Catalysis of CO2 reactions by lung carbonic anhydrase

Steady-state carbon dioxide excretion was studied in isolated bloodless lung preparations perfused with bicarbonate solutions. Addition of acetazolamide produced a prompt, significant decrease in the volume of excreted CO2 under all conditions studied. Excreted CO2 was derived from two sources: CO2 dissolved in the perfusate and CO2 produced by dehydration of bicarbonate in the pulmonary capillary. The relative quantity of these two sources was determined by measurement of the simultaneous excretion of acetylene. Determination of the rate of CO2 production permitted the calculation of capillary blood volume, mean capillary transit time, and the degree of catalysis of CO2 reactions by carbonic anhydrase present in the lung. Contamination of perfusate with blood carbonic anhydrase was ruled out by measuring hemoglobin concentration and carbonic anhydrase activity in pulmonary venous drainage. Comparison of steady-state CO2 production during control conditions and carbonic anhydrase inhibition indicated that bicarbonate in plasma has access to sufficient lung carbonic anhydrase to catalyze the CO2 hydration-dehydration reaction by a factor of five.