REGULATION OF L-ARGININE SYNTHESIS FROM L-CITRULLINE BY L-GLUTAMINE IN ENDOTHELIAL-CELLS

L-Arginine synthesis from L-Citrulline was studied in cultured bovine venular, bovine aortic, human microvascular, and cloned human microvascular endothelial cells (EC). L-Citrulline was actively recycled into L-arginine in all four EC lines, with similar rates between venular and aortic EC. L-Arginine synthesis from L-Citrulline was very sensitive to extracellular L-citrulline concentrations in the range normally found in plasma (50-100 muM). L-Glutamine (0.5 mM) decreased L-arginine synthesis from L-Citrulline, whereas 0.5 MM L-arginine, L-alanine, L-glutamate, or L-lysine had no effect. In contrast to the findings in intact cells, 1 MM L-glutamine had no effect on L-arginine synthesis from L-Citrulline in EC lysates. Similarly, L-glutamine (I mM) had no effect on the conversion of argininosuccinate to arginine in EC lysates. L-Glutamine (0.5 and 1 mM), but not 0.5 MM L-arginine, L-alanine, L-glutamate, or L-lysine, inhibited L-citrulline transport by EC. The inhibition of L-citrulline transport by L-glutamine was dose dependent and competitive in nature. These results suggest that L-glutamine decreased L-arginine synthesis from extracellular L-citrulline by interfering with its transport. In as much as nitric oxide (NO) and L-citrulline are constantly generated from L-arginine, with L-citrulline being actively converted into L-arginine in venular, microvascular, and aortic EC, our data indicate a functioning intracellular arginine-citrulline cycle in these cells. This cycle may function to efficiently scavenge the carbon and alpha-amino group of L-arginine and to maintain a sufficient cellular concentration Of L-arginine during prolonged synthesis of NO in EC. The regulation Of L-arginine synthesis by L-glutamine may play an important role in NO production in EC under such conditions as infection, sepsis, or inflammation.