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.     

Glutamine kinetics and protein turnover in end-stage renal disease

Alanine and glutamine constitute the two most important nitrogen carriers released from the muscle. We studied the intracellular amino acid transport kinetics and protein turnover in nine end-stage renal disease (ESRD) patients and eight controls by use of stable isotopes of phenylalanine, alanine, and glutamine. The amino acid transport kinetics and protein turnover were calculated with a three-pool model from the amino acid concentrations and enrichment in the artery, vein, and muscle compartments. Muscle protein breakdown was more than synthesis (nmol.min(-1).100 ml leg(-1)) during hemodialysis (HD) (169.8 +/- 20.0 vs. 125.9 +/- 21.8, P < 0.05) and in controls (126.9 +/- 6.9 vs. 98.4 +/- 7.5, P < 0.05), but synthesis and catabolism were comparable pre-HD (100.7 +/- 15.7 vs. 103.4 +/- 14.8). Whole body protein catabolism decreased by 15% during HD. The intracellular appearance of alanine (399.0 +/- 47.1 vs. 243.0 +/- 34.689) and glutamine (369.7 +/- 40.6 vs. 235.6 +/- 27.5) from muscle protein breakdown increased during dialysis (nmol.min(-1).100 ml leg(-1), P < 0.01). However, the de novo synthesis of alanine (3,468.9 +/- 572.2 vs. 3,140.5 +/- 467.7) and glutamine (1,751.4 +/- 82.6 vs. 1,782.2 +/- 86.4) did not change significantly intradialysis (nmol.min(-1).100 ml leg(-1)). Branched-chain amino acid catabolism (191.8 +/- 63.4 vs. -59.1 +/- 42.9) and nonprotein glutamate disposal (347.0 +/- 46.3 vs. 222.3 +/- 43.6) increased intradialysis compared with pre-HD (nmol.min(-1).100 ml leg(-1), P < 0.01). The mRNA levels of glutamine synthase (1.45 +/- 0.14 vs. 0.33 +/- 0.08, P < 0.001) and branched-chain keto acid dehydrogenase-E2 (3.86 +/- 0.48 vs. 2.14 +/- 0.27, P < 0.05) in the muscle increased during HD. Thus intracellular concentrations of alanine and glutamine are maintained during HD by augmented release of the amino acids from muscle protein catabolism. Although muscle protein breakdown increased intradialysis, the whole body protein catabolism decreased, suggesting central utilization of amino acids released from skeletal muscle.     

In vivo muscle amino acid transport involves two distinct processes

We have tested the hypothesis that transit through the interstitial fluid, rather than across cell membranes, is rate limiting for amino acid uptake from blood into muscle in human subjects. To quantify muscle transmembrane transport of naturally occurring amino acids, we developed a novel 4-pool model that distinguishes between the interstitial and intracellular fluid compartments. Transport kinetics of phenylalanine, leucine, lysine, and alanine were quantified using tracers labeled with stable isotopes. The results indicate that interstitial fluid is a functional compartment insofar as amino acid kinetics are concerned. In the case of leucine and alanine, transit between blood and interstitial fluid was potentially rate limiting for muscle amino acid uptake and release in the postabsorptive state. For example, in the case of leucine, the rate of transport between blood and interstitial fluid compared with the corresponding rate between interstitial fluid and muscle was 247 +/- 36 vs. 610 +/- 95 nmol.min(-1).100 ml leg(-1), respectively (P < 0.05). Our results are consistent with the process of diffusion governing transit from blood to interstitial fluid without selectivity, and of specific amino acid transport systems with varying degrees of efficiency governing transit from interstitial fluid to muscle. These results imply that changes in factors that affect the transit of amino acids from blood through interstitial fluid, such as muscle blood flow or edema, could play a major role in controlling the rate of muscle amino acid uptake.     

Acute responses of muscle protein metabolism to reduced blood flow reflect metabolic priorities for homeostasis

The present experiment was designed to measure the synthetic and breakdown rates of muscle protein in the hindlimb of rabbits with or without clamping the femoral artery. l-[ring-(13)C(6)]phenylalanine was infused as a tracer for measurement of muscle protein kinetics by means of an arteriovenous model, tracer incorporation, and tracee release methods. The ultrasonic flowmeter, dye dilution, and microsphere methods were used to determine the flow rates in the femoral artery, in the leg, and in muscle capillary, respectively. The femoral artery flow accounted for 65% of leg flow. A 50% reduction in the femoral artery flow reduced leg flow by 28% and nutritive flow by 26%, which did not change protein synthetic or breakdown rate in leg muscle. Full clamp of the femoral artery reduced leg flow by 42% and nutritive flow by 59%, which decreased (P < 0.05) both the fractional synthetic rate from 0.19 +/- 0.05 to 0.14 +/- 0.03%/day and fractional breakdown rate from 0.28 +/- 0.07 to 0.23 +/- 0.09%/day of muscle protein. Neither the partial nor full clamp reduced (P = 0.27-0.39) the intracellular phenylalanine concentration or net protein balance in leg muscle. We conclude that the flow threshold to cause a fall of protein turnover rate in leg muscle was a reduction of 30-40% of the leg flow. The acute responses of muscle protein kinetics to the reductions in blood flow reflected the metabolic priorities to maintain muscle homeostasis. These findings cannot be extrapolated to more chronic conditions without experimental validation.     

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.     

Extremity hyperinsulinemia stimulates muscle protein synthesis in severely injured patients

Insulin has a well-recognized anabolic effect on muscle protein, yet critically ill, severely injured patients are often considered "resistant" to the action of insulin. The purpose of this study was to assess the in vivo effects of hyperinsulinemia on human skeletal muscle in severely injured patients. To accomplish this goal, 14 patients with burns encompassing >40% of their body surface area underwent metabolic evaluation utilizing isotopic dilution of phenylalanine, femoral artery and vein blood sampling, and sequential muscle biopsies of the leg. After baseline metabolic measurements were taken, insulin was infused into the femoral artery at 0.45 mIU.min(-1).100 ml leg volume(-1) to create a local hyperinsulinemia but with minimal systemic perturbations. Insulin administration increased femoral venous concentration of insulin (P < 0.01) but with only a 4% (insignificant) decrease in the arterial glucose concentration and a 7% (insignificant) decrease in the arterial concentration of phenylalanine. Extremity hyperinsulinemia significantly increased leg blood flow (P < 0.05) and the rate of muscle protein synthesis (P < 0.05). Neither the rate of muscle protein breakdown nor the rate of transmembrane transport of phenylalanine was significantly altered with extremity hyperinsulinemia. In conclusion, this study demonstrates that insulin directly stimulates muscle protein synthesis in severely injured patients.     

Protein turnover and amino acid transport kinetics in end-stage renal disease

Protein and amino acid metabolism is abnormal in end-stage renal disease (ESRD). Protein turnover is influenced by transmembrane amino acid transport. The effect of ESRD and hemodialysis (HD) on intracellular amino acid transport kinetics is unknown. We studied intracellular amino acid transport kinetics and protein turnover by use of stable isotopes of phenylalanine, leucine, lysine, alanine, and glutamine before and during HD in six ESRD patients. Data obtained from amino acid concentrations and enrichment in the artery, vein, and muscle compartments were used to calculate intracellular amino acid transport and muscle protein synthesis and catabolism. Fractional muscle protein synthesis (FSR) was estimated by the precursor product approach. Despite a significant decrease in the plasma concentrations of amino acids in the artery and vein during HD, the intracellular concentrations remained stable. Outward transport of the amino acids was significantly higher than the inward transport during HD. FSR increased during HD (0.0521 +/- 0.0043 vs. 0.0772 +/- 0.0055%/h, P < 0.01). Results derived from compartmental modeling indicated that both protein synthesis (118.3 +/- 20.6 vs. 146.5 +/- 20.6 nmol.min-1.100 ml leg-1, P < 0.01) and catabolism (119.8 +/- 18.0 vs. 174.0 +/- 14.2 nmol.min-1.100 ml leg-1, P < 0.01) increased during HD. However, the intradialytic increase in catabolism exceeded that of synthesis (57.8 +/- 13.8 vs. 28.0 +/- 8.5%, P < 0.05). Thus HD alters amino acid transport kinetics and increases protein turnover, with net increase in protein catabolism.     

Amino acid metabolism in leg muscle after an endotoxin injection in healthy volunteers

Decreased plasma amino acid concentrations and increased net release of amino acids from skeletal muscle, especially for glutamine, are common features in critically ill patients. A low dose of endotoxin administered to healthy volunteers was used as a human model for the initial phase of sepsis to study the early metabolic response to sepsis. Six healthy male volunteers were studied in the postabsorptive state. Blood samples from the forearm artery and femoral vein were taken during 4 h before and 4 h after an intravenous endotoxin injection (4 ng/kg body wt). In addition, muscle biopsies from the leg muscle were taken. Plasma concentration of the total sum of amino acids decreased by 19% (P = 0.001) and of glutamine by 25% (P = 0.004) the 3rd h after endotoxin administration. At the same time, muscle concentrations of the sum of amino acids and glutamine decreased by 11% (P = 0.05) and 9% (P = 0.09), respectively. In parallel, the efflux from the leg increased by 35% (P = 0.004) for the total sum of amino acids and by 43% (P = 0.05) for glutamine. In conclusion, intravenous endotoxin administration to healthy volunteers, used as a model for the initial phase of sepsis, resulted in a decrease in plasma amino acid concentrations. At the same time, amino acid concentrations in muscle tissue decreased, whereas the efflux of amino acids from leg skeletal muscle increased.     

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.     

Growth hormone decreases muscle glutamine production and stimulates protein synthesis in hypercatabolic patients

We determined the effects of 24-h recombinant human growth hormone (rhGH) infusion into a femoral artery on leg muscle protein kinetics, amino acid transport, and glutamine metabolism in eight adult hypercatabolic trauma patients. Metabolic pathways were assessed by leg arteriovenous catheterization and muscle biopsies with the use of stable amino acid isotopes. Muscle mRNA levels of selected enzymes were determined by competitive PCR. rhGH infusion significantly accelerated the inward transport rates of phenylalanine and leucine and protein synthesis, whereas the muscle protein degradation rate and cathepsin B and UbB polyubiquitin mRNA levels were not significantly modified by rhGH. rhGH infusion decreased the rate of glutamine de novo synthesis and glutamine precursor availability, total branched-chain amino acid catabolism, and nonprotein glutamate utilization. Thus net glutamine release from muscle into circulation significantly decreased after rhGH administration ( approximately 50%), whereas glutamine synthetase mRNA levels increased after rhGH infusion, possibly to compensate for reduced glutamine precursor availability. We conclude that, after trauma, the anticatabolic action of rhGH is associated with a potentially harmful decrease in muscle glutamine production.     

Role of the ubiquitin-proteasome pathway in sepsis-induced muscle catabolism

Several lines of evidence suggest that the ubiquitin-proteasome pathway is involved in sepsis-induced muscle catabolism. The gene expression of ubiquitin and several of the proteasome subunits was increased in muscle from both septic rats and patients. In other studies, the activity of isolated 20S proteasomes was stimulated in septic muscles. Sepsis-induced increase in muscle total and myofibrillar protein breakdown was inhibited with specific proteasome blockers. Although the ubiquitin-proteasome pathway is upregulated in septic muscle, it is still unclear how the myofibrillar proteins actin and myosin are ubiquitinated and become substrates for the 26S proteasome. Recent studies suggest that a calcium-dependent, calpain-mediated process releases myofilaments from the Z-disks during sepsis. It is possible that this process exposes destabilizing N-terminal residues on actin and myosin, making them suitable substrates for the N-end rule pathway involving the 14 kD ubiquitin-conjugating enzyme E214k and the ubiquitin-protein ligase E3.     

Positive net movements of amino acids in the hindlimb after overnight food deprivation contribute to sustaining the elevated anabolism of neonatal pigs

During the neonatal period, high protein breakdown rate is a metabolic process inherent to elevated rates of protein accretion in skeletal muscle. To determine the relationship between hindlimb net movements of essential and nonessential amino acids in the regulation of hindlimb protein breakdown during an overnight fasting-feeding cycle, we infused overnight-food-deprived 10- and 28-day-old piglets with [1-(13)C]phenylalanine and [ring-(2)H(4)]tyrosine over 7 h (during 3 h of fasting and then during 4 h of feeding). Extraction rates for aspartate and glutamate after an overnight fast were 15% and 51% in the 10-day-old compared with 6% and 25% in the 28-day-old (P < 0.05) piglets, suggesting an altered requirement for precursors of amino acids to shuttle nitrogen to the liver as early life progresses. This occurred simultaneously with marginal positive hindlimb net balance of essential amino acids after an overnight fast, with negative net release of many nonessential amino acids, such as alanine, asparagine, glutamine, glycine, and proline. This suggests that newborn muscle does not undergo significant protein mobilization after a short period of fasting in support of an elevated rate of protein accretion. Furthermore, tyrosine efflux from hindlimb breakdown between overnight fasting and feeding periods was not different in the 10-day-old piglets, for which tyrosine was limiting, but when tyrosine supply balanced requirements in the 28-day-old piglet, hindlimb efflux was increased (P = 0.01). The results of the present study indicate that proteolysis and net movements of amino acids are coordinated mechanisms that sustain the elevated rate of net protein accretion during overnight feeding-fasting cycles in the neonate.     

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.     

Essential amino acids and muscle protein recovery from resistance exercise

This study tests the hypothesis that a dose of 6 g of orally administered essential amino acids (EAAs) stimulates net muscle protein balance in healthy volunteers when consumed 1 and 2 h after resistance exercise. Subjects received a primed constant infusion of L-[(2)H(5)]phenylalanine and L-[1-(13)C]leucine. Samples from femoral artery and vein and biopsies from vastus lateralis were obtained. Arterial EAA concentrations increased severalfold after drinks. Net muscle protein balance (NB) increased proportionally more than arterial AA concentrations in response to drinks, and it returned rapidly to basal values when AA concentrations decreased. Area under the curve for net phenylalanine uptake above basal value was similar for the first hour after each drink (67 +/- 17 vs. 77 +/- 20 mg/leg, respectively). Because the NB response was double the response to two doses of a mixture of 3 g of EAA + 3 g of nonessential AA (NEAA) (14), we conclude that NEAA are not necessary for stimulation of NB and that there is a dose-dependent effect of EAA ingestion on muscle protein synthesis.     

Leg glucose and protein metabolism during an acute bout of resistance exercise in humans.

The present study investigated the responses of leg glucose and protein metabolism during an acute bout of resistance exercise. Seven subjects (5 men, 2 women) were studied at rest and during a strenuous lower body resistance exercise regimen consisting of approximately 8 sets of 10 repetitions of leg press at approximately 75% 1 repetition maximum and 8 sets of 8 repetitions of knee extensions at approximately 80% 1 repetition maximum. L-[ring-2H5]phenylalanine was infused throughout the study for measurement of phenylalanine rates of appearance, disappearance, protein synthesis, and protein breakdown across the leg. Femoral arterial and venous blood samples were collected at rest and during exercise for determination of leg blood flow, concentrations of glucose, lactate, alanine, glutamine, glutamate, leucine, and phenylalanine, and phenylalanine enrichments. Muscle biopsies were obtained at rest and immediately after exercise. Leg blood flow was nearly three times (P <0.009) higher and glucose uptake more than five times higher (P=0.009) during exercise than at rest. Leg lactate release was 86 times higher than rest during the exercise bout. Although whole body phenylalanine rate of appearance, an indicator of whole body protein breakdown, was reduced during exercise; leg phenylalanine rate of appearance, rate of disappearance, protein synthesis, and protein breakdown did not change. Arterial and venous alanine concentrations and glutamate uptake were significantly higher during exercise than at rest. We conclude that lower body resistance exercise potently stimulates leg glucose uptake and lactate release. In addition, muscle protein synthesis is not elevated during a bout of resistance exercise.     

The unfolding and attempted refolding of mitochondrial aspartate aminotransferase from pig heart.

The role of prostaglandins in the regulation of muscle protein breakdown is controversial. We examined the influence of arachidonic acid (5 microM), prostaglandin E2 (PGE2) (2.8 microM) and the prostaglandin-synthesis inhibitor indomethacin (3 microM) on total and myofibrillar protein breakdown in rat extensor digitorum longus and soleus muscles incubated under different conditions in vitro. In other experiments, the effects of indomethacin, administered in vivo to septic rats (3 mg/kg, injected subcutaneously twice after induction of sepsis by caecal ligation and puncture) on plasma levels and muscle release of PGE2 and on total and myofibrillar protein breakdown rates were determined. Total and myofibrillar proteolysis was assessed by measuring production by incubated muscles of tyrosine and 3-methylhistidine respectively. Arachidonic acid or PGE2 added during incubation of muscles from normal rats did not affect total or myofibrillar protein degradation under a variety of different conditions in vitro. Indomethacin inhibited muscle PGE2 production by incubated muscles from septic rats, but did not lower proteolytic rates. Administration in vivo of indomethacin did not affect total or myofibrillar muscle protein breakdown, despite effective plasma levels of indomethacin with decreased plasma PGE2 levels and inhibition of muscle PGE2 release. The present results suggest that protein breakdown in skeletal muscle of normal or septic rats is not regulated by PGE2 or other prostaglandins.     

Gene expression profiles and protein balance in skeletal muscle of burned children after beta-adrenergic blockade

Propranolol, a nonselective beta-blocker, has been shown effective in hypermetabolic burn patients by decreasing cardiac work, protein catabolism, and lipolysis. This study investigates the effect of propranolol on gene and protein expression changes in skeletal muscle of burned children by use of high-density oligonucleotide arrays to establish the genetic profiles and stable isotope technique to quantitate protein synthesis. Thirty-seven children (mean age 9.7 +/- 1.1 yr) were randomized into groups to receive placebo (n = 23) or propranolol (n = 14) titrated to reduce heart rate by 15%. Children had >40% total body surface area burns (mean 43 +/- 5.6%). Protein net balance was determined by stable-isotope infusion technique. Total RNA from muscle biopsies was isolated, labeled, and cRNA hybridized to the HG-U95Av2 Affymetrix array. Mean net balance of protein synthesis and breakdown was -14.3 +/- 12.9 nmol. min-1. 100 ml leg volume-1 for placebo and +69.3 +/- 34.9 nmol. min-1. 100 ml leg volume-1 in the propranolol-treated children (P = 0.012). Comparison of 12,000 genes in burned children receiving placebo showed increased expression of two genes with time, whereas children receiving propranolol showed increased expression of nine genes with a decrease in five genes. We conclude that burned children receiving propranolol showed a significant upregulation in genes involved in muscle metabolism and downregulation of an important enzyme involved in gluconeogenesis and insulin resistance compared with burned children receiving placebo. The upregulation of genes involved in muscle metabolism correlates well with the increase in net protein balance across the leg.     

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.     

Coordinated increase in albumin, fibrinogen, and muscle protein synthesis during hemodialysis: role of cytokines

Serum albumin, fibrinogen levels, and lean body mass are important predictors of outcome in end-stage renal disease (ESRD). We estimated the fractional synthesis rates of albumin (FSR-A), fibrinogen (FSR-F), and muscle protein (FSR-M) in nine ESRD patients and eight controls, using primed constant infusion of l-[ring-(13)C(6)]phenylalanine. Cytokine profile and arteriovenous balance of amino acids were also measured. ESRD patients were studied before (Pre-HD) and during hemodialysis (HD). Plasma IL-6, IL-10, and C-reactive protein increased significantly during HD. Despite a decrease in the delivery of amino acids to the leg, the outflow of the amino acids increased during HD. The net balance of amino acids became more negative during HD, indicating release from the muscle. HD increased leg muscle protein synthesis (45%) and catabolism (108%) but decreased whole body proteolysis (15%). FSR-A during HD (9.7 +/- 0.9%/day) was higher than pre-HD (6.5 +/- 0.9%/day) and controls (5.8 +/- 0.5%/day, P < 0.01). FSR-F increased during HD (19.7 +/- 2.6%/day vs. 11.8 +/- 0.6%/day, P < 0.01), but it was not significantly different from that of controls (14.4 +/- 1.4%/day). FSR-M intradialysis (1.77 +/- 0.19%/day) was higher than pre-HD (1.21 +/- 0.25%/day) and controls (1.30 +/- 0.32%/day, P < 0.001). Pre-HD FSR-A, FSR-F, and FSR-M values were comparable to those of controls. There was a significant and positive correlation between plasma IL-6 and the FSRs. Thus, in ESRD patients without metabolic acidosis, the fractional synthesis rates of albumin, fibrinogen, and muscle protein are not decreased pre-HD. However, HD increases the synthesis of albumin, fibrinogen, and muscle protein. The coordinated increase in the FSRs is facilitated by constant delivery of amino acids derived from the muscle catabolism and intradialytic increase in IL-6.