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 ([Na+](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(+)-Ca-O(2+) 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(+)-Ca-O(2+) 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 A(2) (PLA(2)) activation may be involved in the large influx, since both were completely inhibited by PLA(2) inhibitors. Moreover, melittin (a PLA(2) activator) or lysophosphatidylcholine or arachidonic acid (both PLA(2) activation products) caused similar responses. Inhibition of PLA(2) activity may help prevent the influx of these ions that may result in serious brain injury and oedema during hypoxia/ischaemia.