Figure 1.

Schematic depiction of the anion transport pathways in secretory epithelia. Apical chloride (Cl⁻) and bicarbonate (HCO₃ ⁻) efflux is mediated by CFTR and TMEM16A and probably members of the SLC26A family. Basolateral chloride and bicarbonate entry is conducted by cation cotransporters NKCC1 and NBC1, respectively. Alternatively chloride entry at the basolateral membrane is conducted by chloride-bicarbonate exchange via AE2. Carbonic anhydrase (CA) catalyzes the de novo formation of bicarbonate. Examples for the cytokine-mediated regulation of channel function or expression are indicated.