Dependence of the red blood cell calcium pump on the membrane potential

Abstract 1. (1) It is shown that the rate of calcium extrusion from intact human red cells is faster at a membrane potential of ∼ +50mV (inside) than at ∼ −50mV. 2. (2) The positive potential applied was the chloride potential of KCl cells in a K-gluconate medium when the Ca 2+ sensitive K + channel was blocked by 0.3mM quinidine. The negative potential resulted from the high K + permeability in Ca 2+ loaded cells (the cells were loaded to a Ca 2+ activity in the cell water of about 50μM). 3. (3) It is further demonstrated that the Ca 2+ affinity of the pump ATPase is decreased both at the internal (high affinity) and external (low affinity) site by increasing the proton concentration. Acidification thus inhibits internally and stimulates externally. 4. (4) An indirect effect of the membrane potential on the pump activity via the accompanying pH shifts on either side of the membrane could be ruled out by choosing Ca 2+ concentrations which are fully activating at the internal Ca 2+ binding site at pH 6.5 and not yet inhibitory at the external Ca 2+ binding site at pH 8. 5. (5) The results is compatible with the assumption that the human red cell Ca-pump is exchanging Ca 2+ for protons, yet is electrogenic by virtue of a stoichiometry of 1H + :1Ca 2+ for this exchange.