Nominal Carbonic Anhydrase Activity Mitigates Airway‐Surface Liquid pH Changes During the Respiratory Cycle

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The airway‐surface liquid pH (pHASL) is slightly acidic relative to the plasma pH and becomes more acidic in airway disease (e.g., cystic fibrosis, asthma, and COPD). Pathological pHASL may dysregulate ASL viscosity, mucociliary transport, and bacterial killing. Under constant 5% CO2, porcine large airways are more acidic (~7.0) than small airways (~7.4). However, the small airways are exposed to a constant 5% CO2, whereas the large airways are exposed to wide swings in pCO2 during respiration. Therefore, pHASL in the large airway may oscillate and be more alkaline during respiration. In this study, we investigated how oscillating CO2 affects the large airway pHASL for differentiated porcine tracheal epithelia cultured at the airway‐liquid interface. The apical surface of epithelia was initially exposed to 5% CO2. Apical CO2 was then rapidly switched to 0.04% CO2, allowed to equilibrate, and was then returned to 5% CO2. 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 CO2 affects pHASL in the large airway, we modeled the pHASL during respiration using the pH‐dependent rates (dph/dt) 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 pHASL oscillations were small (peak‐to‐peak pHASL = 0.09 ± 0.02 pH units). In contrast, adding carbonic anhydrase increased the pHASL oscillations (peak‐to‐peak pHASL = 0.56 ± 0.16 pH units). Second, correcting the large airway steady‐state pHASL for the respiratory cycle still yields an acidic pHASL relative to the small airway (pHlarge 7.05 vs. pHsmall 7.45). Third, the inspiratory:expiratory ratio affected the average pHASL, whereas the respiration rate affected the magnitude of pHASL oscillations. Thus, minimal carbonic anhydrase activity in large airways prevents sizeable shifts in pHASL during respiration. Maintaining relatively constant pHASL may optimize airway host defense by antimicrobials and mucociliary transport.Support or Funding InformationNIH HL091842, NIH HL5167, CF Foundation LI1410