Glutamine Transport in Mouse Cerebral Astrocytes

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Abstract
We measured initial influx and exchange of [14C]glutamine in primary astrocyte cultures in the presence and absence of Na+. Kinetic analysis of transport in Na+ -free solution indicated two saturable Na+ -independent components, one of which was identifiable functionally as system L1 transport. In the presence of Na+, multiple hyperbolic components were not resolvable from the kinetic data. Nevertheless, other evidence supported participation by at least three Na+ -dependent neutral amino acid transporters (systems A, ASC, and N). System A transport of glutamine was usually absent or minimal, based on lack of inhibition by alpha-(methylamino) isobutyric acid. However, vigorous system A-mediated transport emerged after derepression by substrate deprivation. Participation by system ASC was indicated by trans-acceleration of Na+ -dependent uptake, preferential inhibition of an Li+ -intolerant component of uptake by cysteine, and inhibition by cysteine of a component resistant to inhibition by histidine and alpha-(methylamino) isobutyric acid. Because nonsaturable transport of glutamine appeared negligible, and system L transport of glutamine was suppressed in the presence of Na+, low-affinity system ASC transport may be the major route of export of glutamine from astrocytes. At 700 microM glutamine, the primary uptake route was system N transport, identified on the basis of selective inhibition by histidine and asparagine, pH sensitivity, and tolerance of Li+ in place of Na+.

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