Characteristics and regulatory mechanisms of the amiloride-blockable Na+ channel.

Studies of active Na+ transport across intact amphibian skin and bladder epithelia and, more recently, epithelial cells in culture have served as prototypes for understanding transport function in other experimentally less accessible epithelia such as renal tubules, lung, and sweat glands. Epithelia of diverse phylogenetic origin contain amiloride-blockable Na+ channels that are undoubtedly involved in the regulation of transepithelial Na+ transport and electrolyte homeostasis. With the advent of the techniques of tissue culture, patch clamp, isotope flux measurements in native vesicles and liposomes, and planar lipid bilayer reconstitution, it has now become possible for the first time to explore the functional operation and regulation of this widespread and important transport protein at the molecular level. Epithelial transport physiology has now reached a point where investigators can embark on studies concerning the cellular and molecular biology of epithelial Na+ channels. In our opinion, concentrated experimental efforts should be directed in three general areas. First, detailed kinetic information concerning the molecular mechanisms of Na+ movement through this channel is required. For example, it is necessary to elucidate the nature (i.e., site and location) of channel block by amiloride and structurally related compounds, the structural determinants of its ion selectivity, the voltage dependence of amiloride and ion blockage, and the minimal number of polypeptide subunits required for channel activity. The second area of study concerns the nature of the regulation of this ion channel. What are the mechanisms of channel regulation and, specifically, how does cAMP and aldosterone activate or recruit these Na+ channels? Does regulation occur at the level of channel synthesis, through posttranslational modifications, or via noncovalent interactions with small molecules or peptides? Third, we feel that the isolation and purification of the Na+ channel is important because it will eventually enable investigators to establish the molecular details of ion movement through individual channels, i.e., structural correlates of ion selectivity, binding and blockade by amiloride, and ion flow. The isolation of the Na+ channel will allow the development of molecular probes of the channel protein. These probes will be useful for immunocytochemical localization studies and, ultimately, will lead to sequencing and site-directed mutagenesis studies. Also, questions concerning the homology between Na+ channels found in different tissues and organisms as well as between the different modes of amiloride-sensitive transporters can be addressed.