Anion Secretion in Murine Proximal Colon

31 Anion secretion by colonic epithelium is dependent on apical CFTR-mediated anion 32 conductance and basolateral ion transport. In many tissues, the NKCC1 Na-K-2Cl 33 cotransporter mediates basolateral Cl uptake. However, additional evidence suggests that the 34 AE2 Cl/HCO3 exchanger, when coupled with the NHE1 Na/H exchanger or a Na-HCO3 35 cotransporter (NBC), contributes to HCO3 and/or Cl uptake. To analyze the secretory 36 functions of AE2 in proximal colon, short-circuit current (Isc) responses to cAMP and inhibitors 37 of basolateral anion transporters were measured in muscle-stripped wild-type (WT) and AE238 null (AE2) proximal colon. In physiologic Ringers, the magnitude of cAMP-stimulated Isc was 39 the same in WT and AE2 colon. However, the Isc response in AE2 colon exhibited 40 increased sensitivity to the NKCC1 inhibitor bumetanide and decreased sensitivity to the 41 distilbene derivative SITS (which inhibits AE2 and some NBCs), indicating that loss of AE2 42 results in a switch to increased NKCC1-supported anion secretion. Removal of HCO3 resulted 43 in robust cAMP-stimulated Isc in both AE2 and WT colon that was largely mediated by 44 NKCC1, whereas removal of Cl resulted in sharply decreased cAMP-stimulated Isc in AE2 45 colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated Isc in AE2 46 colon but caused a switch to NKCC1-supported secretion in WT colon. Thus, in AE2 colon, 47 Cl secretion supported by basolateral NKCC1 is enhanced, while HCO3 secretion is 48 diminished. These results show that AE2 is a component of the basolateral ion transport 49 mechanisms that support anion secretion in the proximal colon. 50 51 52 53