The Role of Carbonic Anhydrase in Respiration, ion Regulation and Acid-Base Balance in the Aquatic Crab Callinectes Sapidus and the Terrestrial Crab Gecarcinus Lateralis

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Abstract
The enzyme carbonic anhydrase (CA), which is concentrated mainly in the osmoregulatory tissue of the gills, appears to be required for ion regulation but not for CO2 excretion. An injection of the CA inhibitor acetazolamide produced an inhibition of between 90 and 100%, which took 6 h to be fully effective, and 48–96 h to wear off. During the period of inhibition in Callinectes sapidus there was no change in either O2 uptake or CO2 excretion, nor was there any increase in blood Pcoco2. In blue crabs acclimated to 250 mosM salinity, at which the animals are ion regulators, inhibition of CA caused both Na+ and Cl− concentrations in the blood to be lowered, with Cl− being lowered to a greater degree. As a result of an increase in the Na+-Cl− difference the animal experienced a ‘metabolic’ alkalosis: elevated blood pH and HCO3− at constant Pco2. The data are consistent with the hypothesis that branchial CA functions in providing H+ and HCO3− as counterions for Na+ and Cl− transport through the hydration of respiratory CO2. In the terrestrial Gecarcinus lateralis, inhibition of CA caused an increase in blood Pco2, but did not alter O2 uptake or CO2 excretion. After an initial acidosis, blood pH and HCO3− increased and remained elevated. Blood osmolality, Na+, Cl− and Ca2+ concentrations all increased, and the animals experienced a high rate of mortality. These data suggest that CA in the land crab is also important in blood ion regulation, probably to combat desiccation.