Activation of NMDA receptors produces large increases in cytosolic Ca 2+ that are taken up into mitochondria. We used recombinant aequorin targeted to mitochondria to report changes in matrix Ca2+ in rat hippocampal neurons in culture. Upon binding Ca2+, aequorin emits a photon in a one-shot reaction that consumes the indicator. Here we show that stimulation with NMDA produced a mitochondrial Ca2+ response that rapidly inactivated. However, following a 30-min recovery period the response was restored, suggesting the presence of a pool of indicator that was not exposed to high Ca2+ during the initial stimulus. We speculate that aequorin distant from the Ca2+ source was protected from microdomains of high Ca2+ near the plasmalemma and that this aequorin moved, either by movement of individual mitochondria or via the mitochondrial tubular network, to replenish consumed indicator during the recovery time. A large Ca2+ increase in a subset of mitochondria could produce local changes in energy metabolism, regional Ca2+ buffering, and foci that initiate neurotoxic processes.
- Ca microdomains