Effects of dietary arginine supplementation on protein turnover and tissue protein synthesis in scald-burn rats

BASIC NUTRITIONAL INVESTIGATION

Nutrition Vol. 15, Nos. 7/8, 1999

Effects of Dietary Arginine Supplementation on Protein Turnover and Tissue Protein Synthesis in Scald-Burn Rats XUE-LIN CUI, MD, PHD,* MASATO IWASA, MD, PHD,* YOSHIE IWASA, MD, PHD,* YOSHINOBU OHMORI, MD,* AKIRA YAMAMOTO, MD,* HIRONORI MAEDA, MD, PHD,* MOTOHIKO KUME, MD,* SHOHEI OGOSHI, MD, PHD,* AKIRA YOKOYAMA, PHD,† TATSUYA SUGAWARA, PHD,† AND TADASHI FUNADA, PHD† From the *Department of Surgery II, Kochi Medical School, Kochi; and the †Tsukuba Research Laboratory, NOF Corporation, Ibaragi, Japan ABSTRACT

We assessed the effects of dietary arginine supplementation on protein turnover and organ protein synthesis in burned rats. Male Wistar rats weighing about 200 g underwent catheter jejunostomy and received scald burns covering 30% of the whole-body surface area. Animals were divided into a control group (n ⫽ 9) and an arginine group (n ⫽ 9) and continuously received total enteral nutrition for 7 d (250 kcal 䡠 kg⫺1 䡠 d⫺1, 1.72 gN 䡠 kg⫺1 䡠 d⫺1). Changes in body weight, plasma total protein, plasma albumin, urinary excretion of polyamines, nitrogen balance, whole-body protein kinetics, and tissue protein synthesis rates were determined. Whole-body protein kinetics and tissue fractional protein synthetic rates (Ks, percent/d) were estimated using a 24-h constant enteral infusion of 15N glycine on the last day. The changes in body weight were not different between the control and arginine groups. The urinary excretion of polyamines was higher in the arginine group than in the control group (P ⬍ 0.01). Burned rats enterally fed arginine-supplemented diet yielded significantly greater cumulative and daily nitrogen balance on days 3 and 5 than those fed a control diet (cumulative, P ⬍ 0.05; day 3, P ⬍ 0.01; day 5, P ⬍ 0.01). Whole-body protein turnover rate was significantly elevated in the arginine group as compared to that in the control group (P ⬍ 0.05). The Ks of rectus abdominis muscles were significantly increased in the arginine group in comparison to the control group (P ⬍ 0.01). We have shown that dietary arginine supplementation improved protein anabolism and attenuated muscle protein catabolism after thermal injury. Nutrition 1999;15:563–569. ©Elsevier Science Inc. 1999 Key words: arginine, enteral diet, nitrogen balance, protein turnover, muscle protein synthesis, polyamines

INTRODUCTION

Arginine is an important amino acid for protein synthesis, biosynthesis of amino acids, and generation of urea by the urea cycle.1 Although in normal circumstances, arginine is not an “essential” amino acid for healthy adults of most mammalian species to maintain positive nitrogen balance,2 it appears that arginine is a conditionally essential amino acid in certain stressful states.3 Several studies have shown that arginine could improve protein metabolism in animals and human beings that underwent severe stress such as operation, trauma, burn, and sepsis. Arginine administration has been shown to improve growth in rats following the trauma of laparotomy or experimental hind limb fracture.3–5 Plasma albumin, histone, and acute-phase protein synthesis are increased with arginine supplementation in traumatized or gram-negative sepsis rats.6,7 Supplemental dietary arginine has

been shown to be of benefit in wound healing by accelerating the increase in wound-breaking strength and the rate of collagen deposition after injury.8 It has also been shown to decrease protein catabolism and increase nitrogen retention in rats with severe trauma and in patients that underwent operation.2,9,10 However, the roles of arginine on whole-body protein turnover and tissue protein synthesis have not been fully determined. Therefore, in this study, we want to evaluate the effects of dietary arginine supplementation on whole-body protein kinetics and tissue protein synthesis in burned rats. MATERIALS AND METHODS

Animals Male Wistar rats weighing about 200 g (obtained from SLC, Shizuoka, Japan) were used in the experiment and were housed in

Correspondence to: Masato Iwasa, MD, PhD, Department of Surgery II, Kochi Medical School, Kohasu, Okoh, Nankoku, Kochi, 783-8505, Japan.

Nutrition 15:563–569, 1999 ©Elsevier Science Inc. 1999 Printed in the USA. All rights reserved.

0899-9007/99/$20.00 PII S0899-9007(99)00086-6

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EFFECT OF ARGININE SUPPLEMENTATION ON PROTEIN METABOLISM

FIG. 1. Experimental design. BSA, body surface area; N, nitrogen; TEN, total enteral nutrition.

stainless steel cages maintained in a temperature-controlled room (22–24°C) with a 12:12-h light-dark cycle. The animals were provided with water and laboratory stock diet (CE-2, Clea Japan, Tokyo, Japan) ad libitum for 4 d before experimentation. All

TABLE I. COMPOSITION OF ENTERAL DIETS* Parameter Amino acids (g/L) Arginine (g/L) Dextrose (g/L) Lipid (g/L) Na (g/L) K (g/L) Ca (g/L) Mg (g/L) Cl (g/L) P (g/L) Vitamin A (IU/L) Vitamin B1 (mg/L) Vitamin B2 (mg/L) Vitamin B6 (mg/L) Vitamin B12 (␮g/L) Vitamin C (mg/L) Vitamin D (IU/L) Vitamin E (mg/L) Niacinimide (mg/L) Folic acid (␮g/L) Pantothenic acid (mg/L) Water (g/L) Total Cal (kcal/L) Total N (g/L) N.P. Cal/Amino N

Control group

Arginine group

46.00 1.50 138.00 30.00 0.98 0.88 0.57 0.22 0.91 0.81 278.72 1.10 1.10 2.60 1.30 42.90 150.10 15.00 27.30 353.80 15.50 860.00 1006.00 7.38 111.38

45.50 9.20 138.00 30.00 0.98 0.88 0.57 0.22 0.91 0.81 278.72 1.10 1.10 2.60 1.30 42.90 150.10 15.00 27.30 353.80 15.50 860.00 1004.00 7.28 112.90

* Enteral diets from NOF Corp., Japan.

procedures conducted in this study were approved by the Kochi Medical School Animal Care Committee. Surgical Procedures and Burning Injury The animals were fasted for 24 h before surgical operation. All surgical procedures were performed under ether anesthesia (diethyl ether, Nacalai Tesque, Kyoto, Japan). After the dorsal side of a rat was shaved, the rat underwent jejunostomy with a silastic catheter (medical grade silicone, 1.0 mm ID, 1.5 mm OD, Nipro, Osaka, Japan). One end of the catheter was inserted into the antrum of the stomach and advanced about 2 cm beyond the pylorus ring. The other end was tunneled through subcutaneously from the abdomen to the skin in midscapular region, passed through a protective coil, and connected to a swivel (Instech, Instech Lab., Philadelphia, PA, USA) fixed on the cover of the metabolic cage. After catheter jejunostomy, a full-thickness scald burn in 30% of body surface was inflicted by immersing the animal’s dorsal surface in boiling water for 10 s. No treatment of the burn wound was necessary. Animals were housed individually in wire-bottom cages. They could move freely in the cage and were allowed free access to water in the experimental period. The enteral feeding was maintained by the infusion pumps and the diet and apparati were replaced daily. Study Design Eighteen male Wistar rats were randomly assigned to one of two kinds of enteral diets provided for the subsequent 7 d following catheter jejunostomy and scald burn injury. Each diet was infused at a rate of calories 250 kcal 䡠 kg⫺1 䡠 d⫺1, nitrogen 1.72 g 䡠 kg⫺1 䡠 d⫺1. Arginine was designed to be given 0.38 g 䡠 kg⫺1 䡠 d⫺1 in the control group and 2.30 g 䡠 kg⫺1 䡠 d⫺1 in the arginine group. The 15N glycine was administered enterally to each animal at a constant dose of 125 mg 䡠 kg⫺1 䡠 d⫺1 with total enteral nutrition on the last day to measure whole-body protein turnover rates and tissue protein synthesis rates. All rats were sacrificed on day 8 to collect blood and organs. The experimental design is shown in Figure 1.

EFFECT OF ARGININE SUPPLEMENTATION ON PROTEIN METABOLISM TABLE II. COMPOSITION OF AMINO ACIDS IN DIETS (g/L) Amino acids

Control group

Arginine group

Arginine Lysine Histidine Phenylalanine Tyrosine Leucine Isoleucine Valine Methionine Alanine Glycine Proline Glutamine Serine Threonine Asparagine Tryptophan Cysteine

1.50 3.40 1.30 2.30 2.00 4.20 2.30 2.80 1.20 1.30 0.90 4.80 9.60 2.60 1.90 3.20 0.50 0.20

9.20 2.70 1.10 1.90 1.80 3.50 1.90 2.40 0.90 1.20 0.80 3.90 7.30 2.20 1.60 2.50 0.40 0.10

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laboratory. Briefly, 125 mg 䡠 kg⫺1 䡠 d⫺1 of 15N glycine (glycine [2–15N, 99AP], Masstrace, Woburn, MA, USA), 99 atom percent excess, was constantly infused for 24 h with total enteral nutrition. Average 15N enrichment of urinary total nitrogen at 22–24 h was used as the plateau value. The amount of urinary total nitrogen and 15 N enrichment were measured automatically by a mass spectrometer (MSI-1,Yanaco, Kyoto, Japan) equipped with a total N analyzer (MT-500, Yanaco), without any particular preparation of urine. Whole-body protein turnover, synthesis, and breakdown rates were calculated according to the formula of Picou and Taylor-Roberts. Q ⫽ I ⫹ B ⫽ S ⫹ E and e-max/d ⫽ E/Q where Q ⫽ whole-body protein turnover rate (mg N 䡠 kg⫺1 䡠 d⫺1); I ⫽ rate of amino acid infusion (mg N 䡠 kg⫺1 䡠 d⫺1); B ⫽ whole-body protein breakdown rate (mg N 䡠 kg⫺1 䡠 d⫺1); S ⫽ whole-body protein synthesis rate (mg N 䡠 kg⫺1 䡠 d⫺1); E ⫽ rate of excretion of total urinary nitrogen (mg N 䡠 kg⫺1 䡠 d⫺1); e ⫽ rate of excretion of urinary 15N (mg N 䡠 kg⫺1 䡠 d⫺1); d ⫽ rate of infusion of 15N (mg N 䡠 kg⫺1 䡠 d⫺1). Determination of Fractional Protein Synthesis Rate in Tissue

Diets Enteral diets involved the following two kinds that were provided by NFO Co., Tsukuba, Japan. The control diet was a standard diet that contained only a little arginine (1.50 g/L). The diet for the arginine group was supplemented with a large amount of arginine (9.20 g/L) in the form of crystalline arginine hydrochloride. Concentrations of dextrose and lipid in the two diets were similar. The same amounts of electrolytes and vitamins were also added to every diet (Table I). The composition of amino acids in the two diets is shown in Table II. Sampling On day 8, rats were weighed and anesthetized. Midline abdominal incision was made. Blood was collected into heparinized specimen tubes from the aorta abdominalis. Plasma was separated and stored frozen at ⫺80°C for total protein, albumin, GOT, and GPT measurement. Immediately after plasma sampling, the liver, rectus abdominis muscle, and kidneys were removed, weighed, and stored frozen at ⫺80°C for determining tissue fractional protein synthesis rate. Urine was collected daily during the experimental period and stored frozen at ⫺80°C for determining uric urea nitrogen and urinary excretion of polyamines. Analytical Methods for Blood Chemistry Total plasma protein and albumin concentrations were measured by using a visible absorption spectrophotometry (Hitachi 7170 Autoanalyser, Hitachi, Tokyo, Japan). Plasma GOT and GPT concentrations were determined by a UV-rate method (Hitachi 7170 Autoanalyser, Hitachi). Analytical Methods for Uric Polyamines Urinary excretion of polyamines was examined by the enzymatic method using the Olympus AU 800 Autoanalyser (Olympus AU 800, Olympus, Tokyo, Japan). Measurement of Whole-Body Protein Turnover On day 7, whole-body protein turnover was measured according to the method of Picou and Taylor-Roberts,11 modified in our

One gram of each tissue of liver, rectus abdominis muscle, and kidney was homogenized in a Potter-Elvehjem homogenizer with 10 mL 10% trichloroacetic acid at 0°C. The homogenate was spun down (3000 rpm for 15 min) and the precipitate was washed three times with 5% trichloroacetic acid, and dried in the exsiccator at 40°C. Simultaneously, after the supernate was concentrated by centrifugation, the free amino acids were separated by the method of Sobel et al.13 The 15N enrichments in the obtained proteins and the free amino acids were measured automatically by a mass spectrometer (Yanagimoto MSI-1, Kyoto, Japan), respectively. The fractional protein synthetic rates (Ks) of liver, rectus abdominis muscle, and kidney proteins were calculated by using the convenient equation of Garlick et al.12 Sb/Si ⫽ 1 ⫺ e⫺kst where Sb ⫽

15

N specific activity of protein N (atom percent excess);

Si ⫽ N specific activity of intracellular fluid amino N (atom percent excess); 15

Ks ⫽ rate of protein synthesis (percent/d); t ⫽ duration of

15

N infusion (d).

Statistical Analysis Data are presented as mean ⫾ SD and were analyzed using the Student’s t test; statistical significance was determined as P ⬍ 0.05. RESULTS

Body Weight Change and Organ Weight The body weights of rats decreased after the experiment (Table III). The alterations of the weights during the experimental period were not statistically different in the two groups.

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EFFECT OF ARGININE SUPPLEMENTATION ON PROTEIN METABOLISM TABLE III.

BODY WEIGHT CHANGE DURING THE STUDY PERIOD AND BLOOD CHEMISTRY ON DAY 7 Parameter Pre-body weight (g) Post-body weight (g) Body weight change (g) Total protein (g/dL) Albumin (g/dL) GOT (IU/L) GPT (IU/L)

Control group

Arginine group

200.87 ⫾ 4.75 198.30 ⫾ 6.36 ⫺2.56 ⫾ 3.69 5.13 ⫾ 0.24 3.46 ⫾ 0.20 66.00 ⫾ 13.14 26.22 ⫾ 5.61

201.49 ⫾ 6.69 198.36 ⫾ 5.17 ⫺3.13 ⫾ 4.73 5.20 ⫾ 0.19 3.44 ⫾ 0.14 61.67 ⫾ 9.34 23.11 ⫾ 2.03

Values are expressed as mean ⫾ SD. GOT, glutamic oxaloacetic transaminase; GPT, glutamic pyruvate transaminase. Each group n ⫽ 9.

Blood Chemistry The plasma levels of total protein and albumin in the two groups did not differ significantly (Table III). The plasma GOT, GPT remained at the same levels in the arginine group as well as in the control group. Nitrogen Balance Figure 2 showed the changes of the daily nitrogen balance and the cumulative nitrogen balance for the period of enteral feeding with the two diets. The burned animals in the two groups all acquired positive nitrogen retention on the examined days, and furthermore, the nitrogen balance in the arginine group on day 3 and 5 were markedly greater than the values for the control group (129.65 ⫾ 39.76 versus 62.22 ⫾ 51.68 mg/d and 138.18 ⫾ 41.58 versus 44.92 ⫾ 24.22 mg/d) (P ⬍ 0.01). Moreover, the cumulative nitrogen balance was also significantly higher in the arginine group than that in the control group (625.06 ⫾ 321.12 versus 368.24 ⫾ 100.71 mg/d) (P ⬍ 0.05).

FIG. 2. The changes of nitrogen balance and the cumulative nitrogen balance in the experimental period in burned rats receiving control diet and arginine-supplemented diet. Values are expressed as mean ⫾ SD. *, P ⬍ 0.05; **, P ⬍ 0.01, versus control group. e: control group, : arginine group.

Whole-Body Protein Turnover Rate The rates of the mean whole-body protein turnover (Q) on day 7 in the control and arginine groups were 5620.22 ⫾ 848.47 and 6505.42 ⫾ 869.97 mg N 䡠 kg⫺1 䡠 d⫺1, respectively (Fig. 3). The whole-body protein turnover rate was significantly increased in the arginine group when compared to the control group (P ⬍ 0.05). The rate of whole-body protein synthesis (S) or the rate of whole-body protein breakdown (B) on the last day in the arginine group was higher than in the control group, but there was no significant difference. Protein synthesis rate was higher than the protein breakdown rates in each group. Fractional Protein Synthesis Rates (Ks) in Tissues Comparison of the Ks of the rectus abdominis muscle in the two groups showed that the arginine group had a significantly higher rate (13.09 ⫾ 5.03 percent/d) when compared to the control group (6.57 ⫾ 3.31 percent/d) (P ⬍ 0.01) (Fig. 4). The Ks in the liver of the arginine group was slightly higher than that of the control group. There was no difference in Ks of the kidney between the two groups. Urinary Excretion of Polyamines The urinary excretion of polyamines was 4.72 ⫾ 0.98, 3.81 ⫾ 1.07, and 4.10 ⫾ 1.41 ␮mol/d in the control group on days 2, 4, and 6, respectively (Fig. 5). The urinary excretion of polyamines was 7.20 ⫾ 1.73, 19.47 ⫾ 15.50, and 29.14 ⫾ 15.65 ␮mol/d in the arginine group on days 2, 4, and 6, respectively. The urinary excretion of polyamines was significantly higher in the arginine group than that in the control group on every examined day (P ⬍ 0.01). Moreover, there was no change in urinary excretion of polyamines in the control group, but in the arginine group, the urinary excretion of polyamines gradually rose significantly according to the extension of the experimental period (P ⬍ 0.01). DISCUSSION

Some studies have demonstrated that supplemental arginine could markedly improve nitrogen retention in seriously trauma-

FIG. 3. The whole-body protein turnover rates, breakdown rates, and synthesis rates on day 7 in burned rats receiving control diet and argininesupplemented diet. Values are expressed as mean ⫾ SD. *, P ⬍ 0.05, versus control group. B, protein breakdown rate; N, nitrogen; Q, wholebody protein turnover rate; S, protein synthesis rate. e: control group, : arginine group.

EFFECT OF ARGININE SUPPLEMENTATION ON PROTEIN METABOLISM

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FIG. 4. The Ks on day 7 in burned rats receiving control diet and arginine-supplemented diet. Values are expressed as mean ⫾ SD. **, P ⬍ 0.01, versus control group. e: control group, : arginine group.

tized animals and patients.9,10,14 –16 In this study, we also found that the nitrogen balance in the arginine group was significantly greater compared to the control group during the experimental period. However, this observation contrasts with that of Nirgiotis et al.6 They reported that there was no statistical difference in nitrogen balance between the arginine-supplemented and the arginine-free enteral diets in the postsurgical rats. Differences in the degree of stress inflicted on the animals may account for the result of this work compared to that of Nirgiotis. In the present study, the rats received scald burn injuries covering 30% of total body surface area and catheter jejunostomy, while in Nirgiotis’s study the stress was only the placement of a wound cylinder on the back of the animals. It is possible that they were not ill enough to need supplemental arginine to maintain nitrogen balance. We usually use the measurement of nitrogen balance to determine the protein metabolism in traumatized animals and patients, but the nitrogen balance is only the sum of nitrogen metabolism and it cannot help us to know the kinetics of protein metabolism in organisms. So, in this investigation, in order to elucidate further the aspects of protein metabolism affected by the arginine-supplemented diet, whole-body protein dynamics were studied after 7 d of enteral feeding following thermal injury. According to our results, we found that the average of the whole-body protein turnover rates was significantly higher in arginine-supplemented animals than in the control animals. Moreover, the whole-body protein synthesis rate was higher than the breakdown rate in each group. Therefore, from these results it can be proposed that the improved nitrogen balance was associated with the increased whole-body protein synthesis in burned rats fed with argininesupplemented diet. In addition, although both the protein synthesis and breakdown rates were not significantly different between the two groups, the increased tendencies were found in argininesupplemented animals. It is difficult to explain the mechanisms concerning the non-significant difference of protein synthesis rate or breakdown rate between the control and arginine groups. One possibility could be that the supplemental arginine was insufficient. But the excess arginine supplementation may produce adverse effects for organisms undergoing severe injury, such as burning.15 Arginine is used by all tissues for protein biosynthesis.16 For the purpose of clarifying the effects of arginine on tissue protein

FIG. 5. The urinary excretions of polyamines on days 2, 4, and 6 in burned rats receiving control diet and arginine-supplemented diet. Values are expressed as mean ⫾ SD. **, P ⬍ 0.01, versus control group; #, P ⬍ 0.05, versus day 2 in arginine group; $, P ⬍ 0.05, versus day 4; P ⬍ 0.01, versus day 2 in arginine group. e: control group, : arginine group.

synthesis, the Ks of rectus abdominis muscle, kidney, and liver were determined. In addition, this method can provide more detailed information on endogenous changes in protein synthesis, which cannot be obtained from nitrogen balance measurement alone. We found that the rectus abdominis muscle in the arginine group had a significantly higher protein synthesis rate when compared to the control group. The Ks of liver or kidney were not statistically different between the control and arginine groups. But Leon et al.7 reported that the Ks of liver in arginine-supplemented sepsis rats were markedly greater than that in the control rats. Skeletal muscle is one of the most important organs for body protein preservation in animals and human beings. It has been known that skeletal muscle protein is first broken down into amino acids for other organs to use under severe stress such as large-area burn injury. Bilmazes et al.17 have reported that the rates of muscle protein breakdown and whole-body protein breakdown in the burned patients were significantly higher than that in healthy adults, and the increase in whole-body protein turnover in children recovering from burn injury was associated with an elevated rate of muscle protein breakdown. Accordingly, it can be said that the role of muscle tissue on body protein metabolism could not be turned back and the nutrients that increase protein synthesis in skeletal muscle were necessary in burn-injured rats. Based on our results, it could be suggested that supplemental arginine has a distinct protein sparing action, at least in part because of the increases on muscle protein synthesis. Polyamines (PA), i.e., putrescine, spermidine, and spermine, are ubiquitous organic cations of low molecular weight and are contained in all cells of higher eukaryotes in significant amounts. In particular, there are higher concentrations of PA in the tissues that regenerate prosperously, and the PA can stimulate tissue

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EFFECT OF ARGININE SUPPLEMENTATION ON PROTEIN METABOLISM

growth and cell differentiation.18 Chung et al.19 have demonstrated that the oral administration of spermidine prevented the inhibition of mucosal growth caused by the attenuation of polyamine biosynthesis and indicated that early increases of polyamine biosynthesis are crucial cellular events for mucosal repair after burn injury. In our laboratory, we have examined the urinary excretion of PA in postoperative patients undergoing gastrointestinal operation and found that urinary excretion of PA in patients who received enteral nutrition was significantly increased compared to patients who received total parenteral nutrition.20 In this paper, we indicated that the urinary excretion of PA in burned rats fed with diet supplemented with arginine was significantly higher than that in control rats on days 2, 4, and 6. There was no change in the urinary excretion of PA in the control group, but in the arginine-supplemented group the urinary excretion of PA increased significantly in proportion to the extension of the experimental period. The increased urinary excretion of PA reflected the enhanced tissue level of PA. Because arginine is an important precursor of PA, the arginine-supplemented diet can increase the level of PA in the tissues, and then the PA can promote the proliferation of tissues and cells in the reversal period of injury. Therefore, it can be supposed that arginine supplementation has beneficial effects on repair of tissues and gut functions in patients who undergo trauma, operation, and burn injury. Our data does not allow us to draw firm conclusions about the underlying mechanism by which arginine-supplemented diet improved protein metabolism. Nevertheless, several possibilities could be considered. First, arginine is relatively insufficient in traumatized or burned animals and human beings. Although arginine is not an essential amino acid for adult species, the requirement for arginine is increased for traumatized hosts, and the increased demand cannot be satisfied by endogenous arginine synthesis. Thus, exogenous arginine supplementation becomes necessary.21 In addition, since arginine occurs in a particularly high concentration in the skin and connective tissue, it is plausible that arginine is required in greater amounts for tissue repair in traumatized and burned rats.5,22 So, arginine may be appropriately thought of as a “conditionally essential” amino acid under severe stressful conditions. The second possibility is that the beneficial effect of arginine on improved nitrogen retention in traumatized rats may be related to increased creatine synthesis and turnover. Arginine is one of the precursors for the synthesis of creatine16 and is necessary to increase the amount of creatine in stressed species. Minuskin et al.4 have reported that muscle creatine content in rats following laparotomy was significantly increased by a diet enriched with arginine plus glycine. They thought that the increased storage of muscle creatine was one factor that may partially have accounted for the enhancement of nitrogen reten-

tion. The third possibility is that arginine plays an important role in the generation of urea by the urea cycle. Arginine is required for the functioning of the urea cycle to detoxify the blood of ammonia,1 and an increased demand for arginine as a substrate for urea cycle results in an improvement of nitrogen retention.4 Finally, arginine as a potent secretagogue promotes the release of endocrine and neuroendocrine hormones such as growth hormone, insulin-like growth factor-1, insulin, glucagon, and pancreatic polypeptide.1,23 These hormones have been reported to play important roles in protein metabolism in humans and animals. Growth hormone, insulin-like growth factor-1, and insulin inhibit protein catabolism and lead to increased synthesis of certain proteins, including liver and muscle protein, albumin, and collagen after injury.24,25 On the other hand, some researchers have demonstrated that arginine supplementation increased protein contents in traumatized animals, such as hepatic protein, wound collagen, and fibrinogen.7,22 In the present study, we found that administration of enteral diet supplemented with arginine resulted in increased whole-body protein turnover and muscle protein synthesis. Therefore, it can be speculated that arginine may beneficially contribute to protein metabolism in injured rats through its hormone secretagogue effects. However, there remains a question as to why the Ks were only significantly increased in muscle tissue, but were not markedly increased in liver and kidney tissue. We think it may be because the arginase activity is greatly different between muscle and liver or kidney. The liver has a 100-fold, and the kidney has an 18-fold arginase activity of muscle.16 When the extra arginine is provided, it is rapidly decomposed in liver and kidney, so that the arginine cannot be utilized to synthesize proteins as a substrate. Dietary supplemental arginine is essential for adult mammals and human beings under certain conditions such as trauma, burn, and sepsis.6,16,22 This experiment, and other studies, have demonstrated that arginine is very important for protein metabolism and wound healing. In addition, it has been reported that arginine could enhance the immune response after several serious stresses in animals and humans.15,16 It might be beneficial to use diets supplemented with adequate arginine for critically ill patients. SUMMARY

The effects of dietary arginine supplementation on nitrogen balance, whole-body protein turnover, and tissue protein synthesis were investigated in burned rats. The supplemental arginine improved protein metabolism and nitrogen balance in burned rats. This observation could be attributed in part to the enhanced muscle protein synthesis and the increased whole-body protein turnover.

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ANNOUNCEMENT

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