Effect of membrane potential on the passive transport of TI+in human red blood cells

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Abstract
Tl+ can easily penetrate the red cell membrane. Tl+ can be used for studying alterations in the membrane potential of human red blood cells exposed to various experimental conditions. After inhibiting active transport by ouabain, both the rate of transmembrane movement and the cell/medium distribution of Tl+ were in a good agreement with the expected changes in membrane potential. Alterations in membrane potential were induced by modifying the cation permeability of the red cell membrane and by varying the cation concentration gradient across the membrane, which were achieved by incubation in an electrolyte-free sucrose solution, by addition of valinomycin or by addition of propranolol. Changes in cation permeability were followed by a 86Rb tracer. Hyperpolarization of the red cell membrane led to accelerated influx and retarded efflux of Tl+. The opposite effect was obtained by depolarization. Quantification of the results was made using the Nernst equation and the cell/medium concentration ratio of Tl+ at equilibrium. The membrane potential of the propranolol-treated cells increased to about -20 mV, negative inside. The mechanism of the propranolol effect is briefly discussed.