Intracellular Ascorbic Acid Inhibits the Na+-Ca2+ Exchanger in Cultured Rat Astrocytes |
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Authors: | Kazuhiro Takuma Toshio Matsuda Shoichi Asano Akemichi Baba |
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Affiliation: | Department of Pharmacology, Faculty of Pharmaceutical Sciences, Osaka University, Suita, Japan |
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Abstract: | Abstract: The effect of ascorbic acid on Ca2+ uptake in cultured rat astrocytes was examined in the presence of ouabain and monensin, which are considered to drive the Na+-Ca2+ exchanger in the reverse mode. Ascorbic acid at 0.1–1 m M inhibited Na+-dependent Ca2+ uptake significantly but not Na+-dependent glutamate uptake in the cells, although the inhibition required pretreatment for more than 30 min. The effect of ascorbic acid on the Ca2+ uptake was blocked by simultaneous addition of ascorbate oxidase (10 U/ml). Na+-dependent Ca2+ uptake was also inhibited by isoascorbate at 1 m M but not by ascorbate 2-sulfate, dehydroascorbate, and sulfhydryl-reducing reagents such as glutathione and 2-mercaptoethanol. The inhibitory effect of ascorbic acid was observed even in the presence of an inhibitor of lipid peroxidation, o -phenanthroline, or a radical scavenger, mannitol, and the degrading enzymes such as catalase and superoxide dismutase. On the other hand, the inhibitory effect was not observed under the Na+-free conditions that inhibited the uptake of ascorbic acid in astrocytes. When astrocytes were cultured for 2 weeks in a medium containing ascorbic acid, the content of ascorbic acid in the cells was increased and conversely Na+-dependent Ca2+ uptake was decreased. These results suggest that an increase in intracellular ascorbic acid results in a decrease of Na+-Ca2+ exchange activity in cultured astrocytes and the mechanism is not related to lipid peroxidation. |
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Keywords: | Na+-Ca2+ exchanger Ascorbic acid Astrocytes Monensin |
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