Citrulline a More Suitable Substrate than Arginine to Restore NO Production and the Microcirculation during Endotoxemia

Abstract

BackgroundImpaired microcirculation during endotoxemia correlates with a disturbed arginine-nitric oxide (NO) metabolism and is associated with deteriorating organ function. Improving the organ perfusion in endotoxemia, as often seen in patients with severe infection or systemic inflammatory response syndrome (SIRS) is, therefore, an important therapeutic target. We hypothesized that supplementation of the arginine precursor citrulline rather than arginine would specifically increase eNOS-induced intracellular NO production and thereby improve the microcirculation during endotoxemia.Methodology/Principal FindingsTo study the effects of L-Citrulline and L-Arginine supplementation on jejunal microcirculation, intracellular arginine availability and NO production in a non-lethal prolonged endotoxemia model in mice. C57/Bl6 mice received an 18 hrs intravenous infusion of endotoxin (LPS, 0.4 µg•g bodyweight−1•h−1), combined with either L-Citrulline (6.25 mg•h-1), L-Arginine (6.25 mg•h−1), or L-Alanine (isonitrogenous control; 12.5 mg•h−1) during the last 6 hrs. The control group received an 18 hrs sterile saline infusion combined with L-Alanine or L-Citrulline during the last 6 hrs. The microcirculation was evaluated at the end of the infusion period using sidestream dark-field imaging of jejunal villi. Plasma and jejunal tissue amino-acid concentrations were measured by HPLC, NO tissue concentrations by electron-spin resonance spectroscopy and NOS protein concentrations using Western blot.Conclusion/SignificanceL-Citrulline supplementation during endotoxemia positively influenced the intestinal microvascular perfusion compared to L-Arginine-supplemented and control endotoxemic mice. L-Citrulline supplementation increased plasma and tissue concentrations of arginine and citrulline, and restored intracellular NO production in the intestine. L-Arginine supplementation did not increase the intracellular arginine availability. Jejunal tissues in the L-Citrulline-supplemented group showed, compared to the endotoxemic and L-Arginine-supplemented endotoxemic group, an increase in degree of phosphorylation of eNOS (Ser 1177) and a decrease in iNOS protein level. In conclusion, L-Citrulline supplementation during endotoxemia and not L-Arginine reduced intestinal microcirculatory dysfunction and increased intracellular NO production, likely via increased intracellular citrulline and arginine availability.

Two weeks of watermelon juice supplementation improves nitric oxide bioavailability but not endurance exercise performance in humans

“…There is evidence to suggest that the plasma [nitrite] sensitively reflects endothelial nitric oxide synthase activity [32] and that L-citrulline supplementation can increase nitric oxide synthase activation [20] and nitric oxide biomarkers [19,21]. [nitrite]) were increased after 4 weeks supplementation with watermelon pomace in Zucker diabetic fatty rats [31], but the current study is the first, to our knowledge, to suggest that watermelon juice supplementation can increase nitric oxide production through nitric oxide synthase in humans.…”

“…It has also been reported that L-citrulline supplementation can increase the activity of nitric oxide synthase [20] and increase nitric oxide biomarkers [19,21], and there is accumulating evidence to suggest that short term supplementation with L-citrulline can improve skeletal muscle oxygenation [22], metabolism [23][24], force production [23] and fatigue resistance [22]. Therefore, short-term supplementation with L-citrulline might represent an effective dietary intervention to increase nitric oxide synthase-derived nitric oxide and to improve skeletal muscle perfusion, metabolism and exercise performance.…”

“…Although traditionally recognised as a product of the nitric oxide synthase-mediated oxidation of L-arginine [3], recent evidence suggests that L-citrulline can be recycled back to L-arginine [17] and is more effective than orally ingested L-arginine at increasing systemic [18,19] and muscle [20] L-arginine.…”

l-Citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans

“…In addition to an increase in NOS substrate provision, there is some evidence to suggest that NO production is enhanced after Cit treatment (46,53,63) and that Cit can restore NO production in conditions where NO production is compromised (16,36). However, there is also a suggestion that Cit ingestion tends to lower NO production, as inferred from plasma [NOx] (26).…”

“…Synthesis of Arg from Cit is prevalent in other tissues (15,23,70), and this process is facilitated since Cit does not compete with ADMA for cell transport (50,69). Importantly, it has been shown that oral Cit supplementation more effectively increases the circulating (32,47,53,62) and tissue (63) [Arg] than an equivalent dose of Arg and that Cit supplementation can increase NOS activation (63) and NO biomarkers (46,53). Therefore, these findings suggest that Cit might serve as an important precursor for NO production.…”