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

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 (Vm-EK). Net effluxes and 42K influxes were determined during an initial period of approximately 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 microM of the protonophore CCCP, monitoring in this way changes in membrane potential as changes in extracellular pH. (Vm-EK) was varied at constant EK by varying the Nernst potential and the conductance of the anion and the conductance of the potassium ion. In another series of experiments EK was varied by suspending cells in salt solutions with different K+ concentrations. At high extracellular K+ concentrations both of the unidirectional fluxes were determined as 42K in- and effluxes in pairs of parallel experiments. Within a range of (Vm-EK) 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 exponent n of 2.7. The magnitude of n was independent of the driving force (Vm-EK), independent of Vm and independent of the conductance gK.