Single-file diffusion through the Ca²⁺-activated K⁺ channel of human red cells

Summary

The ratio between the unidirectional fluxes through the Ca²⁺-activated K⁺-specific ion channel of the human red cell membrane has been determined as a function of the driving force (V _m -E _K ). Net effluxes and⁴²K influxes were determined during an initial period of ∼90 sec on cells which had been depleted of ATP and loaded with Ca. The cells were suspended in buffer-free salt solutions in the presence of 20 μm of the protonophore CCCP, monitoring in this way changes in membrane potential as changes in extracellular pH. (V _m -E _K) was varied at constantE _K by varying the Nernst potential and the conductance of the anion and the conductance of the potassium ion. In another series of experimentsE _K was varied by suspending cells in salt solutions with different K⁺ concentrations. At high extracellular K⁺ concentrations both of the unidirectional fluxes were determined as⁴²K in- and effluxes in pairs of parallel experiments. Within a range of (V _m -E _K) of −6 to 90 mV the ratio between the unidirectional fluxes deviated strongly from the values predicted by Ussing's flux ratio equation. The Ca²⁺-activated K⁺ channel of the human red cell membrane showed single-file diffusion with a flux ratio exponentn of 2.7. The magnitude ofn was independent of the driving force (V _m -E _K), independent ofV _m and independent of the conductanceg _K.