Department of Life Science, National Taiwan Normal University, Taipei, Taiwan.
Am J Physiol Regul Integr Comp Physiol. 2012 Jan 1;302(1):R84-93. doi: 10.1152/ajpregu.00318.2011. Epub 2011 Oct 12.
To investigate whether Na(+) uptake by zebrafish is dependent on NH4(+) excretion, a scanning ion-selective electrode technique was applied to measure Na(+) and NH4(+) gradients at the yolk-sac surface of zebrafish larvae. Low-Na(+) acclimation induced an inward Na(+) gradient (uptake), and a combination of low Na(+) and high NH4(+) induced a larger inward Na(+) gradient. When measuring the ionic gradients, raising the external NH4(+) level (5 mM) blocked NH4(+) excretion and Na(+) uptake; in contrast, raising the external Na(+) level (10 mM) simultaneously enhanced Na(+) uptake and NH4(+) excretion. The addition of MOPS buffer (5 mM), which is known to block NH4(+) excretion, also suppressed Na(+) uptake. These results showed that Na(+) uptake and NH4(+) excretion by larval skin are associated when ambient Na(+) level is low. Knockdown of Rhcg1 translation with morpholino-oligonucleotides decreased both NH4(+) excretion and Na(+) uptake by the skin and Na(+) content of whole larvae. Knockdown of nhe3b translation or inhibitor (5-ethylisopropyl amiloride) treatment also decreased both the NH4(+) excretion and Na(+) uptake. This study provides loss-of-function evidence for the involvement of Rhcg1 and NHE3b in the ammonium-dependent Na(+) uptake mechanism in zebrafish larvae subjected to low-Na(+) water.
为了研究斑马鱼对钠离子的摄取是否依赖于铵离子的排泄，我们采用扫描离子选择性电极技术，测量了斑马鱼胚胎卵黄囊表面的钠离子和铵离子梯度。低盐适应诱导了内向钠离子梯度（摄取），而低盐和高铵的组合诱导了更大的内向钠离子梯度。在测量离子梯度时，升高外源性铵离子水平（5 mM）会阻断铵离子的排泄和钠离子的摄取；相比之下，升高外源性钠离子水平（10 mM）同时增强了钠离子的摄取和铵离子的排泄。添加已知会阻断铵离子排泄的 MOPS 缓冲液（5 mM）也抑制了钠离子的摄取。这些结果表明，当环境钠离子水平较低时，幼虫皮肤的钠离子摄取和铵离子排泄是相关的。用反义寡核苷酸对 Rhcg1 翻译进行敲低，降低了皮肤的铵离子排泄和钠离子摄取以及整个幼虫的钠离子含量。敲低 nhe3b 翻译或用抑制剂（5-乙基异丙基氨氯吡咪）处理也降低了铵离子排泄和钠离子摄取。本研究为 Rhcg1 和 NHE3b 参与低钠水环境中斑马鱼幼虫的铵依赖型钠离子摄取机制提供了功能丧失证据。
