Figure 3

Ion transporters and channels in mammalian choroidal epithelium. CSF secretion results from coordinated transport of ions and water from basolateral membrane to cytoplasm, then sequentially across apical membrane into ventricles. On the plasma-facing membrane is parallel Na⁺-H⁺ and Cl^--HCO₃^- exchange bringing Na⁺ and Cl^- into cells in exchange for H⁺ and HCO₃^-, respectively. Also basolaterally located is Na⁺-HCO₃^- cotransport (NBCn1) and Na-dependent Cl^--HCO₃^- exchange that modulate pH and perhaps CSF formation. Apical Na⁺ pumping maintains a low cell Na⁺ that sets up a favorable basolateral gradient to drive Na⁺ uptake. Na⁺ is extruded into CSF mainly via the Na⁺ pump and, under some conditions, the Na⁺-K⁺-2Cl^- cotransporter. K⁺-Cl^- cotransport helps maintain cell volume. Apical channels facilitate K⁺, Cl^- and HCO₃^- diffusion into CSF. Aquaporin 1 (AQP1) channels on CSF-facing membrane mediate water flux into ventricles. Polarized distribution of carbonic anhydrase (c.a.) and Na⁺-K⁺-ATPase, and aquaporins, enable net ion and water translocation to CSF. Reprinted with permission from [14].