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

1. 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. 2. In isolated cardiac mitochondria, diazoxide and pinacidil decreased the rate and magnitude of Ca²⁺ uptake into the mitochondrial matrix with 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. 3. 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. 4. 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. 5. 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. 6. 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. 7. 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.