Isovolumetric regulation of rat glial cells during development and correction of hypo-osmolality

Rat C6 glioma cells undergo regulatory volume decrease (RVD) following sudden exposure to hypo-osmolality, but little or no regulatory volume increase (RVI) is observed when cells cultured in hypo-osmotic media are suddenly returned to isoosmolality. Because C6 glioma cells would rarely be exposed to sudden large changes in osmolality in vivo, we examined the ability of these cells to maintain their volume, termed 'isovolumetric regulation', when exposed to gradual changes in osmolality. When osmolality was gradually reduced by reduction of NaCl concentration from 300 to 250 mOsmol/kg at a rate of 0.4 mOsmol/kg/min or less cells were able to maintain their volume, while at higher rates, the cells swelled. Cells which were cultured in hypo-osmotic (200 mOsmol/kg) media for 3 days exhibited isovolumetric regulation at rates of osmolality increase of 0.5 mOsmol/kg/min or less over the range of 200-250 mOsmol/kg. We conclude that rat C6 glioma cells can sensitively regulate their volume over the osmolality range of pathophysiologic interest at rates of osmolality change which are faster than those generally seen in clinical conditions.