Membrane-associated carbonic anhydrase in gills of the blue crab, Callinectes sapidus

The presence of the enzyme carbonic anhydrase (CA) on the basal membrane of the branchial endothelial cells in the blue crab and its physiological significance were studied in vivo using a membrane-impermeant CA inhibitor, quaternary ammonium sulfanilamide (QAS). Injection of QAS into the hemolymph of Callinectes sapidus resulted in the rapid development of a respiratory acidosis; PCO2 rose almost 2 Torr, pH was lowered by approximately 0.25 units, and total CO2 rose by 2 mM. These results support the hypothesis that membrane-associated CA exposed to hemolymph is present in the crustacean gill and that it is physiologically significant in mobilizing hemolymph HCO-3 to CO2 to facilitate CO2 excretion across the gill. The recovery from this acidosis coincides with the clearance of the inhibitor from the hemolymph. Hemolymph osmotic and ionic parameters were unaffected by QAS, reconfirming the role of branchial cytoplasmic CA in ion regulation and also providing a convenient bioassay for determining CA inhibitor permeability in the intact organism.