Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore.
J Comp Physiol B. 2012 Aug;182(6):793-812. doi: 10.1007/s00360-012-0664-9. Epub 2012 Apr 22.
This study aimed to clone and sequence the cystic fibrosis transmembrane conductance regulator (cftr) from, and to determine the effects of seawater acclimation or exposure to 100 mmol l⁻¹ NH₄Cl in freshwater on its mRNA and protein expressions in, the gills of Anabas testudineus. There were 4,530 bp coding for 1,510 amino acids in the cftr cDNA sequence from A. testudineus. The branchial mRNA expression of cftr in fish kept in freshwater was low (<50 copies of transcript per ng cDNA), but significant increases were observed in fish acclimated to seawater for 1 day (92-fold) or 6 days (219-fold). Branchial Cftr expression was detected in fish acclimated to seawater but not in the freshwater control, indicating that Cl⁻ excretion through the apical Cftr of the branchial epithelium was essential to seawater acclimation. More importantly, fish exposed to ammonia also exhibited a significant increase (12-fold) in branchial mRNA expression of cftr, with Cftr being expressed in a type of Na⁺/K⁺-ATPase-immunoreactive cells that was apparently different from the type involved in seawater acclimation. It is probable that Cl⁻ excretion through Cftr generated a favorable electrical potential across the apical membrane to drive the excretion of NH₄⁺ against a concentration gradient through a yet to be determined transporter, but it led to a slight loss of endogenous Cl⁻. Since ammonia exposure also resulted in significant decreases in blood pH, [HCO₃⁻] and [total CO₂] in A. testudineus, it can be deduced that active NH₄⁺ excretion could also be driven by the exit of HCO₃⁻ through the apical Cftr. Furthermore, A. testudineus uniquely responded to ammonia exposure by increasing the ambient pH and decreasing the branchial bafilomycin-sensitive V-type H⁺-ATPase activity, which suggests that its gills might have low NH₃ permeability.
本研究旨在从黄颡鱼（Anabas testudineus）克隆和测序囊性纤维化跨膜电导调节因子（cftr），并确定海水驯化或暴露于淡水 100mmol/L NH₄Cl 对其 mRNA 和蛋白表达的影响。从黄颡鱼 cftr cDNA 序列中，编码 1510 个氨基酸的有 4530bp。在淡水饲养的鱼类中，鳃 cftr 的mRNA 表达水平较低（每 ng cDNA 转录本<50 个拷贝），但在适应海水 1 天（92 倍）或 6 天（219 倍）的鱼类中，cftr 的表达显著增加。在适应海水的鱼类中检测到鳃 Cftr 的表达，但在淡水对照组中没有检测到，这表明通过鳃上皮细胞顶端 Cftr 的 Cl⁻排泄对于海水适应至关重要。更重要的是，暴露于氨的鱼类鳃 cftr 的 mRNA 表达也显著增加（12 倍），Cftr 在一种与参与海水适应的类型明显不同的 Na⁺/K⁺-ATPase 免疫反应细胞中表达。很可能是通过 Cftr 的 Cl⁻排泄产生了有利于跨顶端膜的电势能，从而通过尚未确定的转运体将 NH₄⁺逆浓度梯度排出，但这导致了内源性 Cl⁻的轻微损失。由于氨暴露也导致黄颡鱼血液 pH 值、[HCO₃⁻]和[总 CO₂]显著降低，可以推断出通过顶端 Cftr 的 HCO₃⁻的排出也可以驱动主动 NH₄⁺排泄。此外，黄颡鱼通过增加环境 pH 值和降低鳃 bafilomycin 敏感的 V 型 H⁺-ATPase 活性来独特地应对氨暴露，这表明其鳃可能具有低 NH₃通透性。
