Chloride shift in fish blood

When the physically dissolved CO2 is suddenly increased or decreased in whole blood of red snapper at 24° C, or rainbow trout at 14° C, the plasma Cl− falls or rises with a time constant of 0.35 or 0.40 sec, respectively. The reaction is completely blocked by the carbonic anhydrase inhibitor, acetazolamide. This “chloride shift” provides a rapid dehydration pathway for excretion of the CO2 bound as HCO3. These results would seem to contradict the theory that fish RBC's are impermeable to HCO .