Mitochondrial ATP-Sensitive Potassium Channels Attenuate Matrix Ca ²⁺ Overload During Simulated Ischemia and Reperfusion

Abstract

Abstract-Mitochondrial ATP-sensitive potassium (mitoK ATP ) channels play a key role in ischemic preconditioning of the heart. However, the mechanism of cardioprotection remains controversial. We measured rhod-2 fluorescence in adult rabbit ventricular cardiomyocytes as an index of mitochondrial matrix Ca 2ϩ concentration ([Ca 2ϩ ] m ), using time-lapse confocal microscopy. To simulate ischemia and reperfusion (I/R), cells were exposed to metabolic inhibition (50 minutes) followed by washout with control solution. Rhod-2 fluorescence gradually increased during simulated ischemia and rose even further with reperfusion. The mitoK ATP channel opener diazoxide attenuated the accumulation of [Ca 2ϩ ] m during simulated I/R (EC 50 ϭ18 mol/L). These effects of diazoxide were blocked by the mitoK ATP channel antagonist 5-hydroxydecanoate (5HD). In contrast, inhibitors of the mitochondrial permeability transition (MPT), cyclosporin A and bongkrekic acid, did not alter [Ca 2ϩ ] m accumulation during ischemia, but markedly suppressed the surge in rhod-2 fluorescence during reperfusion. Measurements of mitochondrial membrane potential, ⌬⌿ m , in permeabilized myocytes revealed that diazoxide depolarized ⌬⌿ m (by 12% at 10 mol/L, PϽ0.01) in a 5HD-inhibitable manner. Our data support the hypothesis that attenuation of mitochondrial Ca 2ϩ overload, as a consequence of partial mitochondrial membrane depolarization by mitoK ATP channels, underlies cardioprotection. Furthermore, mitoK ATP channels and the MPT differentially affect mitochondrial calcium homeostasis: mitoK ATP channels suppress calcium accumulation during I/R, while the MPT comes into play only upon reperfusion. Key Words: mitochondrial calcium overload Ⅲ cardioprotection Ⅲ ischemia I schemic preconditioning (IPC) 1 is the endogenous mechanism whereby brief periods of ischemia paradoxically protect the myocardium against the damaging effects of subsequent prolonged ischemia. IPC exists in all species examined, including humans. 2 Mitochondrial K ATP (mitoK ATP ) channels feature prominently in the mechanism of cardioprotection 3-9 ; however, their precise role remains controversial. A decrease in the extent of mitochondrial Ca 2ϩ overload during ischemia and reperfusion (I/R) has been proposed to prevent or delay cell death, 10,11 and activation of the mitochondrial permeability transition (MPT) may be involved in this process. MPT opening results in the collapse of oxidative phosphorylation and can be accelerated synergistically by Ca 2ϩ , oxidative stress, and ATP depletion during I/R. 12,13 These observations prompted us to investigate not only whether the cytoprotective effect of mitoK ATP channel activation is related to inhibition of mitochondrial Ca 2ϩ overload during I/R, but also whether mitoK ATP channel opening can depolarize ⌬⌿ m . 4 To test our hypothesis, we have monitored mitochondrial Ca 2ϩ concentration ([Ca 2ϩ ] m ) during simulated I/R and separately measured mitochondrial membrane potential in response to mitoK ATP channel mediators. Our...

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“…Preconditioning, a series of brief periods of ischemia and reperfusion, protects the myocardium from deleterious events associated with extended durations of ischemia and reperfusion (7)(8)(9). Cardioprotection afforded by preconditioning appears mediated, in part, by the production of oxygen radicals (35)(36)(37)(38)(39)(40) and is associated with reductions in Ca 2ϩ overload (7,(41)(42)(43) and free radical production (44) during extended periods of ischemia͞ reperfusion.…”

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