Phospholipids as modulators of K-ATP channels: Distinct mechanisms for control of sensitivity to sulphonylureas, K+ channel openers, and ATP

Recent work has established membrane phospholipids such as phosphatidylinos itol-4,5-bisphosphate (PIP2) as potent regulators of K-ATP channels control ling open probability and ATP sensitivity. We here investigated the effects of phospholipids on the pharmacological properties of cardiac type K-ATP ( Kir6.2/ SUR2A) channels. In excised membrane patches K-ATP channels showed considerable variability in sensitivity to glibenclamide and ATP. Applicati on of the phosphatidylinositol phosphates (PIPs) phosphatidylinositiol-4-ph osphate, PIP2, and phosphatidylinositol-3,4,5-trisphosphate reduced sensiti vity to ATP and glibenclamide closely resembling the native variability. In sertion of the patch back into the oocyte (patch-cramming) restored high AT P and glibenclamide sensitivity, indicating reversible modulation of K-ATP channels via endogenous PIPs-degrading enzymes. Thus, the observed variabil ity seemed to result from differences in the membrane phospholipid content. PIP2 also diminished activation of K-ATP channels by the K+ channel opener s (KCOs) cromakalim and P1075. The properties mediated by the sulphonylurea receptor (sensitivity to sulfonylureas and KCOs) seemed to be modulated by PIPs via a different mechanism than ATP inhibition mediated by the Kir6.2 subunits. First, polycations abolished the effect of PIP2 on ATP inhibition consistent with an electrostatic mechanism but only weakly affected gliben clamide inhibition and activation by KCOs. Second, PIP2 had clearly distinc t effects on the concentration-response curves for ATP and glibenclamide. H owever, PIPs seemed to mediate the different effects via the Kir6.2 subunit s because a mutation in Kir6.2 (R176A) attenuated simultaneously the effect s of PIP2 on ATP and glibenclamide inhibition. Finally, experiments with va rious lipids revealed structural features necessary to modulate K-ATP chann el properties and an artificial lipid (dioleoylglycerol-succinyl-nitriloace tic acid) that mimicked the effects of PIPs on K-ATP channels.