AE2 Cl−/HCO3− exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon

Anion secretion by colonic epithelium is dependent on apical CFTR-mediated anion conductance and basolateral ion transport. In many tissues, the NKCC1 Na+-K+-2Cl− cotransporter mediates basolateral Cl− uptake. However, additional evidence suggests that the AE2 Cl−/HCO3− exchanger, when coupled with the NHE1 Na+/H+ exchanger or a Na+-HCO3− cotransporter (NBC), contributes to HCO3− and/or Cl− uptake. To analyze the secretory functions of AE2 in proximal colon, short-circuit current (Isc) responses to cAMP and inhibitors of basolateral anion transporters were measured in muscle-stripped wild-type (WT) and AE2-null (AE2−/−) proximal colon. In physiological Ringer, the magnitude of cAMP-stimulated Isc was the same in WT and AE2−/− colon. However, the Isc response in AE2−/− colon exhibited increased sensitivity to the NKCC1 inhibitor bumetanide and decreased sensitivity to the distilbene derivative SITS (which inhibits AE2 and some NBCs), indicating that loss of AE2 results in a switch to increased NKCC1-supported anion secretion. Removal of HCO3− resulted in robust cAMP-stimulated Isc in both AE2−/− and WT colon that was largely mediated by NKCC1, whereas removal of Cl− resulted in sharply decreased cAMP-stimulated Isc in AE2−/− colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated Isc in AE2−/− colon but caused a switch to NKCC1-supported secretion in WT colon. Thus, in AE2−/− colon, Cl− secretion supported by basolateral NKCC1 is enhanced, whereas HCO3− secretion is diminished. These results show that AE2 is a component of the basolateral ion transport mechanisms that support anion secretion in the proximal colon.