Ion transporters and channels in mammalian choroidal epithelium. Typical CPe is schematized to localize transporters and channels that transfer ions and water . 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 [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  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 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.