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Bicarbonate‐dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu‐3
Author(s) -
Shan Jiajie,
Liao Jie,
Huang Junwei,
Robert Renaud,
Palmer Melissa L.,
Fahrenkrug Scott C.,
O’Grady Scott M.,
Hanrahan John W.
Publication year - 2012
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2012.236893
Subject(s) - bicarbonate , secretion , ion transporter , cystic fibrosis transmembrane conductance regulator , chemistry , chloride channel , epithelium , amiloride , forskolin , cell culture , transepithelial potential difference , biophysics , chloride , sodium , stimulation , medicine , endocrinology , biochemistry , biology , membrane , genetics , organic chemistry , gene
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.