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

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Abstract
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.

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