Temperature-related changes in the erythrocytic carbonic anhydrase (acetazolamide-sensitive esterase) activity of goldfish, Carassius auratus.

1. Carbonic anhydrase activity in 'membrane' and 'cytosol' fractions of goldfish erythrocytes was assayed by the p-nitrophenyl acetate procedure following thermal acclimation. 2. The thermal sensitivity of "membrane"-associated activity was apparently unaltered by acclimation. "Cytosol" activity in warm-acclimated specimens was somewhat more thermosensitive than that animals maintained at low temperature. 3. Significant increases in specific activity, and activity per unit volume of packed cells and blood were observed at higher temperatures when assays were conducted at the temperatures at which the system actually functions in the fish. By contrast, when determinations were carried out at a standard temperature (41 degrees C) corresponding to the upper incipient lethal for this species, activity was either unaffected, or declined as acclimation temperatures increased. 4. Changes in carbonic anhydrase activity following acclimation are consistent with the hypothesis that this system is implicated in the maintenance of stable plasma chloride levels, and the suggestion that alternations in red cell chloride levels with temperature are, in part at least, attributable to concomitant variations in enzyme activity.