CO2 diffusing capacity in isolated dog lung lobes and the role of carbonic anhydrase.

CO2 diffusing capacities (DmCO2) were measured at 22 degrees C on 12 isolated perfused dog lung lobes before and after inhibition of lung tissue carbonic anhydrase (CA) by acetazolamide (Diamox). The hypothesis is that CA in the alveolar-capillary tissue enhances overall transport of CO2 by converting CO2 to HCO-3 within aqueous portions of the tissue. HCO-3 diffuses simultaneously with molecular CO2, increasing the overall CO2 flux, and then converts back to molecular CO2 at the end of the aqueous pathway. To ensure at least partial diffusion limitation, lobes were perfused with phosphate buffer at high pH (7.7) and high flow rates. Plant CA (which is not inhibited significantly by Diamox) was added to the perfusate to provide rapid uptake of CO2 via conversion to HCO-3. After Diamox, DCO2 decreased 39.6%, indicating that CA does increase CO2 transport through lung tissue. Surprisingly, DCO2 exceeds CO diffusing capacity by only 9.3 +/- 2.1 times (without Diamox inhibition) rather than by the factor of 24 predicted by Graham's law on the basis of solubilities and molecular weights of the gases.