Early metabolic inhibition-induced intracellular sodium and calcium increase in rat cerebellar granule cells

1. Possible mechanisms responsible for the increases in intracellular calcium ([Ca²⁺](i)) and sodium ([Na²⁺](i)) levels seen during metabolic inhibition were investigated by continuous [Ca²⁺](i) and [Na⁺](i) measurement in cultured rat cerebellar granule cells. An initial small mitochondrial Ca²⁺ release was seen, followed by a large influx of extracellular Ca²⁺. A large influx of extracellular Na⁺ was also seen. 2. The large [Ca²⁺](i) increase was not due to opening of voltage-dependent or voltage-independent calcium channels, activation of NMDA/non-NMDA channels, activation of the Na⁺(i)-Ca²⁺(o) exchanger, or inability of plasmalemmal Ca²⁺-ATPase to extrude, or mitochondria to take up, calcium. 3. The large [Na⁺](i) increase was not due to activation of the TTX-sensitive Na⁺ channel, the Na⁺(i)-Ca²⁺(o) exchanger, the Na⁺-H⁺ exchanger, or the Na⁺-K⁺-2Cl^- cotransporter, or an inability of Na⁺-K⁺-ATPase to extrude the intracellular sodium. 4. Phospholipase A₂ (PLA₂) activation may be involved in the large influx, since both were completely inhibited by PLA₂ inhibitors. Moreover, melittin (a PLA₂ activator) or lysophosphatidylcholine or arachidonic acid (both PLA₂ activation products) caused similar responses. Inhibition of PLA₂ activity may help prevent the influx of these ions that may result in serious brain injury and oedema during hypoxia/ischaemia.