Cytosolic sodium regulation in mouse cortical astrocytes and its dependence on potassium and bicarbonate.

Abstract

Sodium plays a major role in different astrocytic functions, including maintenance of ion homeostasis and uptake of neurotransmitters and metabolites, which are mediated by different Na+ -coupled transporters. In the current study, the role of an electrogenic sodium-bicarbonate cotransporter (NBCe1), a sodium-potassium-chloride transporter 1 (NKCC1) and sodium-potassium ATPase (Na+ -K+ -ATPase) for the maintenance of [Na+ ]i was investigated in cultured astrocytes of wild-type (WT) and of NBCe1-deficient (NBCe1-KO) mice using the Na+ -sensitive dye, asante sodium green-2. Our results suggest that cytosolic Na+ was higher in the presence of CO2 /HCO3- (15 mM) thanCO2 /HCO3- -free, HEPES-buffered solution in WT, but not in NBCe1-KO astrocytes (12 mM). Surprisingly, there was a strong dependence of cytosolic [Na+ ] on the extracellular [HCO3- ] attributable to NBCe1 activity. Pharmacological blockage of NKCC1 with bumetanide led to a robust drop in cytosolic Na+ in both WT and NBCe1-KO astrocytes by up to 6 mM. There was a strong dependence of the cytosolic [Na+ ] on the extracellular [K+ ]. Inhibition of the Na+ -K+ -ATPase led to larger increase in cytosolic Na+ , both in the absence of K+ as compared with the presence of ouabain and in NBCe1-KO astrocytes as compared with WT astrocytes. Our results show that cytosolic Na+ in mouse cortical astrocytes can vary considerably and depends greatly on the concentrations of HCO3- and K+ , attributable to the activity of the Na+ -K+ -ATPase, of NBCe1 and NKCC1.