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A Reevaluation of the Role of Mitochondria in Neuronal Ca2+ Homeostasis

Authors


Address correspondence and reprint requests to Dr. D. G. Nicholls at Department of Pharmacology, Neurosciences Institute, Ninewells Medical School, University of Dundee, Dundee DD1 9SY, Scotland, U.K.

Abstract

Abstract: The ability of mitochondrial Ca2+ transport to limit the elevation in free cytoplasmic Ca2+ concentration in neurones following an imposed Ca2+ load is reexamined. Cultured cerebellar granule cells were monitored by digital fura-2 imaging. Following KCI depolarization, addition of the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP) to depolarize mitochondria released a pool of Ca2+ into the cytoplasm in both somata and neurites. No CCCP-releasable pool was found in nondepolarized cells. Although the KCI-evoked somatic and neurite Ca2+ concentration elevations were enhanced when CCCP was present during KCI depolarization, this was associated with a collapsed ATP/ADP ratio. In the presence of the ATP synthase inhibitor oligomycin, glycolysis maintained high ATP/ADP ratios for at least 10 min. The further addition of the mitochondrial complex I inhibitor rotenone led to a collapse of the mitochondrial membrane potential, monitored by rhodamine-123, but had no effect on ATP/ADP ratios. In the presence of rotenone/oligomycin, no CCCP-releasable pool was found subsequent to KCI depolarization, consistent with the abolition of mitochondrial Ca2+ transport; however, paradoxically the KCI-evoked Ca2+ elevation is decreased. It is concluded that the CCCP-induced increase in cytoplasmic Ca2+ response to KCI is due to inhibition of nonmitochondrial ATP-dependent transport and that mitochondrial Ca2+ transport enhances entry of Ca2+, perhaps by removing the cation from cytoplasmic sites responsible for feedback inhibition of voltage-activated Ca2+ channel activity.

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