title: Glutamate up-regulates alpha 1 and alpha 2 subunits of the sodium pump in astrocytes of mixed telencephalic cultures but not in pure astrocyte cultures.
abstract: Prior work employing an in vitro model of the cerebral cortex has shown that sodium pump activity is a critical determinant for neuronal survival of glutamate stimulation. We have hypothesized that up-regulation of total brain sodium pump activity will protect against potential excitotoxins. Increased sodium pump activity could theoretically occur by changes in the reaction rate (short-term) and/or by increased levels of sodium pump protein (long-term) and is potentially complex since the three catalytic (a) subunit isoforms of the sodium pump are distributed in a highly variable, cell-specific pattern in the brain. Short-term regulation (seconds to minutes) has been well studied: brain sodium pump exhibits a large dynamic range. In contrast, the possibility of long-term modulation of sodium pump activity has not been extensively explored. We used isoform specific antibodies and [3H]ouabain binding to determine whether prolonged stimulation of sodium pump activity in rodent telencephalic cultures increased total sodium pump enzyme. Exposure of mixed neuronal-glial cultures to high levels of glutamate (10 mM) for 18 h, which is highly toxic to neurons, was associated with an approximately 80% increase in alpha 1 and alpha 2 subunit expression by glia. Induction of alpha 2 subunit immunoreactivity was also associated with comparable changes in [3H]ouabain binding, suggesting that the up-regulation corresponded to functional alpha 2 protein. Shorter (30 min) glutamate treatments, which also killed neurons, did not produce similar changes in sodium pump expression. In contrast to mixed cultures, pure astrocyte cultures had undetectable alpha 2 and alpha 3 and moderate levels of alpha 1 protein, as confirmed by low levels of [3H]ouabain binding. Glutamate treatment using this protocol was associated with a decrease in alpha 1 sodium pump expression. We conclude that long-term regulation of the sodium pump can be demonstrated in glia which have developed in the presence of neurons. Both alpha 1 and alpha 2 isoforms of the sodium pump are involved in this response to glutamate.
authors.affiliation: Department of Internal Medicine, Yale University School of Medicine, New Haven, CT 06510.
authors.forename: M L
authors.initials: ML
authors.lastname: Brines
authors.forename: R J
authors.initials: RJ
authors.lastname: Robbins
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D003584:Cytological Techniques; D005971:Glutamates / Q000494:pharmacology*; D018698:Glutamic Acid; D015151:Immunoblotting; D008856:Microscopy, Fluorescence; D009457:Neuroglia / Q000187:drug effects / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects*; D013687:Telencephalon / Q000166:cytology / Q000378:metabolism*; D013997:Time Factors
chemical_list: D005971:Glutamates; D018698:Glutamic Acid; D000254:Sodium-Potassium-Exchanging ATPase