Uphill and downhill water transport in rat glioma cells

The cell water content determines the cell volume, which in turn controls numerous cellular functions. The mean volume of rat glioma cells was electronically measured under isotonic and anisotonic conditions. Two types of isotonic solutions were used containing either high or low concentrations of NaCl, KCl or N-methylglucamineCl. In low salt solutions, osmolarity was maintained constant by the addition of sucrose or mannitol. Anisotonicity was induced by changing the concentration of electrolytes. As expected, the cell volume increased when the concentration of electrolytes was decreased from a high (165 mM) monovalent cation concentration. In contrast, the cell volume decreased when the concentration of electrolytes was decreased from a low (85 mM) monovalent cation concentration. Reciprocally and unexpectedly, the cell volume increased during a hyperosmotic challenge when the initial cation concentration was low, whereas it decreased when the initial cation concentration was high. These opposite volume changes observed during similar anisotonic challenges but starting from different electrolyte concentrations provide the first evidence that H2O is not only passively transported (downhill) through aquaporins but also follows ion fluxes (uphill).