A full-length cDNA cloning and tissue distribution of epithelial sodium channel (ENaC) protein were studied during ontogeny by immunohistochemistry in the external gills, and the kidney, pronephros and mesonephros, of the Japanese black salamander, Hynobius nigrescens (Family Hynobiidae; a primitive caudate species). The amino acid sequence of Hynobius ENaC alpha is 64 and 63% identical to Bufo ENaC alpha and Rat ENaC alpha, respectively. In aquatic larva salamander at the digit differentiation stage, Hynobius ENaC alpha mRNA was expressed in the external gills and pronephros. In the adult, the mRNA was expressed in the skin and the mesonephros. In the larvae, juvenile, and adult specimens, Hynobius ENaC alpha immunoreactivity was observed at the apical cell membrane of the external gills, late parts of the distal tubules, and mesonephric duct in the kidney. Colocalization of the apical Hynobius ENaC alpha and the basolateral Na+,K+-ATPase was observed in the tubular cells of pronephros and mesonephros. These results suggest that Hynobius ENaC alpha plays an important role in the regulation of sodium transport in the external gills and pronephros of aquatic larvae, and in the skin and mesonephros of terrestrial adult. This is the first study to indicate ENaC expression during ontogeny in amphibians. Since no orthologs or paralogs for ENaC have been found, so far, in databases of the genomes of teleosts, it is assumed that ENaC might have played a role in terrestriality during the evolution of early tetrapods, the origin of lissamphibians. J. Exp. Zool. (Mol. Dev. Evol.) 316:135-145, 2017. (C) 2010 Wiley-Liss, Inc.
