Mechanisms of hypoosmotic volume regulation in glioma cells.

Rat C6 glioma cells undergo regulatory volume decrease (RVD) following hypoosmotic exposure. RVD was inhibited by the K+ channel blockers barium (10 mM) and quinine (1 mM). The mechanism of activation of the volume regulatory process was studied. Volume regulation was not observed following incubation of cells in Ca(2+)-free medium. Fluorescent measurement of intracellular free Ca2+ revealed no change following hypoosmotic exposure. Okadaic acid, an inhibitor of protein phosphatase type 1 and 2A inhibited VRD in C6 glioma cells. These results suggest that hypoosmotic RVD in C6 glioma cells involves a loss of K+ (and anion) from the cell. The activation of K+ loss is dependent on the presence of extracellular calcium (but not an increase in intracellular free calcium); and on protein dephosphorylation, either of a transport protein or another protein in the signalling pathway.