Transport of L-citrulline in neural cell cultures

Uptake of L-[C-14]citrulline was studied in cell culture models of the main neural cell populations, in astroglia-rich primary cultures derived from n eonatal rat brain, in rat glioma cells C6-BU-1, in cells of the murine micr oglial clone N11 and in the glioma x neuroblastoma hybrid cell line 108CC15 with neuronal properties. For comparison, cells of the peripheral macropha ge cell line RAW 264.7 were also investigated. A saturable component of upt ake was found in all cases with KM values between 0.4 and 3.4 mM and V-max values between 15 and 35 nmol min(-1). (mg protein)(-1). A nonsaturable com ponent dominated uptake at high concentrations of extracellular citrulline, Rates of uptake of L-citrulline were not affected when Na+ or Cl- were omi tted from the incubation medium or in the presence of depolarizing concentr ations of K+. Saturable uptake of citrulline was strongly inhibited by an e xcess of histidine or beta -2-aminobicyclo(2.2.1)-heptane-2-carboxylic acid ; excess amounts of arginine, creatine, glutamate, cysteic acid or N-methyl -craminoisobutyric acid did not reduce citrulline uptake. Preincubation of the cells with bacterial lipopolysaccharide and interferon gamma did not st imulate transport of citrulline. The results suggest that at physiological concentrations citrulline is taken up by neural cells with the help of tran sport system L for large neutral amino acids. Therefore, in the brain, effe ctive utilization of extracellular citrulline as part of an intercellular t rafficking of intermediates of an NO/citrulline cycle depends on the concen trations of all neutral amino acids present. Copyright (C) 2000 S. Karger A G, Basel.