Acidic pH increases airway surface liquid viscosity in cystic fibrosis
Xiao Xiao Tang, … , David A. Stoltz, Michael J. Welsh
Xiao Xiao Tang, … , David A. Stoltz, Michael J. Welsh
Published January 25, 2016
Citation Information: J Clin Invest. 2016;126(3):879-891. https://doi.org/10.1172/JCI83922.
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Research Article Pulmonology

Acidic pH increases airway surface liquid viscosity in cystic fibrosis

Abstract

Cystic fibrosis (CF) disrupts respiratory host defenses, allowing bacterial infection, inflammation, and mucus accumulation to progressively destroy the lungs. Our previous studies revealed that mucus with abnormal behavior impaired mucociliary transport in newborn CF piglets prior to the onset of secondary manifestations. To further investigate mucus abnormalities, here we studied airway surface liquid (ASL) collected from newborn piglets and ASL on cultured airway epithelia. Fluorescence recovery after photobleaching revealed that the viscosity of CF ASL was increased relative to that of non-CF ASL. CF ASL had a reduced pH, which was necessary and sufficient for genotype-dependent viscosity differences. The increased viscosity of CF ASL was not explained by pH-independent changes in HCO3– concentration, altered glycosylation, additional pH-induced disulfide bond formation, increased percentage of nonvolatile material, or increased sulfation. Treating acidic ASL with hypertonic saline or heparin largely reversed the increased viscosity, suggesting that acidic pH influences mucin electrostatic interactions. These findings link loss of cystic fibrosis transmembrane conductance regulator–dependent alkalinization to abnormal CF ASL. In addition, we found that increasing Ca2+ concentrations elevated ASL viscosity, in part, independently of pH. The results suggest that increasing pH, reducing Ca2+ concentration, and/or altering electrostatic interactions in ASL might benefit early CF.

Authors

Xiao Xiao Tang, Lynda S. Ostedgaard, Mark J. Hoegger, Thomas O. Moninger, Philip H. Karp, James D. McMenimen, Biswa Choudhury, Ajit Varki, David A. Stoltz, Michael J. Welsh

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Figure 5

Increasing ASL pH reduces ASL viscosity.

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Increasing ASL pH reduces ASL viscosity. (A) 21 mM NaHCO3 (3 μl, 5% CO2)...
(A) 21 mM NaHCO3 (3 μl, 5% CO2) or 68 mM NaHCO3 (3 μl, 15% CO2) was added to the apical surface of non-CF cultured airway epithelia; HCO3– concentration and CO2 were balanced to achieve the same pH. τASL/τsaline and ASL pH were measured 5 minutes later. n = 9 epithelia per genotype, each from a different pig. (B) 24 mM NaHCO3 (3 μl with 5% or 15% CO2) was applied to the apical surface of non-CF cultured airway epithelia. Five minutes later, τASL/τsaline and ASL pH were measured. n = 6 epithelia per genotype, each from a different pig. (C) Non-CF cultured airway epithelia were exposed to 5% or 15% CO2 in a humidified chamber at 37°C. τASL/τsaline and ASL pH were measured 5 minutes later. n = 6 epithelia per genotype, each from a different pig. (D) To eliminate HCO3–/CO2, HEPES buffer (3 μl, 20 mM in saline) at a pH of 6.8 or 7.8 was applied to the apical surface of non-CF cultured airway epithelia. τASL/τsaline and ASL pH were measured 5 minutes later (see also Supplemental Figure 5, which indicates that changes in viscosity were not due to HEPES per se). n = 6 epithelia per genotype, each from a different pig. (E) Methacholine-stimulated ASL was collected from newborn non-CF (blue) and CF (red) pigs after methacholine stimulation and immediately assayed for τASL/τsaline and pH in a humidified chamber containing either 5% or 15% CO2. The line is a linear regression. n = 5 pigs per genotype; littermate controls were used. The dashed horizontal lines indicate the viscosity of saline. *P < 0.05 by unpaired Student’s t test. Mean ± SEM.