Abstract
The inwardly rectifying K+ (Kir) current in mammalian retinal pigment epithelial (RPE) cells, which is largely mediated by Kir7.1 channels, is stable in cells dialyzed with MgATP but runs down when intracellular ATP is depleted. A potential mechanism for this rundown is a decrease in phosphatidylinositol 4,5-bisphosphate (PIP2) regeneration by ATP-dependent lipid kinases. Here, we used the whole cell voltage-clamp technique to investigate the membrane PIP2 dependence of Kir channels in isolated bovine RPE cells. When RPE cells were dialyzed with ATP-free solution containing PIP2 (25-50 microM), rundown persisted but was markedly reduced. Removal of Mg2+ from the pipette solution also slowed rundown, indicating that elevated intracellular Mg2+ concentration contributes to rundown. Cell dialysis with the PIP2 scavenger neomycin in MgATP solution diminished Kir current in a voltage-dependent manner, suggesting that it acted at least in part by blocking the Kir channel. Kir current in MgATP-loaded cells was partially inhibited by bath application of quercetin (100 microM), phenylarsine oxide (100 microM), or wortmannin (50 microM), inhibitors of phosphatidylinositol (PI) kinases, and was completely inhibited by cell dialysis with 2 mM adenosine, a PI4 kinase inhibitor. Both LY-294002 (100 microM), an inhibitor of PI3 kinases, and its inactive analog LY-303511 (100 microM) rapidly and reversibly inhibited Kir current, suggesting that these compounds act as direct channel blockers. We conclude that the activity of Kir channels in the RPE is critically dependent on the regeneration of membrane PIP2 by PI4 kinases and that this may explain the dependence of these channels on hydrolyzable ATP.