Pharmacological evidence that the activation of the Na +-Ca 2+ exchanger protects C6 glioma cells during chemical hypoxia

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