Exogenous arginine in sepsis

Yvette C. Luiking, PhD; Nicolaas E. P. Deutz, MD, PhD

Sepsis is a major complicationof an acute infection, triggeredby a systemic inflammatory re-action. According to its pro-

gressive injury process, the sepsis syn-drome can be classified as mild or severesepsis or septic shock. Severe sepsis co-incides with multiple organ failure, whileseptic shock is characterized by addi-tional cardiovascular failure and the needfor blood pressure supportive therapy (1).Sepsis is a highly frequent condition inthe intensive care unit with a high mor-bidity and mortality rate, the latter vary-ing between 20% and 50% within thefirst month of disease (2, 3), still furtherincreasing thereafter (4). Risk factors for

sepsis are age (70% of patients are �60yrs), gender (63% male vs. 37% female),and comorbidities like malignancies, re-spiratory failure, and diabetes (70% of allpatients) (2–5). Most common sites ofinfection are the lungs (47%) and theabdomen (34%) (5).

Arginine is considered a conditionallyessential amino acid: not essential undernormal healthy conditions but essentialin disease states like sepsis (6–8). This ispartly because arginine is a key aminoacid in several metabolic pathways ofwhich synthesis of nitric oxide is consid-ered of main importance during disease(for recent reviews, see Refs. 9–14). Thisreview focuses on arginine metabolism insepsis and implications for arginine avail-ability and related functions, arginineand arginine-related therapies, and po-tential mechanisms by which exogenousarginine may improve the condition ofthe patient with sepsis/multiple organfailure.

Arginine Deficiency in Sepsis:Rationale

A deficiency of arginine can be consid-ered from changes in arginine metabolismthat may compromise the availability ofarginine with functional consequences.Both are described in detail later. Forextensive information on arginine metab-

olism in normal health, we refer to exist-ing reviews (9, 10, 12–14).

Changes in Arginine Metabolism inSepsis. Sepsis is characterized by a reduc-tion in plasma and tissue arginine levelscompared with healthy individuals ornonseptic critically ill patients (15–18).In addition, plasma amino acid levels arein general lower during sepsis, which ispartly related to starvation due to limitednutritional protein supply as well as toincreased amino acid clearance (17)through gluconeogenesis, oxidation forenergy supply, and protein synthesis inespecially the liver and immune cells(19–21). This negative amino acid bal-ance apparently cannot be compensatedfor by the excessive protein catabolism(protein breakdown is increased by 50%in septic patients (YC Luiking, unpub-lished data), mainly from muscle but alsofrom the gut (21).

In addition, recent evidence suggeststhat arginine metabolism in sepsis is dis-turbed in various aspects, probably re-lated to the severity of the inflammatoryresponse and induced by inflammatorymediators. In sepsis, de novo arginineproduction, that is, the endogenous syn-thesis of arginine from the amino acidcitrulline (22–26), is reduced to one thirdof the normal level (18). Studies in ananimal model of sepsis demonstrated ac-cordingly that impaired de novo arginine

From the Center for Translational Research onAging & Longevity, Donald W. Reynolds Institute onAging, University of Arkansas for Medical Sciences,Little Rock, AR.

Supported, in part, by Novartis Consumer Health,St. Louis Park, MN.

The authors have not disclosed any potential con-flicts of interest.

Address requests for reprint to: N. E. P. Deutz, MD,PhD, Center for Translational Research on Aging &Longevity, Room 3.121, Donald W. Reynolds Instituteon Aging, University of Arkansas for Medical Sciences,4301 W. Markham Street, Slot 806, Little Rock, AR.E-mail: deutzNEP@uams.edu

Copyright © 2007 by the Society of Critical CareMedicine and Lippincott Williams & Wilkins

DOI: 10.1097/01.CCM.0000279191.44730.A2

Sepsis is a severe condition in critically ill patients and isconsidered an arginine deficiency state. The rationale for argininedeficiency in sepsis is mainly based on the reduced argininelevels in sepsis that are associated with the specific changes inarginine metabolism related to endothelial dysfunction, severecatabolism, and worse outcome.

Exogenous arginine supplementation in sepsis shows con-troversial results with only limited data in humans and variableresults in animal models of sepsis. Since in these studies theseverity of sepsis varies but also the route, timing, and dose ofarginine, it is difficult to draw a definitive conclusion for sepsisin general without considering the influence of these factors.

Enhanced nitric oxide production in sepsis is related tosuggested detrimental effects on hemodynamic instability and

enhanced oxidative stress. Potential mechanisms for beneficialeffects of exogenous arginine in sepsis include enhanced(protein) metabolism, improved microcirculation and organfunction, effects on immune function and antibacterial effects,improved gut function, and an antioxidant role of arginine. Werecently performed a study indicating that arginine can be givento septic patients without major effects on hemodynamics, sug-gesting that more studies can be conducted on the effects ofarginine supplementation in septic patients. (Crit Care Med 2007;35[Suppl.]:S557–S563)

KEY WORDS: arginine; sepsis; supplementation; critically ill;nitric oxide; citrulline; nutrition; metabolism; amino acid; pro-tein

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synthesis in the kidney is a major factorresponsible for the rapid decrease in ex-tracellular arginine content after lipo-polysaccharide injection (27). Anothermetabolic change in sepsis is the four-fold increase in arginase activity (YC Lu-iking, unpublished data), that is, thecatabolism of arginine to urea and orni-thine, probably through up-regulation ofarginase I activity in macrophages by cy-tokines (interleukin-4, interleukin-10,and transforming growth factor �) se-creted by T-helper lymphocyte cells (28).Changes in arginine metabolism can sub-sequently affect availability of argininefor nitric oxide (NO) synthase (NOS) andtherefore NO production. NO productionis essential for smooth muscle relaxationand vasodilation via NOS-3 (or eNOS) andas a neurotransmitter via NOS-1 (ornNOS) (29, 30), and NO production isstimulated by inflammatory mediatorsthrough induction of NOS-2 (or iNOS)(10). Besides arginine availability, otherfactors that probably affect NO produc-tion in sepsis are 1) coupling of NOSwith other enzymes involved in argi-nine metabolism (31–33); 2) the effectsof bacterial endotoxins and host cyto-kines on intracellular cationic trans-port systems (34, 35); and 3) increasedpresence of endogenous NOS inhibitorslike asymmetric dimethylarginine, a by-product of protein breakdown (36). Al-though dramatic increases in NO pro-duction have been suggested in sepsisand are ascribed to stimulation ofNOS-2 by cytokines (interleukin-1, in-terleukin-2, tumor necrosis factor, and�-interferon) produced by T-helper 1cells (28), evidence was mainly based onreported increases in plasma nitratelevels and increased NOS gene expres-sion (increased NOS-2, but unchangedor even lowered NOS-1 and NOS-3)(37– 45). Stable isotope studies could,however, not confirm this dramatic in-crease in NO production in septic ani-mal models (46 – 48) and humans (49)(YC Luiking, unpublished data). Thediscrepancy in measures of NO in sepsismay be due partly to an effect of renalfailure on plasma nitrate levels (39),since nitrate is excreted via the urine(50) and its concentration correlateswith the glomerular filtration rate inseptic patients (49). However, time-specific changes in NOS enzyme activ-ity in sepsis may also play a role (32,51–53), as well as diversity of the septicpopulation related to, for example, theinitial presence of trauma (42).

The previously described changes inarginine availability and arginine me-tabolism in sepsis therefore make sepsisan arginine deficiency disease withfunctional consequences. It can be hy-pothesized that this is a time-relatedmechanism represented by an earlyphase with adaptive changes in activityof metabolic pathways to preserveamino acids and a late adaptive phaseaimed at cell survival with enhancedprotein breakdown before the onset ofarginine deficiency when all adaptivemechanisms are exhausted (Fig. 1).

Functional Consequences of ArginineDeficiency. Changes in arginine metabo-lism in sepsis have mainly focused on NOproduction. The presumed excessive NOproduction in sepsis related to enhancedNOS-2 activity is thought to contribute tothe systemic hypotension and vascularhyporeactivity in sepsis (42, 54, 55). How-ever, inhibition of NOS turned out to bedetrimental also in terms of diminishedblood flow (56), and, although resolutionfrom shock was promoted (57), mortalityand especially cardiovascular deaths werehigher in patients treated with a nonse-lective NOS inhibitor in a multicentertrial (58). On the other hand, basal NOproduction (by endothelial NOS orNOS-3) is limited due to endotoxemia(59) and related to diminished vasodila-tion as observed in aortic rings (60) butprobably also in the microcirculation.This latter hypothesis is supported by theobservation that administration of the

NO donor nitroglycerin in septic patientsimproves the microcirculation (61). A re-lation between de novo synthesis of argi-nine in the kidneys and renal perfusionwas observed in an animal model of sepsis(27) and suggests a functional relationbetween de novo arginine synthesis andNOS-3 activity, which was also demon-strated by the colocalization of the enzy-matic pathways involved (33).

Other metabolic changes in sepsiswith functional consequences are the se-vere catabolic state of sepsis. This cata-bolic state is related to impaired proteinsynthesis and increased protein break-down and may result in loss of musclemass and function and impaired recovery(62). Besides diminished substrate supplydue to impaired (microvascular) flow,other factors like cytokines and hor-mones probably affect protein turnover insepsis (62). Loss of skeletal muscle masscan result in fatigue and reduced qualityof life after recovery, but loss of musclemass from the diaphragm may also affectrespiratory muscle during sepsis (63). Onthe other hand, however, muscle fatiguehas also been associated with the forma-tion of peroxynitrite due to access of NO(64). Muscle catabolism is not specific forlack of arginine (62), as muscle proteincomprises other amino acids besides ar-ginine in larger amounts (65). Besides, ithas been demonstrated that for the syn-thesis of acute phase protein, which isgreatly elevated in sepsis, an estimateddouble amount of muscle protein has to

Figure 1. Hypothesis: Time-related changes in arginine availability and arginine metabolism in sepsis(1) or sepsis with optimal nutritional supply (2) with functional consequences. This time-relatedmechanism in sepsis is represented by an early phase with adaptive changes in activity of metabolicpathways to preserve amino acids and a late adaptive phase aimed at cell survival with enhancedprotein breakdown before the onset of arginine deficiency when all adaptive mechanisms are ex-hausted. Optimal nutritional supply may preserve metabolism and function.

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be broken down to supply aromaticamino acids in adequate amounts (66).

Regarding the role of arginine inwound healing (67), which seems to oc-cur via enhanced ornithine and subse-quent collagen formation (68) or via aNO-mediated mechanism (69), it is feasi-ble that arginine deficiency may impairwound healing, if present, in sepsis. Fi-nally, lower plasma arginine levels alsocorrelated with a poorer prognosis andhigher mortality rate from sepsis (15).

In summary, the rationale for argininedeficiency in sepsis is mainly based on thereduced arginine levels in sepsis that re-flect the specific changes in arginine me-tabolism with functional consequencesregarding endothelial dysfunction, severecatabolism, impaired wound healing, andworse outcome. However, we need tomention that direct effects of cytokinesand hormones are also involved in thesepathophysiological processes and thatother amino acids may also be present ininadequate amounts (70).

Exogenous ArginineSupplementation in Sepsis:Where Do We Stand and Is ItSafe?

Arginine has been supplied in variousforms, of which incorporation into theso-called immunonutrition has beenmost widely used and described. Only afew studies supplied exogenous arginineas a monotherapy. Moreover, the doseand route of exogenous arginine supply(i.e., intravenous or enteral) vary. Thissection gives an overview of arginine sup-plementation in patients with sepsis,which is still subject of debate regardingharm or benefit (71–73).

Immunonutrition Containing Argi-nine in Sepsis. Immunonutrition con-taining arginine has been investigated inseveral clinical trials (74–79). In addi-tion, several published meta-analyses, re-views, and opinion papers have focusedon arginine-containing immunonutritionin sepsis (80–87). Despite apparent ben-efits of immunonutrition on infectionrate and length of hospital stay, meta-analyses identified no beneficial effect onmortality rate (84) or even suggested ahigher mortality rate with immunonutri-tion (80). However, benefits of immunonu-trition were considered most marked insurgical patients (84). Especially theseoutcomes caused the greatest contro-versy and discussion, which also resultedin a recommendation by the Canadian

Clinical Practice Guidelines against sup-plemental arginine in critically ill adultsand especially nonsurgical patients withsepsis (82).

Immune-enhancing nutrients have astheir principal components several nutri-ents with putative benefits. Besides argi-nine, these include omega-3 fatty acids andnucleotides, and some contain also glu-tamine. Although all these components areconsidered beneficial, adequate evaluationof individual effects of components andpossible mechanisms of (combined) actionare largely missing. Concerning the latter,counterregulating effects of other compo-nents in immunonutrition on the efficacyof arginine have been considered but notyet identified (88).

Animal Studies in Septic Models. Var-ious animal models of sepsis are avail-able, using different animals as well asdifferent challenges to induce sepsis.Moreover, the route of arginine supply(i.e., intravenous or enteral) also varies.

The effects of arginine supplementationon survival are not homogeneous in animalmodels of sepsis. While some studies re-ported improved survival (89, 90), othersshowed no effect (91) or even reported in-creased mortality (92, 93). In the latterstudy, this coincided with increased plasmaornithine and serum levels of nitrate andnitrite, a lowered mean arterial pressure,and worsened organ injury (93). In septicsheep, arginine supplementation also re-sulted in decreased blood pressure (94). Areduction of the production of inflamma-

tory mediators at the site of infection (91),as well as an increase in albumin, histone,and liver protein synthesis (95), was ob-served with intravenous arginine in septicrats. In a hyperdynamic pig model of sepsis,arginine supplementation increased organNO production (47, 96), with decreasedliver protein turnover and increased mus-cle protein turnover (97). Arginine supple-mentation nonspecifically (i.e., no differ-ence between L-arginine and D-arginine)prevented endothelial dysfunction by re-storing endothelial histologic injury in rab-bit endotoxic shock, while it did not affectacidosis, coagulation, or monocyte tissuefactor expression (98). From this study itwas questionable whether the action of ar-ginine supplementation was via enhancedNO synthesis or otherwise.

Exogenous Arginine Supplementationas a Monotherapy in Human Sepsis. Onlya few studies have investigated the effectsof arginine per se in patients with sepsis.Lorente et al. (99) supplied seven septicshock patients with 200 mg/kg L-arginineas an intravenous bolus, which resultedin immediate but transient hemodynamicchanges consistent with systemic andpulmonary vasodilation. In a dose-response pilot study in eight patientswith septic shock (100), intravenous L-arginine infusion in doses increasingfrom 0.6 to 1.8 �mol/kg·min (each dosefor 2 hrs, equal to 11–33 g daily for a75-kg adult) resulted in plasma argininelevels of four times baseline at the high-est dose. No changes in systemic blood

Figure 2. Dose-response pilot study of intravenous L-arginine supplementation in septic shock patients(n � 8). Each dose was supplied for 2 hrs in a stepwise increasing order. No significant effect onsystemic blood pressure was observed. BP, blood pressure; MAP, mean arterial pressure. Adapted fromLuiking et al (100).

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pressure or use of vasopressive medica-tion were observed with any dose, butstroke volume increased (Fig. 2) (100). Inaddition, a recent randomized controlledtrial with 3-day intravenous continuousarginine supplementation at 1.2 �mol/kg·min also could not demonstrate aneffect on hemodynamic parameters (101).

Factors That Potentially Affect theEffectiveness of Exogenous Arginine.First, differences between enteral and in-travenous supply of exogenous argininemay influence the outcome of treatment.Continuous supply of arginine via the en-teral route will result in a high first-passuptake of arginine in the gut mucosa andthe liver, regarding the 40% splanchnicextraction that is reported in healthy sub-jects using stable isotope techniques(102). Intravenous arginine supplemen-tation, on the other hand, bypasses thelarge first-pass extraction of arginine inthe splanchnic area. In addition, the im-paired absorptive gut function in sepsis(103, 104) may further negatively affectbioavailability of enteral arginine. Thiswas also demonstrated in a peritonitis ratmodel, in which oral arginine supple-mentation did not increase plasma argi-nine levels and survival, whereas intrave-nous arginine improved survival whenadministered after initiation of sepsis(90). In addition, differences in arginine-stimulated NO production between intra-venous (105) and dietary arginine supple-mentation (106) in healthy subjectssuggest that the route of arginine admin-istration is probably also important forthe NO-stimulating effect of arginine.

Second, it is well possible that bolusadministration of arginine differs fromcontinuous administration with regard tobioavailability. After an oral bolus of ar-ginine (10 g), a bioavailability of about20% in healthy subjects has been re-ported (107), which is substantially lowerthan the 60% availability to the periph-eral circulation after continuous intra-gastric arginine supply (102). Intrave-nous bolus arginine supply (30 g in 30mins) that causes a rapid peak in plasmaarginine of 8 mmol/L results in urinaryexcretion of arginine, as the threshold forrenal reabsorption probably is exceeded(107). Intravenous arginine supply as abolus in sepsis, like 200 mg/kg (about15 g) arginine administered in the studyby Lorente et al. (99), caused transient hy-potension, while continuous arginine sup-plementation at 12.5 mg/kg·hr (reachingplasma levels of about 300 �mol/L) did not

affect blood pressure in septic patients(101).

Third, most of the survival benefits ofimmunotherapy occurred in groups withthe lowest Acute Physiology and ChronicHealth Evaluation II scores (10–15), re-flecting moderate severity of illness, withlittle difference in outcome in the moreseverely ill (74, 84). This suggests thatsubgroups of septic patients may benefitfrom arginine supplementation whileothers do not.

In summary, exogenous arginine sup-plementation in sepsis shows controver-sial results with only limited data in hu-mans and various results in animalmodels of sepsis. Since the severity ofsepsis varies, and the route, timing, anddose of arginine differ between studies, itis difficult to draw a definitive conclusionfor the effect of exogenous arginine sup-plementation in sepsis in general withoutconsidering these factors.

Possible Risks and Benefits ofExogenous Arginine in Sepsis

Although mechanistic studies on argi-nine supplementation in septic patientsare scarce, major risks of exogenous ar-ginine are ascribed to the suggested in-crease of NO synthesis. Stimulated NOproduction is related to reduced bloodpressure (105, 108) and is suggested toimpair cardiac contractility, induce liverdamage, and increase vascular permeabil-ity and bacterial translocation from theintestine (109). In addition, oxidativestress (through production of peroxyni-trite, a harmful metabolite formed fromNO and superoxide that nitrates the ty-rosine residues in proteins to nitroty-rosine) (110) and mitochondrial dysfunc-tion (111) are considered further riskfactors of increased NO and, therefore,indirect results of exogenous arginine sup-ply. However, in a recent placebo-con-trolled study in septic patients, we couldnot demonstrate an effect of 3-day argininesupplementation on plasma nitrotyrosinein patients with severe sepsis (101).

Besides the suggested detrimental ef-fects of NO, benefits of exogenous argi-nine can also be considered. First, from ametabolic point of view, exogenous argi-nine could compensate for the increasedarginine need and could diminish theneed for endogenous arginine sourceslike body protein and, thereby, poten-tially reduce catabolism. Exogenous ar-ginine supply indeed reduced proteinbreakdown in a pilot study in septic

patients (YC Luiking et al., unpublisheddata). Moreover, exogenous arginineenhanced protein synthesis and degra-dation across the hindquarter and si-multaneously reduced protein synthesisand degradation in the liver at equalrates in a pig sepsis model (97).

A second mechanism is related to theimportance of NOS-3 in the regulation ofthe microcirculation and endothelialfunction, as demonstrated for argininesupplementation in other vascular dis-eases (112–118). Stimulation of NOS-3-mediated NO production may be benefi-cial, specifically regarding the fact thatNOS-3 is down-regulated in sepsis (44,45). In vitro studies also confirm that NOproduction by NOS-3 can be stimulatedby exogenous arginine, but this occurredonly when L-arginine stores were de-pleted (119). The mechanism for im-paired microcirculation in sepsis is, how-ever, multifactorial, and other factors likemechanic capillary occlusion through ex-tensive edema, occlusion from leuko-cytes, and impaired mitochondrial func-tion may be involved. Improvement ofthe microcirculation and local vasodila-tion may contribute to restoration of or-gan function in sepsis (multiple organfailure).

Arginine may also affect the immuneresponse in sepsis (120, 121) with an ag-gravated inflammatory reaction and in-creased tissue neutrophil infiltration(121). In addition, restored intestinal mo-tility was observed with arginine supple-mentation in a pig model of sepsis (96).

Finally, under conditions of impairedsubstrate (arginine) availability, the en-dothelial NOS-3 may undergo structuraldisarrangement, resulting in the conver-sion of this enzyme from a NO synthe-sizer into a generator of superoxide anion(122). This uncoupled reaction can beantagonized by excess L-arginine, as wasdemonstrated in a rabbit model of hyper-cholesterolemia (123) and in cells in vitro(124). It may, therefore, be suggestedthat patients with sepsis lack antioxidantdefense strategies, which may contributeto compromised NO availability, with ar-ginine acting as an antioxidant.

While enhanced NO production insepsis is related to suggested detrimentaleffects of hemodynamic instability andenhanced oxidative stress, potentialmechanisms for beneficial effects of exog-enous arginine supplementation in sepsisinclude enhanced (protein) metabolism,improved (micro)circulation and organfunction, effects on immune function and

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antibacterial effects, improved gut func-tion, and an antioxidant role of arginine.

CONCLUSION

Sepsis is considered an arginine defi-ciency state with reduced arginine levelsand specific changes in arginine metabo-lism related to endothelial dysfunction,severe catabolism, and worse outcome.Regarding the multiple pathways thatmay benefit from arginine supplementa-tion and taking into account our recentstudy indicating that arginine can begiven to septic patients without majoreffects on hemodynamics, we recom-mend more studies on the effects of argi-nine supplementation in septic patients.

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