Interaction of vanadate with the cloned beta cell K-ATP channel

Vanadate is used as a tool to trap magnesium nucleotides in the catalytic s ite of ATPases. However, is has also been reported to activate ATP-sensitiv e potassium (K-ATP) channels in the absence of nucleotides. K-ATP channels comprise Kir6.2 and sulfonylurea receptor subunits (SUR1 in pancreatic beta cells, SUR2A in cardiac and skeletal muscle, and SUR2B in smooth muscle). We explored the effect of vanadate (2 mM), in the absence and presence of m agnesium nucleotides, on different types of cloned K-ATP channels expressed in Xenopus oocytes. Currents were recorded from inside-out patches. Vanada te inhibited Kir6.2/SUR1 currents by approximately-50% but rapidly activate d Kir6.2/SUR2A (approximately-4-fold) and Kir6.2/SUR2B (approximately-2-fol d) currents. Mutations in SUR abolish channel activation by magnesium nucle otides did not prevent the effects of vanadate. Studies with chimeric SUR i ndicate that the first six transmembrane domains account for the difference in both the kinetics and the vanadate response of Kir6.2/SUR2A. Boiling th e vanadate solution, which removes the decavanadate polymers, largely aboli shed both stimulatory and inhibitory actions of vanadate. Our results demon strate that decavanadate modulates K-ATP channel activity via the SUR subun it, that this modulation varies with the type of SUR, that it differs from that produced by magnesium nucleotides, and that it involves transmembrane domains 1-6 of SUR.