Asymmetric distribution of carbonic anhydrase in the alveolar-capillary barrier

Pulmonary carbonic anhydrase (CA) activity was studied by injecting 0.5 ml of a buffered solution of NaH14CO3 and 3H2O into the distal airways of perfused rabbit lungs. The early recovery of 14C in the pulmonary venous outflow averaged 0.28 +/- 0.02 (SE) that of 3H under control circumstances. Incorporation of a CA inhibitor (20 mg/l acetazolamide) in the perfusate reduced 14C recovery relative to 3H by two-thirds, whereas this inhibitor had no effect when placed in the airway injection fluid. Addition of 100 mg/l Ca to the perfusate did not increase 14C recovery, but addition of the enzyme to the bronchial fluid increased 14C recovery relative to 3H by a factor of more than 5. It is concluded that CA is associated with the endothelial side of the alveolar-capillary barrier but is absent on the airway surfaces.