Cyclic AMP-dependent protein kinase opens chloride channels in normal but not cystic fibrosis airway epithelium

Abstract

Chloride (Cl^–) secretion by the airway epithelium regulates, in part, the quantity and composition of the respiratory tract fluid, thereby facilitating mucociliary clearance. The rate of Cl^– secretion is controlled by apical membrane Cl^– channels¹. Apical Cl^– channels are opened and Cl^– secretion is stimulated by a variety of hormones and neurotransmitters that increase intracellular levels of cyclic AMP (cAMP)^1–2. In cystic fibrosis (CF), a common lethal genetic disease of Caucasians, airway^3,4, sweat-gland duct⁵, secretory-coil⁶ and possibly other epithelia⁷ are anion impermeable. This abnormality may explain several of the clinical manifestations of the disease. The Cl^– impermeability in CF-airway epithelia has been localized to the apical cell membrane⁴, where regulation of Cl^– channels is abnormal^8,9: hormonal secretagogues stimulate cAMP accumulation appropriately but Cl^– channels fail to open. Here we report that the purified catalytic subunit of cAMP-dependent protein kinase plus ATP opens Cl^– channels in excised, cell-free patches of membrane from normal cells, but fails to open Cl^–; channels in CF cells. These results indicate that in normal cells, the cAMP-dependent protein kinase phosphorylates the Cl^– channel or an associated regulatory protein, causing the channel to open. The failure of CF Cl^– channels to open suggests a defect either in the channel or in such an associated regulatory protein.