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

Abstract

Abstract: The ability of mitochondrial Ca 2+ transport to limit the elevation in free cytoplasmic Ca 2+ concentration in neurones following an imposed Ca 2+ 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 Ca 2+ into the cytoplasm in both somata and neurites. No CCCP‐releasable pool was found in nondepolarized cells. Although the KCI‐evoked somatic and neurite Ca 2+ 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 Ca 2+ transport; however, paradoxically the KCI‐evoked Ca 2+ elevation is decreased. It is concluded that the CCCP‐induced increase in cytoplasmic Ca 2+ response to KCI is due to inhibition of nonmitochondrial ATP‐dependent transport and that mitochondrial Ca 2+ transport enhances entry of Ca 2+, perhaps by removing the cation from cytoplasmic sites responsible for feedback inhibition of voltage‐activated Ca 2+ channel activity.