Calcium uptake and cytochrome c release from normal and ischemic brain mitochondria.

At abnormally elevated levels of intracellular Ca, mitochondrial Ca uptake may compromise mitochondrial electron transport activities and trigger membrane permeability changes that allow for release of cytochrome c and other mitochondrial apoptotic proteins into the cytosol. In this study, a clinically relevant canine cardiac arrest model was used to assess the effects of global cerebral ischemia and reperfusion on mitochondrial Ca uptake capacity, Ca uptake-mediated inhibition of respiration, and Ca-induced cytochrome c release, as measured in vitro in a K-based medium in the presence of Mg, ATP, and NADH-linked oxidizable substrates. Maximum Ca uptake by frontal cortex mitochondria was significantly lower following 10 min cardiac arrest compared to non-ischemic controls. Mitochondria from ischemic brains were also more sensitive to the respiratory inhibition associated with accumulation of large levels of Ca. Cytochrome c was released from brain mitochondria in vitro in a Ca-dose-dependent manner and was more pronounced following both 10 min of ischemia alone and following 24 h reperfusion, in comparison to mitochondria from non-ischemic Shams. These effects of ischemia and reperfusion on brain mitochondria could compromise intracellular Ca homeostasis, decrease aerobic and increase anaerobic cerebral energy metabolism, and potentiate the cytochrome c-dependent induction of apoptosis, when re-oxygenated mitochondria are exposed to abnormally high levels of intracellular Ca.