Mitochondrial control of acute glutamate excitotoxicity in cultured cerebellar granule cells

Mitochondria within cultured rat cerebellar granule cells have a complex in fluence on cytoplasmic free Ca2+ ([Ca2+](c)) responses to glutamate. A decr eased initial [Ca2+](c) elevation in cells whose mitochondria are depolariz ed by inhibition of the ATP synthase and respiratory chain (conditions whic h avoid ATP depletion) was attributed to enhanced Ca2+ extrusion from the c ell rather than inhibited Ca2+ entry via the NMDA receptor. Even in the pre sence of elevated extracellular Ca2+, when [Ca2+](c) responses were restore d to control values, such cells showed resistance to acute excitotoxicity, defined as a delayed cytoplasmic Ca2+ deregulation (DCD) during glutamate e xposure. DCD was a function of the duration of mitochondrial polarization i n the presence of glutamate rather than the total period of glutamate expos ure. Once initiated, DCD could not be reversed by NMDA receptor inhibition. In the absence of ATP synthase inhibition, respiratory chain inhibitors pr oduced an immediate Ca2+ deregulation (ICD), ascribed to an ATP deficit. In contrast to DCD, ICD could be reversed by subsequent ATP synthase inhibiti on with or without additional NMDA receptor blockade. DCD could not be ascr ibed to the failure of an ATP yielding metabolic pathway. It is concluded t hat mitochondria can control Ca2+ extrusion from glutamate-exposed granule cells by the plasma membrane in three ways: by competing with efflux pathwa ys for Ca2+, by restricting ATP supply, and by inducing a delayed failure o f Ca2+ extrusion. Inhibitors of the mitochondrial permeability transition o nly marginally delayed the onset of DCD.