INTERACTIONS BETWEEN L-ARGININE AND L-GLUTAMINE CHANGE ENDOTHELIAL NO PRODUCTION - AN EFFECT INDEPENDENT OF NO SYNTHASE SUBSTRATE AVAILABILITY

The effect of extracellular L-arginine and L-glutamine on nitric oxide (NO) release was studied in cultured bovine aortic endothelial cells and in rabbit aortic rings, Increasing L-arginine (0.01 to 10 mM) did not alter NO release from cultured endothelial cells or modify endothelium-dependent relaxation to acetylcholine in isolated vessels, L-Glutamine (0.6 and 2 mM) inhibited NO release from cultured cells (in response to bradykinin) and from aortic rings (in response to acetylcholine or ADP), L-Arginine (0.1-10 mM) dose-dependently reversed the L-glutamine inhibition of receptor-stimulated NO release in both models, In contrast to its inhibitory response to receptor-mediated stimuli, glutamine alone slightly potentiated NO release in both models when the calcium ionophore, A23187, was added, Furthermore, cultured cells incubated with L-arginine (0.01-10 mM), in the presence or absence of glutamine, released similar amounts of NO in response to A23187, L-Glutamine did not affect intracellular L-arginine levels, Neither D-glutamine nor D-arginine affected NO release or endothelium-dependent vascular relaxation, L-Glutamine had no effect on the activity of endothelial NOS assessed by L-arginine to L-citrulline conversion, These findings show that in the absence of L-glutamine, manipulating intracellular L-arginine levels over a wide range does not affect NO release, L-Glutamine in concentrations circulating in vivo may tonically inhibit receptor-mediated NO release by interfering with signal transduction. One mechanism by which L-arginine may enhance NO release is via reversal of the inhibitory effect of L-glutamine, but apparently independently of enhancing NO synthase substrate.