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J. Gen. Physiol.,
Volume 113, Number 5, May 1, 1999 743-760
From the Department of Cell Biology and Physiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
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 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 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 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 forces 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.
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