Reduced effectiveness of HMR 1098 in blocking cardiac sarcolemmal KATP channels during metabolic stress

ATP-sensitive K+ (KATP) channels are involved in ischemic cardioprotection induced by preconditioning (IPC), though the relative role of sarcolemmal (sKATP) and mitochondrial (mitoKATP) channels remains controversial. The sKATP-selective sulphonylthiourea HMR 1098 has often been reported to be without effect on ischemic cardioprotection, suggesting minimal involvement of sKATP. Since some sulphonylureas show reduced potency under conditions of metabolic stress, we used patch clamp to assess the ability of HMR 1098 to block sKATP currents of adult rat ventricular myocytes activated by metabolic inhibition (MI, NaCN + iodoacetate). In contrast to the prototype sulphonylurea glibenclamide, HMR 1098 (10 μM) was without effect on sKATP currents, and also did not inhibit MI-induced action potential shortening. However, HMR 1098 blocked sKATP current induced by the KATP opener pinacidil (IC50 = 0.36 ± 0.02 μM), and reversed pinacidil-induced action potential shortening. In inside-out patches, block by HMR 1098 was relieved by increasing MgADP concentrations (1-100 μM). HMR 1098 inhibited pinacidil-activated recombinant Kir6.2/SUR2A channels with a similar IC50 (0.30 ± 0.04 μM), but was less effective when channels were activated by low intracellular ATP. HMR 1098 displaced binding of the pinacidil analogue [3H]P1075 to native cardiac membranes with a biphasic inhibition curve. Our results show that HMR 1098 becomes a much less effective inhibitor of sKATP during metabolic stress, and suggest that the lack of effect of HMR 1098 on ischemic cardioprotection reported in some studies may represent loss of block by the drug under these conditions rather than a lack of involvement of sKATP channels.