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

Gawenis LR, Bradford EM, Alper SL, Prasad V, Shull GE. AE2 Cl-/HCO3- exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon. Am J Physiol Gastrointest Liver Physiol 298: G493-G503, 2010. First published January 28, 2010; doi: 10.1152/ajpgi.00178.2009.-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 (I-sc) 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 I-sc was the same in WT and AE2(-/-) colon. However, the I-sc 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 I-sc in both AE2(-/-) and WT colon that was largely mediated by NKCC1, whereas removal of Cl- resulted in sharply decreased cAMP-stimulated I-sc in AE2(-/-) colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated I-sc 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.