Activity and inhibition of carbonic anhydrase in Amia calva, a bimodal-breathing holostean fish

Carbonic anhydrase (CA) activity was measured in the respiratory swimbladder, gill filaments and red blood cells of the primitive air-breathing holostean fish, the bowfin, Amia calva . The activity of swimbladder CA, relative to gill and red cell CA activities, was within the range reported previously for unimodally-breathing fishes and was comparable to the CA activities reported for the air-breathing organs of teleosts. It is unlikely that carbon dioxide excretion across the bowfin swimbladder is limited to the uncatalyzed rate of HCO₃-dehydration. Bowfin blood plasma lacked any endogenous inhibitor(s) of CA, in contrast with information on teleostean fishes. This absence may have interesting phylogenetic implications and may offer some potential for investigating the nature and physiological role of plasma CA inhibitors.