EXTRACELLULAR CARBONIC-ANHYDRASE ACTIVITY AND CARBONIC-ANHYDRASE INHIBITORS IN THE CIRCULATORY-SYSTEM OF FISH

Carbonic anhydrase activity in the extracellular fluid of lower verteb rates is considered to be minimal, either because of the absence of ca rbonic anhydrase or because of the presence of naturally occurring inh ibitors. The presence of carbonic anhydrase activity and circulating i nhibitors was measured in plasma and subcellular fractions of gill tis sue in elasmobranchs and teleosts. Plasma carbonic anhydrase activity was confirmed in the former but in extremely low amounts, especially c ompared with activity in red cells. The activity was correlated with p lasma iron concentration and red cell hemolysis, which suggests that i t is a by-product of endogenous hemolysis during red cell turnover. A subcellular fraction of dogfish gills rich in microsomes contained sig nificantly higher carbonic anhydrase activity than previously found in teleosts, making elasmobranchs the only aquatic lower vertebrates to possess putative basolateral membrane-associated carbonic anhydrase in the gill vasculature. It is suggested that branchial membrane-associa ted carbonic anhydrase is correlated more with a pH and/or CO2-sensiti ve ventilatory drive than with the maintenance of resting CO2 excretio n. The occurrence and effectiveness of plasma carbonic anhydrase inhib itors were highly species-specific with the salmonids having the most potent inhibitor. Cross-reactivity of inhibitor to red cell carbonic a nhydrase appeared to be related to phylogenetic proximity. Selection f or the presence of carbonic anhydrase inhibitors in fish plasma appear s to be the result of multiple physiological pressures, including pres ervation of red cell intracellular pH, ventilatory control, and red ce ll fragility.