Relative contributions of the slippage and tunneling mechanisms to anion net efflux from human erythrocytes

Abstract

The rates of anion net efflux from gramicid in-treated erythrocytesin the presence of a K gradient were measured at 25°C, pH 7.8, as rates of loss of Ki. The experiments served to estimate the relative contributions of two hypothetical mechanisms to Cl net efflux at low extra cellular Cl concentrations. Cl, Br, and NO3 net effluxes were measured into media of different Cl, Br, or NO3 concentrations, respectively, to determine and compare the relative rates of the extra cellular anion-inhibitable components. They were 48, 160, and 230 mmol/(kg Hb · min), respectively, at a membrane potential of about-90 mV. This indicates that the anion-inhibitable efflux is not due solely to the return trans-location of the empty transport site ("slippage") because slippage should be independent of the chemical nature of the anion. Cl net efflux was also measured as a function of the intracellular Cl concentration into media containing either 0 or 50 mM Cl. Under both conditions, net efflux was linearly dependent on Cli between 30 and 300 mM Cli and was 0 when back-extrapolated to 0 Cli. This observation is not compatible with the slippage process, whichunder these conditions would have been expected to be independent of Cliabove 15 mM Cli. It was concluded that slippage contributes negligibly to Cl net efflux even at low extra cellular anion concentrations and that the alternative process of "tunneling"-that is, movement of the anion through the anion transporter without a conformational change in a channel-type behavior-is the major, if not the sole, mechanism underlying Cl conductance. © 1984, Rockefeller University Press., All rights reserved.