Pharmacological evidence that the activation of the Na(+)-Ca2+ exchanger protects C6 glioma cells during chemical hypoxia

In C6 glioma cells exposed to chemical hypoxia a massive release of lactate dehydrogenase (LDH) occurred at 3 and 6 h, coupled with an increased number of propidium-iodide positive dead cells. Extracellular Na+ removal, which activates the Na+-Ca2+ exchanger as a Na+ efflux pathway and prevents Na+ entrance, significantly reduced LDH release and the number of propidium iodide positive C6 cells. During chemical hypoxia, in the presence of extracellular Na+ ions, a progressive increase of [Ca2+]i occurred; in the absence of extracellular Na+ ions [Ca2+]i was enhanced to a greater extent. The blockade of the Na+-Ca2+ exchanger by the amiloride derivative 5-(N-4-chlorobenzyl)-2',4'-dimethylbenzamil (CB-DMB), lanthanum (La3+) and the Ca2+ chelator EGTA, completely reverted the protective effect exerted by the removal of Na+ ions on C6 glioma cells exposed to chemical hypoxia. The inhibition of the Na+-Ca2+ antiporter enhanced chemical hypoxia-induced LDH release when C6 glioma cells were incubated in the presence of physiological concentrations of extracellular Na+ ions (145 mM), suggesting that the blockade of the Na+-Ca2+ antiporter during chemical hypoxia can lead to increased cell damage. Collectively, these results suggest that activation of the Na+-Ca2+ exchanger protects C6 glioma cells exposed to chemical hypoxia, whereas its pharmacological blockade can exacerbate cellular injury.