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Volume regulation in rat brain glial cells: lack of a substantial contribution of free amino acids.

Volume regulation of C6 glioma cells was studied while the bath osmolality was reduced from 300 to 150 mosmol/kg. Exposure to a hyposmotic challenge elicited a typical regulatory volume decrease (RVD). No regulatory volume increase was observed upon restoration of isosmotic conditions. During a second subsequent hyposmotic challenge, the cells did not respond with RVD. High extracellular K+ concentration and the K+ channel blockers Ba2+ and quinine inhibited the RVD. RVD was abolished after Cl- was replaced by gluconate and by the Cl- channel blocker 5-nitro-2(3-phenylpropylamino)benzoic acid. Amino acid (AA) concentration in cell and perfusate was determined. In control, cell content was only 26 mmol/l. Hypotonicity increased the efflux of AA from 0.14 to 0.60 mmol/min. During the second hyposmotic challenge, the release was 0.32 mmol/min. The data show that C6 cells adjust their volume under hyposmotic conditions but lose the ability to restore their volume during a subsequent hyposmotic treatment. K+ and Cl- are the main osmolytes involved in volume adjustment through conductive pathways. AA do not contribute substantially to cell volume regulation.