Calcium and cAMP activate different chloride channels in the apical membrane of normal and cystic fibrosis epithelia.
The genetic disease cystic fibrosis (CF) causes decreased Cl- transport in several epithelia. cAMP-dependent regulation of apical membrane Cl- channels is defective in CF airway epithelia; as a result, CF epithelia fail to secrete Cl-. In contrast, Ca(2+)-stimulated Cl- secretion is intact in CF airway epithelia and thus has the potential to bypass the CF Cl- secretory defect. For a Cl- channel to govern Cl- secretion, it must be located in the apical membrane. To specifically investigate apical membrane Cl- channels, we studied cells grown on permeable filter supports and measured Cl- currents across the apical membrane. We found that Ca2+ and cAMP activate different Cl- channels in the apical membrane. (i) Ca(2+)-activated Cl- channels were present in the apical membrane of airway but not in intestinal epithelia. (ii) cAMP- but not Ca(2+)-activated Cl- channels were defective in CF airway epithelia. (iii) Ca(2+)- but not cAMP-activated Cl- channels were blocked by 4,4'-diisothiocyanato-2,2'-stilbenedisulfonate. (iv) Ca(2+)- and cAMP-activated apical channels had different anion permeabilities. (v) An increase in both second messengers produced an additive increase in Cl- current. These results also explain the puzzling observation that Ca(2+)-stimulated Cl- secretion is defective in CF intestine: the Ca(2+)-activated Cl- channels that could circumvent the Cl- secretory defect in CF airway are missing from the apical membrane of intestinal epithelia.
Anderson
M P
MP
Howard Hughes Medical Institute, Department of Internal Medicine, University of Iowa College of Medicine, Iowa City 52242.
Welsh
M J
MJ
D002118:Calcium / Q000494:pharmacology*; D002460:Cell Line; D002462:Cell Membrane / Q000502:physiology; D002478:Cells, Cultured; D018118:Chloride Channels; D000242:Cyclic AMP / Q000494:pharmacology*; D003550:Cystic Fibrosis / Q000503:physiopathology*; D004848:Epithelium / Q000502:physiology / Q000503:physiopathology; D006801:Humans; D007422:Intestines; D007473:Ion Channels / Q000502:physiology*; D015759:Ionomycin / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008565:Membrane Proteins / Q000187:drug effects / Q000502:physiology*; D008954:Models, Biological; D012016:Reference Values; D012143:Respiratory Physiological Phenomena; D012137:Respiratory System / Q000503:physiopathology
D018118:Chloride Channels; D007473:Ion Channels; D008565:Membrane Proteins; D015759:Ionomycin; D000242:Cyclic AMP; D002118:Calcium