Whole body nitric oxide synthesis in healthy men determined from [15N] arginine-to-[15N]citrulline labeling.

Abstract

The rates of whole body nitric oxide (NO) synthesis, plasma arginine flux, and de novo arginine synthesis and their relationships to urea production, were examined in a total of seven healthy adults receiving an L-amino acid diet for 6 days. NO synthesis was estimated by the rate of conversion of the ['5N] indicating that -11% of the plasma arginine flux originates via conversion of plasma citrulline to arginine. Thus, the fraction of the plasma arginine flux associated with NO and also urea synthesis in healthy humans is small, although the plasma arginine compartment serves as a significant precursor pool (54%) for whole body NO formation. This tracer model should be useful for exploring these metabolic relationships in vivo, under specific pathophysiologic states where the L-arginine-NO pathway might be altered.Arginine serves various important metabolic functions, including roles in protein synthesis, nitrogen transport and elimination (1, 2), and as the precursor of nitric oxide (NO), which is a current area of considerable research interest (3-5). Oxidation of arginine by a homodimeric group of nitric oxide synthases yields citrulline and nitric oxide in stoichiometric amounts, with each containing a nitrogen atom derived from the guanidino moiety of arginine. NO undergoes oxidative degradation to the stable end products nitrite (NO-) and nitrate (NO-). In the in vivo system, NO-is readily oxidized to NO{ via hemoglobin, and so NO is eventually detected in plasma or urine as NOK (5) and its measurement has been used to estimate the rate of whole body NO synthesis.It has been suggested that therapeutic modulation of nitric oxide production may be achieved by supplying the precursor arginine (6-9) or by inhibiting NO production with L-arginine analogs (10-12). However, before profound changes in exogenous arginine intake levels or a pharmacologic inhibition of NO formation can be safely recommended, it is desirable to gain a better understanding of the regulation of whole body arginine metabolism in human subjects and its quantitative interrelationships with the L-arginine-NO pathway.In our previous studies, using stable isotopic tracer techniques, we have investigated whole body arginine metabolism and the use of arginine for NO synthesis, by measurement of the transfer of the guanidino nitrogen of plasma arginine to urinary NO-. These studies have been carried out in healthy humans receiving variable levels of arginine intake (13-15), and in infants with pulmonary hypertension (16). We have now extended these earlier investigations of whole body arginine homeostasis and the metabolism of urea cycle intermediates by examining the kinetics of arginine metabolism throughout a continuous 24-35-hr period. Using this design, it has been possible, for the first time to our knowledge, to determine the in vivo rate of conversion of [15N]guanidino arginine to [15N]ureido citrulline and NO, presumably due to the NO synthase reaction. We compared this estimate with the rate of transfer of the guanidino nitroge...

Arginine metabolism: nitric oxide and beyond

“…Endogenous synthesis of arginine plays an important role in regulating arginine homoeostasis in neonatal and growing pigs [42,61], but does not play a major role in regulating arginine homoeostasis in healthy adult humans [54]. Virtually all flux of plasma citrulline is associated with endogenous synthesis of arginine in adult humans [62] and rats [20], but apparently not in adult dogs [60]. Endogenous synthesis of arginine from citrulline is not responsive to reduced dietary intake of arginine in adult rats [59] or humans [54].…”

“…In healthy adult humans, for example, rates of urea production (239 µmol\h per kg in the fed state and 184 µmol\h per kg in the fasted state) are vastly greater than rates of NO synthesis (0.91 µmol\h per kg in the fed state and 1.00 µmol\h per kg in the fasted state) [62], and also much higher than rates of creatine synthesis (7.9 µmol\h per kg) [123]. Net arginine synthesis by the liver is only possible if the urea cycle is replenished by necessary intermediates, such as ornithine.…”

“…However, our understanding of this subject is being expanded by recent tracer studies using stable isotopes. Only 5 % of urea production is derived from plasma arginine [62], reflecting very low uptake of arginine by the liver and the strict segregation of hepatic and plasma arginine pools. Relative rates of NO synthesis from plasma arginine are low.…”

“…Relative rates of NO synthesis from plasma arginine are low. For example, in infants [52] and adult humans [62], NO synthesis represent only 0.48 % and 1.2 % respectively of the flux of plasma arginine. This accounts for about half of total NO production, because plasma arginine provides only 54 % of the arginine used in NO synthesis [62] ; the remainder is presumably derived from endogenous sources, such as protein degradation and endogenous arginine synthesis at sites of NO synthesis.…”

Adaptation to a long term (4weeks) arginine- and precursor (glutamate, proline and aspartate)-free diet

“…The procedures followed have been described previously. 8,10 Each infusion study period lasted a total of 24 h. During the first 12 h (0-720 min) the subjects received their daily protein and energy intake, provided as small, isonitrogenous, isocaloric meals every hour. For the following 12 h (720-1440 min) the subjects remained fasting.…”