Ion exchange mechanisms for the control of volume and pH in fish and amphibian erythrocytes
Creators: name: Tufts, Bruce Laurie
Publisher: name: University of British Columbia
Year: 2010

description: The characteristics of the ion exchange mechanisms which regulate volume and pH in fish and amphibian erythrocytes were investigated and compared. Experiments were carried out under steady state conditions and also following adrenergic stimulation both in vivo and in vitro. Under steady state conditions, a decrease in extracellular pH caused an increase in the volume of rainbow trout erythrocytes, and a decrease in the intracellular pH. These pH-induced volume changes were mainly associated with movements of chloride across the chloride/bicarbonate exchange pathway. The sodium/proton exchange mechanism is quiescent at all pH's studied under steady state conditions. Beta adrenergic stimulation of rainbow trout erythrocytes promoted cell swelling and proton extrusion from the erythrocytes. Amiloride inhibited both the volume and pH changes associated with adrenergic stimulation indicating that this response is associated with an increase in the activity of the sodium/proton exchange mechanism on the erythrocyte membrane. The adrenergic swelling and pH responses are enhanced by a decrease in extracellular pH. An increase in bicarbonate concentration reduces the adrenergic pH response, but it is still significant even at 10 mM bicarbonate. DIDS markedly enhanced the beta adrenergic effect on the erythrocyte pH, but abolished the increase in erythrocyte volume. The adrenergic response was independent of temperature between 10 and 18°C. These results support a loosely coupled sodium/proton and chloride/bicarbonate exchange model for the adrenergic response in rainbow trout erythrocytes. The increases in erythrocyte pH and volume following adrenergic stimulation are associated with increases in the haemoglobin:oxygen affinity. The oxygen carrying capacity of the blood is, therefore, increased following adrenergic stimulation in rainbow trout. Carbon dioxide excretion, however, was not significantly affected by adrenergic stimulation. The functional significance of the adrenergic response of fish erythrocytes may be to offset the effects of the Root shift on the oxygen carrying capacity of the blood during acute changes in extracellular pH. In contrast to fish erythrocytes, the sodium/proton exchange mechanism in amphibian erythrocytes is active under steady state conditions. In the presence of bicarbonate movements, this exchange significantly affects the erythrocyte volume, but not the erythrocyte pH. Similar to fish erythrocytes, protons are passively distributed in amphibian erythrocytes under steady state conditions and in Donnan equilibrium with chloride ions. The erythrocyte volume also increases with decreases in extracellular pH as in fish erythrocytes, due to changes in the chloride distribution across the erythrocyte membrane. Adrenergic stimulation does not affect the volume or pH of amphibian erythrocytes either in vivo or in vitro. These animals, therefore, do not appear to regulate erythrocyte pH adrenergically. Amphibians are able to efficiently utilize oxygen stores via both central and peripheral shunting. In addition, the blood of these animals does not have a Root shift. Adrenergic regulation of erythrocyte pH in order to enhance oxygen transport during fluctuations in ambient and internal gas tensions, therefore, is probably less important than it would be in fish.