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 osmola lity was reduced from 300 to 150 mosmol/kg. Exposure to a hyposmotic c hallenge elicited a typical regulatory volume decrease (RVD). No regul atory volume increase was observed upon restoration of isosmotic condi tions. During a second subsequent hyposmotic challenge, the cells did not respond with RVD. High extracellular K+ concentration and the K+ c hannel blockers Ba2+ and quinine inhibited the RVD. RVD was abolished after Cl- was replaced by gluconate and by the Cl- channel blocker 5-n itro-2(3-phenylpropylamino)benzoic acid. Amino acid (AA) concentration in cell and perfusate was determined. In control, cell content was on ly 26 mmol/l. Hypotonicity increased the efflux of AA from 0.14 to 0.6 0 mmol/min. During the second hyposmotic challenge, the release was 0. 32 mmol/min. The data show that C6 cells adjust their volume under hyp osmotic 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 c ontribute substantially to cell volume regulation.