REVERSAL OF THE ATP-LIGANDED STATE OF ATP-SENSITIVE K-ACTIVITY( CHANNELS BY ADENYLATE KINASE)

The mechanism that promotes transition from the ATP- to the ADP-ligand ed state of ATP-sensitive K+ (K-ATP) channels and consequent channel o pening in a cytosolic environment of high ATP concentration has yet to be understood. A mechanism examined here that could reverse the ATP-i nhibited state is based on the action of adenylate kinase to catalyze phosphoryl transfer between ATP and AMP, resulting in transformation o f ATP into ADP. In membrane patches excised from guinea pig cardiomyoc ytes, AMP alone did not affect channel behavior but increased the open probability of ATP-inhibited K-ATP channels. This required MgCl2 and a hydrolyzable form of ATP and was prevented by P1,P5-di-adenosine-5'- pentaphosphate, an inhibitor of adenylate kinase, The single channel a mplitude and kinetics of channel openings induced by the ADP-generatin g substrates of adenylate kinase, AMP and MgATP, were indistinguishabl e from the biophysical properties of the channel exhibited after addit ion of MgADP. In whole cell voltage-clamped cardiomyocytes, introducti on of exogenous adenylate kinase along with millimolar MgATP and AMP i nduced a K+ current that was suppressed by a sulfonylurea blocker of K -ATP channels. Enriched sarcolemmal membrane preparations were found t o possess ATP . AMP phosphotransferase activity with properties attrib utable to an extramitochondrial isoform of adenylate kinase, These res ults indicate that adenylate kinase is a naturally occurring component of sarcolemmal membranes that could provide dynamic governance of K-A TP channel opening through its phosphoryl transfer catalytic action in the microenvironment of the channel.