Takei Yoshio
Laboratory of Physiology, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.
Zoolog Sci. 2015 Jan;32(1):1-7. doi: 10.2108/zs140142.
It is generally accepted that ancient fishes first experienced freshwater (FW), and then variably by lineage moved onto the land or re-entered the seas during evolution. As both land and sea are desiccating environments, animals must change their strategies for body fluid regulation from protecting against overhydration in FW to coping with dehydration in seawater (SW) or on land. The evolution of the mechanisms for acquisition of water surely must have accompanied these dramatic environmental changes. The major route for water acquisition is by oral drinking in terrestrial tetrapods (represented here by mammals) and in SW fishes (represented by teleosts as they are dehydrated in SW), but the regulation is contrasting between the two groups; mechanisms inducing thirst have developed in mammals, whereas inhibitory mechanisms are dominant in marine teleosts as observed in FW teleosts. Thus, the apparent difference was found not between hydrating and dehydrating habitat, but rather between terrestrial and aquatic habitats. This contrast is also reflected in regulatory hormones; dipsogenic hormones such as angiotensin II play pivotal roles in water homeostasis in mammals, whereas antidipsogenic hormones such as atrial natriuretic peptide are essential in teleosts. Imbibed water becomes body fluid only after absorption by the intestine, and there is a distinct difference in the mechanisms for water absorption between mammals and teleosts. Like regulation of drinking, we found that the inhibitory mechanisms are dominant for intestinal water absorption in teleosts. In the initial part of this short review, interesting differences in the body fluid regulation between mammals and teleosts are introduced, particularly with regard to water acquisition (drinking and intestinal absorption). Then an attempt was made to discuss the evolution of the mechanisms from the two perspectives; transitions from aquatic to terrestrial habitats and from hydrating (FW) to dehydrating (land and SW) habitats.
人们普遍认为，古代鱼类最初生活在淡水中，然后在进化过程中，不同的谱系以不同方式迁移到陆地或重新进入海洋。由于陆地和海洋都是干燥的环境，动物必须改变其体液调节策略，从在淡水中防止水分过多，转变为在海水或陆地上应对脱水。获取水分机制的进化肯定伴随着这些巨大的环境变化。获取水分的主要途径是通过口腔饮水，在陆生四足动物（此处以哺乳动物为代表）和海水鱼类（以硬骨鱼为代表，因为它们在海水中会脱水）中都是如此，但两组动物的调节方式截然不同；哺乳动物中已形成了诱发口渴的机制，而在硬骨鱼中，抑制机制占主导地位，这与淡水硬骨鱼的情况类似。因此，明显的差异并非存在于补水和脱水的栖息地之间，而是存在于陆地和水生栖息地之间。这种差异也反映在调节激素上；血管紧张素 II 等致渴激素在哺乳动物的水平衡中起关键作用，而心房利钠肽等抗利尿激素在硬骨鱼中至关重要。摄入的水只有在被肠道吸收后才会成为体液，哺乳动物和硬骨鱼在水吸收机制上存在明显差异。与饮水调节一样，我们发现硬骨鱼肠道水吸收的抑制机制占主导地位。在这篇简短综述的开头部分，介绍了哺乳动物和硬骨鱼在体液调节方面的有趣差异，特别是在水分获取（饮水和肠道吸收）方面。然后试图从两个角度讨论这些机制的进化；从水生栖息地到陆地栖息地的转变，以及从补水（淡水）栖息地到脱水（陆地和海水）栖息地的转变。
