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Blocking sodium current reduces the rise in intracellular calcium concentration during hypoxia in rat hippocampal neurons

It is believed a marked rise in intracellular Ca²⁺ concentration ([Ca²⁺]_i) is the leading cause of irreversible cell damage during hypoxia. There is increasing evidence that an increase in intracellular Na⁺ concentration ([Na⁺]_i) is also involved. Whether there is a relationship between the rises in [Na⁺]_i and [Ca²⁺]_i remains controversial. We have studied this relationship in cultured hippocampal neurons (cells were obtained from rapidly decapitated newborn rats) by recording [Ca²⁺]_i before and during hypoxia or exposure to ion channel blockers in cultured hippocampal neurons using the Ca²⁺ indicator fluo-3-AM. The fluorescence of fluo-3 was monitored within single cells before and during hypoxia to track changes in [Ca²⁺]_i. It has been shown that low concentrations of the Na⁺ channel blockers TTX (1 nM) or lidocaine (10 nM) block persistent Na⁺ current but not the transient, inactivating Na⁺ current responsible for action potentials¹. We found that these drugs effectively blocked the hypoxic rise in [Ca²⁺]_i. In contrast, blocking Ca²⁺ channels with cadmium (100µM) did not prevent the hypoxic rise in [Ca²⁺]_i. These results suggest that the persistent Na⁺ influx is making a major contribution to the [Ca²⁺]_i rise during hypoxia. A rise in [Na⁺]_i could influence [Ca²⁺]_i by influencing removal of Ca²⁺ from cells by the Na⁺-Ca²⁺ exchanger. To test this hypothesis, we examined the effect of an Na⁺-Ca²⁺ exchanger inhibitor, KB-R7943 (5 ì M), and found that it reduced the hypoxic rise in [Ca²⁺]_i. These results support the hypothesis that hypoxia causes an increase in [Ca²⁺]_i by increasing persistent Na⁺ current and consequently [Na⁺]_i and this then depresses removal of Ca²⁺ so that [Ca²⁺]_i rises.

(1) 1. Hammarstrom AK and Gage PW. Inhibition of oxidative metabolism increases persistent sodium current in rat CA1 hippocampal neurons. J Physiol 510 ( Pt 3): 735-741, 1998.