ABSTRACT
Mitochondrial ATP-sensitive potassium channels (mitoKATP) locate in the inner mitochondrial membrane and possess protective cellular properties. mitoKATP opening-induced cardioprotection (using the pharmacological agent diazoxide) is preventable by antagonists, such as glibenclamide. However, the mechanisms of action of these drugs and how mitoKATP respond to them are poorly understood. Here, we show data that reinforce the existence of a mitochondrial sulfonylurea receptor (mitoSUR) as part of the mitoKATP. We also show how diazoxide and glibenclamide compete for the same binding site in mitoSUR. A glibenclamide analog that lacks its cyclohexylurea portion (IMP-A) loses its ability to inhibit diazoxide-induced swelling. These results suggest that the cyclohexylureia portion of glibenclamide is indispensable for mitoKATP inhibition. Moreover, IMP-A did not suppress diazoxide-induced preconditioning (EC50 10.66 µM) in a rat model of a cardiac ischemia/reperfusion. Importantly, glibenclamide inhibited both diazoxide-induced cardioprotection (IC50 86 nM). We suggest that IMP-A must be used with caution since we found this drug possesses significant inhibitory effects on mitochondrial respiration. We characterized the binding of glibenclamide and diazoxide using a molecular simulation (docking) approach. Using the molecular structure of the ATP binding protein ABCB8 (pointed by others as the mitoSUR) we demonstrate that glibenclamide competitively inhibits diazoxide actions. This was reinforced (pharmacologically) in a competitive antagonism test. Taken together, these results bring valuable and novel insights into the pharmacological/biochemical aspects of mitokATP activation and cardioprotection. This study may lead to the discovery of novel therapeutic strategies that may impact ischemia-reperfusion injury.
