Substrate supply for nitric-oxide synthase in macrophages and endothelial cells: role of cationic amino acid transporters.

The current study was designed to investigate the importance of cationic amino acid transporters (CATs) for the L-arginine supply to nitric oxide (NO) synthases in mouse J774A.1 macrophages and human EA.hy926 endothelial cells. CAT-1 was expressed in both cell types, whereas CAT-2B was only expressed in activated macrophages. Apparent K(M) values for transport of L-arginine in both cell types was consistent with the expression of the system y(+) carriers CAT-1 (and CAT-2B in macrophages). In addition, L-arginine transport was Na(+) independent and sensitive to trans-stimulation. A 2-h preincubation of activated macrophages in 2 mM L-lysine (which is exchanged for L-arginine by the CATs) reduced the intracellular L-arginine concentration from 2 mM to 160 microM. At the same time, nitric-oxide synthase (NOS) II activity was completely abolished. NOS II activity could be restored with extracellular L-arginine. No difference in NO production was seen between macrophages preincubated in L-arginine-containing buffer and incubated either with or without L-arginine during the 2-min NO assay. Incubation of endothelial cells in 2 mM L-lysine for up to 24 h decreased the intracellular L-arginine concentration from 3.5 mM to about 600 microM but did not reduce the NOS III activity. Our results suggest that both activated macrophages and endothelial cells have an L-arginine pool that is not freely exchangeable with the extracellular space. This pool seems to be accessible to NOS III in endothelial cells but not to NOS II in macrophages.