Interactions between ion and gas transfer in freshwater teleost fish.

Carbonic anhydrase and proton ATPase are co-distributed, being restricted to the apical regions of the gill epithelium of freshwater teleosts. Carbonic anhydrase supplies protons to the apical proton ATPase. Carbonic anhydrase is absent from the basal regions of the gill epithelium. Plasma flowing through the gills has no available carbonic anhydrase activity and plasma CO2/bicarbonate reactions are uncatalyzed. Thus, bicarbonate dehydration in plasma is negligible, and catalyzed bicarbonate dehydration occurs in erythrocytes in blood flowing through the gills. This results in tight coupling of carbon dioxide excretion to oxygen uptake and the evolution of hemoglobins with large Haldane effects but low buffering capacities, typical of many freshwater teleosts. Tight coupling of carbon dioxide and oxygen transfer in these fish also ensures that the Root shift does not impair oxygen uptake at the gills. Under these conditions, there is a selective advantage for hemoglobins with a Root shift. The presence of a Root shift augments oxygen transfer to the tissues in general and the eye and swimbladder in particular.