Bicarbonate-dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu-3
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 derive...
Published in: | The Journal of Physiology |
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Main Authors: | , , , , , , , |
Format: | Text |
Language: | English |
Published: |
Blackwell Science Inc
2012
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Subjects: | |
Online Access: | http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3515819 http://www.ncbi.nlm.nih.gov/pubmed/22777674 https://doi.org/10.1113/jphysiol.2012.236893 |
Summary: | 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. |
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