The Distribution of Branchial Carbonic Anhydrase and the Effects of Gill and Erythrocyte Carbonic Anhydrase Inhibition in the Channel Catfish Ictalurus Punctatus

Carbonic anhydrase (CA) activity was assayed in lysed erythrocytes and in branchial cytoplasm, mitochondria and microsomes of the channel catfish, Ictalurus punctatus. Branchial CA activity was highest in the cytoplasmic fraction, but activity was very low in mitochondria and microsomes. Erythrocyte CA activity was over four-fold greater than that in the gills. Intact animals were injected with the CA inhibitors acetazolamide and benzolamide. Slow, intra-arterial injection of both inhibitors elicited transient side effects of apnoea, bradycardia and hypoxaemia. Acetazolamide and benzolamide induced a mixed but primarily respiratory acidosis. The onset and the time course of the acidosis were correlated with the inhibition of erythrocyte CA; acetazolamide acted faster because it is more freely diffusible than benzolamide. The acid-base disturbance in the blood reached its maximum after 2 h; compensation was delayed until 24 h, when CA inhibition began to disappear. We conclude from these results that there is very little, if any, membrane-associated CA in the gill, and that the branchial enzyme is not quantitatively important in directly converting plasma HCO3- to CO2 for excretion. Rather, CO2 excretion is accomplished via the traditional chloride shift, followed by intracellular dehydration of HCO3- by erythrocyte CA. These results also suggest that branchial cytoplasmic CA inhibition might impair ion transport processes that are used to compensate blood acid-base disturbances and thus delay compensation of the respiratory acidosis.