Bijvelds MJ, Velden JA, Kolar ZI, Flik G
Department of Animal Physiology, Faculty of Science, University of Nijmegen, Toernooiveld, The Netherlands and Department of Radiochemistry, Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, The Netherlands. mbyv.
J Exp Biol. 1998 Jul;201 (Pt 13):1981-90. doi: 10.1242/jeb.201.13.1981.
The magnesium handling of freshwater teleost fish is discussed, with an emphasis on the role of branchial, intestinal and renal transport. In response to the eminent threat of constant diffusive losses of minerals such as magnesium, freshwater fish have developed efficient mechanisms for magnesium homeostasis. Magnesium losses are overcome by the uptake of magnesium from the food, making the intestine an important route for magnesium uptake. Some evidence suggests that intestinal magnesium uptake in fish is a regulated, cellular process. The ambient water is an additional magnesium source for fish, implicating the gills as a secondary route for magnesium uptake. Certainly, in some species, direct uptake from the water, probably via branchial routes, ameliorates the effects of a low-magnesium diet. The hard tissues, representing over 50 % of the total body magnesium pool, form a reservoir from which magnesium can be recruited to perform its functions in the cellular metabolism of soft tissues such as muscle. In fish, as in terrestrial vertebrates, the balance of a variety of elements becomes disturbed when the magnesium homeostasis of the soft tissues is disrupted. However, fish appear to be less sensitive than terrestrial vertebrates to these perturbations. Magnesium is reabsorbed in the kidneys to minimise losses. For renal cells, part of a cellular pathway has been elucidated that would allow absorptive magnesium transport (a magnesium conductive pathway in renal brush-border membranes). In some euryhaline teleosts, the kidneys appear to switch instantaneously to rapid magnesium secretion upon magnesium loading, a response common to marine fish that are threatened by diffusive magnesium entry. This enigmatic mechanism underlies the capacity of some euryhaline species to acclimate rapidly to sea water. Despite the progress made over the last decade, much of the cellular and molecular basis of magnesium transport in the gills, intestine and kidneys remains obscure. The application of fluorescent, radioactive and molecular probes, some of which have only recently become available, may yield rapid progress in the field of magnesium research.
本文讨论了淡水硬骨鱼的镁处理过程，重点强调了鳃、肠道和肾脏转运的作用。由于面临镁等矿物质持续扩散流失的严重威胁，淡水鱼已形成了有效的镁稳态维持机制。通过从食物中摄取镁来克服镁的流失，这使得肠道成为镁摄取的重要途径。一些证据表明，鱼类肠道对镁的摄取是一个受调控的细胞过程。周围水体是鱼类额外的镁来源，这意味着鳃是镁摄取的第二条途径。当然，在某些物种中，直接从水中摄取镁（可能通过鳃途径）可减轻低镁饮食的影响。硬组织占全身镁库的50%以上，形成了一个储存库，镁可从中被调用以在肌肉等软组织的细胞代谢中发挥作用。与陆生脊椎动物一样，当软组织的镁稳态被破坏时，鱼类体内各种元素的平衡也会受到干扰。然而，鱼类似乎比陆生脊椎动物对这些干扰不太敏感。镁在肾脏中被重吸收以尽量减少流失。对于肾细胞，已阐明了部分细胞途径，该途径允许进行吸收性镁转运（肾刷状缘膜中的镁传导途径）。在一些广盐性硬骨鱼中，肾脏在镁负荷时似乎会立即切换到快速分泌镁，这是受扩散性镁进入威胁的海水鱼类常见的反应。这种神秘机制是一些广盐性物种能够迅速适应海水的基础。尽管在过去十年中取得了进展，但鳃、肠道和肾脏中镁转运的许多细胞和分子基础仍然不清楚。荧光、放射性和分子探针的应用（其中一些直到最近才可用）可能会在镁研究领域取得快速进展。
