Ion transport its regulation in the endolymphatic sac: suggestions for clinical aspects of Meniere's disease

Ion transport and its regulation in the endolymphatic sac (ES) are reviewed on the basis of recent lines of evidence. The morphological and physiological findings demonstrate that epithelial cells in the intermediate portion of the ES are more functional in ion transport than those in the other portions. Several ion channels, ion transporters, ion exchangers, and so on have been reported to be present in epithelial cells of ES intermediate portion. An imaging study has shown that mitochondria-rich cells in the ES intermediate portion have a higher activity of Na⁺, K⁺-ATPase and a higher Na⁺ permeability than other type of cells, implying that molecules related to Na⁺ transport, such as epithelial sodium channel (ENaC), Na⁺-K⁺-2Cl^- cotransporter 2 (NKCC2) and thiazide-sensitive Na⁺-Cl^- cotransporter (NCC), may be present in mitochondria-rich cells. Accumulated lines of evidence suggests that Na⁺ transport is most important in the ES, and that mitochondria-rich cells play crucial roles in Na⁺ transport in the ES. Several lines of evidence support the hypothesis that aldosterone may regulate Na⁺ transport in ES, resulting in endolymph volume regulation. The presence of molecules related to acid/base transport, such as H⁺-ATPase, Na⁺-H⁺ exchanger (NHE), pendrin (SLC26A4), Cl^--HCO₃^- exchanger (SLC4A2), and carbonic anhydrase in ES epithelial cells, suggests that acid/base transport is another important one in the ES. Recent basic and clinical studies suggest that aldosterone may be involved in the effect of salt-reduced diet treatment in Meniere's disease.