[Distribution of amiloride-sensitive sodium channel in epithelial tissue]

Sodium transport across the apical membrane, via amiloride sensitive sodium channels, is the limiting step of sodium absorption in transporting epithelia with high intercellular electrical resistance, such as the distal parts of the colon and of the renal tubule. Several types of amiloride sensitive sodium channels have been functionally characterized: one of them (type I) with high selectivity and low conductance for sodium is under the control of aldosterone and antidiuretic hormone. This channel has been cloned (2): it is formed of three subunits, alpha, beta and gamma. The distribution of these subunits has been examined in several epitheliums at the mRNA (in situ hybridization) and protein (immunocytochemistry) levels. All three subunits are expressed in the most superficial cells of the distal colon, in principal cells of the renal distal tubule and cortical collecting duct, in striated ducts of serous acini of salivary glands, and in excretory ducts of sweat glands. Immunocytochemistry established the apical localization of the channel subunit proteins. No expression was detected in other cell types of these tissues. These results highlight the crucial role of the type I amiloride sensitive sodium channel in the control of sodium homeostasis at the level of tight, aldosterone-sensitive epitheliums. Furthermore, novel questions are opened, in view of the sodium channel being a member of a highly conserved family of mechanoreceptors, and of its implication in some human genetic diseases.