Direct evidence of participation of rat lung carbonic anhydrase in CO2 reactions

Isolated rat lungs were ventilated with air and perfused with a blood-free Krebs-Ringer bicarbonate solution under conditions of net CO2 elimination in the lung. Some of the effluent perfusate was drawn through a stop-flow pH electrode apparatus, arriving at the electrode within 4 s after passing through the pulmonary capillaries. pH and temperature of the fluid in the electrode chamber were continuously monitored both before and after withdrawal was suddenly stopped. Little or no change was observed in the pH of the perfusate after flow was stopped, despite the fact that CO2 was eliminated in the lung, suggesting that the conversion of H2CO3 to CO2 in the blood-free perfusion fluid was markedly accelerated and the rise in pH was complete by the time the perfusate reached the electrode. Because the effluent perfusate was shown to be free of carbonic anhydrase activity, the catalysis must have occurred during transit through the isolated lung. When acetazolamide was added to the perfusate, a rise in the pH of the perfusate after stopping flow was consistently seen. These results suggest that the carbonic anhydrase of isolated lungs accelerates the conversion of H2CO3 to CO2 and enhances COW elimination as perfusate passes through the pulmonary capillaries, and that the enzyme may be present on the capillary endothelial surface.