Bicarbonate and Chloride Secretion in Calu-3 Human Airway Epithelial Cells

Serous cells are the predominant site of cystic fibrosis transmembrane conductance regulator expression in the airways, and they make a significant contribution to the volume, composition, and consistency of the submucosal gland secretions. We have employed the human airway serous cell line Calu-3 as a model system to investigate the mechanisms of serous cell anion secretion. Forskolin-stimulated Calu-3 cells secrete HCO(−) (3) by a Cl (−)-independent, serosal Na(+)-dependent, serosal bumetanide-insensitive, and serosal 4,4′-dinitrostilben-2,2′-disulfonic acid (DNDS)–sensitive, electrogenic mechanism as judged by transepithelial currents, isotopic fluxes, and the results of ion substitution, pharmacology, and pH studies. Similar studies revealed that stimulation of Calu-3 cells with 1-ethyl-2-benzimidazolinone (1-EBIO), an activator of basolateral membrane Ca(2+)-activated K(+) channels, reduced HCO(−) (3) secretion and caused the secretion of Cl (−) by a bumetanide-sensitive, electrogenic mechanism. Nystatin permeabilization of Calu-3 monolayers demonstrated 1-EBIO activated a charybdotoxin- and clotrimazole- inhibited basolateral membrane K(+) current. Patch-clamp studies confirmed the presence of an intermediate conductance inwardly rectified K(+) channel with this pharmacological profile. We propose that hyperpolarization of the basolateral membrane voltage elicits a switch from HCO(−) (3) secretion to Cl (−) secretion because the uptake of HCO(−) (3) across the basolateral membrane is mediated by a 4,4 ′-dinitrostilben-2,2′-disulfonic acid (DNDS)–sensitive Na(+):HCO(−) (3) cotransporter. Since the stoichiometry reported for Na (+):HCO(−) (3) cotransport is 1:2 or 1:3, hyperpolarization of the basolateral membrane potential by 1-EBIO would inhibit HCO(−) (3) entry and favor the secretion of Cl (−). Therefore, differential regulation of the basolateral membrane K(+) conductance by secretory agonists could provide a means of stimulating HCO(−) (3) and Cl (−) secretion. In this context, cystic fibrosis transmembrane conductance regulator could serve as both a HCO(−) (3) and a Cl (−) channel, mediating the apical membrane exit of either anion depending on basolateral membrane anion entry mechanisms and the driving forces that prevail. If these results with Calu-3 cells accurately reflect the transport properties of native submucosal gland serous cells, then HCO(−) (3) secretion in the human airways warrants greater attention.