Sequential Opening of Mitochondrial Ion Channels as a Function of Glutathione Redox Thiol Status

Mitochondrial membrane potential (ΔΨm) depolarization contributes to cell death and electrical and contractile dysfunction in the post-ischemic heart. An imbalance between mitochondrial reactive oxygen species production and scavenging was previously implicated in the activation of an inner membrane anion channel (IMAC), distinct from the permeability transition pore (PTP), as the first response to metabolic stress in cardiomyocytes. The glutathione redox couple, GSH/GSSG, oscillated in parallel with ΔΨm and the NADH/NAD+ redox state. Here we show that depletion of reduced glutathione is an alternative trigger of synchronized mitochondrial oscillation in cardiomyocytes and that intermediate GSH/GSSG ratios cause reversible ΔΨm depolarization, although irreversible PTP activation is induced by extensive thiol oxidation. Mitochondrial dysfunction in response to diamide occurred in stages, progressing from oscillations in ΔΨm to sustained depolarization, in association with depletion of GSH. Mitochondrial oscillations were abrogated by 4′-chlorodiazepam, an IMAC inhibitor, whereas cyclosporin A was ineffective. In saponin-permeabilized cardiomyocytes, the thiol redox status was systematically clamped at GSH/GSSG ratios ranging from 300:1 to 20:1. At ratios of 150:1-100:1, ΔΨm depolarized reversibly, and a matrix-localized fluorescent marker was retained; however, decreasing the GSH/GSSG to 50:1 irreversibly depolarized ΔΨm and induced maximal rates of reactive oxygen species production, NAD(P)H oxidation, and loss of matrix constituents. Mitochondrial GSH sensitivity was altered by inhibiting either GSH uptake, the NADPH-dependent glutathione reductase, or the NADH/NADPH transhydrogenase, indicating that matrix GSH regeneration or replenishment was crucial. The results indicate that GSH/GSSG redox status governs the sequential opening of mitochondrial ion channels (IMAC before PTP) triggered by thiol oxidation in cardiomyocytes.