Figure 1. The time course and dependence on Na+of glutamate uptake by astrocytes. (Upper) Total uptake of L-[(3) H]glutamic acid (100 [micro sign]M, 37 [degree sign]C) by astrocytes incubated in medium that was Na+replete, Na+free (N-methyl-D-glucamine was substituted as the principal cation), or contained a saturating concentration of unlabeled glutamate (1 mM). (Lower) Na+-dependentuptake of glutamate, which was linear for at least 1 min because it can be fitted by linear regression with r = 0.9990. Plotted are the mean +/- SEM values for three independent experiments performed in triplicate.