VOLUME-ACTIVATED RB+ TRANSPORT IN ASTROCYTES IN CULTURE

The involvement of K+ on the volume regulatory process in astrocytes w as investigated by characterizing the hyposmolarity-induced efflux of K+ using Rb-86 as a tracer. About 70 and 30% of the intracellular cont ent of Rb-86 was released after reductions in osmolarity from 320 to 1 60 or 220 mosM, respectively, during the time in which cells exhibit a volume regulatory response subsequent to swelling. No significant inc rease in Rb-86 efflux was observed with lower reductions in osmolarity . The Rb-86 efflux was Ca2+ independent and insensitive to temperature . It was inhibited by furosemide but not by bumetanide and was unaffec ted when nitrate, but not gluconate, replaced intracellular Cl-. The e fflux was markedly inhibited by quinidine and by 4,4'-diisothiocyanost ilbene-2,2'-disulfonic acid. Quinidine also prevented the volume regul atory decrease of cells, and this effect was overcome when a large cat ion permeability was imposed by gramicidine. In isosmotic conditions R b-86 efflux was not activated by N-ethylmaleimide, but this drug stron gly inhibited the hyposmolarity-activated release. These findings sugg est that Rb-86 efflux from astrocytes associated to cell swelling is n ot mediated by an electroneutral cotransporter and rather favor the im plication of a conductive exit pathway that may be a Ca2+-independent K+ channel.