Figure 10. Diagram of ion transport in the endolymphatic sac.

The endolymphatic sac epithelium consists mainly of ribosomal-rich cells that are interspersed by mitochondrial-rich cells. Mitochondrial-rich cells express H⁺ ATPase and the Cl⁻/HCO₃ ⁻ exchanger pendrin in their apical membrane. Ribosomal-rich cells express Na⁺ channels including ENaC. A current generated by the H⁺ ATPase drives Na⁺ reabsorption via Na⁺ channels. The role of the Cl⁻/HCO₃ ⁻ exchanger pendrin is to export HCO₃ ⁻ that is generated by carbonic anhydrase (CA) in the reaction that leads to the generation of H⁺.