Dept. of Biological Sciences, Tokyo Institute of Technology, 4259-B-19 Nagatsuta-cho, Midori-ku, Yokohama 226-8501, Japan.
Am J Physiol Regul Integr Comp Physiol. 2011 Feb;300(2):R284-97. doi: 10.1152/ajpregu.00725.2009. Epub 2010 Nov 17.
The process of NaCl reabsorption in the distal nephron allows freshwater fishes to excrete hypotonic urine and seawater fishes to excrete urine containing high concentrations of divalent ions; the relevant transporters, however, have not yet been identified. In the mammalian distal nephron, NaCl absorption is mediated by Na(+)-K(+)-Cl(-) cotransporter 2 (NKCC2, Slc12a1) in the thick ascending limb, Na(+)-Cl(-) cotransporter (NCC, Slc12a3) in the distal convoluted tubule, and epithelial sodium channel (ENaC) in the collecting duct. In this study, we compared the expression profiles of these proteins in the kidneys of euryhaline and seawater pufferfishes. Mining the fugu genome identified one NKCC2 gene and one NCC gene, but no ENaC gene. RT-PCR and in situ hybridization analyses demonstrated that NKCC2 was highly expressed in the distal tubules and NCC was highly expressed in the collecting ducts of euryhaline pufferfish (mefugu, Takifugu obscurus). On the other hand, the kidney of seawater pufferfish (torafugu, Takifugu rubripes), which lacked distal tubules, expressed very low levels of NCC, and, in the collecting ducts, high levels of NKCC2. Acclimation of mefugu to seawater resulted in a 2.7× decrease in NCC expression, whereas NKCC2 expression was not markedly affected. Additionally, internalization of NCC from the apical surface of the collecting ducts was observed. These results suggest that NaCl reabsorption in the distal nephron of the fish kidney is mediated by NCC and NKCC2 in freshwater and by NKCC2 in seawater.
在远端肾单位中氯化钠的重吸收过程使淡水鱼类能够排出低渗尿液，而海水鱼类则能排出含有高浓度二价离子的尿液；然而，相关的转运蛋白尚未被鉴定。在哺乳动物的远端肾单位中，氯化钠的吸收是由升支粗段中的钠离子-钾离子-氯离子共转运蛋白 2（NKCC2，Slc12a1）、远端卷曲小管中的钠离子-氯离子共转运蛋白（NCC，Slc12a3）和收集管中的上皮钠离子通道（ENaC）介导的。在这项研究中，我们比较了这些蛋白质在广盐性和海水河豚鱼肾脏中的表达谱。对河豚基因组的挖掘鉴定出一个 NKCC2 基因和一个 NCC 基因，但没有 ENaC 基因。RT-PCR 和原位杂交分析表明，NKCC2 在远端小管中高度表达，NCC 在广盐性河豚鱼（暗纹东方鲀，Takifugu obscurus）的收集管中高度表达。另一方面，缺乏远端小管的海水河豚鱼（红鳍东方鲀，Takifugu rubripes）肾脏中 NCC 的表达水平非常低，而在收集管中 NKCC2 的表达水平很高。暗纹东方鲀适应海水后，NCC 的表达下降了 2.7 倍，而 NKCC2 的表达则没有明显变化。此外，还观察到 NCC 从收集管的顶膜内化。这些结果表明，鱼类肾脏远端肾单位中的氯化钠重吸收是由淡水鱼类中的 NCC 和 NKCC2 以及海水鱼类中的 NKCC2 介导的。
