Interaction Between the Glutamine Cycle and the Uptake of Large Neutral Amino Acids in Astrocytes

Authors

  • Neville Brookes

    Corresponding author
    1. Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland, U.S.A.
    Search for more papers by this author

Address correspondence and reprint requests to Dr. N. Brookes at Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine. 655 West Baltimore Street, Baltimore. MD 21201. U.S.A.

Abstract

Abstract: When astrocyte cultures are incubated with glutamate and ammonium, the clearance of these substrates followed by the formation and export of glutamine simulates the action of the “glutamine cycle” that is believed to function in the CNS. In the present study this process was found to increase the uptake of large neutral amino acids (LNAAs), namely, histidine, kynurenine, leucine, phenylalanine, and tryptophan, by two-to threefold in mouse cerebral astrocytes. The enhancement of kynurenine uptake was shown to depend on the formation of glutamine and to saturate at low levels of glutamine. LNAAs transiently lowered the glutamine content of astrocytes that were incubated with glutamate and ammonium, but they did not affect net export of glutamine to the solution at normal physiological pH. However, on adjustment of the pH of the solution to 7.8, which causes a large increase in glutamine content without affecting export, kynurenine now significantly increased net glutamine export. These findings relate to proposed mechanisms of cerebral dysfunction in hyperammonemia.

Ancillary