Saskia J. H. Brinkmann

Intravenous glutamine supplementation enhances renal de novo arginine synthesis in humans: a stable isotope study

BACKGROUND Arginine plays a role in many different pathways in multiple cell types. Consequently, a shortage of arginine, caused by pathologic conditions such as cancer or injury, has the potential to disturb many cellular and organ functions. Glutamine is the ultimate source for de novo synthesis of arginine in humans via the intestinal-renal axis. Therefore, we hypothesized that parenteral glutamine supplementation may stimulate the interorgan pathway of arginine production. OBJECTIVES The objectives were to quantify arginine production from its precursor glutamine and to establish the contribution of the kidneys to de novo synthesis of arginine in patients receiving intravenous supplementation of glutamine dipeptide during major abdominal surgery. DESIGN Whole-body and renal metabolism of glutamine, citrulline, and arginine was assessed by stable isotope techniques in 7 patients receiving a perioperative supplement of intravenous alanyl-glutamine (0.5 g · kg(-1) · d(-1)). RESULTS Plasma glutamine, citrulline, and arginine concentrations increased significantly in patients receiving intravenous glutamine dipeptide. At whole-body level, 91% of total citrulline turnover was derived from glutamine, whereas 49% of whole-body citrulline turnover was used for de novo synthesis of arginine. The kidneys were responsible for 75% of whole-body arginine production from citrulline. CONCLUSIONS Glutamine and citrulline are important sources for de novo arginine synthesis. The kidneys are the main production site for endogenous arginine. After comparison of these results with previous similar studies, our data suggest that an intravenous glutamine supplement doubles renal arginine production from citrulline. This trial was registered at www.trialregister.nl as NTR2914.

Asymmetric dimethylarginine and critical illness.

Purpose of reviewAsymmetric dimethylarginine (ADMA) is an analog of arginine and functions as an endogenous inhibitor of the nitric oxide synthase, which forms nitric oxide. Nitric oxide is crucial for perfusion of vital organs and is an important signaling agent in the development of critical illness. The role of ADMA in the pathophysiological mechanisms underlying critical illness is widely studied in the last decades, and recently it has become clear that ADMA should not be overlooked by clinicians working at the ICU. The aim of this review is to describe new insights into the role of ADMA in critical illness and its clinical relevance. Recent findingsHigh levels of ADMA are found in critically ill patients, because of higher levels of protein methylation, increased rate of protein turnover, decreased activity of dimethylamine dimethylaminohydrolase, and impaired renal and hepatic clearance capacity. These high levels are an independent risk factor for cardiac dysfunction, organ failure, and ICU mortality. The arginine : ADMA ratio in particular is of clinical importance and the restoration of this ratio is expedient to restore several functions that are disturbed during critical illness. SummaryElevated ADMA levels occur in critically ill patients, which is detrimental for morbidity and mortality. The arginine : ADMA ratio should be restored to maintain nitric oxide production and therewith improve the clinical outcome of the patient.

Novel nutritional substrates in surgery.

Pharmaco-nutrients have beneficial effects on protective and immunological mechanisms in patients undergoing surgery, which are important for recovery after injury and in combating infectious agents. The aim of this review article was to outline the potential of the administration of nutritional substrates to surgical patients and the underlying mechanisms that make them particularly important in peri-operative care. Surgery causes a stress response, which has catabolic effects on the bodys substrate stores. The amino acid glutamine is a stimulating agent for immune cells. It activates protective mechanisms through its role as a precursor for antioxidants and it improves the barrier function of the gut. Arginine also enhances the function of the immune system, since it is the substrate for T-lymphocytes. Furthermore, n-3 PUFA stabilise surgery-induced hyper-inflammation. Taurine is another substrate that may counteract the negative effects of surgical injury on acid-base balance and osmotic balance. These pharmaco-nutrients rapidly become deficient under the influence of surgical stress. Supplementation of these nutrients in surgical patients may restore their protective and immune-enhancing actions and improve clinical outcome. Moreover, pre-operative fasting is still common practice in the Western world, although fasting has a negative effect on the patients condition and the recovery after surgery. This may be counteracted by a simple intervention such as administering a carbohydrate-rich supplement just before surgery. In conclusion, there are various nutritional substrates that may be of great value in improving the condition of the surgical patient, which may be beneficial for post-operative recovery.

Perioperative glutamine supplementation restores disturbed renal arginine synthesis after open aortic surgery: a randomized controlled clinical trial

Postoperative renal failure is a common complication after open repair of an abdominal aortic aneurysm. The amino acid arginine is formed in the kidneys from its precursor citrulline, and citrulline is formed from glutamine in the intestines. Arginine enhances the function of the immune and cardiovascular systems, which is important for recovery after surgery. We hypothesized that renal arginine production is diminished after ischemia-reperfusion injury caused by clamping of the aorta during open abdominal aortic surgery and that parenteral glutamine supplementation might compensate for this impaired arginine synthesis. This open-label clinical trial randomized patients who underwent clamping of the aorta during open abdominal aortic surgery to receive a perioperative supplement of intravenous alanyl-glutamine (0.5 g·kg(-1)·day(-1); group A, n = 5) or no supplement (group B, n = 5). One day after surgery, stable isotopes and tracer methods were used to analyze the metabolism and conversion of glutamine, citrulline, and arginine. Whole body plasma flux of glutamine, citrulline, and arginine was significantly higher in group A than in group B (glutamine: 391 ± 34 vs. 258 ± 19 μmol·kg(-1)·h(-1), citrulline: 5.7 ± 0.4 vs. 2.8 ± 0.4 μmol·kg(-1)·h(-1), and arginine: 50 ± 4 vs. 26 ± 2 μmol·kg(-1)·h(-1), P < 0.01), as was the synthesis of citrulline from glutamine (4.8 ± 0.7 vs. 1.6 ± 0.3 μmol·kg(-1)·h(-1)), citrulline from arginine (2.3 ± 0.3 vs. 0.96 ± 0.1 μmol·kg(-1)·h(-1)), and arginine from glutamine (7.7 ± 0.4 vs. 2.8 ± 0.2 μmol·kg(-1)·h(-1)), respectively (P < 0.001 for all). In conclusion, the production of citrulline and arginine is severely reduced after clamping during aortic surgery. This study shows that an intravenous supplement of glutamine increases the production of citrulline and arginine and compensates for the inhibitory effect of ischemia-reperfusion injury.

The Arginine/ADMA Ratio Is Related to the Prevention of Atherosclerotic Plaques in Hypercholesterolemic Rabbits When Giving a Combined Therapy with Atorvastatine and Arginine

Supplementation with arginine in combination with atorvastatin is more efficient in reducing the size of an atherosclerotic plaque than treatment with a statin or arginine alone in homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits. We evaluated the mechanism behind this feature by exploring the role of the arginine/asymmetric dimethylarginine (ADMA) ratio, which is the substrate and inhibitor of nitric oxide synthase (NOS) and thereby nitric oxide (NO), respectively. Methods: Rabbits were fed either an arginine diet (group A, n = 9), standard rabbit chow plus atorvastatin (group S, n = 8), standard rabbit chow plus an arginine diet with atorvastatin (group SA, n = 8) or standard rabbit chow (group C, n = 9) as control. Blood was sampled and the aorta was harvested for topographic and histological analysis. Plasma levels of arginine, ADMA, cholesterol and nitric oxide were determined and the arginine/ADMA ratio was calculated. Results: The decrease in ADMA levels over time was significantly correlated to fewer aortic lesions in the distal aorta and total aorta. The arginine/ADMA ratio was correlated to cholesterol levels and decrease in cholesterol levels over time in the SA group. A lower arginine/ADMA ratio was significantly correlated to lower NO levels in the S and C group. Discussion: A balance between arginine and ADMA is an important indicator in the prevention of the development of atherosclerotic plaques.

Preoperative oral nutritional interventions in surgery, including arginine- and glutamine-enhanced supplements

The patients’ condition prior to surgery is of major importance for clinical outcome. It is believed nowadays that artificial nutrition in the form of a preoperative drink may improve postoperative outcome. Until now, a clear overview concerning the effects of preoperative supplementation on patients’ well-being has been lacking. The aim of this review is to summarize the results of randomized clinical trials investigating the effects of different preoperative supplements such as carbohydrate (CHO)-rich beverages, oral nutritional supplements (ONSs), immunonutrition and lemonades on clinical, metabolic and immunological parameters. We reviewed the relevant articles published between 1995 and 2012. Preoperative CHO-rich drinks appear to be safe, do not affect gastric emptying time and switch the fasted state to a fed state. Moreover, a significant reduction of postoperative insulin resistance, an improved well-being of the patient and prevention of surgery-induced immunodepression was found. For ONSs, beneficial effects were found according to muscle metabolism and strength, glucose storage and cost-effectiveness. Preoperative supplementation of immunonutrition resulted in improved immune function, restricted inflammatory response after surgery, prevention of organ damage, improved nutritional status with subsequently decreased postoperative complication rates and reduced length of hospital stay. Preoperative oral supplements have beneficial effects on many aspects of patient’s well-being and recovery. Different supplements, preferably in combination with each other, can help to reduce postoperative complications after surgery and length of hospital stay. Nutritional support must not be overlooked when considering optimal surgical care.

Enteral Glutamine Administration in Critically Ill Nonseptic Patients Does Not Trigger Arginine Synthesis

Glutamine supplementation in specific groups of critically ill patients results in favourable clinical outcome. Enhancement of citrulline and arginine synthesis by glutamine could serve as a potential mechanism. However, while receiving optimal enteral nutrition, uptake and enteral metabolism of glutamine in critically ill patients remain unknown. Therefore we investigated the effect of a therapeutically relevant dose of L-glutamine on synthesis of L-citrulline and subsequent L-arginine in this group. Ten versus ten critically ill patients receiving full enteral nutrition, or isocaloric isonitrogenous enteral nutrition including 0.5 g/kg L-alanyl-L-glutamine, were studied using stable isotopes. A cross-over design using intravenous and enteral tracers enabled splanchnic extraction (SE) calculations. Endogenous rate of appearance and SE of glutamine citrulline and arginine was not different (SE controls versus alanyl-glutamine: glutamine 48 and 48%, citrulline 33 versus 45%, and arginine 45 versus 42%). Turnover from glutamine to citrulline and arginine was not higher in glutamine-administered patients. In critically ill nonseptic patients receiving adequate nutrition and a relevant dose of glutamine there was no extra citrulline or arginine synthesis and glutamine SE was not increased. This suggests that for arginine synthesis enhancement there is no need for an additional dose of glutamine when this population is adequately fed. This trial is registered with NTR2285.

Reply to GC Ligthart-Melis et al.

generated can be incorporated into the ureido group of citrulline and subsequently the guanidino group of arginine, thus labeling the third nitrogen group of these compounds. Furthermore, [2-N]glutamine can contribute the other nitrogen in the guanidino group of arginine via aspartate through transamination. These pathways are shown in Figure 1. In addition, the infusion of [2-N]glutamine also generates [5-N] glutamine (8). By using LC–tandem MS (LC-MS/MS) and looking at different fragments we have shown this scrambling of the amino nitrogen of glutamine and the labeling of the 3 nitrogen groups of citrulline in mouse models (8). We recently confirmed using the same analytic approach these findings in postprandial surgical patients who received alanyl-[2-N]glutamine as a stable isotope to investigate the precursor relation between glutamine, citrulline, and arginine (GC Ligthart-Melis GC, unpublished results, 2014). Had Buijs et al. used a similar LCMS/MS analytic approach that allows for the measurement of the positional isomers of glutamine and citrulline (10), they would have observed this same phenomenon. For this reason and because every single assumption on the use of [2-N]glutamine to determine precursor-product relations is violated, the conversion data generated by Buijs et al. cannot be interpreted. We agree with Buijs et al. that “the development of optimal nutrition strategies relies on distinct insights into the effect of a supplemental component on metabolic pathways.” For these reasons, we believe that the correct approach to the interpretation of glutamine supplementation in surgical patients has to be done on the basis of fluxes and that the precursor-product relation between glutamine and citrulline can only be determined utilizing tracers that follow the carbon skeleton of these compounds.

Strict glucose control and artificial regulation of the NO–ADMA–DDAH system in order to prevent endothelial dysfunction

Asymmetric dimethylarginine (ADMA) is a naturally occurring analogue of the conditionally essential amino acid arginine and a metabolic by-product of continual protein turnover processes in the cytoplasm of all human cells. Arginine is the precursor for nitric oxide (NO), an important regulator of immune function and organ circulation. Physiologically, NO is known to regulate the transport of insulin and uptake of glucose by several tissues including the endothelium, liver, pancreas, and skeletal muscle. ADMA functions as an endogenous inhibitor of the enzyme nitric oxide synthase (NOS) which forms NO and can therefore impair its bioavailability (see Fig. 1). The elimination of ADMA occurs via both urinary excretion and degradation by the enzyme dimethylarginine dimethylaminohydrolase (DDAH) (Brinkmann et al. 2014). The discovery of ADMA and the observation of its reductive effects on NO synthesis in vitro and in vivo led to a large body of research attempting to discover its role in human metabolism. In insulin resistant (IR) subjects, the body fails to properly respond to endogenous insulin and as a result glucose accumulates extracellularly instead of being transported into the cell. Low levels of intracellular glucose diminish or even eliminate enzyme systems such as the NOS–ADMA–DDAH pathway (Lin et al. 2002; Lai & Ghebremariam, 2016). As consequence, IR results in a disturbed NO metabolism and subsequent endothelial dysfunction in the vascular wall, manifested by impaired endothelium-dependent vasodilatation. In addition to alteration of the NOS–ADMA–DDAH pathway, some evidence points to a disruption of the