EFFECT OF INTRACELLULAR GLUTAMINE ON THE UPTAKE OF LARGE NEUTRAL AMINO-ACIDS IN ASTROCYTES - CONCENTRATIVE NA+-INDEPENDENT TRANSPORT EXHIBITS METASTABILITY

To examine whether the concentration gradient of glutamine (Gln) drives concentrative Na+-independent uptake of neutral amino acids (NAA) in mouse cerebral astrocytes, uptake was compared in "Gln-depleted" and "Gln-replete" cultures. Uptake (30 min in Na+-free buffer) of histidine, kynurenine, leucine, tyrosine, and a model substrate for System L transport was 70-150% greater in Gln-replete cultures. Phenylalanine uptake was not affected. All of these NAA trans-stimulated the export of Gln from astrocytes. However, the increase in NAA uptake was sustained even though the Gln content of Gln-replete cultures declined. Also, uptake of Gln itself was enhanced in Gln-replete cultures. Thus, countertransport of Gln was insufficient to explain the enhancement of NAA uptake. Enhanced uptake was restored, and could be magnified, by reloading Gln-depleted cultures either with Gln or with histidine. It is suggested that substrate-induced asymmetry and molecular hysteresis in the Na+-independent carrier could account for the sustained enhancement of NAA uptake. Only histidine and kynurenine were concentrated comparably to Gln (15- to 29-fold at 1 mM in Na+-free buffer). The other NAA were four to six times less concentrated. At least two Na+-dependent transport systems also supported the concentration gradient of Gln in regular buffer.