Oxygen and carbon dioxide transport in vertebrate erythrocytes: an evolutionary change in the role of membrane transport.

Two major strategies are apparent for the regulation of gas transport by vertebrate blood except in the myxinoids, which seem to have little scope for such regulation. In lampreys and teleost fish, haemoglobins have low buffering capacities and large Bohr/Haldane effects. Na+/H+ exchange plays an important role in the control of haemoglobin oxygen-affinity in these vertebrate groups. The large Bohr/Haldane effect also facilitates carbon dioxide transport: the blood (or erythrocyte) pH increases upon deoxygenation, thus increasing the concentration of bicarbonate formed at a given carbon dioxide tension. In lampreys, the bicarbonate permeability of the erythrocyte membrane is low. As a consequence, extracellular acid loads cannot be buffered by haemoglobin. In contrast, teleost erythrocytes possess a functional anion exchange, allowing extracellular proton loads to be buffered by haemoglobin. However, because the buffering capacity of teleost haemoglobins is low, buffering of extracellular acid loads is less effective in teleost fish than in elasmobranch fish and in air-breathing vertebrates whose haemoglobins have a high buffering capacity. However, the high buffering capacity of the haemoglobins diminishes the possibility of regulating haemoglobin oxygen-affinity via secondarily active Na+/H+ exchange, because intracellular pH changes, caused by proton efflux, remain small.