CULTURED RAT ASTROCYTES POSSESS NA-CA2+ EXCHANGER()

Na+-Ca2+ exchange activity in its reverse mode was demonstrated in cul tured rat astrocytes. Combination of ouabain (1 mM) and monensin (20 m u M) caused a marked increase in Ca-45(2+) uptake in astrocytes. Ca-45 (2+) uptake was also stimulated by lowering the external Na+ concentra tion. Ouabain plus monensin-stimulated Ca-45(2+) uptake was blocked by 3,4-dichlorobenzamil (IC50, 16 mu M), an inhibitor of Na+-Ca2+ exchan ger, but not by nifedipine (0.1 mu M). The stimulated-Ca-45(2+) uptake was observed even in K+-free medium, and external K+ at 5-10 mM. caus ed a 2.2-fold increase in the uptake. Microspectrofluorimetry using th e Ca2+-sensitive dye fura-2 showed that ouabain plus monensin increase d intracellular Ca2+ concentration in single astrocytes. The Ca2+ sign al was dependent on external Ca2+ (EC(50), 1.4 mM), and blocked by 20 mu M 3,4-dichlorobenzamil, but not by Ca2+ channel blockers (Cd2+, 20 mu M; Ni2+, 100 mu M). Antiserum of cardiac Na+-Ca2+ exchanger recogni zed 160 and 120-135 kDa proteins on SDS-polyacrylamide gel electrophor esis of astrocyte homogenate. Northern blot analysis revealed the pres ence of mRNA for the exchanger protein in astrocytes. These findings i ndicate that Na+-Ca2+ exchanger which is modulated by K+ is present in cultured rat astrocytes. (C) 1994 Wiley-Liss, Inc.