Ion Channels: ENaC

Abstract

Amiloride-sensitive sodium channels in the lung play an important role in lung fluid balance. Particularly in the alveoli, sodium transport is closely regulated to maintain an appropriate fluid layer on the surface of the alveoli. Both alveolar type I and II cells (T1 and T2) have several different amiloride-sensitive, sodium-permeable channels in their apical membranes. This diversity appears to play a role in both normal lung physiology and pathological states. In many epithelial tissues, amiloride-sensitive epithelial sodium channels (ENaC) are formed from three subunit proteins designated, α, β, and γ ENaC. Besides these three homologous subunits, other members of the ENaC/Degenerin family of proteins apparently can form ion channels when associated with ENaC subunits leading to a diverse assortment of sodium-permeable channels in lung with different biophysical properties and different mechanisms for regulation. This leads to epithelial tissue in the lung that has enormous flexibility to alter the magnitude and regulation of salt and water transport. In lung, ENaC and other sodium-permeable channels are regulated by many transmitters and hormonal agents. The mechanism of regulation depends upon which type of sodium channel is present but involves either controlling the number of channels present in the apical membrane or controlling the activity of individual channels.