Intracellular ADP activates K+ channels that are inhibited by ATP in an insulin-secreting cell line.

The effect of ADP on ATP-sensitive K+ channels in the insulin-secreting RINm5F cell line has been investigated with the help of single-channel current recording from saponin-permeabilized cells. ADP (100-500 microM) markedly activates K+ channels when added to the bath solution in contact with the membrane inside. ADP-beta-S cannot mimick this effect. During sustained ATP (500 microM)-evoked inhibition of K+ channel opening, 500 microM ADP markedly and reversibly activates the channels. Conversely ATP markedly reduces the opening probability of ADP-activated channels. It is suggested that the physiological control of K+ channel opening in the insulin-secreting cells is mediated by changes in ATP/ADP ratio rather than being solely determined by the ATP concentration.