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.
Brookes
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Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201.
D000596:Amino Acids / Q000737:chemistry / Q000493:pharmacokinetics*; D000818:Animals; D001253:Astrocytes / Q000378:metabolism*; D001692:Biological Transport; D002478:Cells, Cultured; D005973:Glutamine / Q000378:metabolism*; D006632:Histamine / Q000494:pharmacology*; D007425:Intracellular Membranes / Q000378:metabolism*; D009994:Osmolar Concentration; D012964:Sodium / Q000494:pharmacology
D000596:Amino Acids; D005973:Glutamine; D006632:Histamine; D012964:Sodium