Large Na+ influx and high Na+, K+–ATPase activity in mitochondria-rich epithelial cells of the inner ear endolymphatic sac

Fluid in the mammalian endolymphatic sac (ES) is connected to the endolymph in the cochlea and the vestibule. Since the dominant ion in the ES is Na+, it has been postulated that Na+ transport is essential for regulating the endolymph pressure. This study focused on the cellular mechanism of Na+ transport in ES epithelial cells. To evaluate the Na+ transport capability of the ES epithelial cells, changes in intracellular Na+ concentration ([Na+]i) of individual ES cells were measured with sodium-binding benzofurzan isophthalate in a freshly dissected ES sheet and in dissociated ES cells in response to either the K+-free or ouabain-containing solution. Analysis of the [Na+]i changes by the Na+ load and mitochondrial staining with rhodamine 123 showed that the ES cells were classified into two groups; one exhibited an intensive [Na+]i increase, higher Na+, K+–ATPase activity, and intensive mitochondrial staining (mitochondria-rich cells), and the other exhibited a moderate [Na+]i increase, lower Na+, K+–ATPase activity, and moderate mitochondrial staining (filament-rich cells). These results suggest that mitochondria-rich ES epithelial cells (ca. 30% of ES cells) endowed with high Na+ permeability and Na+, K+-ATPase activity potentially contribute to the transport of Na+ outside of the endolymphatic sac.