Localization of ENaC subunit mRNAs in adult bullfrog skin

Adult amphibian skin has served as a model for the investigation of Na+-transporting epithelia, such as mammalian renal tubules. The amiloride-blockable epithelial Na+ channel (ENaC), which is located in the apical membrane of the outer living cell layer, regulates Na+ transport across the epithelium. ENaC is thought to develop during the terminal differentiation of epidermal cells, but the details are unclear. Here, we used in situ hybridization to examine the localization of the ENaC subunit mRNAs in skin of adult bullfrogs, to clarify the development of ENaC. We found that α-ENaC mRNA was expressed within the cells of the Stratum granulosum, the Stratum spinosum, and the Stratum germinativum, while β-ENaC mRNA was expressed within the cells of the S. granulosum and the S. spinosum. However, we could not detect expression of γ-ENaC mRNA, possibly for technical reasons. α- and β-ENaC mRNAs, at least, were present in the sub-apical cells, in which ENaC protein is not necessary for amphibian skin to possess its Na+-transport function. Our results may mean that the sub-apical cells are already producing the ENaC subunit mRNAs prior to the final step in their differentiation.