Dominant role of mitochondria in protection against a delayed neuronal Ca2+ overload induced by endogenous excitatory amino acids following a glutamate pulse.

The objective of this study was to evaluate the contribution of mitochondria to the clearance of Ca2+ loads induced by glutamate or 25 mM K+ pulses. The mitochondrial Ca2+ uptake was suppressed by application of 0.5 microM antimycin A or 3-5 mM NaCN in combination with 2.5 micrograms/ml oligomycin. In most cells such treatments both in the presence and in the absence of external Na+ failed to abolish the early fast phase of [Ca2+]i recovery following a 1-min 100 microM glutamate pulse. However, the late slow phase of [Ca2+]i recovery in the presence of mitochondrial poisons was transformed into a delayed [Ca2+]i elevation culminating in the neuronal Ca2+ overload. Suppression of the Na+/Ca2+ exchange caused by glutamate-induced [Na+]i elevation promoted the development of delayed Ca2+ increase. Under identical conditions, the high [Ca2+]i transient induced by 25 mM K+ was never accompanied by a delayed Ca2+ elevation. The glutamate-induced delayed Ca2+ increase could be readily abolished by the removal of external Ca2+ or by application in the post-glutamate period of the antagonist of NMDA receptors, 100-200 microM AP-5. The results obtained suggest that mitochondria play a dominant role in the protection against the neuronal Ca2+ overload induced by endogenous excitatory amino acids released in response to a short-term glutamate challenge.