In the mitochondrial permeability transition (MPT), large conductance permeability transition (PT) pores open that make the mitochondrial inner membrane abruptly permeable to solutes up to 1500 Da. After the MPT, mitochondria depolarize, uncouple and undergo large amplitude swelling. If enough mitochondria undergo the MPT within a single cell, profound ATP depletion occurs, which inhibits activation of apoptotic pathways but promotes onset of necrotic cell death. If some ATP is preserved, outer membrane rupture causes cytochrome c release, caspase activation and apoptosis. Ca
2+ is an important MPT inducer, but its role in MPT induction varies with circumstance. Ca
2+ overload with ionophores is sufficient to induce the MPT, leading both to apoptotic and necrotic cell death. By contrast after ischemia–reperfusion to cardiac myocytes, mitochondrial Ca
2+ overload occurs as the consequence of bioenergetic failure. In other models, Ca
2+ appears to be permissive to MPT onset, such as cytotoxicity from Reye-related agents and storage-reperfusion injury to liver grafts. Lastly in oxidative stress, Ca
2+ and reactive oxygen species (ROS) act synergistically to produce the MPT and cell death. Another cation important for MPT onset is iron. Chelatable (loosely bound) iron catalyzes hydroxyl radical formation from superoxide and hydrogen peroxide. Lysosomes sequester chelatable iron and release Fe
2+ into the cytosol after inhibition of the vacuolar proton-pumping ATPase. During oxidative stress, hypoxia/ischemia and acetaminophen hepatotoxicity, lysosomes release chelatable iron with consequent pro-oxidant injury. Mitochondria accumulate cytosolic Fe
2+ released from lysosomes via the mitochondrial electrogenic Ca
2+ uniporter. Inside mitochondria, chelatable iron catalyzes ROS formation. The iron chelators, desferal and starch-desferal, and the uniporter inhibitors, Ru360 and minocycline, prevent mitochondrial iron loading, blunt ROS formation and decrease cell death in oxidative stress, hypoxia/ischemia and photodynamic therapy. Thus, Ca
2+ and Fe
2+ are dynamic mediators of cell injury in several pathophysiological settings. However, reevaluation of earlier work is needed to determine whether injury attributed to mitochondrial Ca
2+ uptake is actually due to Fe
2+ uptake.
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