Dual role of CFTR in cAMP-stimulated HCO 3 − secretion across murine duodenum

The role of the cystic fibrosis transmembrane conductance regulator (CFTR) in cAMP-stimulated HCO3- secretion across the murine duodenum was investigated. Serosal-to-mucosal flux of HCO3- (Js-->m, in mu eq.cm-2.h-1) and short-circuit current (Isc; in mu eq.cm-2.h-1) were measured by the pH stat method in duodenum from CFTR knockout [CFTR(-)] and normal [CFTR(+)] mice. Under control conditions, forskolin increased Js-->m and Isc (+1.7 and +3.5, respectively) across the CFTR(+) but not CFTR(-) duodenum. Both the forskolin-stimulated delta Js-->m and delta Isc were abolished by the CFTR channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate, whereas inhibition of luminal Cl-/HCO3- exchange by luminal Cl- removal or DIDS reduced the Js-->m by approximately 18% without a consistent effect on the delta Isc. Methazolamide also reduced the Js-->m by 39% but did not affect the delta Isc. When carbonic anhydrase-dependent HCO3- secretion was isolated by using a CO2-gassed, HCO3(-)-free Ringer bath, forskolin stimulated the Js-->m and Isc (+0.7 and +2.0, respectively) across CFTR(+) but not CFTR(-) duodenum. Under these conditions, luminal Cl- substitution or DIDS abolished the Js-->m but not the delta Isc. It was concluded that cAMP-stimulated HCO3- secretion across the duodenum involves 1) electrogenic secretion via a CFTR HCO3- conductance and 2) electroneutral secretion via a CFTR-dependent Cl-/HCO3- exchange process that is closely associated with the carbonic anhydrase activity of the epithelium.