Intracellular sodium homeostasis in rat hippocampal astrocytes

Research output: Journal Publications and Reviews (RGC: 21, 22, 62)21_Publication in refereed journalpeer-review

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Original languageEnglish
Pages (from-to)291-305
Journal / PublicationJournal of Physiology
Issue number2
Publication statusPublished - 1 Mar 1996
Externally publishedYes


1. We determined the intracellular Na+ concentration (Na+]1 and mechanisms of its regulation in cultured rat hippocampal astrocytes using fluorescence ratio imaging of the Na+ indicator SBFI-AM (acetoxymethylester of sodium-binding benzofuran isophthalate, 10 μM). Dye signal calibration within the astrocytes showed that the ratiometric dye signal changed monotonically with changes in [Na+]1 from 0 to 140 mM. The K+ sensitivity of the dye was negligible; intracellular pH changes, however, slightly affected the 'Na+' signal.

2. Baseline [Na+]1 was 14.6 ± 4.9 mM (mean ± S.D.) in CO2/HCO3-,-containing saline with 3 mM K+. Removal of extracellular Na+ decreased [Na+](i) in two phases: a rapid phase of [Na+]1, reduction (0.58) ± 0.32 mM min-1) followed by a slower phase (0.15 ± 0.09 mM min-1).

3. Changing from CO2/HCO3-,-free to CO2/HCO3-,-buffered saline resulted in a transient increase in [Na+](i) of ~5 mM, suggesting activation of inward Na+-HCO3-, cotransport by CO2/HCO3-. During furosemide (frusemide, 1 mM) or bumetanide (50 μM) application, a slow decrease in [Na+]1, of ~2 mM was observed, indicating a steady inward transport of Na+ via Na+-K+-2Cl- cotransport under control conditions. Tetrodotoxin (100 μM) did not influence [Na+]1 in the majority of cells (85%), suggesting that influx of Na+ through voltage-gated Na+ channels contributed to baseline [Na+]1 in only a small subpopulation of hippocampal astrocytes.

4. Blocking Na+,K+-ATPase activity with cardiac glycosides (ouabain or strophanthidin, 1 mM) or removal. of extracellular K+ led to an increase in [Na+]1 of about 2 and 4 mM min-1, respectively. This indicated that Na+,K+--ATPase activity was critical in maintaining low [Na+]1 in the face of a steep electrochemical gradient, which would favour a much higher [Na+]1.

5. Elevation of extracellular K+ concentration ([K+](o)) by as little as 1 mM (from 3 to 4 mM) resulted in a rapid and reversible decrease in [Na+](i). Both the slope and the amplitude of the [K+](o)-induced reductions in [[Na+]1, were sensitive to bumetanide. A reduction of [K+](o) by 1mM increased [Na+]1 by 3.0 ± 2.3 mM . In contrast, changing extracellular Na+ concentration by 20 mM resulted in changes in [Na+]1 of less than 3 mM.

6. These results implied that in hippocampal astrocytes low baseline [Na+]1 is determined by the action of Na+-HCO3-, cotransport, Na+-K+-2Cl- cotransport and Na+,K+-ATPase, and that both Na+,K+-ATPase and inward Na+-K+-2Cl- cotransport are activated by small, physiologically relevant increases in [K+]o. These mechanisms are well suited to help buffer increases in [K+]o associated with neural activity.

Citation Format(s)

Intracellular sodium homeostasis in rat hippocampal astrocytes. / Rose, Christine R.; Ransom, Bruce R.

In: Journal of Physiology, Vol. 491, No. 2, 01.03.1996, p. 291-305.

Research output: Journal Publications and Reviews (RGC: 21, 22, 62)21_Publication in refereed journalpeer-review