Transport of L-Citrulline in Neural Cell Cultures

Abstract

Uptake of L-[¹⁴C]citrulline was studied in cell culture models of the main neural cell populations, in astroglia-rich primary cultures derived from neonatal rat brain, in rat glioma cells C6-BU-1, in cells of the murine microglial clone N11 and in the glioma × neuroblastoma hybrid cell line 108CC15 with neuronal properties. For comparison, cells of the peripheral macrophage cell line RAW 264.7 were also investigated. A saturable component of uptake was found in all cases with K_M values between 0.4 and 3.4 mM and V_max values between 15 and 35 nmol· min^–1·(mg protein)^–1. A nonsaturable component dominated uptake at high concentrations of extracellular citrulline. Rates of uptake of L-citrulline were not affected when Na⁺ or Cl^– were omitted from the incubation medium or in the presence of depolarizing concentrations of K⁺. Saturable uptake of citrulline was strongly inhibited by an excess of histidine or β-2-aminobicyclo-(2.2.1)-heptane-2-carboxylic acid; excess amounts of arginine, creatine, glutamate, cysteic acid or N-methyl-α-aminoisobutyric acid did not reduce citrulline uptake. Preincubation of the cells with bacterial lipopolysaccharide and interferon γ did not stimulate transport of citrulline. The results suggest that at physiological concentrations citrulline is taken up by neural cells with the help of transport system L for large neutral amino acids. Therefore, in the brain, effective utilization of extracellular citrulline as part of an intercellular trafficking of intermediates of an NO/citrulline cycle depends on the concentrations of all neutral amino acids present.