Moreau, Christophe

ABSTRACT ATP‐sensitive K + (K ATP ) channels are unique metabolic sensors formed by association of Kir6.2, an inwardly rectifying K + channel, and the sulfonylurea receptor SUR, an ATP binding cassette protein. We identified an ATPase activity in immunoprecipitates of cardiac K ATP channels and in purified fusion proteins containing nucleotide binding domains NBD1 and NBD2 of the cardiac SUR2A isoform. NBD2 hydrolyzed ATP with a twofold higher rate compared to NBD1. The ATPase required Mg 2+ and was insensitive to ouabain, oligomycin, thapsigargin, or levamisole. K1348A and D1469N mutations in NBD2 reduced ATPase activity and produced channels with increased sensitivity to ATP. K ATP channel openers, which bind to SUR, promoted ATPase activity in purified sarcolemma. At higher concentrations, openers reduced ATPase activity, possibly through stabilization of MgADP at the channel site. K1348A and D1469N mutations attenuated the effect of openers on K ATP channel activity. Opener‐induced channel activation was also inhibited by the creatine kinase/creatine phosphate system that removes ADP from the channel complex. Thus, the K ATP channel complex functions not only as a K + conductance, but also as an enzyme regulating nucleotide‐dependent channel gating through an intrinsic ATPase activity of the SUR subunit. Modulation of the channel ATPase activity and/or scavenging the product of the ATPase reaction provide novel means to regulate cellular functions associated with K ATP channel opening.—Bienengraeber, M., Alekseev, A. E., Abraham, M. R., Carrasco, A. J., Moreau, C., Vivaudou, M., Dzeja, P. P., Terzic, A. ATPase activity of the sulfonylurea receptor: a catalytic function for the K ATP channel complex. FASEB J. 14, 1943–1952 (2000)

The pharmacological phenotype of ATP-sensitive potassium (K ATP ) channels is defined by their tissue-specific regulatory subunit, the sulfonylurea receptor (SUR), which associates with the pore-forming channel core, Kir6.2. The potassium channel opener diazoxide has hyperglycemic and hypotensive properties that stem from its ability to open K ATP channels in pancreas and smooth muscle. Diazoxide is believed not to have any significant action on cardiac sarcolemmal K ATP channels. Yet, diazoxide can be cardioprotective in ischemia and has been found to bind to the presumed cardiac sarcolemmal K ATP channel-regulatory subunit, SUR2A. Here, in excised patches, diazoxide (300 μM) activated pancreatic SUR1/Kir6.2 currents and had little effect on native or recombinant cardiac SUR2A/Kir6.2 currents. However, in the presence of cytoplasmic ADP (100 μM), SUR2A/Kir6.2 channels became as sensitive to diazoxide as SUR1/Kir6.2 channels. This effect involved specific interactions between MgADP and SUR, as it required Mg 2+ , but not ATP, and was abolished by point mutations in the second nucleotide-binding domain of SUR, which impaired channel activation by MgADP. At the whole-cell level, in cardiomyocytes treated with oligomycin to block mitochondrial function, diazoxide could also activate K ATP currents only after cytosolic ADP had been raised by a creatine kinase inhibitor. Thus, ADP serves as a cofactor to define the responsiveness of cardiac K ATP channels toward diazoxide. The present demonstration of a pharmacological plasticity of K ATP channels identifies a mechanism for the control of channel activity in cardiac cells depending on the cellular ADP levels, which are elevated under ischemia.