The
amino acid arginine is the sole precursor for
nitric oxide (NO) synthesis. We recently demonstrated that an acute reduction of circulating
arginine does not compromise basal or LPS-inducible NO production in mice. In the present study, we investigated the importance of
citrulline availability in
ornithine transcarbamoylase-deficient spf(ash) (OTCD) mice on NO production, using stable
isotope techniques and C57BL6/J (wild-type) mice controls. Plasma
amino acids and tracer-to-tracee ratios were measured by LC-MS. NO production was measured as the in vivo conversion of l-[guanidino-(15)N(2)]
arginine to l-[
guanidine-(15)N]
citrulline; de novo
arginine production was measured as conversion of l-[ureido-(13)C-5,5-(2)H(2)]
citrulline to l-[guanidino-(13)C-5,5-(2)H(2)]
arginine.
Protein metabolism was measured using l-[ring-(2)H(5)]
phenylalanine and l-[ring-(2)H(2)]
tyrosine.
OTC deficiency caused a reduction of systemic
citrulline concentration and production to 30-50% (P < 0.001), reduced de novo
arginine production (P < 0.05), reduced whole-body NO production to 50% (P < 0.005), and increased net
protein breakdown by
a factor of 2-4 (P < 0.001). NO production was twofold higher in female than in male OTCD mice in agreement with the X-linked location of the OTC gene. In response to LPS treatment (10 mg/kg ip), circulating
arginine increased in all groups (P < 0.001), and NO production was no longer affected by the
OTC deficiency due to increased net
protein breakdown as a source for
arginine. Our study shows that reduced
citrulline availability is related to reduced basal NO production via reduced de novo
arginine production. Under basal conditions this is probably cNOS-mediated NO production. When sufficient
arginine is available after LPS stimulated net
protein breakdown, NO production is unaffected by
OTC deficiency.