Mitochondrial calcium uptake during ischemia is regulated by cytosolic ATP:ADP ratio

Abstract Prolonged ischemia causes functional impairment of mitochondria by irreversible Ca 2+ overload, which leads to induction of permeability transition and initiation of apoptosis. It has been reported that cytosolic adenine nucleotides regulate mitochondrial Ca 2+ uptake in a concentration dependent manner, through a mechanism which does not require hydrolysis. In ischemia, cytosolic [ATP] is decreased and [ADP] is increased. We hypothesized that during ischemia, the fluctuations in the ATP:ADP ratio, rather than absolute changes in either, govern mitochondrial Ca 2+ uptake. Methods: Mitochondria were isolated from rat livers by differential centrifugation and incubated at 37°C in Mg 2+ -free medium containing 0.2 μM 45 Ca 2+ , 1 mM pyruvate, and 1 mM malate. Mitochondrial Ca 2+ uptake was determined by scintillation counting in the presence of varying ATP:ADP (3:1, 1:1, 1:3) and each alone (0, 0.1, 1, 3 mM). All measurements were performed in triplicate on mitochondria prepared from 4 separate rats. Statistical analysis was performed using Student t-test, with p Results: 1) ATP and ADP alone at low concentration each had a strong regulatory effect on mitochondrial Ca 2+ accumulation: both ATP and ADP increased Ca 2+ uptake at concentrations 0.1 mM and [ADP] > 1 mM, however, there was a decrease in Ca 2+ uptake. 3) The individual effects of ATP and ADP on mitochondrial Ca 2+ uptake were not additive when combined: Ca 2+ uptake was increased by about 12-fold at ATP:ADP ratio of 1:3, while it was only increased by 3-fold at 3:1. Conclusion: These data indicate that mitochondrial Ca 2+ uptake is significantly enhanced when the extramitochondrial ATP:ADP is low; thus, ischemic conditions facilitate mitochondrial Ca 2+ uptake. Maintenance of a cytosolic ATP:ADP > 3 during ischemia, however, will minimize mitochondrial Ca 2+ overload and may minimize subsequent cellular damage. Download high-res image (69KB) Download full-size image