Facilitation of CO2 excretion by carbonic anhydrase located on the surface of the basal membrane of crab gill epithelium.

An isolated perfused crab gill preparation was used to test the hypotheses that crab gill carbonic anhydrase (CA) catalyzes the efflux of CO2 from the hemolymph, which lacks the enzyme, to the ambient medium and that the CA is localized on the luminal surface of the basal membrane. It was found that the efflux of CO2 from the internal perfusate was sensitive to the flow rate of the internal perfusate through the gill (and thus the residence time within the gill). The sensitivity of the CO2 efflux to residence time was nearly abolished upon treatment of the gill with an impermeable dextran-bound CA inhibitor. It is concluded that CA present on the luminal surface of the gill epithelium facilitates CO2 excretion by catalyzing the dehydration of the large hemolymph bicarbonate pool to the more diffusible molecular CO. The action of the enzyme is important in maintaining a CO2 gradient between hemolymph and water in a situation where hemolymph PCO2 is normally low, water PCO2 is variable, and the gills themselves are a source of metabolic CO2.