Analysis of the all or nothing behaviour of Ca-dependent K channels in one-step inside-out vesicles from human red cell membranes.

The all or nothing behaviour of Ca2+-dependent K+ channels has been analyzed in one-step inside-out vesicles. There is a threshold for Ca2+ below which the K+ channels remain silent, and which ranges between the 10(-6) and 10(-8) M for different vesicles under the experimental conditions tested, in the absence of Mg2+. The increase of Ca2+ concentration within this range recruits a larger fraction of the vesicles to the active (permeable to 86Rb+) state. The apparent rate of 86Rb+ transport through each individual channel was found to increase, however, with Ca2+ concentration. This finding is not an artefact due to size heterogeneity of the vesicle population, and it is consistent with the variations of the mean open time of the channels with Ca2+ concentration reported previously in patch-clamp experiments. The electron donor system ascorbate + phenazine-methosulphate increases the rate of 86Rb+ transport through the channels whereas oxidized cytochrome c has the opposite effect.