Tok Chia Y, Chew Shit F, Peh Wendy Y X, Loong Ai M, Wong Wai P, Ip Yuen K
Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore.
J Exp Biol. 2009 May;212(Pt 9):1248-58. doi: 10.1242/jeb.025395.
The swamp eel, Monopterus albus, is an air-breathing teleost which typically lives in freshwater but can also be found in estuaries, where it has to deal with ambient salinity fluctuations. Unlike other teleosts, its gills are highly degenerate. Hence, it may have uncommon osmoregulatory adaptations, but no information is available on its osmoregulatory capacity and mechanisms at present. In this study M. albus was exposed to a 5 day progressive increase in salinity from freshwater (1 per thousand) to brackish water (25 per thousand) and subsequently kept in 25 per thousand water for a total of 4 days. The results indicate that M. albus switched from hyperosmotic hyperionic regulation in freshwater to a combination of osmoconforming and hypoosmotic hypoionic regulation in 25 per thousand water. Exposure to 25 per thousand water resulted in relatively large increases in plasma osmolality, [Na(+)] and [Cl(-)]. Consequently, fish exposed to 25 per thousand water had to undergo cell volume regulation through accumulation of organic osmolytes and inorganic ions. Increases in tissue free amino acid content were apparently the result of increased protein degradation, decreased amino acid catabolism, and increased synthesis of certain non-essential amino acids. Here we report for the first time that glutamine is the major organic osmolyte in M. albus. Glutamine content increased to a phenomenal level of > 12 micromol g(-1) and > 30 micromol g(-1) in the muscle and liver, respectively, of fish exposed to 25 per thousand water. There were significant increases in glutamine synthetase (GS) activity in muscle and liver of these fish. In addition, exposure to 25 per thousand water for 4 days led to significant increases in GS protein abundance in both muscle and liver, indicating that increases in the expression of GS mRNA could have occurred.
黄鳝（Monopterus albus）是一种呼吸空气的硬骨鱼，通常生活在淡水中，但也能在河口发现，在那里它必须应对环境盐度的波动。与其他硬骨鱼不同，它的鳃高度退化。因此，它可能具有不寻常的渗透调节适应性，但目前尚无关于其渗透调节能力和机制的信息。在本研究中，将黄鳝暴露于盐度从淡水（千分之一）逐渐增加到半咸水（千分之二十五）的环境中5天，随后在千分之二十五的水中饲养4天。结果表明，黄鳝从淡水中的高渗高离子调节转变为在千分之二十五的水中的渗透顺应和低渗低离子调节的组合。暴露于千分之二十五的水中导致血浆渗透压、[Na⁺]和[Cl⁻]相对大幅增加。因此，暴露于千分之二十五水中的鱼必须通过积累有机渗透物和无机离子来进行细胞体积调节。组织中游离氨基酸含量的增加显然是蛋白质降解增加、氨基酸分解代谢减少以及某些非必需氨基酸合成增加的结果。在此我们首次报道谷氨酰胺是黄鳝中的主要有机渗透物。暴露于千分之二十五水中的鱼，其肌肉和肝脏中的谷氨酰胺含量分别增加到惊人的水平，即>12微摩尔/克和>30微摩尔/克。这些鱼的肌肉和肝脏中谷氨酰胺合成酶（GS）活性显著增加。此外，暴露于千分之二十五的水中4天导致肌肉和肝脏中GS蛋白丰度显著增加，表明可能发生了GS mRNA表达的增加。
