Role of L ‐citrulline transport in nitric oxide synthesis in rat aortic smooth muscle cells activated with LPS and interferon‐ γ

L ‐citrulline, a coproduct of nitric oxide synthase (NOS)‐catalysed metabolism of L ‐arginine to nitric oxide (NO), is an important intermediate of the urea cycle and a precursor for L ‐arginine biosynthesis in vascular cells. In the present study, we have examined the characteristics of L ‐citrulline transport, regulation by lipopolysaccharide (LPS) and interferon‐ γ (IFN‐ γ ) and the ability of L ‐citrulline to sustain NO synthesis in rat cultured aortic smooth muscle cells.L ‐citrulline transport was saturable with an apparent K m =1.6±0.2 m M and V max =5.9±0.6 pmol μ g −1 protein min −1 . Transport was pH‐insensitive, partially Na + ‐dependent and markedly inhibited by substrates selective for amino‐acid transport systems L and N but not by L ‐arginine or substrates for systems A, ASC, x c ‐ or X AG . Moreover, transport was not altered in cells treated with LPS (100 μ g ml −1 ) and IFN‐ γ (50 U ml −1 ) for 0–24 h. Unlike L ‐arginine, L ‐citrulline could not sustain maximal NO production in cells expressing iNOS. Our findings provide the first evidence in vascular smooth muscle cells that L ‐citrulline transport is mediated via a low‐affinity carrier with characteristics resembling systems L and N. Moreover, in L ‐arginine‐deprived rat aortic smooth muscle cells, L ‐citrulline cannot sustain maximal NO release via iNOS.British Journal of Pharmacology (2003) 140 , 179–185. doi: 10.1038/sj.bjp.0705407