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The epithelial sodium channel (short: ENaC, also: sodium channel non-neuronal 1 (SCNN1) or amiloride-sensitive sodium channel (ASSC)) is a membrane-bound ion-channel that is permeable to Li+-ions, protons, and especially Na+-ions. It is a constitutively active ion-channel. It can be argued that it is the most selective ion channel. The apical membranes of many tight epithelia contains sodium channels that are characterized primarily by their high affinity for the diuretic blocker amiloride.〔 These channels mediate the first step of active sodium reabsorption essential for the maintenance of body salt and water homeostasis.〔 In vertebrates, the channels control reabsorption of sodium in kidney, colon, lung and sweat glands; they also play a role in taste perception. == Location and function == ENaC is located in the apical membrane of polarized epithelial cells in particular in the kidney (primarily in the distal tubule), the lung, and the colon. It is involved in the transepithelial Na+-ion transport, which it accomplishes together with the Na+/K+-ATPase. It plays a major role in the Na+- and K+-ion homeostasis of blood and epithelia and extraepithelial fluids by resorption of Na+-ions. The activity of ENaC in colon and kidney is modulated by the mineralcorticoid aldosterone. It can be blocked by either triamterene or amiloride, which are used medically to serve as diuretics. In the kidney, it is inhibited by atrial natriuretic peptide, causing natriuresis and diuresis. ENaC can furthermore be found in taste receptor cells, where it plays an important role in salt taste perception. In rodents, virtually the entire salt taste is mediated by ENaC, whereas it seems to play a less significant role in humans: About 20 percent can be accredited to the epithelial sodium channel. In cells with motile cilia, ENaC is located along the entire length of the cilia indicating that ENaC functions as a regulator of osmolarity of the periciliary fluid. In contrast to ENaC, CFTR is not found on cilia. These findings contradict previous hypothesis that stated that, under normal circumstances, ENaC is downregulated by direct interaction with CFTR and that, in CF patients, CFTR cannot downregulate ENaC, causing hyper-absorption in the lungs and recurrent lung infections. It has been suggested that it may be a ligand-gated ion channel. Epithelial Na+ channel (ENaC)/degenerin family members are involved in mechanosensation, blood pressure control, pain sensation, and the expression of fear. They display a form of desensitization ((Roy et al. 2013 )). Members all exhibit the same apparent topology, each with N- and C-termini on the inside of the cell, two amphipathic transmembrane spanning segments, M1 and M2, and a large extracellular loop. The extracellular domains contain numerous highly conserved cysteine residues. They are proposed to serve a receptor function. Welsh et al. ((2002 )) present three models whereby members of the ENaC family sense mechanostimulation. Their preferred model involves tethering the channel protein to extracellular matrix proteins such as collagens and/or intracellular cystoskeletal proteins such as α- and β-tubulins. (Carnally et al., 2008 ) have presented evidence, based on the X-ray crystal structure, that ASIC1a assembles as a heterotrimer. Carattino ((2011 )) has reviewed the structural mechanisms underlying the function of epithelial sodium channel/acid-sensing ion channel. Opening of the ion conductive pathway involves coordinated rotation of the second transmembrane-spanning domains ((Tolino et al., 2011 )). The second TMS modulates channel gating in response to shear stress ((Abi-Antoun et al., 2011 )). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Epithelial sodium channel」の詳細全文を読む スポンサード リンク
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