Why is there no carbonic anhydrase activity available to fish plasma?

Carbonic anhydrase (CA) is absent in the plasma of vertebrates. In vitro, CA in fish plasma will short-circuit the effect of catecholamines on the increase in red blood cell (RBC) pH and volume, both of which increase the hemoglobin affinity for O₂. CA was infused into trout for a period of 6h during which the animal was submitted to deep hypoxia (PO₂= 30-35 torr) and during recovery after exhaustive exercise. During hypoxia, O₂ content, lactate, catecholamines, hematocrit, hemoglobin and pHi were similar to the saline infused control group. On the other hand, cell volume was significantly higher and PHe, total CO₂ and organic phosphates were significantly lower than the control group. The concentration of CA was not high enough to completely short-circuit the increase in pHi and RBC volume caused by catecholamines. The lower pH in the CA infused animals could enhance the activity of the Na+/H+ pump which would keep the NTP low. CA in plasma, during hypoxia, did not cause the expected reduction in blood oxygen content but did have a marked effect on plasma total CO₂. During the recovery period of exhaustive exercise, lactate, catecholamines, hematocrit, hemoglobin, MCHC, PO₂, HbO₂, and pHi were similar to the saline infused animals. Total CO₂ and PHe were significantly higher in the CA infused fish than in the saline infused ones. CA infusion, in this case, probably caused acid retention in the muscle. Acid efflux from the muscle would decrease pH or if the acid was excreted at the gills, bicarbonate would be titrated and the stores would be lower than in control animals. CA activity available to plasma would mean greater fluctuation of plasma pH, at least in hypoxic conditions, and red blood cell pH in general. pH is a balance between acid loading at the muscle and acid excretion at the gills or the kidneys, we cannot distinguish between a decrease and an increase of one of the two which resulted in a decrease of plasma pH. Fish have a large Haldane effect and HCO₃ flux through the red cell ensures that the protons are excreted as CO₂ and cannot bind again to hemoglobin. The absence of CA in the plasma ensures that HCO₃ flux through the red cell is maintained.Science, Faculty ofZoology, Department ofGraduat