Bicarbonate‐dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu‐3

Key points•  The mechanisms of anion and fluid transport by airway submucosal glands are not well understood and may differ from those in surface epithelium. •  The Calu‐3 cell line is often used as a model for submucosal gland serous cells and has cAMP‐stimulated fluid secretion; however, it does not actively transport chloride under short‐circuit conditions. •  In this study we show that fluid secretion requires chloride, bicarbonate and sodium, that chloride is the predominant anion in Calu‐3 secretions, and that a large fraction of the basolateral chloride loading during cAMP stimulation occurs by Cl − /HCO 3 − exchange. •  The results suggest a novel cellular model for anion and fluid secretion by Calu‐3 and submucosal gland acinar cellsAbstract  Anion and fluid secretion are both defective in cystic fibrosis (CF); however, the transport mechanisms are not well understood. In this study, Cl − and HCO 3 − secretion was measured using genetically matched CF transmembrane conductance regulator (CFTR)‐deficient and CFTR‐expressing cell lines derived from the human airway epithelial cell line Calu‐3. Forskolin stimulated the short‐circuit current ( I sc ) across voltage‐clamped monolayers, and also increased the equivalent short‐circuit current ( I eq ) calculated under open‐circuit conditions. I sc was equivalent to the HCO 3 − net flux measured using the pH‐stat technique, whereas I eq was the sum of the Cl − and HCO 3 − net fluxes. I eq and HCO 3 − fluxes were increased by bafilomycin and ZnCl 2 , suggesting that some secreted HCO 3 − is neutralized by parallel electrogenic H + secretion. I eq and fluid secretion were dependent on the presence of both Na + and HCO 3 − . The carbonic anhydrase inhibitor acetazolamide abolished forskolin stimulation of I eq and HCO 3 − secretion, suggesting that HCO 3 − transport under these conditions requires catalysed synthesis of carbonic acid. Cl − was the predominant anion in secretions under all conditions studied and thus drives most of the fluid transport. Nevertheless, 50–70% of Cl − and fluid transport was bumetanide‐insensitive, suggesting basolateral Cl − loading by a sodium–potassium–chloride cotransporter 1 (NKCC1)‐independent mechanism. Imposing a transepithelial HCO 3 − gradient across basolaterally permeabilized Calu‐3 cells sustained a forskolin‐stimulated current, which was sensitive to CFTR inhibitors and drastically reduced in CFTR‐deficient cells. Net HCO 3 − secretion was increased by bilateral Cl − removal and therefore did not require apical Cl − /HCO 3 − exchange. The results suggest a model in which most HCO 3 − is recycled basolaterally by exchange with Cl − , and the resulting HCO 3 − ‐dependent Cl − transport provides an osmotic driving force for fluid secretion.