Developmental and cellular expression pattern of epithelial sodium channelalpha, beta and gamma subunits in the inner ear of the rat

Endolymphatic ion composition in the adult inner ear is characterized by hi gh K+ and low Na+ concentration. This unique ion composition is essential f or proper functioning of sensory processing. Although a lot has been learne d in recent years about molecules involved in K+ transport in inner ear, th e molecules involved in Na+ transport are only beginning to emerge. The epi thelial Na+ channel (ENaC) is a highly selective Na+ channel that is expres sed in many Na+-reabsorbing tissues. The aim of our study was to investigat e whether ENaC is expressed in inner ear of rats and could account for Nareabsorption from endolymph. We detected mRNA for the three channel-forming subunits (alpha, beta and gamma ENaC) in cochlea, vestibular system and en dolymphatic sac. mRNA abundance increased during the first 12 days of life in cochlea and vestibular system, coinciding with decreasing Na+ concentrat ion in endolymph. Expression was strongest in epithelial cells lining scala media, most notably Claudius' cells. As these cells are characterized by a very negative resting potential they would be ideally suited for reabsorpt ion of Na+. mRNA abundance in endolymphatic sac decreased during the first 6 days of life, suggesting that ENaC might be implicated in reabsorption of endolymph in the endolymphatic sac of neonatal animals. Together, our resu lts suggest that the epithelial Na+ channel is a good candidate for a molec ule involved in Na+ homeostasis in inner ear.