Figure 5

Ion transporters and channels in mammalian choroidal epithelium. Typical CPe is schematized to localize transporters and channels that transfer ions and water [290]. 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 [38] bringing Na⁺ and Cl^- into cells in exchange for H⁺ and HCO₃^-, respectively. Also basolaterally located is Na⁺-HCO₃^- cotransport (NBCn1) [41] and Na-dependent Cl^--HCO₃^- exchange [42] that modulate pH and perhaps CSF formation. Apical Na⁺ pumping [49, 110] 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 [110] and, under some conditions, the Na⁺-K⁺-2Cl^- cotransporter [46]. K⁺-Cl^- cotransport helps maintain cell volume [50]. Apical channels facilitate K⁺, Cl^- and HCO₃^- diffusion into CSF [68]. Aquaporin 1 channels on CSF-facing membrane [77] mediate water flux into ventricles. Polarized distribution of carbonic anhydrase (c.a.) and Na⁺-K⁺-ATPase [51], and aquaporins, enable net ion and water translocation to CSF.