Citrulline a More Suitable Substrate than Arginine to Restore NO Production and the Microcirculation during Endotoxemia
"To 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.""An imbalance in Nitric Oxide (NO) metabolism at the cellular level, especially the endothelium, is thought to play a crucial role in the development of endotoxemic-related microcirculatory disturbances, such as in sepsis [15]–[18]. NO is produced by the conversion of arginine into citrulline by one of the three isoforms of nitric-oxide synthase (NOS) and is thought to depend largely on extracellular arginine availability, which is decreased during endotoxemia and sepsis [15], [17], [19]. Several mechanisms were proposed to explain arginine deficiency in diseases with an increased arginine catabolism such as endotoxemia and sepsis. Especially, enhanced activity of Arginase-1 [20], [21] and iNOS (NOS2), but also diminished de novo arginine synthesis from citrulline and a diminished de novo citrulline synthesis are relevant mechanism which contribute to arginine deficiency [17], [19], [22]. iNOS can produce excessive amounts of NO during endotoxemia, which contributes to the decreased arterial pressure and abnormal distribution of the blood flow in the microcirculation [18], [23], [24]. The resulting low plasma arginine levels may lead to decreased intracellular substrate availability in the microvasculature for eNOS (NOS3) [17]. This decreased substrate availability in combination with endotoxemia-induced eNOS downregulation results in decreased eNOS-mediated NO production, which is considered to result in the microcirculatory disturbances in sepsis [25]. Interestingly, L-Arginine supplementation in prolonged endotoxemia and sepsis failed to increase the intracellular NO production despite increased arginine plasma availability [26]–[30], hypothetically because the intracellular arginine does not increase during L-Arginine supplementation [15], [22]."
"These characteristic clinical manifestations of endotoxemia (piloerection/erection of the fur, exudates around the eyes and nostrils, lethargy, hypothermia, diarrhea and diminished locomotor activity) were scored by the animal care taker, blinded for treatment. A 2-points (absent/present) score was used for lethargy, hypothermia and diarrhea, and a 3-points score (absent/moderate/severe) for the evaluation of piloerection/erection of the fur, exudates around the eyes/nostrils and diminished locomotor activity."(Funnily enough my presentation when I was ill)
"The L-Citrulline and L-Arginine dosages were extrapolated from the arginine dosages used in both porcine endotoxemia [34] and a human arginine supplementation study (Luiking, Poeze, Deutz, unpublished data), taking in consideration that the nitrogen requirements in rodents are ten times higher than in humans"
"The iNOS protein concentration changed in an opposite detection, with significantly lower levels in the control (P<0.05) and LPS-Cit group (P<0.01) than in the LPS-Ala and LPS-Arg group. (C) "
"Endotoxemia resulted in a significantly decreased total number of perfused vessels (1 µm≤diameter≤50 µm) in the LPS-Ala group compared to control (n = 8; P<0.01; Figure 2A). L-Arginine administration during endotoxemia did not result in a restoration of the number of perfused vessels (n = 8; P<0.05). In contrast, L-Citrulline supplementation during endotoxemia restored the total number of perfused vessels to nearly the number of vessels seen in the control group (n = 8; P = 1.0). "
"Here we show that L-Arginine supplementation increased tissue NO production as assessed with ESR spectroscopy combined with in vivo NO-trapping, while the jejunal microcirculation did not improve. We consider that a disbalance between iNOS and eNOS-mediated NO release could explain the lack of improvement in microcirculation. Under normal physiological conditions, eNOS activity is central to the microcirculatory homeostasis. However, during endotoxemia and inflammation, iNOS is upregulated whereas eNOS expression is downregulated [25] resulting in a disrupted microvascular perfusion, a redistribution of the microcirculatory flow, hypoxia, and eventually organ failure [2], [25]. In line, the present study demonstrated a disturbed microcirculation and low degree in tissue phosphorylation of eNOS-protein levels in the LPS-Ala group, which were both not restored by supplementation of L-Arginine. Moreover, L-Arginine supplementation did not affect iNOS protein levels compared to the LPS-Ala group. Interestingly, L-Citrulline supplementation resulted in a similar tissue NO increase, while both arginine and citrulline tissue levels were increased. We consider that the reduced iNOS protein levels combined with the enhanced levels of active eNOS and the enhanced arginine concentrations in tissue following L-Citrulline supplementation are responsible for the improved microcirculation. These data strongly suggest that the citrulline availability appears to be the key factor rather than arginine to enhance the eNOS-derived NO production, to reduce iNOS protein levels and thereby improve organ perfusion during endotoxemia....In line with these data, Shen et al. [27] reported that citrulline is the major arginine pool for eNOS-mediated NO production, while extracellular arginine forms the only arginine pool for iNOS-induced NO production [27]...."
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Please forgive my obsession with endotoxin, iNOS and sepsis, throwing up blood for several years changes your outlook on quite a few things.
