Uptake of leucine, lysine, aspartic acid, and glycine into isolated neurons and astrocytes.

The uptake of tritium-labeled L-leucine, L-lysine, L-aspartic acid, and glycine by neurons and astrocytes isolated from the cerebral cortex of 3-week-old rats was followed for varying periods up to 40 min at amino acid concentrations from 1 to 2000 micromol/liter in medium. The effects of a low-sodium (15.5 mmol/liter) medium on the uptake were also studied. The influx of the amino acids was faster into astrocytes than into neurons. Leucine penetrated into the cells faster than the other amino acids. Amino acids transport was mainly saturable at the lowest amino acid concentrations studied, whereas nonsaturable penetration into the cells dominated in the millimolar concentration range. The saturable transport comprised only one transport system with relatively small transport constants, resembling in nature the so-called high-affinity transport. The maximal velocities of transport were about two times higher in astrocytes than in neurons. In neurons the partial substitution of sodium by choline in medium had the most effect in reducing the influx of glycine and aspartic acid. In astrocytes the effects were generally less pronounced. The results suggest that extracellular amino acids generally penetrate more readily into astrocytes than into neurons. Both cell types transport essential amino acids more effectively than other amino acids.