Dynamic Sensitivity of ATP-sensitive K⁺ Channels to ATP

ATP and MgADP regulate K_ATP channel activity and hence potentially couple cellular metabolism to membrane electrical activity in various cell types. Using recombinant K_ATP channels that lack sensitivity to MgADP, expressed in COSm6 cells, we demonstrate that similar on-cell activity can be observed with widely varying apparent submembrane [ATP] ([ATP]_sub). Metabolic inhibition leads to a biphasic change in the channel activity; activity first increases, presumably in response to a fast decrease in [ATP]_sub, and then declines. The secondary decrease in channel activity reflects a marked increase in ATP sensitivity and is correlated with a fall in polyphosphoinositides (PPIs), including phosphatidylinositol 4,5-bisphosphate, probed using equilibrium labeling of cells with [³H]myo-inositol. Both ATP sensitivity and PPIs rapidly recover following removal of metabolic inhibition, and in both cases recovery is blocked by wortmannin. These data are consistent with metabolism having a dual effect on K_ATP channel activity: rapid activation of channels because of relief of ATP inhibition and much slower reduction of channel activity mediated by a fall in PPIs. These two mechanisms constitute a feedback system that will tend to render K_ATP channel activity transiently responsive to a change in [ATP]sub over a wide range of steady state concentrations.