Takei Y
Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo, 164-8639 Japan.
Jpn J Physiol. 2000 Apr;50(2):171-86. doi: 10.2170/jjphysiol.50.171.
The origin of life took place in the ancient sea where the ionic concentration is thought to have been somewhat lower than that of the present day seas. This may partly explain why most vertebrate species have plasma ionic concentrations roughly one-third of seawater. Exceptions are primitive marine cyclostomes whose plasma is almost identical to seawater, and marine cartilaginous fishes that accumulate urea in plasma to increase osmolarity to a seawater level. The mechanisms for regulation of water and electrolyte balance should have evolved from these animals into those of more advanced ones in which plasma ions are regulated to one-third of seawater irrespective of the habitat. Although most extant terrestrial and aquatic animals maintain similar plasma osmolarity and ionic concentrations, the mechanisms of regulation differ greatly among different groups of animals according to their habitat. An outstanding difference is that while plasma Na(+) concentration is a primary factor of regulation in terrestrial mammals and birds, blood volume is most strictly regulated in aquatic teleost fishes. Consistently, while an increase in plasma osmolarity (cellular dehydration) is a major dipsogenic stimulus for birds and mammals, hypovolemia (extracellular dehydration) is a much stronger stimulus for elicitation of drinking in teleost fishes. Furthermore, fish cells in culture are tolerant to changes in environmental osmolarity compared with mammalian cells, further suggesting a secondary role of plasma osmolarity as a target of regulation in fishes. A secondary role of blood volume for body fluid regulation in birds is further assessed by the fact that volume receptors for thirst, salt gland secretion, and vasotocin secretion are localized in the extravascular, interstitial space in some species of birds. All terrestrial animals including mammals have derived from the fishes in phylogeny, during which the mechanisms for body fluid regulation underwent adaptive evolution in the course of transition from aquatic to terrestrial life. Therefore, much can be learned from comparative studies of body fluid regulation that reveals the diversity and uniformity of the mechanisms. In this review, important comparative studies that may contribute to an understanding of body fluid regulation throughout vertebrate species will be summarized.
生命起源于古代海洋，据认为当时的离子浓度略低于现代海洋。这或许可以部分解释为何大多数脊椎动物物种的血浆离子浓度约为海水的三分之一。例外情况是原始的海洋圆口纲动物，其血浆几乎与海水相同，以及海洋软骨鱼类，它们在血浆中积累尿素以将渗透压提高到海水水平。水和电解质平衡的调节机制应该是从这些动物进化到更高级动物的机制，在更高级动物中，无论栖息地如何，血浆离子都被调节到海水的三分之一。尽管大多数现存的陆生和水生动物保持相似的血浆渗透压和离子浓度，但根据栖息地的不同，不同动物群体的调节机制差异很大。一个显著的差异是，虽然血浆Na(+)浓度是陆生哺乳动物和鸟类调节的主要因素，但在硬骨鱼类中，血容量受到最严格的调节。同样，虽然血浆渗透压升高（细胞脱水）是鸟类和哺乳动物主要的致渴刺激，但血容量减少（细胞外脱水）对硬骨鱼类引发饮水的刺激要强得多。此外，与哺乳动物细胞相比，培养的鱼类细胞对环境渗透压的变化具有耐受性，这进一步表明血浆渗透压在鱼类中作为调节靶点的次要作用。鸟类血容量在体液调节中的次要作用通过以下事实进一步得到评估：在某些鸟类物种中，口渴、盐腺分泌和血管升压素分泌的容量感受器位于血管外的间质空间。在系统发育过程中，包括哺乳动物在内的所有陆生动物都起源于鱼类，在此期间，体液调节机制在从水生生活向陆生生活的转变过程中经历了适应性进化。因此，通过对体液调节的比较研究可以学到很多东西，这些研究揭示了调节机制的多样性和统一性。在这篇综述中，将总结可能有助于理解整个脊椎动物物种体液调节的重要比较研究。
