Molecular biology of the amiloride-sensitive epithelial Na+ channel

The amiloride-sensitive epithelial Na+ channel is formed by the assembly of three homologous subunits, alpha, beta and gamma. The channel is characterized by its sensitivity to amiloride and to some amiloride derivatives, such as phenamil and benzamil, by its small unitary conductance (approximately 5 pS), by its high selectivity for lithium and sodium, and by its slow kinetics. The alpha-, beta-, and gamma-proteins share significant identity with degenerins, a family of proteins found in the mechanosensory neurons and interneurons of the nematode Caenorhabditis elegans. They are also homologous to FaNaCh, a protein from Helix aspersa nervous tissues, which corresponds to a neuronal ionotropic receptor for the Phe-Met-Arg-Phe-NH2 peptide. All these proteins contain a large extracellular loop, located between two transmembrane alpha-helices. The NH2 and COOH terminal segments are cytoplasmic and contain potential regulatory segments that are able to modulate the activity of the channel. Accordingly, in Liddle syndrome, in which patients develop a form of genetic hypertension, mutations within the cytoplasmic COOH terminal of the beta- and gamma-chains of the epithelial Na+ channel lead to a hyperactivity of the channel. Epithelial Na+ channel activity is tightly controlled by several distinct hormonal systems, including corticosteroids and vasopressin. In kidney and colon, aldosterone is the major sodium-retaining hormone, acting by stimulation of Na+ reabsorption through the epithelium. In the distal colon from steroid-treated animals, a large increase in beta- and gamma-subunit transcription is observed, whereas the alpha-subunit remains constitutively transcribed. In kidney, RNA levels of the three subunits are not altered by aldosterone, suggesting that other mechanisms control Na+ channel activity in that tissue. In lung, the glucocorticoids are positive regulators of the channel activity, especially around birth, and act via an increased transcription of the three subunits.