We studied the roles of gill and erythrocyte
carbonic anhydrase in normal CO2 transfer (metabolic CO2 elimination) and in HCO3- excretion during metabolic
alkalosis in the resting and swimming dogfish shark, Squalus acanthias. Gill
carbonic anhydrase was selectively inhibited (greater than 98.5%) by 1 mg/kg
benzolamide, which caused no physiologically significant red cell
carbonic anhydrase inhibition (approximately 40%).
Enzyme in both tissues was inhibited by 30 mg/kg
methazolamide (greater than 99%). Both drugs caused equivalent reductions in HCO3- excretion following an infusion of 9 mmol/kg NaHCO3 as measured by the rate of fall in plasma HCO3- and by transfer into seawater.
Methazolamide (red cell and gill
carbonic anhydrase inhibition) caused a
respiratory acidosis in fish with normal
acid-base status, whereas
benzolamide (gill
carbonic anhydrase inhibition) did not. The only effect observed with
benzolamide in these fish was a small elevation in plasma HCO3-. These findings, taken together, suggest that red cell
carbonic anhydrase is required for normal metabolic CO2 elimination by the gill. Although
carbonic anhydrase is located in the respiratory epithelium, it appears to have no quantitative role in transfer of metabolic CO2 to the environment, a pattern similar to all terrestrial vertebrates. However,
carbonic anhydrase in the gill is crucial to this organ's function in
acid-base regulation, both in the excretion of H+ or HCO3- generated in normal metabolism and in various
acid-base disturbances.