Modulation of nucleotide sensitivity of ATP-sensitive potassium channels by phosphatidylinositol-4-phosphate 5-kinase

ATP-sensitive potassium channels (K-ATP channels) regulate cell excitabilit y in response to metabolic changes. K-ATP channels are formed as a complex of a sulfonylurea receptor (SURx), a member of the ATP-binding cassette pro tein family, and an inward rectifier K+ channel subunit (Kir6.x). Membrane phospholipids, in particular phosphatidylinositol (PI) 4,5-bisphosphate (PI P2), activate KATP channels and antagonize ATP inhibition of K-ATP channels when applied to inside-out membrane patches. To examine the physiological relevance of this regulatory mechanism, we manipulated membrane PIP2 levels by expressing either the wild-type or an inactive form of PI-4-phosphate 5 -kinase (PIP5K) in COSm6 cells and examined the ATP sensitivity of coexpres sed KATP channels. Channels from cells expressing the wild-type PIP5K have a 6-fold lower ATP sensitivity (K-1/2, the half maximal inhibitory concentr ation, 60 mu M) than the sensitivities from control cells (K-1/2 approximat e to 10 mu M) An inactive form of the PIP5K had little effect on the K-1/2 of wild-type channels but increased the ATP-sensitivity of a mutant K-ATP c hannel that has an intrinsically lower ATP sensitivity (from K-1/2 approxim ate to 450 mu M to K-1/2 approximate to 100 mu M), suggesting a decrease in membrane PIP2 levels as a consequence of a dominant-negative effect of the inactive PIP5K. These results show that PIP5K activity, which regulates PI P2 and PI-3,4,5-P-3 levels, is a significant determinant of the physiologic al nucleotide sensitivity of K-ATP channels.