HCO 3 − /Cl− exchange across the human erythrocyte membrane: Effects of pH and temperature

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Abstract
Changes in extracellular pH (pHo) in red cell suspensions were monitored in a stopped-flow rapid reaction apparatus under conditions wheredpHo/dt was determined by the rate of HCO 3 − /Cl− exchange across the membrane. Experiments were performed at 5°C<T0 and temperature. The rate of HCO 3 − /Cl− exchange exhibits a minimum at about pHo 5 and a maximum at about pH0 7.4 at all temperatures. A transition temperature of 17°C was observed in the Arrhenius relationship for all pH0. The activation energies (E a) in kcal/mol are 19.6 below and 11.7 above 17°C for 50− self-exchange, suggest that: (i) a change in the rate-limiting step for HCO 3 − /Cl− exchange occurs at 17°C, possibly due to an altered interaction between the transport pathway and membrane lipids; (ii) the carrier system can be titrated by either H+ or SITS from the outside of the membrane, but the untitrated sites continue to transport normally; (iii) the pH0 dependence of the rate of exchange is consistent with the titratable carrier having its most alkaline pK in the range expected for amino groups; and (iv) below pH0 5, the nature of the exchange is markedly altered.