UREA CYCLE INTERMEDIATE KINETICS AND NITRATE EXCRETION AT NORMAL AND THERAPEUTIC INTAKES OF ARGININE IN HUMANS

We investigated the effects of a high dietary supplement of arginine o n plasma arginine, ornithine, and leucine kinetics and on urea product ion and excretion in five healthy young adult men. Subjects received e ither 56 or 561 mg arginine . kg(-1). day(-1) for 6 days via a complet e L-amino acid diet, and on day 7 a tracer protocol (first 3 h fasted; next 5 h fed) was conducted, involving primed constant intragastric i nfusions of L-[N-15(2)-guanidino,5,5-H-2(2)]arginine, L-[5-C-13]ornith ine, L-[5,5,5-H-2(3)]leucine, and [N-15(2)]urea, with a prime of (HCO3 )-C-13. Plasma arginine and ornithine fluxes increased significantly ( P < 0.05) with arginine supplementation, as did the rate of conversion of plasma labeled arginine to ornithine (P < 0.05) and rate of ornith ine oxidation (P < 0.001). However, absolute changes in ornithine kine tics were less than those for arginine or those based on changes expec ted from the change in arginine intake, implying a complex compartment ation in both whole body arginine and ornithine metabolism. The plasma NO3 concentration, daily output of total NO3, and conversion of [N-15 ]arginine to NO3 did not differ between the diets. Urea production and excretion were reduced significantly with arginine supplementation, s uggesting an anabolic effect on the whole body nitrogen economy, possi bly via the raised plasma insulin levels (P = 0.013) during the prandi al phase.