Calcium induced release of mitochondrial cytochrome c by different mechanisms selective for brain versus liver

Abstract

Increased mitochondrial Ca²⁺ accumulation is a trigger for the release of cytochrome c from the mitochondrial intermembrane space into the cytosol where it can activate caspases and lead to apoptosis. This study tested the hypothesis that Ca²⁺-induced release of cytochrome c in vitro can occur by membrane permeability transition (MPT)-dependent and independent mechanisms, depending on the tissue from which mitochondria are isolated. Mitochondria were isolated from rat liver and brain and suspended at 37°C in a K⁺-based medium containing oxidizable substrates, ATP, and Mg²⁺. Measurements of changes in mitochondrial volume (via light scattering and electron microscopy), membrane potential and the medium free [Ca²⁺] indicated that the addition of 0.3–3.2 μmol Ca²⁺ mg⁻¹ protein induced the MPT in liver but not brain mitochondria. Under these conditions, a Ca²⁺ dose-dependent release of cytochrome c was observed with both types of mitochondria; however, the MPT inhibitor cyclosporin A was only capable of inhibiting this release from liver mitochondria. Therefore, the MPT is responsible for cytochrome c release from liver mitochondria, whereas an MPT-independent mechanism is responsible for release from brain mitochondria.

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