Cholesterol-enriched diet inhibits cardioprotection by ATP-sensitive K+ channel activators cromakalim and diazoxide

It has been previously shown that hyperlipidemia interferes with cardioprotective mechanisms. Here we investigated the interaction of hyperlipidemia with cardioprotection induced by pharmacological activators of ATP-sensitive potassium channels (KATP). Hearts isolated from rats fed 2% cholesterol-enriched diet or normal diet for 8 weeks were subjected to 30 min global ischemia and 120 min reperfusion in the presence or absence of KATP modulators. After normal diet, either the non-selective KATP activator cromakalim at 10-5 M or the selective mitochondrial KATP (mitoKATP) opener diazoxide at 3x10-5 M significantly decreased infarct size when compared to vehicle-treated controls. Their cardioprotective effect was abolished by co-administration of the non-selective KATP blocker glibenclamide or the selective mitoKATP blocker 5-hydroxydecanoate, respectively. However, in cholesterol-fed rats, the cardioprotective effect of cromakalim or diazoxide was not observed. Therefore, we further investigated how cholesterol-enriched diet may influence cardiac KATP channels. Cardiac expression of KATP subunit gene (Kir6.1) was significantly down-regulated in cholesterol-fed rats, however, protein levels of Kir6.1 and 6.2 were not changed. Cholesterol diet significantly decreased cardiac ATP and increased lactate content and enhanced myocardial oxidative stress as shown by increased cardiac superoxide and dityrosine formation. This is the first demonstration that cardioprotection by KATP channel activators is impaired in cholesterol-enriched diet-induced hyperlipidemia. The background mechanism may include hyperlipidemia-induced attenuation of mitoKATP function by altered energy metabolism and increased oxidative stress in the heart

ATP-sensitive K+ channel openers prevent Ca2+ overload in rat cardiac mitochondria

Mitochondrial dysfunction, secondary to excessive accumulation of Ca2+, has been implicated in cardiac injury. We here examined the action of potassium channel openers on mitochondrial Ca2+ homeostasis, as these cardioprotective ion channel modulators have recently been shown to target a mitochondrial ATP-sensitive K+ channel.In isolated cardiac mitochondria, diazoxide and pinacidil decreased the rate and magnitude of Ca2+ uptake into the mitochondrial matrix with an IC50 of 65 and 128 μm, respectively. At all stages of Ca2+ uptake, the potassium channel openers depolarized the mitochondrial membrane thereby reducing Ca2+ influx through the potential-dependent mitochondrial uniporter.Diazoxide and pinacidil, in a concentration-dependent manner, also activated release of Ca2+ from mitochondria. This was prevented by cyclosporin A, an inhibitor of Ca2+ release through the mitochondrial permeability transition pore.Replacement of extramitochondrial K+ with mannitol abolished the effects of diazoxide and pinacidil on mitochondrial Ca2+, while the K+ ionophore valinomycin mimicked the effects of the potassium channel openers.ATP and ADP, which block K+ flux through mitochondrial ATP-sensitive K+ channels, inhibited the effects of potassium channel openers, without preventing the action of valinomycin.In intact cardiomyocytes, diazoxide also induced mitochondrial depolarization and decreased mitochondrial Ca2+ content. These effects were inhibited by the mitochondrial ATP-sensitive K+ channel blocker 5-hydroxydecanoic acid.Thus, potassium channel openers prevent mitochondrial Ca2+ overload by reducing the driving force for Ca2+ uptake and by activating cyclosporin-sensitive Ca2+ release. In this regard, modulators of an ATP-sensitive mitochondrial K+ conductance may contribute to the maintenance of mitochondrial Ca2+ homeostasis

Pathophysiological and protective roles of mitochondrial ion channels

Mitochondria possess a highly permeable outer membrane and an inner membrane that was originally thought to be relatively impermeable to ions to prevent dissipation of the electrochemical gradient for protons. Although recent evidence has revealed a rich diversity of ion channels in both membranes, the purpose of these channels remains incompletely determined. Pores in the outer membrane are fundamental participants in apoptotic cell death, and this process may also involve permeability transition pores on the inner membrane. Novel functions are now being assigned to other ion channels of the inner membrane. Examples include protection against ischaemic injury by mitochondrial KATP channels and the contribution of inner membrane anion channels to spontaneous mitochondrial oscillations in cardiac myocytes. The central role of mitochondria in both the normal function of the cell and in its demise makes these channels prime targets for future research and drug development