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 HCO3- 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 Ca2+-activated K+ channels, reduced HCO3- secretion and caused the secretion of Cl- by a bumetanide-sensitive, electrogenic mechanism. Nystatin permeabilization of Calu-3 monolayers demonstrated 1-EBIO activated 3 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 HCO3- secretion to Cl- secretion because the uptake of HCO3- across the basolateral membrane is mediated by a 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS)-sensitive Na+:HCO3- cotransporter. Since the stoichiometry reported for Na+:HCO3- cotransport is 1:2 or 1:3, hyperpolarization of the basolateral membrane potential by 1-EBIO would inhibit HCO3- 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 HCO3- and Cl- secretion. In this context, cystic fibrosis transmembrane conductance regulator could serve as both a HCO3- and a Cl- channel, mediating the apical membrane exit of either anion depending on basolateral membrane anion entry mechanisms and the driving for ces that prevail. If these results with Calu-3 cells accurately reflect the transport properties of native submucosal gland serous cells, then HCO3-, secretion in the human airways warrants greater attention.