SALT STRESS INCREASES ABUNDANCE AND GLYCOSYLATION OF CFTR LOCALIZED AT APICAL SURFACES OF SALT-GLAND SECRETORY-CELLS

Osmotic stress elicits hypertonic NaCl secretion and promotes structur al and biochemical differentiation in avian salt glands. In addition t o cholinergic control, Cl- secretion is stimulated by vasoactive intes tinal peptide (VIP), suggesting that the cystic fibrosis transmembrane conductance regulator (CFTR) may be present and that its expression m ay be regulated by chronic salt stress. Anion efflux, assayed by 6-met hoxy-N-(3-sulfopropyl)quinolinium fluorescence changes in single cells , was stimulated by VIP or 8-(4-chlorophenylthio)adenosine 3',5'-cycli c monophosphate. Immunoblots with a COOH-terminal peptide antibody to human CFTR revealed similar to 170- and similar to 180-kDa bands in ly sates from control and salt-stressed glands, respectively. Both varian ts reduced to similar to 140 kDa after N-glycanase digestion and gave identical tryptic phosphopeptide maps after immunoprecipitation and ph osphorylation by protein kinase A. CFTR was localized to apical membra nes by immunofluorescence and, additionally, to subapical vesicles by immunoelectron microscopy. Salt stress induced an approximately twofol d increase in CFTR abundance/cell protein (similar to 5-fold/cell) and intensified apical membrane immunofluorescence. For comparison, Na+ p ump expression increased approximately fourfold per cell protein with little change in actin. Thus differentiation induced by salt stress is accompanied by alteration in CFTR abundance and glycosylation. Upregu lation of CFTR likely contributes to increased efficiency of Cl- secre tion.