Kinetics of Chloride-Bicarbonate Exchange Across the Human Red Blood Cell Membrane

Abstract.

We use a fluorescent probe of [Cl⁻], 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), to study Cl⁻/HCO⁻ ₃ exchange in human erythrocyte ghosts in a stopped-flow apparatus at 4°C. The quench constant of SPQ in our Cl⁻/HCO⁻ ₃/HPO⁼ ₄ system at pH 7.4 is 0.065 ± 0.005 mm ⁻¹. The time course of Cl⁻/HCO⁻ ₃ exchange does not follow a single exponential function at 4°C and we propose an extended ping-pong model in which slippage is explicitly considered in order to account for this phenomenon. The solution of the system of equations generated by our model is a double exponential function which fits the time course of Cl⁻/HCO⁻ ₃ exchange. Our results confirm the predictions of the model concerning the functional dependence of the two rate constants. One rate constant (k ₁) is independent of medium composition; it is determined by the sum of the two slippage rate constants and its value is 1.04 ± 0.14 sec⁻¹. The other rate constant (k ₂) varies inversely with [Cl⁻]; the regression line is 1/k ₂= 18.8 sec − 0.095 mm ⁻¹sec [Cl⁻].