Effect of carbon dioxide on the temperature dependence of anion exchange in human red cells

Anion exchange across the red cell membrane was studied by measuring the rate of shrinkage of cells when transferred from a medium of low pH to one of higher pH. Removal of trace amounts of CO2/bicarbonate from the media by degassing and the inhibition of carbonic anhydrase with 5 microM ethoxzolamide slowed the shrinkage rate. Arrhenius plots were linear over a temperature range of 40 degrees C, both in the presence of trace amounts of CO2/bicarbonate and in their absence, and gave an apparent activation energy for Cl- exchange of 73.9 +/- 8.9 kJ mole-1 when CO2 was present but this increased to 135.8 +/- 7.7 kJ mole-1 in its absence. From the results it is concluded that the lower value for the activation energy is determined by the rate of formation of bicarbonate ions while the high value represents hydroxyl:anion exchange and is a truer measure of the activation energy of the exchange process.