Citrulline modulates muscle protein metabolism in old malnourished rats.

Protein energy malnutrition is common in the elderly, especially in hospitalized patients. The development of strategies designed to correct such malnutrition is essential. Our working hypothesis was that poor response to nutrition with advancing age might be related to splanchnic sequestration of amino acids, which implies that fewer amino acids reach the systemic circulation. Administration of citrulline, which is not taken up by the liver, can offer a means of increasing whole body nitrogen availability and, hence, improve nutritional status. Thirty old (19 mo) rats were submitted to dietary restriction (50% of food intake) for 12 wk. They were randomized into three groups: 10 rats (R group) were killed and 20 others refed (90% of food intake) for 1 wk with a standard diet (NEAA group) or a citrulline-supplemented diet (Cit group). Before being killed, the rats were injected with [(13)C]valine, and the absolute protein synthesis rate (ASR) was measured in the tibialis using the flooding-dose method. When the rats were killed, the tibialis was removed for protein content analysis. Blood was sampled for amino acid and insulin analysis. The standard diet did not have any effect on protein synthesis or on the protein content in the muscle. Citrulline supplementation led to higher protein synthesis and protein content in muscle (117 +/- 9, 120 +/- 14, and 163 +/- 4 mg/organ for protein content in R, NEAA, and Cit groups, P < 0.05). The ASR were 0.30 +/- 0.04, 0.31 +/- 0.04, and 0.56 +/- 0.10 mg/h in the three groups, respectively (R and NEAA vs. Cit, P < 0.05). Insulinemia was significantly higher in the Cit group. For the first time, a realistic therapeutic approach is proposed to improve muscle protein content in muscle in frail state related to malnutrition in aging.

Almost all about citrulline in mammals.

Citrulline (Cit, C6H13N3O3), which is a ubiquitous amino acid in mammals, is strongly related to arginine. Citrulline metabolism in mammals is divided into two fields: free citrulline and citrullinated proteins. Free citrulline metabolism involves three key enzymes: NO synthase (NOS) and ornithine carbamoyltransferase (OCT) which produce citrulline, and argininosuccinate synthetase (ASS) that converts it into argininosuccinate. The tissue distribution of these enzymes distinguishes three "orthogonal" metabolic pathways for citrulline. Firstly, in the liver, citrulline is locally synthesized by OCT and metabolized by ASS for urea production. Secondly, in most of the tissues producing NO, citrulline is recycled into arginine via ASS to increase arginine availability for NO production. Thirdly, citrulline is synthesized in the gut from glutamine (with OCT), released into the blood and converted back into arginine in the kidneys (by ASS); in this pathway, circulating citrulline is in fact a masked form of arginine to avoid liver captation. Each of these pathways has related pathologies and, even more interestingly, citrulline could potentially be used to monitor or treat some of these pathologies. Citrulline has long been administered in the treatment of inherited urea cycle disorders, and recent studies suggest that citrulline may be used to control the production of NO. Recently, citrulline was demonstrated as a potentially useful marker of short bowel function in a wide range of pathologies. One of the most promising research directions deals with the administration of citrulline as a more efficient alternative to arginine, especially against underlying splanchnic sequestration of amino acids. Protein citrullination results from post-translational modification of arginine; that occurs mainly in keratinization-related proteins and myelins, and insufficiencies in this citrullination occur in some auto-immune diseases such as rheumatoid arthritis, psoriasis or multiple sclerosis.

Effects of an immune-enhancing diet in endotoxemic rats.

OBJECTIVE: This work compared the nutritional efficiency of a recently available enteral formula enriched with arginine, omega-3 fatty acids, and antioxidants and supplied nitrogen as peptides (Crucial, Nestle Clinical Nutrition) with that of a standard polymeric formula (Sondalis HP, Nestle Clinical Nutrition) in endotoxemic rats. METHODS: Male Wistar rats (209 +/- 2 g) underwent catheter gastrostomy and received Sondalis HP until they recovered their preoperative weight. At that time (day 0), an endotoxemic shock was induced by an intraperitoneal injection of lipopolysaccharide (Escherichia coli, 8 mg/kg) and rats then received 290 kcal x kg(-1) x d(-1) and 3.29 g of nitrogen x kg(-1) x d(-1) in the form of Crucial (IED group, n = 7) or Sondalis HP (S group, n = 6) for 3 d. Another group underwent no treatment and was fed ad libitum (AL group). Rats were killed on day 3. Results are presented as mean +/- standard error of the mean (analysis of variance and Newman-Keuls test). RESULTS: The endotoxemic shock induced a weight loss in group S on days 1 and 2 and a weight gain in group IED (-3.5 +/- 1.3 g in group S versus +6.0 +/- 2.2 g in group IED, P < 0.05). In the same way, atrophy of extensor digitorum longus muscle was observed in group S, whereas wasting was limited in group IED (102 +/- 4 mg in group IED versus 90 +/- 3 mg in group S versus 119 +/- 3 mg in group AL, P < 0.05). Muscular atrophy was associated with muscular glutamine depletion and correlated with hyperphenylalaninemia (R = 0.60), with the latter being blunted in group IED (57 +/- 1 microM/L in group AL versus 77 +/- 4 microM/L in group S versus 66 +/- 2 microM/L in group IED, P < 0.05). No difference was observed between the experimental groups of endotoxemic rats with respect to nitrogen balance, urinary excretion of 3-methyl histidine, or total tissue protein content. CONCLUSION: Crucial counteracts injury-mediated weight loss, extensor digitorum longus muscle atrophy, and hyperphenylalaninemia in endotoxemic rats.

Citrulline increases arginine pools and restores nitrogen balance after massive intestinal resection.

OBJECTIVE: Arginine supplementation seems logical in situations where this amino acid becomes essential, for example after massive intestinal resection. Arginine is taken up and metabolised by the liver to a large extent and its supplementation is potentially unsafe. Citrulline is not captured by the liver and passes freely to the kidneys where it is metabolised to arginine, and so is a good candidate to generate arginine and thereby improve nutritional status. METHODS: Twenty four rats were assigned to four groups: citrulline, arginine, control, and sham. The sham group underwent transection and the three other groups resection of 80% of the small intestine. All rats were fed by enteral nutrition and its composition was as follows: supplementation with citrulline in the citrulline group, supplementation with arginine in the arginine group, and standard polymeric enteral nutrition in the control and sham groups. All groups received isonitrogenous nutrition and citrulline and arginine intakes were equimolar in their respective groups. After 10 days, the rats were sacrificed. RESULTS: Arginine concentration was higher (p<0.05) in plasma and muscle in the citrulline group than in the three other groups. Plasma levels of arginine were 110 (12), 79 (7), 167 (22), and 228 (13) mumol/l in the sham, control, arginine, and citrulline groups respectively. Arginine concentrations in the gastrocnemius were: 0.15 (0.02), 0.16 (0.02), 0.40 (0.05), and 0.94 (0.20) mumol/g, respectively. Citrulline preserved nitrogen balance in resected rats but not in arginine supplemented rats (mean J10: 2.27 (0.29), 1.67 (0.15), 1.98 (0.29), and 2.43 (0.41) g/24 hours in the sham, control, arginine, and citrulline groups, respectively). CONCLUSION: Supplementing the diet with citrulline is a very efficient means of increasing arginine levels and improving nitrogen balance after massive intestinal resection. The results of this study form a strong rationale for citrulline supplementation in these patients.