Figure 1.

Ion transporters in mouse cortical collecting duct. Mouse cortical collecting duct is made up of principal cells, which mediate electrogenic Na⁺ absorption through the benzamil-sensitive epithelial Na⁺ channel (ENaC) on the apical plasma membrane. Na⁺ exits principal cells across the basolateral plasma membrane through the Na,K-ATPase. ENaC-mediated Na⁺ absorption creates a lumen-negative voltage, which provides the driving force for K⁺ secretion. Type B ICs mediate electroneutral NaCl absorption and HCO₃⁻ secretion through an apical plasma membrane Na⁺-independent electroneutral Cl⁻/HCO₃⁻ exchanger (pendrin) that acts in tandem with an Na⁺-dependent Cl⁻/HCO₃⁻ exchanger. NaCl exits the cell through a basolateral plasma membrane Cl⁻ channel and an NaHCO₃ cotransporter (AE4). Net H⁺ equivalents exit across the basolateral plasma membrane through the H⁺-ATPase. The type A IC mediates uptake of H⁺ equivalents and Cl⁻ across the basolateral plasma membrane through Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1), Cl⁻/HCO₃⁻ exchange (AE1), and possibly, a Cl⁻ channel. This cell type secretes HCl through an apical H⁺-ATPase and an apical Cl⁻ channel or Cl⁻/HCO₃⁻ exchanger.