Gas transfer in dogfish: A unique model of CO2 excretion

Carbonic anhydrase (CA) is a zinc metalloenzyme that catalyzes the reversible hydration–dehydration reactions of CO2. It is present in high abundance in the cytoplasm of vertebrate red blood cells, where it contributes to CO2 excretion. A membrane-bound CA isoform (CA IV) is also present in the lungs of mammals and reptiles, but plays little role in CO2 excretion. The gills of teleost fish appear to lack plasma-accessible CA activity. In elasmobranchs, however, evidence gathered using a variety of physiological, biochemical and molecular approaches suggests that CA IV is present in the gills, and that at least in dogfish, this CA IV makes a significant contribution to CO2 excretion by catalyzing the dehydration of plasma HCO3−. The contribution of CA IV to CO2 excretion is favoured by unusually high relative plasma buffering that aids in the provision of protons for HCO3− dehydration. Moreover, reduced emphasis on HCO3− flux through the red blood cell may reflect the occurrence of a slower turnover cytosolic CA in dogfish. This model of CO2 excretion, in which HCO3− dehydration in the red blood cell catalyzed by cytosolic CA and HCO3− dehydration in the plasma catalyzed by membrane-bound CA IV are of comparable importance, has been described for the dogfish. Further work is required to determine whether it applies to elasmobranch fish as a group.