Changes in taurine transport evoked by hyperosmolarity in cultured astrocytes

Cultured astrocytes grown chronically (1-3 days) in medium made hyperosmotic (450 mOsm) with NaCl or sucrose showed an increase in taurine concentration from 294 to 501 nmol/mg protein in NaCl and to 382 nmol/mg protein in sucrose. The effect of hyperosmolarity on taurine uptake and release was examined to investigate whether or not changes in these processes may account for the increase observed in cell taurine content. Hyperosmolarity significantly affected the two components of taurine uptake (i.e., the Na(+)-dependent and the diffusional component). The Vmax of the Na(+)-dependent, active transport increased 50%, whereas no change was observed in the Km. The diffusion coefficient was markedly decreased by hyperosmolarity, being 2.2 x 10(-4) and 6.6 x 10(-6) ml/min/mg protein in isosmotic and hyperosmotic conditions, respectively, indicating a blockade of the leak pathway. These changes in the active and passive components of taurine transport were opposite to those induced by hyperosmolarity. The effect of hyperosmolarity increasing cell taurine content was insensitive to cycloheximide and colchicine. The basal efflux of taurine from astrocytes also decreased in cells exposed to hyperosmotic medium, indicating that alterations in both influx and efflux of taurine are involved in the mechanism responsible for the increase in taurine levels induced by hyperosmolarity in astrocytes.