Nominal carbonic anhydrase activity minimizes airway-surface liquid pH changes during breathing

The airway-surface liquid pH (pH_ASL ) is slightly acidic relative to the plasma and becomes more acidic in airway diseases, leading to impaired host defense. CO₂ in the large airways decreases during inspiration (0.04% CO₂ ) and increases during expiration (5% CO₂ ). Thus, we hypothesized that pH_ASL would fluctuate during the respiratory cycle. We measured pH_ASL on cultures of airway epithelia while changing apical CO₂ concentrations. Changing apical CO₂ produced only very slow pH_ASL changes, occurring in minutes, inconsistent with respiratory phases that occur in a few seconds. We hypothesized that pH changes were slow because airway-surface liquid has little carbonic anhydrase activity. To test this hypothesis, we applied the carbonic anhydrase inhibitor acetazolamide and found minimal effects on CO₂ -induced pH_ASL changes. In contrast, adding carbonic anhydrase significantly increased the rate of change in pH_ASL . Using pH-dependent rates obtained from these experiments, we modeled the pH_ASL during respiration to further understand how pH changes with physiologic and pathophysiologic respiratory cycles. Modeled pH_ASL oscillations were small and affected by the respiration rate, but not the inspiratory:expiratory ratio. Modeled equilibrium pH_ASL was affected by the inspiratory:expiratory ratio, but not the respiration rate. The airway epithelium is the only tissue that is exposed to large and rapid CO₂ fluctuations. We speculate that the airways may have evolved minimal carbonic anhydrase activity to mitigate large changes in the pH_ASL during breathing that could potentially affect pH-sensitive components of ASL.