Physiological and molecular characterization of urea transport by the gills of the Lake Magadi tilapia (Alcolapia grahami).
Physiological and molecular characterization of urea transport by the gills of the Lake Magadi tilapia (Alcolapia grahami).
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发表时间：
2001-02
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通讯作者：
P. J. Walsh;Martin Grosell;Gregory G Goss;Harold L. Bergman;Annie N. Bergman;Paul Wilson;Pierre Laurent;Seth L Alper;Craig P. Smith;Collins Kamunde;Chris M. Wood
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作者：
P. J. Walsh;Martin Grosell;Gregory G Goss;Harold L. Bergman;Annie N. Bergman;Paul Wilson;Pierre Laurent;Seth L Alper;Craig P. Smith;Collins Kamunde;Chris M. Wood
马加迪湖罗非鱼（Alcolapia grahami）是一种不寻常的鱼，由于其高度碱性和缓冲的水生栖息地，所有含氮废物都以尿素的形式排出。在这里，使用生理和分子研究，我们描述了鳃尿素排泄在这个物种的机制。在体内，重复的短间隔采样显示尿素排泄是连续的。计算了A. grahami鳃为4.74x10（-）（5）+/-0.38x10（-）（5）cm s（-）（1）（平均值+/- s.e.m.，N=11），一些10倍以上的被动渗透性通过脂质双层和一些5倍以上，甚至最尿素渗透硬骨鱼研究的日期（如海湾蟾鱼）。尿素的运输是双向的，如实验所示，其中外部[尿素]升高。此外，尿素转运被哺乳动物和鱼类尿素转运蛋白的经典抑制剂抑制，顺序为硫脲> N-甲基脲>乙酰胺。对马加迪罗非鱼尿素转运蛋白（mtUT）的1700 bp cDNA进行克隆、测序，发现其与哺乳动物、两栖动物和其它鱼类的尿素转运蛋白具有高度同源性。当从mtUT cDNA转录的cRNA注射到非洲爪蟾卵母细胞中时，根皮素可吸收尿素的摄取相对于注水对照增强了3.4倍。用mtUT的一部分作为探针对鳃、红细胞、肝脏、肌肉和脑进行的北方分析表明，鳃是唯一表达mtUT RNA的组织。马加迪罗非鱼鳃铺层细胞表现出密集的核心囊泡之间发达的高尔基池和顶膜的贩运。没有这种贩运和高尔基体系统的非ureotelic相对（尼罗罗非鱼）的发展不佳表明，囊泡贩运可能与尿素排泄Alcolapia grahami。综上所述，上述研究结果表明，这种碱性湖泊适应物种的鳃分泌尿素组成通过特定的易化尿素转运蛋白mtUT。
The Lake Magadi tilapia (Alcolapia grahami) is an unusual fish, excreting all its nitrogenous waste as urea because of its highly alkaline and buffered aquatic habitat. Here, using both physiological and molecular studies, we describe the mechanism of branchial urea excretion in this species. In vivo, repeated short-interval sampling revealed that urea excretion is continuous. The computed urea permeability of A. grahami gill is 4.74x10(-)(5)+/-0.38x10(-)(5 )cm s(-)(1) (mean +/- s.e.m., N=11), some 10 times higher than passive permeability through a lipid bilayer and some five times higher than that of even the most urea-permeable teleosts studied to date (e.g. the gulf toadfish). Transport of urea was bidirectional, as demonstrated by experiments in which external [urea] was elevated. Furthermore, urea transport was inhibited by classic inhibitors of mammalian and piscine urea transporters in the order thiourea>N-methylurea>acetamide. A 1700 base pair cDNA for a putative Magadi tilapia urea transporter (mtUT) was cloned, sequenced and found to display high homology with urea transporters from mammals, amphibians and other fishes. When cRNA transcribed from mtUT cDNA was injected into Xenopus laevis oocytes, phloretin-inhibitable urea uptake was enhanced 3.4-fold relative to water-injected controls. Northern analysis of gill, red blood cells, liver, muscle and brain using a portion of mtUT as a probe revealed that gill is the only tissue in which mtUT RNA is expressed. Magadi tilapia gill pavement cells exhibited a trafficking of dense-cored vesicles between the well-developed Golgi cisternae and the apical membrane. The absence of this trafficking and the poor development of the Golgi system in a non-ureotelic relative (Oreochromis niloticus) suggest that vesicle trafficking could be related to urea excretion in Alcolapia grahami. Taken together, the above findings suggest that the gills of this alkaline-lake-adapted species excrete urea constitutively via the specific facilitated urea transporter mtUT.