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

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.