Enhancement of the ATP-sensitive K+ current by extracellular ATP in rat ventricular myocytes - Involvement of adenylyl cyclase-induced subsarcolemmal ATP depletion

ATP-sensitive K+ (K-ATP) channels are present at high density in membranes of cardiac cells, where they regulate cardiac function during metabolic impairment. The present study analyzes the effects of extracellular ATP (ATP(e)), a P-2-purinergic agonist that can be released under various conditions in the myocardial cell bed, on K-ATP current (IK-ATP) in rat ventricular myocytes. Under the whole-cell patch-clamp configuration at a physiological level of intracellular ATP, applying ATP(e) in the micromolar range did not activate IK-ATP. However, dialyzing the cell with a low-ATP (100 mu mol/L) pipette solution elicited a slowly, quasilinearly increasing IK-ATP that was markedly enhanced by applying ATP(c) in the presence of a P-1-purinergic antagonist. The effect was reversible on washing out the agonist. The IK-ATP enhancement was inhibited by cholera toxin treatment of the myocytes, suggesting that a G(s) protein was involved to, mediate the effect. Experiments on excised patches allowed us to exclude a membrane-delimited G protein-dependent pathway. Rather, the results suggested that ATP(e) activates the adenylyl cyclase, since its inhibition by 2'-deoxyadenosine 3'-monophosphate and SQ-22536, which respectively interact-with the purine and catalytic site of the cyclase, strongly reduced the ATP(e)-induced IK-ATP enhancement, whereas neither compound affected IK-ATP in inside-out patches. Inhibition of cAMP-dependent protein kinase by protein kinase inhibitor peptide 5-24 did not alter the puriner gic effect. The findings suggests that ATP(e) triggers the activation of adenylyl cyclase, which causes a subsarcolemmal ATP depletion sufficient to enhance IK-ATP as it develops during low-ATP dialysis of rat ventricular myocytes.