Intracellular ascorbic acid inhibits the Na(+)-Ca2+ exchanger in cultured rat astrocytes.
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 mM 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 mM 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.
Takuma
K
K
Department of Pharmacology, Faculty of Pharmaceutical Sciences, Osaka University, Suita, Japan.
Matsuda
T
T
Asano
S
S
Baba
A
A
D000818:Animals; D000831:Animals, Newborn; D001205:Ascorbic Acid / Q000494:pharmacology*; D001253:Astrocytes / Q000187:drug effects* / Q000378:metabolism*; D002118:Calcium / Q000493:pharmacokinetics; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002352:Carrier Proteins / Q000037:antagonists & inhibitors*; D002478:Cells, Cultured; D007425:Intracellular Membranes / Q000378:metabolism*; D007552:Isotonic Solutions / Q000494:pharmacology; D008985:Monensin / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D019831:Sodium-Calcium Exchanger
D002121:Calcium Channel Blockers; D002352:Carrier Proteins; C072670:Hanks Balanced Salt Solution; D007552:Isotonic Solutions; D019831:Sodium-Calcium Exchanger; D010042:Ouabain; D008985:Monensin; D001205:Ascorbic Acid; D002118:Calcium