Tang Lin, Fatehi Mohammad, Linsdell Paul
Department of Physiology & Biophysics, Dalhousie University, Nova Scotia, Canada.
J Cyst Fibros. 2009 Mar;8(2):115-21. doi: 10.1016/j.jcf.2008.10.004. Epub 2008 Nov 18.
CFTR contributes to HCO(3)(-) transport in epithelial cells both directly (by HCO(3)(-) permeation through the channel) and indirectly (by regulating Cl(-)/HCO(3)(-) exchange proteins). While loss of HCO(3)(-) transport is highly relevant to cystic fibrosis, the relative importance of direct and indirect HCO(3)(-) transport it is currently unknown.
Patch clamp recordings from membrane patches excised from cells heterologously expressing wild type and mutant forms of human CFTR were used to isolate directly CFTR-mediated HCO(3)(-) transport and characterize its functional properties.
The permeability of HCO(3)(-) was approximately 25% that of Cl(-) and was invariable under all ionic conditions studied. CFTR-mediated HCO(3)(-) currents were inhibited by open channel blockers DNDS, glibenclamide and suramin, and these inhibitions were affected by mutations within the channel pore. Cystic fibrosis mutations previously associated with disrupted cellular HCO(3)(-) transport did not affect direct HCO(3)(-) permeability.
Cl(-) and HCO(3)(-) share a common transport pathway in CFTR, and selectivity between Cl(-) and HCO(3)(-) is independent of ionic conditions. The mechanism of transport is therefore effectively identical for both ions. We suggest that mutations in CFTR that cause cystic fibrosis by selectively disrupting HCO(3)(-) transport do not impair direct CFTR-mediated HCO(3)(-) transport, but may predominantly alter CFTR regulation of other HCO(3)(-) transport pathways.
囊性纤维化跨膜传导调节因子（CFTR）通过直接（通过通道对HCO₃⁻的通透）和间接（通过调节Cl⁻/HCO₃⁻交换蛋白）两种方式参与上皮细胞中的HCO₃⁻转运。虽然HCO₃⁻转运的丧失与囊性纤维化高度相关，但目前尚不清楚直接和间接HCO₃⁻转运的相对重要性。
从异源表达人CFTR野生型和突变型的细胞中切除的膜片进行膜片钳记录，以直接分离CFTR介导的HCO₃⁻转运并表征其功能特性。
HCO₃⁻的通透性约为Cl⁻的25%，并且在所有研究的离子条件下均保持不变。CFTR介导的HCO₃⁻电流受到开放通道阻滞剂DNDS、格列本脲和苏拉明的抑制，并且这些抑制作用受到通道孔内突变的影响。先前与细胞内HCO₃⁻转运破坏相关的囊性纤维化突变并不影响直接HCO₃⁻通透性。
Cl⁻和HCO₃⁻在CFTR中共享一条共同的转运途径，并且Cl⁻和HCO₃⁻之间的选择性与离子条件无关。因此，两种离子的转运机制实际上是相同的。我们认为，通过选择性破坏HCO₃⁻转运而导致囊性纤维化的CFTR突变不会损害直接CFTR介导的HCO₃⁻转运，但可能主要改变CFTR对其他HCO₃⁻转运途径的调节。
