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- Title
Bicarbonate-dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu-3.
- Authors
Shan, Jiajie; Liao, Jie; Huang, Junwei; Robert, Renaud; Palmer, Melissa L.; Fahrenkrug, Scott C.; O'Grady, Scott M.; Hanrahan, John W.
- Abstract
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−/HCO3− exchange., The results suggest a novel cellular model for anion and fluid secretion by Calu-3 and submucosal gland acinar cells, Abstract Anion and fluid secretion are both defective in cystic fibrosis (CF); however, the transport mechanisms are not well understood. In this study, Cl− and HCO3− 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 ( Isc) across voltage-clamped monolayers, and also increased the equivalent short-circuit current ( Ieq) calculated under open-circuit conditions. Isc was equivalent to the HCO3− net flux measured using the pH-stat technique, whereas Ieq was the sum of the Cl− and HCO3− net fluxes. Ieq and HCO3− fluxes were increased by bafilomycin and ZnCl2, suggesting that some secreted HCO3− is neutralized by parallel electrogenic H+ secretion. Ieq and fluid secretion were dependent on the presence of both Na+ and HCO3−. The carbonic anhydrase inhibitor acetazolamide abolished forskolin stimulation of Ieq and HCO3− secretion, suggesting that HCO3− 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 HCO3− 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 HCO3− secretion was increased by bilateral Cl− removal and therefore did not require apical Cl−/HCO3− exchange. The results suggest a model in which most HCO3− is recycled basolaterally by exchange with Cl−, and the resulting HCO3−-dependent Cl− transport provides an osmotic driving force for fluid secretion.
- Subjects
PHYSIOLOGICAL effects of chlorides; EPITHELIAL cells; CELL lines; CYCLIC adenylic acid; ANIONS
- Publication
Journal of Physiology, 2012, Vol 590, Issue 21, p5273
- ISSN
0022-3751
- Publication type
Article
- DOI
10.1113/jphysiol.2012.236893