Localization of carbonic anhydrase on pulmonary artery endothelial cells in culture

Bovine pulmonary artery endothelial cells in culture possess carbonic anhydrase activity and immunoreactivity. The intact cells and cell homogenates lower the pH of 25 mM triethanolamine sulfate buffer saturated with CO2 (starting pH 8.1). The intact cells are more reactive than the cell homogenates, and the enzymic activity is enriched in association with the plasma membrane fraction. Specific immunofluorescence is obtained when the cells are incubated with rabbit antibovine erythrocyte carbonic anhydrase B and then with goat antirabbit immunoglobulin G coupled to fluorescein. At the level of electron microscopy, antibodies to carbonic anhydrase B are reactive with sites along the plasma membrane and associated caveolae. Multivesicular bodies are the only intracellular sites labeled and appear to correspond to the globular sites of intracellular immunofluorescence. Cells maintained and propagated in culture in the absence of an exogenous source of carbonic anhydrase nonetheless possess carbonic anhydrase activity, suggesting that the cells are capable of synthesizing the enzyme. Taken together, our results indicate that pulmonary artery endothelial cells possess carbonic anhydrase situated so that the enzyme could readily catalyze the dehydration of plasma HCO-3 to facilitate CO2 excretion and participate in the regulation of blood pH as central venous blood is converted into systemic arterial blood.