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To test for the presence of HCO3 - transport across airway epithelia, we measured short-circuit current in primary cultures of canine and human airway epithelia bathed in a Cl- -free, HCO3 - /CO2-buffered solution. cAMP agonists stimulated a secretory current that was likely carried by HCO3 - because it was absent in HCO3 - -free solutions. In addition, the cAMP-stimulated current was inhibited by the carbonic anhydrase inhibitor, acetazolamide, and by the apical addition of a blocker of cystic fibrosis transmembrane conductance regulator (CFTR), diphenylamine-2-carboxylate. The current was dependent on Na+ because it was inhibited by removing Na+ from the submucosal solution and by inhibition of the Na+ -K+ -ATPase with ouabain. The cAMP-stimulated current was absent in cystic fibrosis (CF) airway epithelia. These data suggest that cAMP agonists can stimulate HCO3 - secretion across airway epithelia and that CFTR may provide a conductive pathway for HCO3 - movement across the apical membrane.