TAURINE TRANSPORT IN RAT ASTROCYTES ADAPTED TO HYPEROSMOTIC CONDITIONS

[H-3]Taurine uptake and release was characterized in astrocytes from r at 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. T he uptake rate in 1 mM taurine was not affected by exposure to hyperos motic conditions. The mean +/- S.E.M. apparent binding constant for ca rrier-mediated taurine transport, K(m), was not altered by hyperosmoti c conditions (22.8 +/- 5.1 muM in iso-osmotic media, 21.3 +/- 11.9 muM in hyperosmotic media). However, maximal velocity of uptake, V(max) ( mean +/- S.E.M.), of taurine was significantly lower in hyperosmotical ly treated astrocytes (0.175 +/- 0.035 nmol/mg protein.min) compared w ith the V(max) of iso-osmotically treated astrocytes (0.299 +/- 0.026 nmol/mg protein.min). The diffusional transport rate, K(diff), was not affected by growth in hyperosmotic conditions (0.221 +/- 0.033 mul/mg protein.min in iso-osmotic media, 0.295 +/- 0.043 mul/mg protein.min in hyperosmotic media). Taurine release rate, expressed as a percent o f the total cell content, was not affected by hyperosmotic exposure. H owever, astrocytes grown in hyperosmotic conditions contain nearly 60% more taurine than control cells. Thus, the absolute rate of taurine r elease (mean +/- S.E.M.) was significantly larger (P<0.05) in hyperosm otic cells (0.1592 +/- 0.0082 nmol/mg protein.min) compared with contr ol cells (0.0943 +/- 0.0096 nmol/mg protein.min). Quantitative analysi s 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.