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Nominal Carbonic Anhydrase Activity Mitigates Airway‐Surface Liquid pH Changes During the Respiratory Cycle
Author(s) -
Thornell Ian M,
Li Xiaopeng,
Tang Xiao Xiao,
Zabner Joseph L,
Welsh Michael J
Publication year - 2017
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.31.1_supplement.702.5
Subject(s) - acetazolamide , carbonic anhydrase , chemistry , bicarbonate , carbonic anhydrase inhibitor , mucociliary clearance , respiration , respiratory system , intracellular ph , cystic fibrosis , biochemistry , biophysics , lung , enzyme , medicine , anatomy , biology , extracellular , organic chemistry
The airway‐surface liquid pH (pH ASL ) is slightly acidic relative to the plasma pH and becomes more acidic in airway disease (e.g., cystic fibrosis, asthma, and COPD). Pathological pH ASL may dysregulate ASL viscosity, mucociliary transport, and bacterial killing. Under constant 5% CO 2 , porcine large airways are more acidic (~7.0) than small airways (~7.4). However, the small airways are exposed to a constant 5% CO 2 , whereas the large airways are exposed to wide swings in pCO 2 during respiration. Therefore, pH ASL in the large airway may oscillate and be more alkaline during respiration. In this study, we investigated how oscillating CO 2 affects the large airway pH ASL for differentiated porcine tracheal epithelia cultured at the airway‐liquid interface. The apical surface of epithelia was initially exposed to 5% CO 2 . Apical CO 2 was then rapidly switched to 0.04% CO 2 , allowed to equilibrate, and was then returned to 5% CO 2 . The pH was monitored by SNARF‐1 dextran fluorescence. Relative to a breath (~4 s), the time constant for pH changes were unexpectedly slow (τ = 51.9 ± 5.8 s, n = 12). We hypothesize that this slow time constant might be due to low carbonic anhydrase activity, which catalyzes the carbonic buffering reaction. Consistent with this hypothesis, the carbonic anhydrase inhibitor acetazolamide did not appreciably change the time constant (τ = 64.1 ± 6.1 s, n = 10; p = 0.21 vs. non‐treated). In contrast, adding exogenous carbonic anhydrase strikingly decreased the mean time constant (τ = 17.3 ± 3.1 s, n = 16; p < 0.01 vs. non‐treated). Curated mRNA microarray data indicated nominal expression for all extracellular and secreted carbonic anhydrase isoforms in large airway epithelia. To further understand how CO 2 affects pH ASL in the large airway, we modeled the pH ASL during respiration using the pH‐dependent rates ( d ph/ d t) obtained from our functional experiments. There were three major findings from our simulations. First, under physiological conditions (inspiratory:expiratory ratio = 1:2; respiration rate = 15 breathes per minute), the pH ASL oscillations were small (peak‐to‐peak pH ASL = 0.09 ± 0.02 pH units). In contrast, adding carbonic anhydrase increased the pH ASL oscillations (peak‐to‐peak pH ASL = 0.56 ± 0.16 pH units). Second, correcting the large airway steady‐state pH ASL for the respiratory cycle still yields an acidic pH ASL relative to the small airway (pH large 7.05 vs. pH small 7.45). Third, the inspiratory:expiratory ratio affected the average pH ASL , whereas the respiration rate affected the magnitude of pH ASL oscillations. Thus, minimal carbonic anhydrase activity in large airways prevents sizeable shifts in pH ASL during respiration. Maintaining relatively constant pH ASL may optimize airway host defense by antimicrobials and mucociliary transport. Support or Funding Information NIH HL091842, NIH HL5167, CF Foundation LI1410