Bize Vincent, Horisberger Jean-Daniel
Department of Pharmacology and Toxiology, University of Lausanne, Lausanne, Switzerland.
Am J Physiol Renal Physiol. 2007 Oct;293(4):F1137-46. doi: 10.1152/ajprenal.00100.2007. Epub 2007 Aug 1.
The epithelial Na(+) channel (ENaC) is present in the apical membrane of "tight" epithelia in the distal nephron, distal colon, and airways. Its activity controls the rate of transepithelial sodium transport. Among other regulatory factors, ENaC activity is controlled by the concentration of extracellular Na(+), a phenomenon named self-inhibition. The molecular mechanism by which extracellular Na(+) concentration is detected is not known. To investigate the properties of the extracellular Na(+) sensing site, we studied the effects of extracellular cations on steady-state amiloride-sensitive outward currents in Na(+)-loaded oocytes expressing human ENaC and compared them with self-inhibition of inward current after fast solution changes. About half of the inhibition of outward Na(+) currents was due to self-inhibition itself and the rest might be attributed to conduction site saturation. Self-inhibition by extracellular Li(+) was similar to that of Na(+) except for slightly slower kinetics. Ionic selectivity of the inhibition for steady-state outward current was Na(+) > or = Li(+) > K(+). We estimated an apparent inhibitory constant (K(I)) of approximately 40 mM for extracellular Na(+) and Li(+) and found no evidence for a voltage dependence of the K(I). Protease treatment induced the expected increase of the amiloride-sensitive current measured in high-Na(+) concentrations which was due, at least in part, to abolition of self-inhibition. These results demonstrate that both self-inhibition and saturation play a significant role in the inhibition of ENaC by extracellular Na(+) and that Na(+) and Li(+) interact in a similar way with the extracellular cation sensing site.
上皮钠通道（ENaC）存在于远端肾单位、远端结肠和气道“紧密”上皮的顶端膜中。其活性控制跨上皮钠转运的速率。在其他调节因子中，ENaC活性受细胞外Na⁺浓度控制，这一现象称为自我抑制。细胞外Na⁺浓度被检测的分子机制尚不清楚。为了研究细胞外Na⁺传感位点的特性，我们研究了细胞外阳离子对表达人ENaC的Na⁺负载卵母细胞中稳态氨氯地平敏感外向电流的影响，并将其与快速溶液变化后内向电流的自我抑制进行了比较。外向Na⁺电流抑制的大约一半是由于自我抑制本身，其余可能归因于传导位点饱和。细胞外Li⁺的自我抑制与Na⁺相似，只是动力学稍慢。稳态外向电流抑制的离子选择性为Na⁺≥Li⁺＞K⁺。我们估计细胞外Na⁺和Li⁺的表观抑制常数（K₁）约为40 mM，且未发现K₁存在电压依赖性的证据。蛋白酶处理导致在高Na⁺浓度下测得的氨氯地平敏感电流预期增加，这至少部分是由于自我抑制的消除。这些结果表明，自我抑制和饱和在细胞外Na⁺对ENaC的抑制中都起重要作用，并且Na⁺和Li⁺与细胞外阳离子传感位点的相互作用方式相似。
