Role of l-citrulline transport in nitric oxide synthesis in rat aortic smooth muscle cells activated with LPS and interferon - gamma

1 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.

2 In the present study, we have examined the characteristics Of L-Citrulline transport, regulation by lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) and the ability Of L-citrulline to sustain NO synthesis in rat cultured aortic smooth muscle cells.

3 L-citrulline transport was saturable with an apparent K-m = 1.6 +/- 0.2 mM and V-max = 5.9 +/- 0.6 pmol mug(-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 mug ml(-1)) and IFN-gamma (50 U ml(-1)) for 0-24 h.

4 Unlike L-arginine, L-citrulline could not sustain maximal NO production in cells expressing iNOS.

5 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.