Effect of Intracellular Glutamine on the Uptake of Large Neutral Amino Acids in Astrocytes: Concentrative Na+‐Independent Transport Exhibits Metastability

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Abstract
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 “Gin‐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 Gin‐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 Gin‐replete cultures declined. Also, uptake of Gln itself was enhanced in Gln‐re‐plete cultures. Thus, countertransport of Gln was insufficient to explain the enhancement of NAA uptake. Enhanced uptake was restored, and could be magnified, by reloading Gin‐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 Gin (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.