Nagel G, Barbry P, Chabot H, Brochiero E, Hartung K, Grygorczyk R
Max-Planck-Institute of Biophysics, Frankfurt am Main, Germany.
J Physiol. 2005 May 1;564(Pt 3):671-82. doi: 10.1113/jphysiol.2004.079046. Epub 2005 Mar 3.
The cystic fibrosis transmembrane conductance regulator (CFTR) plays a crucial role in regulating fluid secretion by the airways, intestines, sweat glands and other epithelial tissues. It is well established that the CFTR is a cAMP-activated, nucleotide-dependent anion channel, but additional functions are often attributed to it, including regulation of the epithelial sodium channel (ENaC). The absence of CFTR-dependent ENaC inhibition and the resulting sodium hyperabsorption were postulated to be a major electrolyte transport abnormality in cystic fibrosis (CF)-affected epithelia. Several ex vivo studies, including those that used the Xenopus oocyte expression system, have reported ENaC inhibition by activated CFTR, but contradictory results have also been obtained. Because CFTR-ENaC interactions have important implications in the pathogenesis of CF, the present investigation was undertaken by our three independent laboratories to resolve whether CFTR regulates ENaC in oocytes and to clarify potential sources of previously reported dissimilar observations. Using different experimental protocols and a wide range of channel expression levels, we found no evidence that activated CFTR regulates ENaC when oocyte membrane potential was carefully clamped. We determined that an apparent CFTR-dependent ENaC inhibition could be observed when resistance in series with the oocyte membrane was not low enough or the feedback voltage gain was not high enough. We suggest that the inhibitory effect of CFTR on ENaC reported in some earlier oocyte studies could be attributed to problems arising from high levels of channel expression and suboptimal recording conditions, that is, large series resistance and/or insufficient feedback voltage gain.
囊性纤维化跨膜传导调节因子（CFTR）在调节气道、肠道、汗腺及其他上皮组织的液体分泌中起关键作用。CFTR是一种cAMP激活的、核苷酸依赖性阴离子通道，这一点已得到充分证实，但它常被认为还具有其他功能，包括对上皮钠通道（ENaC）的调节。CF患者的上皮组织中，CFTR依赖性ENaC抑制作用缺失，导致钠过度吸收，这被认为是一种主要的电解质转运异常。包括使用非洲爪蟾卵母细胞表达系统的研究在内的多项离体研究报告了激活的CFTR对ENaC的抑制作用，但也得到了相互矛盾的结果。由于CFTR-ENaC相互作用在CF发病机制中具有重要意义，我们三个独立实验室开展了本研究，以确定CFTR是否在卵母细胞中调节ENaC，并阐明先前报道的不同观察结果的潜在原因。使用不同的实验方案和广泛的通道表达水平，我们发现，当卵母细胞膜电位被精确钳制时，没有证据表明激活的CFTR调节ENaC。我们确定，当与卵母细胞膜串联的电阻不够低或反馈电压增益不够高时，可以观察到明显的CFTR依赖性ENaC抑制作用。我们认为，一些早期卵母细胞研究中报道的CFTR对ENaC的抑制作用可能归因于通道高表达和记录条件不理想所产生的问题，即大的串联电阻和/或反馈电压增益不足。
