The Role of Carbonic Anhydrase in Blood Ion and Acid-Base Regulation

Abstract

The role of carbonic anhydrase (CA) in ion transport processes of aquatic and terrestrial arthropod species is reviewed. In both insects and crustaceans CA is found in a variety of ion transporting tissues. The bulk of CA activity in crustaceans is concentrated in the posterior gills, which are morphologically and biochemically adapted for ion transport. The enzyme can be specifically localized to gill lamellae which contain large populations of salt transporting chloride cells. Enzyme activity in the posterior gills of species having the ability to regulate blood ion concentrations increases when these organisms are acclimated to environmental salinities in which they ion regulate. In stenohaline, ion conforming species branchial CA activity is uniformly low, being only 5–10% that in regulating species. Studies on the blue crab, Callinectes sapidus , using the specific CA inhibitor acetazolamide have shown that the enzyme is indeed important in blood ion regulation. Blood Na$ and Cl− concentrations are both severely lowered in drug-treated animals acclimated to low salinity, while they remain virtually unaffected in animals acclimated to high salinity, in which the animal is an ion conformer. High salinity acclimated crabs treated with acetazolamide do not survive transfer to low salinity, and mortality is related to a breakdown in the ion regulatory mechanism. Branchial CA most likely functions in the hydration of respiratory CO2 to H$ and HCO3−, which serve as counterions for the active uptake of Na$ and Cl−, respectively. In terrestrial species the role of CA is unclear and merits further investigation.