effects on CO transport. Teleosts are characterized by highly pH sensitive Hbs, which can fail to become fully O -saturated at low pH. An adrenergically stimulated Na /H exchanger has evolved in their erythrocyte membrane, and plasma-accessible carbonic anhydrase can be differentially distributed among their tissues. Together, and differing from other vertebrates, these features can maximize O unloading in muscle while ensuring O loading in gills.
脊椎动物的气体运输概念自然是基于人类血液来构建的。然而，最早的脊椎动物是水生的，鱼类和四足动物在数亿年前就已经分化。用肺呼吸的脊椎动物生活在氧气水平低且变化大的环境中，这使得环境氧气成为鱼类进化选择的重要压力，它们的气体运输的各种特征与人类不同。鱼类的红细胞功能目前受到关注，因为当前的环境变化会影响气体运输，而且由于斑马鱼特别被用作生物医学研究的模型，因此了解鱼类和哺乳动物之间气体运输的差异对于进行有意义的研究非常重要。在近 3 万种鱼类中，硬骨鱼类是种类最多的群体。然而，还有另外两个辐射分支被讨论：无颌类和软骨鱼类。软骨鱼类的气体运输可能与四足动物的祖先最为接近。它们的血红蛋白（Hb）功能与人类的主要区别在于其对尿素的高耐受性。无颌类与其他脊椎动物的不同之处在于它们具有 Hb，其中协同作用是通过单体-寡聚平衡来实现的。它们的红细胞也缺乏阴离子交换途径，这对 CO 运输有深远的影响。硬骨鱼类的特点是其 Hb 对 pH 值非常敏感，在低 pH 值下可能无法完全 O 饱和。它们的红细胞膜中已经进化出一种肾上腺素能刺激的 Na+/H 交换器，而血浆可接触的碳酸酐酶可以在它们的组织中进行差异分布。这些特征与其他脊椎动物不同，它们可以在肌肉中最大限度地释放 O2，同时确保在鳃中摄取 O2。
