Differential Actions of Cardioprotective Agents on the Mitochondrial Death Pathway

Abstract

We examined the effect of cardioprotective agents on three distinct phases of the H 2 O 2 -induced response that leads to loss of mitochondrial membrane potential (ΔΨ m ) and cell death in cultured cardiac myocytes: (1) priming, consisting of calcium-dependent morphological changes in mitochondria (swelling and loss of cristae), with preserved ΔΨ m , (2) depolarization, the rapid ΔΨ m depolarization caused by mitochondrial permeability transition pore (PTP) opening, and (3) cell fragmentation. The mitochondrial ATP-sensitive potassium (mitoK ATP ) channel opener diazoxide markedly decreased the likelihood that cells would undergo priming: many mitochondria remained fully polarized and morphologically intact. Diazoxide not only decreased the number of cells undergoing ΔΨ m depolarization but also delayed the onset of ΔΨ m loss, whereas it did not change the duration of depolarization in unprotected cells. The adenine nucleotide translocase inhibitor bongkrekic acid mimicked the effect of diazoxide to suppress priming, except that its effects were not blocked by the mitoK ATP channel blocker 5-hydroxydecanoate. In contrast, the PTP inhibitor cyclosporin A (CsA) did not prevent priming: neither latency for ΔΨ m depolarization nor mitochondrial morphological changes were affected. However, CsA slowed the process of depolarization and blunted its severity. Importantly, coapplication of diazoxide and CsA exhibited additive effects, improving the efficacy of protection. Activation of mitoK ATP channels suppresses the cell death process at its earliest stage, by preserving mitochondrial integrity during oxidative stress. By virtue of its pharmacology and its phenotypic consequences, this mode of action is distinguishable from that of other cardioprotective interventions.