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

From the Division of Cardiovascular Diseases, Department of Medicine and Pharmacology, Mayo Clinic, Mayo Foundation, Rochester, Minn.

Correspondence to Andre Terzic, MD, PhD, Guggenheim 7, Mayo Clinic, Rochester, MN 55905. E-mail terzic.andre{at}mayo.edu

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.

Key Words: ATP-sensitive K⁺ channel • sulfonylurea • nucleotide diphosphates • ischemia • heart