Plasma arginine and citrulline kinetics in adults given adequate and arginine-free diets

The fluxes of arginine and citrulline through plasma and the rate of conversion of labeled citrulline to arginine were estimated in two pilot studies (with a total of six adult subjects) and in a dietary study with five healthy young men. These latter subjects received an L-amino acid-based diet that was arginine-rich or arginine-free each for 6 days prior to conduct, on day 7, of an 8-hr (first 3 hr, fast; final 5 hr, fed) primed continuous intravenous infusion protocol using L-[guanidino-¹³C]arginine, L-[5,5-²H₂]citrulline, and L-[5,5,5-²H₃]leucine, as tracers. A pilot study indicated that citrulline flux was about 20% higher (P < 0.05) when determined with [ureido-¹³C]citrulline compared with [²H₂]citrulline, indicating recycling of the latter tracer. Mean citrulline fluxes were about 8-11 μmol·kg^-1·hr^-1 for the various metabolic/diet groups and did not differ significantly between fast and fed states or arginine-rich and arginine-free periods. Arginine fluxes (mean ± SD) were 60.2 ± 5.4 and 73.3 ± 13.9 μmol·kg^-1·hr^-1 for fast and fed states during the arginine-rich period, respectively, and were significantly lowered (P < 0.05), by 20-40%, during the arginine-free period, especially for the fed state, where this was due largely to reduced entry of dietary arginine into plasma. The conversion of plasma citrulline to arginine approximated 5.5 μmol·kg^-1·hr^-1 for the various groups and also was unaffected by arginine intake. Thus, endogenous arginine synthesis is not markedly responsive to acute alterations in arginine intake in healthy adults. We propose that arginine homeostasis is achieved largely via modulating arginine intake and/or the net rate of arginine degradation.