Cultured rat astrocytes possess Na+‐Ca2+ exchanger

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Abstract
Na+‐Ca2+ exchange activity in its reverse mode was demonstrated in cultured rat astrocytes. Combination of ouabain (1 mM) and monensin (20 ±M) caused a marked increase in 45Ca2+ uptake in astrocytes. 45Ca2+ uptake was also stimulated by lowering the external Na+ concentration. Ouabain plus monensin‐stimulated 45Ca2+ uptake was blocked by 3,4‐dichlorobenzamil (IC50, 16 ±M), an inhibitor of Na+‐Ca2+ exchanger, but not by nifedipine (0.1 ±M). The stimulated‐45Ca2+ uptake was observed even in K+‐free medium, and external K+ at 5‐10 mM caused a 2.2‐fold increase in the uptake. Microspectrofluorimetry using the Ca2+‐sensitive dye fura‐2 showed that ouabain plus monensin increased intracellular Ca2+ concentration in single astrocytes. The Ca2+ signal was dependent on external Ca2+ (EC50, 1.4 mM), and blocked by 20 ±M 3,4‐dichlorobenzamil, but not by Ca2+ channel blockers (Cd2+, 20 ±M; Ni2+, 100 ±M). Antiserum of cardiac Na+‐Ca2+ exchanger recognized 160 and 120‐135 kDa proteins on SDS‐polyacrylamide gel electrophoresis of astrocyte homogenate. Northern blot analysis revealed the presence of mRNA for the exchanger protein in astrocytes. These findings indicate that Na+‐Ca2+ exchanger which is modulated by K+ is present in cultured rat astrocytes.