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Anion transport of the red cell under non‐equilibrium conditions.

1. The exchange of sulphate for chloride across the human red cell membrane was measured in both directions, i.e. by sulphate influx and sulphate efflux. The influence of the concomitant transient pH changes was minimized by phosphate buffer and by choosing experimental conditions of moderate pH sensitivity (pH 6.4 and 7.8). Sulphate self exchange was determined in chloride‐free erythrocyte suspensions. 2. The transport of external sulphate into red cells proceeded at a 15‐fold greater rate if its initial concentration was raised from 5 to 95 mM. In contrast the velocity constant of sulphate efflux into sodium chloride medium increased only twofold when the intracellular sulphate concentration was increased. To explain this asymmetry it is proposed that external sulphate ions are more able to complete with chloride for the anion transport sites that those present in the cell interior. 3. The transient membrane potential due to the uneven distribution of sulphate and chloride was shown by the rapid introduction of chromate into the cells. When the erythrocytes contained chloride and the external anion was sulphate, the cells took up chromate 50 times (16.5 degrees C) faster than with equilibrium chloride distribution. 4. Chloride efflux into sodium sulphate media was measured by a chloride‐sensitive electrode. Under buffered conditions in neutral and alkaline media two kinetic components were observed as the result of chloride exchange against hydroxyl and sulphate ions. At pH 6.4, chloride efflux was characterized by a single velocity constant identical to that of sulphate movement in the opposite direction. The results show that under appropriate circumstances net chloride efflux measurements can provide comparative data on the anion permeability of the red cell membrane.