Mitochondrial Calcium Transport and Mitochondrial Dysfunction After Global Brain Ischemia in Rat Hippocampus

Here we report effect of ischemia-reperfusion on mitochondrial Ca(2+) uptake and activity of complexes I and IV in rat hippocampus. By performing 4-vessel occlusion model of global brain ischemia, we observed that 15 min ischemia led to significant decrease of mitochondrial capacity to accumulate Ca(2+) to 80.8% of control whereas rate of Ca(2+) uptake was not significantly changed. Reperfusion did not significantly change mitochondrial Ca(2+) transport. Ischemia induced progressive inhibition of complex I, affecting final electron transfer to decylubiquinone. Minimal activity of complex I was observed 24 h after ischemia (63% of control). Inhibition of complex IV activity to 80.6% of control was observed 1 h after ischemia. To explain the discrepancy between impact of ischemia on rate of Ca(2+) uptake and activities of both complexes, we performed titration experiments to study relationship between inhibition of particular complex and generation of mitochondrial transmembrane potential (Delta I(m)). Generation of a threshold curves showed that complex I and IV activities must be decreased by approximately 40, and 60%, respectively, before significant decline in Delta I(m) was documented. Thus, mitochondrial Ca(2+) uptake was not significantly affected by ischemia-reperfusion, apparently due to excess capacity of the complexes I and IV. Inhibition of complex I is favourable of reactive oxygen species (ROS) generation. Maximal oxidative modification of membrane proteins was documented 1 h after ischemia. Although enhanced formation of ROS might contribute to neuronal injury, depressed activities of complex I and IV together with unaltered rate of Ca(2+) uptake are conditions favourable of initiation of other cell degenerative pathways like opening of mitochondrial permeability transition pore or apoptosis initiation, and might represent important mechanism of ischemic damage to neurones.