Operative Condition–Dependent Response of Cardiac ATP-Sensitive K + Channels Toward Sulfonylureas

Abstract

Abstract —A defining property of ATP-sensitive K + (K ATP ) channels is inhibition by sulfonylurea drugs, yet the response of cardiac K ATP channels toward sulfonylureas during myocardial ischemia is not consistent. Altered channel sensitivity toward sulfonylureas has, in part, been ascribed to antagonism by cytosolic nucleotide diphosphates, although the mechanism of interaction remains unclear. Herein, in inside-out patches excised from cardiomyocytes, we observed a dual response of K ATP channels toward the sulfonylurea drug, glyburide, in the presence of cytosolic UDP. Specifically, glyburide failed to inhibit spontaneous K ATP channel activity in the presence of UDP but inhibited UDP-induced channel activity after rundown of spontaneous channel openings. Such behavior of K ATP channels cannot be explained by differences in the level of channel activity or by UDP-induced displacement of glyburide. Rather, the dual response toward the sulfonylurea could be attributed to a property of K ATP channels to switch between operative conditions (spontaneous versus UDP-induced) each associated with a distinct responsiveness toward ligands. Conversion of post-rundown K ATP channels to the spontaneously operative channel condition, by Mg-ATP, restored the ability of UDP to antagonize the inhibitory action of glyburide lost after rundown, suggesting that the response of the channel to glyburide is phosphorylation dependent. The existence of distinct operative conditions of cardiac K ATP channels could be the basis for the inconsistent response of the channel toward sulfonylurea drugs and should be considered when sulfonylureas are used to implicate the opening of K ATP channels in the myocardium.