Taurine transport in rat astrocytes adapted to hyperosmotic conditions

[3H]Taurine uptake and release was characterized in astrocytes from rat cerebral cortex grown in normal and hyperosmotic culture conditions to investigate mechanisms of cell volume regulation and adaptation to states of altered osmolality. In high concentrations of taurine (1 mM), uptake was linear in both osmotic conditions for at least 30 min. The uptake rate in 1 mM taurine was not affected by exposure to hyperosmotic conditions. The mean +/- S.E.M. apparent binding constant for carrier-mediated taurine transport, Km, was not altered by hyperosmotic conditions (22.8 +/- 5.1 microM in iso-osmotic media, 21.3 +/- 11.9 microM in hyperosmotic media). However, maximal velocity of uptake, Vmax (mean +/- S.E.M.), of taurine was significantly lower in hyperosmotically treated astrocytes (0.175 +/- 0.035 nmol/mg protein.min) compared with the Vmax of iso-osmotically treated astrocytes (0.299 +/- 0.026 nmol/mg protein.min). The diffusional transport rate, Kdiff, was not affected by growth in hyperosmotic conditions (0.221 +/- 0.033 microliter/mg protein.min in iso-osmotic media, 0.295 +/- 0.043 microliter/mg protein.min in hyperosmotic media). Taurine release rate, expressed as a percent of the total cell content, was not affected by hyperosmotic exposure. However, astrocytes grown in hyperosmotic conditions contain nearly 60% more taurine than control cells. Thus, the absolute rate of taurine release (mean +/- S.E.M.) was significantly larger (P < 0.05) in hyperosmotic cells (0.1592 +/- 0.0082 nmol/mg protein.min) compared with control cells (0.0943 +/- 0.0096 nmol/mg protein.min). Quantitative analysis of these data indicate that maintenance of elevated taurine contents by cultured cerebral astrocytes exposed to hyperosmotic conditions is not due to alterations in rates of transport.