A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly

This study was designed to evaluate the effects of enriching an essential amino acid (EAA) mixture with leucine on muscle protein metabolism in elderly and young individuals. Four (2 elderly and 2 young) groups were studied before and after ingestion of 6.7 g of EAAs. EAAs were based on the composition of whey protein [26% leucine (26% Leu)] or were enriched in leucine [41% leucine (41% Leu)]. A primed, continuous infusion of L-[ring-2H5]phenylalanine was used together with vastus lateralis muscle biopsies and leg arteriovenous blood samples for the determinations of fractional synthetic rate (FSR) and balance of muscle protein. FSR increased following amino acid ingestion in both the 26% (basal: 0.048 +/- 0.005%/h; post-EAA: 0.063 +/- 0.007%/h) and the 41% (basal: 0.036 +/- 0.004%/h; post-EAA: 0.051 +/- 0.007%/h) Leu young groups (P < 0.05). In contrast, in the elderly, FSR did not increase following ingestion of 26% Leu EAA (basal: 0.044 +/- 0.003%/h; post-EAA: 0.049 +/- 0.006%/h; P > 0.05) but did increase following ingestion of 41% Leu EAA (basal: 0.038 +/- 0.007%/h; post-EAA: 0.056 +/- 0.008%/h; P < 0.05). Similar to the FSR responses, the mean response of muscle phenylalanine net balance, a reflection of muscle protein balance, was improved (P < 0.05) in all groups, with the exception of the 26% Leu elderly group. We conclude that increasing the proportion of leucine in a mixture of EAA can reverse an attenuated response of muscle protein synthesis in elderly but does not result in further stimulation of muscle protein synthesis in young subjects.     

Hypercortisolemia alters muscle protein anabolism following ingestion of essential amino acids

Debilitating injury is accompanied by hypercortisolemia, muscle wasting, and disruption of the normal anabolic response to food. We sought to determine whether acute hypercortisolemia alters muscle protein metabolism following ingestion of a potent anabolic stimulus: essential amino acids (EAA). A 27-h infusion (80 microg. kg(-1). h(-1)) of hydrocortisone sodium succinate mimicked cortisol (C) levels accompanying severe injury (>30 microg/dl), (C + AA; n = 6). The control group (AA) received intravenous saline (n = 6). Femoral arteriovenous blood samples and muscle biopsies were obtained during a primed (2.0 micromol/kg) constant infusion (0.05 micromol. kg(-1). min(-1)) of l-[ring-(2)H(5)]phenylalanine before and after ingestion of 15 g of EAA. Hypercortisolemia [36.5 +/- 2.1 (C + AA) vs. 9.0 +/- 1.0 microg/dl (AA)] increased postabsorptive arterial, venous, and muscle intracellular phenylalanine concentrations. Hypercortisolemia also increased postabsorptive and post-EAA insulin concentrations. Net protein balance was blunted (40% lower) following EAA ingestion but remained positive for a greater period of time (60 vs. 180 min) in the C + AA group. Thus, although differences in protein metabolism were evident, EAA ingestion improved muscle protein anabolism during acute hypercortisolemia and may help minimize muscle loss following debilitating injury.     

Method for the determination of the arteriovenous muscle protein balance during non-steady-state blood and muscle amino acid concentrations

We describe a method based on the traditional arteriovenous balance technique in conjunction with muscle biopsies for the determination of leg muscle protein balance during the nonsteady state in blood amino acid concentrations. Six young, healthy individuals were studied in the postabsorptive state (pre-Phe) and after a bolus ingestion of approximately 0.5 g phenylalanine (post-Phe). Post-Phe free phenylalanine concentrations in blood and muscle increased (P < 0.05), but the respective concentrations of the amino acid threonine did not change. The average post-Phe leg net balance (NB) for threonine decreased from basal (P < 0.05), but that for phenylalanine did not change. A volume of distribution for free phenylalanine in the leg was calculated based on the leg lean mass and the relative muscle water content and used to estimate the rate of accumulation of free phenylalanine in the leg. When the post-Phe NB for phenylalanine was corrected for the rate of accumulation of free phenylalanine in the leg, the post-Phe NB for phenylalanine decreased from basal (P < 0.05). This corrected value was not different (P > 0.05) from the value predicted for the phenylalanine NB based on the pre- and post-Phe NB responses for threonine. We conclude that the protein NB in non-steady-state blood phenylalanine concentrations can be determined from the arteriovenous phenylalanine NB by accounting for changes in free phenylalanine within its volume of distribution.     

Amino acid ingestion improves muscle protein synthesis in the young and elderly

We recently demonstrated that muscle protein synthesis was stimulated to a similar extent in young and elderly subjects during a 3-h amino acid infusion. We sought to determine if a more practical bolus oral ingestion would also produce a similar response in young (34 +/- 4 yr) and elderly (67 +/- 2 yr) individuals. Arteriovenous blood samples and muscle biopsies were obtained during a primed (2.0 micromol/kg) constant infusion (0.05 micromol.kg(-1).min(-1)) of L-[ring-2H5]phenylalanine. Muscle protein kinetics and mixed muscle fractional synthetic rate (FSR) were calculated before and after the bolus ingestion of 15 g of essential amino acids (EAA) in young (n = 6) and elderly (n = 7) subjects. After EAA ingestion, the rate of increase in femoral artery phenylalanine concentration was slower in elderly subjects but remained elevated for a longer period. EAA ingestion increased FSR in both age groups by approximately 0.04%/h (P < 0.05). However, muscle intracellular (IC) phenylalanine concentration remained significantly higher in elderly subjects at the completion of the study (young: 115.6 +/- 5.4 nmol/ml; elderly: 150.2 +/- 19.4 nmol/ml). Correction for the free phenylalanine retained in the muscle IC pool resulted in similar net phenylalanine uptake values in the young and elderly. EAA ingestion increased plasma insulin levels in young (6.1 +/- 1.2 to 21.3 +/- 3.1 microIU/ml) but not in elderly subjects (3.0 +/- 0.6 to 4.3 +/- 0.4 microIU/ml). Despite differences in the time course of plasma phenylalanine kinetics and a greater residual IC phenylalanine concentration, amino acid supplementation acutely stimulated muscle protein synthesis in both young and elderly individuals.     

Nonessential amino acids are not necessary to stimulate net muscle protein synthesis in healthy volunteers

The present study was performed to test the hypothesis that orally administered essential amino acids, in combination with carbohydrate, will stimulate net muscle protein synthesis in resting human muscle in vivo. Four volunteers ingested 500 mL of a solution containing 13.4 g of essential amino acids and 35 g sucrose (EAA). Blood samples were taken from femoral arterial and venous catheters over a 2-hour period following the ingestion of EAA to measure arteriovenous concentrations of amino acids across the muscle. Two muscle biopsies were taken during the study, one before administration of the drink and one approximately 2 hours after consumption of EAA. Serum insulin increased from normal physiologic levels at baseline (9.2 +/- 0.8 microU/mL) and peaked (48 +/- 7.1 microU/mL) 30 minutes after EAA ingestion. Arterial essential amino acid concentrations increased approximately 100 to 400% above basal levels between 10 and 30 minutes following drink ingestion. Net nitrogen (N) balance changed from negative (-495 +/- 128 nmol/mL) prior to consumption of EAA to a peak positive value (416 +/- 140 nmol/mL) within 10 minutes of ingestion of the drink. EAA resulted in an estimated positive net N uptake of 307.3 mg N above basal levels over the 2-hour period. Muscle amino acid concentrations were similar prior to and 2 hours following ingestion of EAA. We conclude that ingestion of a solution composed of carbohydrates to stimulate insulin release and a small amount of essential amino acids to increase amino acid availability for protein synthesis is an effective stimulator of muscle protein anabolism.     

Acute response of net muscle protein balance reflects 24-h balance after exercise and amino acid ingestion

The purpose of this study was to determine if the acute anabolic muscle response to resistance exercise and essential amino acids (EAA) reflects the response over 24 h. Seven subjects participated in the following two 24-h studies: 1) resting (REST) and 2) rest plus resistance exercise and consumption of EAA (ES). Net balance (NB) across the leg was determined for four amino acids. [(13)C(6)]phenylalanine was infused to determine mixed muscle fractional synthetic rate (FSR). Twenty-four-hour FSR was significantly greater for ES than for REST (P = 0.003). Exchange of phenylalanine across the leg was -194 +/- 74 (SE) mg for ES and -371 +/- 88 mg for REST (P = 0.07) over 24 h and 229 +/- 42 mg (ES) and 28 +/- 15 mg (REST; P < 0.01) over 3 h corresponding to exercise and EAA consumption for ES. The difference in phenylalanine exchange between REST and ES was not different for measurements over 24 and 3 h. Increases in NB during ES were primarily the result of increases in protein synthesis. Results for other amino acids were similar. The acute anabolic response of muscle to EAA intake and exercise is additive to the response at rest and thus reflects the 24-h response.     

鸡肉粉完全替代鱼粉饲料中补充包被氨基酸对凡纳滨对虾生长、体成分及组织氨基酸含量的影响

为了探讨鸡肉粉完全替代鱼粉时饲料氨基酸的平衡性以及外源氨基酸的添加方式与凡纳滨对虾生长、体成分、血浆游离氨基酸及肌肉氨基酸含量的关系, 本试验采用26因子试验设计进行了为期56d的饲养试验。2个饲料蛋白质水平分别为40%和32%, 6个饲料处理分别为鱼粉组(对照组)、鸡肉粉组、鸡肉粉+晶体EAA组、鸡肉粉+晶体EAA+晶体NEAA组、鸡肉粉+包被EAA组、鸡肉粉+包被EAA+包被NEAA组, 配制12组饲料。将凡纳滨对虾(0.300.01) g随机分配到36个圆桶(150 L)中, 每桶30尾, 每3个桶为一个处理组, 饲喂一种饲料, 每天饱食投喂三次。在每一饲料蛋白质水平下, 无论是补充晶体氨基酸(CAA)组还是包被氨基酸组对虾的增重率均显著高于鸡肉粉组(P0.05), 且在32%蛋白质水平下, 包被EAA组对虾增重率达到了鱼粉组水平(P0.05); 补充晶体EAA+NEAA组对虾增重率与补充晶体EAA组无差异(P0.05), 但均显著低于补充包被氨基酸组(P0.05); 补充包被EAA组对虾增重率显著高于补充包被EAA+NEAA组(P0.05)。饲料系数的变化正好与增重率变化相反(P0.05)。饲喂高蛋白质水平饲料较之饲喂低蛋白质饲料明显提高对虾增重率、虾体蛋白含量(P0.05), 但降低虾体脂肪含量(P0.05)。包被氨基酸组凡纳滨对虾血浆游离氨基酸含量总体显著低于CAA组(P0.05)。除谷氨酸、甘氨酸以及脯氨酸外, 各组对虾肌肉氨基酸含量无显著差异(P0.05)。结果表明, 在32%饲料蛋白质水平下, 用鸡肉粉完全替代鱼粉时, 饲料中补充包被EAA可明显促进凡纳滨对虾的生长, 且达到了鱼粉组的饲喂效果。     

Exogenous amino acids stimulate human muscle anabolism without interfering with the response to mixed meal ingestion

We sought to determine whether ingestion of a between-meal supplement containing 30 g of carbohydrate and 15 g of essential amino acids (CAA) altered the metabolic response to a nutritionally mixed meal in healthy, recreationally active male volunteers. A control group (CON; n = 6, 38 +/- 8 yr, 86 +/- 10 kg, 179 +/- 3 cm) received a liquid mixed meal [protein, 23.4 +/- 1.0 g (essential amino acids, 14.7 +/- 0.7 g); carbohydrate, 126.6 +/- 4.0 g; fat, 30.3 +/- 2.8 g] every 5 h (0830, 1330, 1830). The experimental group (SUP; n = 7, 36 +/- 10 yr, 87 +/- 12 kg, 180 +/- 3 cm) consumed the same meals but, in addition, were given CAA supplements (1100, 1600, 2100). Net phenylalanine balance (NB) and fractional synthetic rate (FSR) were calculated during a 16-h primed constant infusion of L-[ring-2H5]phenylalanine. Ingestion of a combination of CAA supplements and meals resulted in a greater mixed muscle FSR than ingestion of the meals alone (SUP, 0.099 +/- 0.008; CON, 0.076 +/- 0.005%/h; P < 0.05). Both groups experienced an improvement in NB after the morning (SUP, -2.2 +/- 3.3; CON, -1.5 +/- 3.5 nmol x min(-1) x 100 ml leg volume(-1)) and evening meals (SUP, -9.7 +/- 4.3; CON, -6.7 +/- 4.1 nmol x min(-1) x 100 ml leg volume(-1)). NB after CAA ingestion was significantly greater than after the meals, with values of 40.2 +/- 8.5 nmol x min(-1) x 100 ml leg volume(-1). These data indicate that CAA supplementation produces a greater anabolic effect than ingestion of intact protein but does not interfere with the normal metabolic response to a meal.     

Combined ingestion of protein and carbohydrate improves protein balance during ultra-endurance exercise

The aims of this study were to compare different tracer methods to assess whole body protein turnover during 6 h of prolonged endurance exercise when carbohydrate was ingested throughout the exercise period and to investigate whether addition of protein can improve protein balance. Eight endurance-trained athletes were studied on two different occasions at rest (4 h), during 6 h of exercise at 50% of maximal O2 uptake (in sequential order: 2.5 h of cycling, 1 h of running, and 2.5 h of cycling), and during subsequent recovery (4 h). Subjects ingested carbohydrate (CHO trial; 0.7 g CHO.kg(-1.)h(-1)) or carbohydrate/protein beverages (CHO + PRO trial; 0.7 g CHO.kg(-1).h(-1) and 0.25 g PRO.kg(-1).h(-1)) at 30-min intervals during the entire study. Whole body protein metabolism was determined by infusion of L-[1-13C]leucine, L-[2H5]phenylalanine, and [15N2]urea tracers with sampling of blood and expired breath. Leucine oxidation increased from rest to exercise [27 +/- 2.5 vs. 74 +/- 8.8 (CHO) and 85 +/- 9.5 vs. 200 +/- 16.3 mg protein.kg(-1).h(-1) (CHO + PRO), P < 0.05], whereas phenylalanine oxidation and urea production did not increase with exercise. Whole body protein balance during exercise with carbohydrate ingestion was negative (-74 +/- 8.8, -17 +/- 1.1, and -72 +/- 5.7 mg protein.kg(-1).h(-1)) when L-[1-13C]leucine, L-[2H5]phenylalanine, and [15N2]urea, respectively, were used as tracers. Addition of protein to the carbohydrate drinks resulted in a positive or less-negative protein balance (-32 +/- 16.3, 165 +/- 4.6, and 151 +/- 13.4 mg protein.kg(-1).h(-1)) when L-[1-13C]leucine, L-[2H5]phenylalanine, and [15N2]urea, respectively, were used as tracers. We conclude that, even during 6 h of exhaustive exercise in trained athletes using carbohydrate supplements, net protein oxidation does not increase compared with the resting state and/or postexercise recovery. Combined ingestion of protein and carbohydrate improves net protein balance at rest as well as during exercise and postexercise recovery.     

Short-term insulin and nutritional energy provision do not stimulate muscle protein synthesis if blood amino acid availability decreases

Muscle protein synthesis requires energy and amino acids to proceed and can be stimulated by insulin under certain circumstances. We hypothesized that short-term provision of insulin and nutritional energy would stimulate muscle protein synthesis in healthy subjects only if amino acid availability did not decrease. Using stable isotope techniques, we compared the effects on muscle phenylalanine kinetics across the leg of an amino acid-lowering, high-energy (HE, n = 6, 162 +/- 20 kcal/h) hyperglycemic hyperlipidemic hyperinsulinemic clamp with systemic insulin infusion to a low-energy (LE, n = 6, 35 +/- 3 kcal/h, P < 0.05 vs. HE) euglycemic hyperinsulinemic clamp with local insulin infusion in the femoral artery. Basal blood phenylalanine concentrations and phenylalanine net balance, muscle protein breakdown, and synthesis (nmol.min(-1).100 g leg muscle(-1)) were not different between groups. During insulin infusion, femoral insulinemia increased to a similar extent between groups and blood phenylalanine concentration decreased 27 +/- 3% in the HE group but only 9 +/- 2% in the LE group (P < 0.01 HE vs. LE). Phenylalanine net balance increased in both groups, but the change was greater (P < 0.05) in the LE group. Muscle protein breakdown decreased in the HE group (58 +/- 12 to 35 +/- 7 nmol.min(-1).100 g leg muscle(-1)) and did not change in the LE group. Muscle protein synthesis was unchanged in the HE group (39 +/- 6 to 30 +/- 7 nmol.min(-1).100 g leg muscle(-1)) and increased (P < 0.05) in the LE group (41 +/- 9 to 114 +/- 26 nmol.min(-1).100 g leg muscle(-1)). We conclude that amino acid availability is an important factor in the regulation of muscle protein synthesis in response to insulin, as decreased blood amino acid concentrations override the positive effect of insulin on muscle protein synthesis even if excess energy is provided.     

Non-essential amino acids play an important role in adaptation of the rat exocrine pancreas to high nitrogen feeding

We have previously demonstrated that feeding a diet with a high amino acid (60% AA diet) content, as a mixture simulating casein, induced pancreatic growth and pancreatic protease production in rats. In the present study, we examined the effects of an increasing dietary content of essential amino acids (EAA, x1 - x3 in exp. 1 and x1 - x3.3 in exp. 2) and non-essential amino acids (NEAA, x1 - x3 in exp. 1 and x1 - x5.2 in exp. 2) on pancreatic growth, amylase and protease adaptation using casein-type amino acid mixtures (exp. 1, basal diet; 20% AA diet) and egg white-type amino acid mixtures (exp. 2, basal diet; 12% AA diet). Pancreatic growth and trypsin activity were induced as the dietary content of NEAA was increased in experiments 1 and 2. Amylase activity in the pancreas was also induced as the dietary content of NEAA was increased, even with the decrease in dietary carbohydrate in experiment 2. The values of all pancreatic variables decreased with the increase in dietary EAA (x2 and x3) without an increase in NEAA. The changes in the pancreas were coincident with increases in plasma arginine and lysine concentrations and a decrease in the plasma alanine concentration. In rats fed a 60% AA diet (EAA and NEAA x3), in the case of which the EAA content was balanced with the NEAA content, pancreatic growth and protease production increased and reached maximum levels as the plasma amino acid concentrations decreased, except for alanine. These results show that NEAA, not EAA, are associated with induction of pancreatic growth and protease production upon feeding a diet with a high AA content, and that some metabolites may be involved in the induction process. The suppression of pancreatic growth and protease production in rats fed the high EAA diets without balanced NEAA may be associated with impairment of amino acid metabolism rather than the increments in the concentration of one or more essential amino acids. Our results also suggest that there is an unknown mechanism or unknown factors involved in regulating pancreatic amylase.     

Effect of carbohydrate intake on net muscle protein synthesis during recovery from resistance exercise.

The purpose of this study was to determine the effect of ingestion of 100 g of carbohydrates on net muscle protein balance (protein synthesis minus protein breakdown) after resistance exercise. Two groups of eight subjects performed a resistance exercise bout (10 sets of 8 repetitions of leg presses at 80% of 1-repetition maximum) before they rested in bed for 4 h. One group (CHO) received a drink consisting of 100 g of carbohydrates 1 h postexercise. The other group (Pla) received a noncaloric placebo drink. Leg amino acid metabolism was determined by infusion of 2H5- or 13C6-labeled phenylalanine, sampling from femoral artery and vein, and muscle biopsies from vastus lateralis. Drink intake did not affect arterial insulin concentration in Pla, whereas insulin increased several times after the drink in CHO (P < 0.05 vs. Pla). Arterial phenylalanine concentration fell slightly after the drink in CHO. Net muscle protein balance between synthesis and breakdown did not change in Pla, whereas it improved in CHO from -17 +/- 3 nmol.ml(-1).100 ml leg(-1) before drink to an average of -4 +/- 4 and 0 +/- 3 nmol.ml(-1).100 ml leg(-1) during the second and third hour after the drink, respectively (P < 0.05 vs. Pla during last hour). The improved net balance in CHO was due primarily to a progressive decrease in muscle protein breakdown. We conclude that ingestion of carbohydrates improved net leg protein balance after resistance exercise. However, the effect was minor and delayed compared with the previously reported effect of ingestion of amino acids.     

Effect of insulin on human skeletal muscle protein synthesis is modulated by insulin-induced changes in muscle blood flow and amino acid availability

Insulin promotes muscle anabolism, but it is still unclear whether it stimulates muscle protein synthesis in humans. We hypothesized that insulin can increase muscle protein synthesis only if it increases muscle amino acid availability. We measured muscle protein and amino acid metabolism using stable-isotope methodologies in 19 young healthy subjects at baseline and during insulin infusion in one leg at low (LD, 0.05), intermediate (ID, 0.15), or high (HD, 0.30 mUxmin(-1)x100 ml(-1)) doses. Insulin was infused locally to induce muscle hyperinsulinemia within the physiological range while minimizing the systemic effects. Protein and amino acid kinetics across the leg were assessed using stable isotopes and muscle biopsies. The LD did not affect phenylalanine delivery to the muscle (-9 +/- 18% change over baseline), muscle protein synthesis (16 +/- 26%), breakdown, or net balance. The ID increased (P < 0.05) phenylalanine delivery (+63 +/- 38%), muscle protein synthesis (+157 +/- 54%), and net protein balance, with no change in breakdown. The HD did not change phenylalanine delivery (+12 +/- 11%) or muscle protein synthesis (+9 +/- 19%), and reduced muscle protein breakdown (-17 +/- 15%), thus improving net muscle protein balance but to a lesser degree than the ID. Changes in muscle protein synthesis were strongly associated with changes in muscle blood flow and phenylalanine delivery and availability. In conclusion, physiological hyperinsulinemia promotes muscle protein synthesis as long as it concomitantly increases muscle blood flow, amino acid delivery and availability.     

The effects of starvation and refeeding on plasma amino acid levels in carp, Cyprinus carpio L., 1758

Plasma amino acid (AA) levels of carp, Cyprinus carpio L., 1758, were analysed after various periods of starvation as well as after 12 days of refeeding. The levels were compared to control groups, which had been previously fasted for 24 h. A positive correlation between dietary and plasma essential amino acid (EAA) concentrations was observed in all of the control groups.
The effect of starvation on the dynamics of AA concentration was different according to the period of starvation. Fasting already produced a decrease of total α-AA levels at 2 days, and these low levels were maintained until 5 and 8 days. These short periods of fasting affected the levels of EAA (especially branched-chain AA) more than those of non-essential amino acids (NEAA). The only AA that increased was Ala, which rose at 5 days of starvation, surpassing the levels of the control group. These high levels were maintained until 19 days of starvation.
A different situation was observed at 19 days, when an important increase of total α-AA levels was produced, the branched-chain AA being the most notable among EAA and Glu/Gln among NEAA. Later, at 50 days of starvation, total α-AA, EAA (except Leu and Ile) and NEAA decreased.
After 50 days of starvation, 12 days of refeeding did not modify the levels of EAA and NEAA, and their concentrations were lower than those of the control group, which presented an increase of total AA at this time. The differences observed on the changes of individual AA levels are discussed.     

Amino acid repletion does not decrease muscle protein catabolism during hemodialysis

Intradialytic protein catabolism is attributed to loss of amino acids in the dialysate. We investigated the effect of amino acid infusion during hemodialysis (HD) on muscle protein turnover and amino acid transport kinetics by using stable isotopes of phenylalanine, leucine, and lysine in eight patients with end-stage renal disease (ESRD). Subjects were studied at baseline (pre-HD), 2 h of HD without amino acid infusion (HD-O), and 2 h of HD with amino acid infusion (HD+AA). Amino acid depletion during HD-O augmented the outward transport of amino acids from muscle into the vein. Increased delivery of amino acids to the leg during HD+AA facilitated the transport of amino acids from the artery into the intracellular compartment. Increase in muscle protein breakdown was more than the increase in synthesis during HD-O (46.7 vs. 22.3%, P < 0.001). Net balance (nmol.min(-1).100 ml (-1)) was more negative during HD-O compared with pre-HD (-33.7 +/- 1.5 vs. -6.0 +/- 2.3, P < 0.001). Despite an abundant supply of amino acids, the net balance (-16.9 +/- 1.8) did not switch from net release to net uptake. HD+AA induced a proportional increase in muscle protein synthesis and catabolism. Branched chain amino acid catabolism increased significantly from baseline during HD-O and did not decrease during HD+AA. Protein synthesis efficiency, the fraction of amino acid in the intracellular pool that is utilized for muscle protein synthesis decreased from 42.1% pre-HD to 33.7 and 32.6% during HD-O and HD+AA, respectively (P < 0.01). Thus amino acid repletion during HD increased muscle protein synthesis but did not decrease muscle protein breakdown.     

Combined ingestion of protein and free leucine with carbohydrate increases postexercise muscle protein synthesis in vivo in male subjects

The present study was designed to determine postexercise muscle protein synthesis and whole body protein balance following the combined ingestion of carbohydrate with or without protein and/or free leucine. Eight male subjects were randomly assigned to three trials in which they consumed drinks containing either carbohydrate (CHO), carbohydrate and protein (CHO+PRO), or carbohydrate, protein, and free leucine (CHO+PRO+Leu) following 45 min of resistance exercise. A primed, continuous infusion of L-[ring-13C6]phenylalanine was applied, with blood samples and muscle biopsies collected to assess fractional synthetic rate (FSR) in the vastus lateralis muscle as well as whole body protein turnover during 6 h of postexercise recovery. Plasma insulin response was higher in the CHO+PRO+Leu compared with the CHO and CHO+PRO trials (+240 +/- 19% and +77 +/- 11%, respectively, P < 0.05). Whole body protein breakdown rates were lower, and whole body protein synthesis rates were higher, in the CHO+PRO and CHO+PRO+Leu trials compared with the CHO trial (P < 0.05). Addition of leucine in the CHO+PRO+Leu trial resulted in a lower protein oxidation rate compared with the CHO+PRO trial. Protein balance was negative during recovery in the CHO trial but positive in the CHO+PRO and CHO+PRO+Leu trials. In the CHO+PRO+Leu trial, whole body net protein balance was significantly greater compared with values observed in the CHO+PRO and CHO trials (P < 0.05). Mixed muscle FSR, measured over a 6-h period of postexercise recovery, was significantly greater in the CHO+PRO+Leu trial compared with the CHO trial (0.095 +/- 0.006 vs. 0.061 +/- 0.008%/h, respectively, P < 0.05), with intermediate values observed in the CHO+PRO trial (0.0820 +/- 0.0104%/h). We conclude that coingestion of protein and leucine stimulates muscle protein synthesis and optimizes whole body protein balance compared with the intake of carbohydrate only.     

Protein coingestion stimulates muscle protein synthesis during resistance-type exercise

In contrast to the effect of nutritional intervention on postexercise muscle protein synthesis, little is known about the potential to modulate protein synthesis during exercise. This study investigates the effect of protein coingestion with carbohydrate on muscle protein synthesis during resistance-type exercise. Ten healthy males were studied in the evening after they consumed a standardized diet throughout the day. Subjects participated in two experiments in which they ingested either carbohydrate or carbohydrate with protein during a 2-h resistance exercise session. Subjects received a bolus of test drink before and every 15 min during exercise, providing 0.15 g x kg(-1) x h(-1) carbohydrate with (CHO + PRO) or without (CHO) 0.15 g x kg(-1) x h(-1) protein hydrolysate. Continuous intravenous infusions with l-[ring-(13)C(6)]phenylalanine and l-[ring-(2)H(2)]tyrosine were applied, and blood and muscle biopsies were collected to assess whole body and muscle protein synthesis rates during exercise. Protein coingestion lowered whole body protein breakdown rates by 8.4 +/- 3.6% (P = 0.066), compared with the ingestion of carbohydrate only, and augmented protein oxidation and synthesis rates by 77 +/- 17 and 33 +/- 3%, respectively (P < 0.01). As a consequence, whole body net protein balance was negative in CHO, whereas a positive net balance was achieved after the CHO + PRO treatment (-4.4 +/- 0.3 vs. 16.3 +/- 0.4 micromol phenylalanine x kg(-1) x h(-1), respectively; P < 0.01). In accordance, mixed muscle protein fractional synthetic rate was 49 +/- 22% higher after protein coingestion (0.088 +/- 0.012 and 0.060 +/- 0.004%/h in CHO + PRO vs. CHO treatment, respectively; P < 0.05). We conclude that, even in a fed state, protein coingestion stimulates whole body and muscle protein synthesis rates during resistance-type exercise.     

Metabolic adjustments to varying protein intake in harbor seals (Phoca vitulina): evidence from serum free amino acids

Effects of varying dietary protein intake on serum free amino acid (FAA) concentrations were studied in harbor seals (Phoca vitulina) fed two different prey fish diets: either exclusively low-fat, high-protein walleye pollock (Theragra chalcogramma) or high-fat, relatively high-energy-density Pacific herring (Clupea pallasi). Significant differences in FAA concentrations and patterns were observed between the two diets. All essential amino acids (EAA), except methionine and phenylalanine, and two nonessential amino acids (NEAA), glycine and tyrosine, decreased when the diet was switched from herring to pollock and increased on switching back to herring. Both total EAA concentrations and EAA : NEAA ratios decreased with the elevated protein intake typical of a low-fat pollock diet, indicating an inverse correlation between EAA concentrations and dietary protein intake levels. We propose that differing dietary protein intake, caused by differences in macronutrient composition of the two prey fish species, induced a change in protein metabolism that was reflected in blood-circulating amino acids. These findings suggest that surveys of amino acid profiles may be useful to partially determine the protein metabolic status of harbor seals.     

Whole body and skeletal muscle glutamine metabolism in healthy subjects

We measured glutamine kinetics using L-[5-15N]glutamine and L-[ring-2H5]phenylalanine infusions in healthy subjects in the postabsorptive state and during ingestion of an amino acid mixture that included glutamine, alone or with additional glucose. Ingestion of the amino acid mixture increased arterial glutamine concentrations by approximately 20% (not by 30%; P < 0.05), irrespective of the presence or absence of glucose. Muscle free glutamine concentrations remained unchanged during ingestion of amino acids alone but decreased from 21.0 +/- 1.0 to 16.4 +/- 1.6 mmol/l (P < 0.05) during simultaneous ingestion of glucose due to a decrease in intramuscular release from protein breakdown and glutamine synthesis (0.82 +/- 0.10 vs. 0.59 +/- 0.06 micromol x 100 ml leg(-1) x min(-1); P < 0.05). In both protocols, muscle glutamine inward and outward transport and muscle glutamine utilization for protein synthesis increased during amino acid ingestion; leg glutamine net balance remained unchanged. In summary, ingestion of an amino acid mixture that includes glutamine increases glutamine availability and uptake by skeletal muscle in healthy subjects without causing an increase in the intramuscular free glutamine pool. Simultaneous ingestion of glucose diminishes the intramuscular glutamine concentration despite increased glutamine availability in the blood due to decreased glutamine production.     

NOS3 is involved in the increased protein and arginine metabolic response in muscle during early endotoxemia in mice

Sepsis is a severe catabolic condition. The loss of skeletal muscle protein mass is characterized by enhanced release of the amino acids glutamine and arginine, which (in)directly affects interorgan arginine and the related nitric oxide (NO) synthesis. To establish whether changes in muscle amino acid and protein kinetics are regulated by NO synthesized by nitric oxide synthase-2 or -3 (NOS2 or NOS3), we studied C57BL6/J wild-type (WT), NOS2-deficient (NOS2-/-), and NOS3-deficient (NOS3-/-) mice under control (unstimulated) and lipopolysaccharide (LPS)-treated conditions. Muscle amino acid metabolism was studied across the hindquarter by infusing the stable isotopes L-[ring-2H5]phenylalanine, L-[ring-2H2]tyrosine, L-[guanidino-15N2]arginine, and L-[ureido-13C,2H2]citrulline. Muscle blood flow was measured using radioactive p-aminohippuric acid dilution. Under baseline conditions, muscle blood flow was halved in NOS2-/- mice (P < 0.1), with simultaneous reductions in muscle glutamine, glycine, alanine, arginine release and glutamic acid, citrulline, valine, and leucine uptake (P < 0.1). After LPS treatment, (net) muscle protein synthesis increased in WT and NOS2-/- mice [LPS vs. control: 13 +/- 3 vs. 8 +/- 1 (SE) nmol.10 g(-1).min(-1) (WT), 18 +/- 5 vs. 7 +/- 2 nmol.10 g(-1).min(-1) (NOS2-/-); P < 0.05 for LPS vs. control]. This response was absent in NOS3-/- mice (LPS vs. control: 11 +/- 4 vs. 10 +/- 2 nmol.10 g(-1).min(-1)). In agreement, the increase in muscle arginine turnover after LPS was also absent in NOS3-/- mice. In conclusion, disruption of the NOS2 gene compromises muscle glutamine release and muscle blood flow in control mice, but had only minor effects after LPS. NOS3 activity is crucial for the increase in muscle arginine and protein turnover during early endotoxemia.