The hormonal control of protein metabolism

While all the hormones described have regulatory effects on the rates of protein synthesis and breakdown there is a complex interaction between them in this control process. Insulin, GH and IGF-I play a dominant role in the day-to-day regulation of protein metabolism. In humans insulin appears to act primarily to inhibit proteolysis while GH stimulates protein synthesis. In the post-absorptive state IGF-I has acute insulin-like effects on proteolysis but in the fed state, or when substrate is provided for protein synthesis in the form of an amino acid infusion, IGF-I has been shown to stimulate protein synthesis. Growth hormone and testosterone have an important role during growth but continue to be required to maintain body protein during adulthood. Thyroid hormones are also required for normal growth and development. The hormones glucagon, glucocorticoids and adrenaline are all increased in catabolic states and may work in concert to increase protein breakdown in muscle tissue and to increase amino acid uptake in liver for gluconeogenesis. While increased glucocorticoids result in reduced muscle mass the effects of glucagon may be predominantly in the liver resulting in increased uptake of amino acids. In contrast to the catabolic effect of adrenaline on glucose and lipid metabolism, studies to date suggest that adrenaline may have an anti-catabolic effect on protein metabolism. Despite this adrenaline increases the production of the gluconeogenic amino acid alanine by muscle and its uptake by the splanchnic bed. There is considerable interest in the use of anabolic hormones, either alone or in combination, in the treatment of catabolic states. GH combined with insulin has been shown to improve whole-body and skeletal muscle kinetics while GH combined with IGF-I has a greater positive effect on protein metabolism in catabolic states than either hormone alone. If catabolic states are to be treated successfully a greater understanding of the role of the catabolic hormones in these states and the possible treatment of these states with anabolic hormones is required.     

A nitrogen-free hypocaloric diet and recombinant human growth hormone stimulate postoperative protein synthesis: fasted and fed leucine kinetics in the surgical patient

Twelve otherwise healthy patients undergoing elective surgery for resection of rectosigmoid adenocarcinoma were randomly allocated to two groups: one group receiving intravenous dextrose 5% 600 to 800 kcal.d-1 (DX, n = 6) and the other group receiving the same amount of dextrose intravenously plus recombinant human growth hormone (DX + rGH, n = 6). Supplementation with rGH started on the day of surgery and continued postoperatively for 5 days. No nitrogen was provided in the diet. This regimen was started 3 days before surgery and continued for 5 days after surgery. Protein kinetics were studied over a period of 8 hours in all patients. Following an overnight fast, a primed constant infusion of L-[1-13C]leucine was maintained for 4 hours (fasted state) and continued for a further 4 hours (fed state) during which 5% beet dextrose (low 13C content) with or without rGH was administered. The isotope studies were performed on the day before surgery and 6 days after surgery. Other measurements included urinary nitrogen excretion, gaseous exchange, and plasma concentrations of insulin, GH, and insulin-like growth factor-I (IGF-I). Addition of rGH to the dextrose diet had a significant positive effect on protein synthesis (P = .02). Surgery was responsible for a significant increase in postoperative whole-body protein breakdown and synthesis and leucine oxidation (P < .01), although lesser changes were observed in the DX group. An interaction between rGH and surgery was associated with a significant increase in protein synthesis (P = .009), but not with changes in either protein breakdown or leucine oxidation. Carbohydrate provision in the form of beet dextrose during the fed state of the isotopic study did not attenuate the significant decrease in protein synthesis (P = .01) or breakdown (P = .003) either before or after surgery, probably reflecting the absence of nitrogen in the diet. No significant interaction was found between rGH and feeding. These results of leucine kinetics indicate that addition of rGH to a low-dextrose intake in the absence of dietary nitrogen can actually promote protein synthesis. The low levels of leucine oxidation could be explained by the fact that amino acids resulting from protein degradation were directed preferentially toward resynthesis of new proteins rather than to oxidative pathways. There was a significant increase in plasma insulin and GH in the group receiving rGH (P < .05). The postoperative plasma concentration of IGF-I did not change in the latter group compared with the DX group, in which IGF-I concentration decreased significantly (P < .05) as part of the response to combined surgery and dietary restriction. Although both IGF-I and insulin are independently capable of stimulating protein synthesis, elevated levels of either hormone or GH itself may primarily modulate protein synthesis, even with a low intake of carbohydrates.     

Heritable disorder resembling neuronal storage disease in mice expressing prion protein with deletion of an alpha-helix

Ethanol abuse is frequently associated with protein malnutrition. To assess the acute effects of ethanol on whole-body protein metabolism, [1-13C]leucine kinetics were measured in eight postabsorptive normal male subjects three times, ie, during administration of two doses of ethanol (dose 1, 0.52 g/kg during 2 hours and 0.3 g/kg during 3 hours; dose 2, 0.69 g/kg during 2 hours and 0.3 g/kg during 3 hours) and during saline (controls). During the last 2 hours of the studies, glucose, insulin, and amino acids were infused to assess the effects of ethanol on protein kinetics under anabolic conditions (euglycemic clamp). The decreases in leucine flux (reflecting whole-body protein breakdown) and nonoxidative leucine disappearance (a parameter of protein synthesis) during saline infusion were abolished in both ethanol protocols (P < .05 or less v saline). The rate of leucine oxidation decreased during the higher dose of ethanol compared with saline (P < .005), indicating an anticatabolic effect. During anabolic conditions (clamp), leucine flux and nonoxidative leucine disappearance were significantly higher in both ethanol studies compared with saline (P < .05). Resting energy expenditure (REE) and oxygen consumption (VO2) during the euglycemic clamp increased to a greater degree during both ethanol studies than during saline (P < .05 or less). Thus, an elevation of blood ethanol concentrations to the levels observed in social drinking results in a net anticatabolic effect (diminished leucine oxidation) when ethanol is administered alone. However, during administration of other nutritional substrates, the anticatabolic effect was not detectable, possibly because ethanol enhanced nutrient-induced thermogenesis.     

Slow and fast dietary proteins differently modulate postprandial protein accretion

The speed of absorption of dietary amino acids by the gut varies according to the type of ingested dietary protein. This could affect postprandial protein synthesis, breakdown, and deposition. To test this hypothesis, two intrinsically 13C-leucine-labeled milk proteins, casein (CAS) and whey protein (WP), of different physicochemical properties were ingested as one single meal by healthy adults. Postprandial whole body leucine kinetics were assessed by using a dual tracer methodology. WP induced a dramatic but short increase of plasma amino acids. CAS induced a prolonged plateau of moderate hyperaminoacidemia, probably because of a slow gastric emptying. Whole body protein breakdown was inhibited by 34% after CAS ingestion but not after WP ingestion. Postprandial protein synthesis was stimulated by 68% with the WP meal and to a lesser extent (+31%) with the CAS meal. Postprandial whole body leucine oxidation over 7 h was lower with CAS (272 +/- 91 micromol.kg-1) than with WP (373 +/- 56 micromol.kg-1). Leucine intake was identical in both meals (380 micromol.kg-1). Therefore, net leucine balance over the 7 h after the meal was more positive with CAS than with WP (P < 0.05, WP vs. CAS). In conclusion, the speed of protein digestion and amino acid absorption from the gut has a major effect on whole body protein anabolism after one single meal. By analogy with carbohydrate metabolism, slow and fast proteins modulate the postprandial metabolic response, a concept to be applied to wasting situations.     

Acute growth hormone effects on amino acid and lipid metabolism

The anabolic actions of GH are well known, although specific tissue responses and the mechanism of nitrogen conservation are less well understood. This study was designed to examine the acute metabolic effects of GH on whole body and regional protein metabolism, using an experimental protocol which controlled for confounding perturbations in other hormones by a simultaneous infusion of somatostatin. Control subjects received replacement doses of insulin, glucagon, and GH for the entire 7-h study period, whereas GH subjects received an identical protocol, except for an increased dose of GH sufficient to increase serum concentrations into the high-physiological range (12-20 ng/mL) for the final 3.5 h of the study (P < 0.001). Thirteen young, healthy male subjects were studied in the postabsorptive period; five served as control subjects and eight as treatment (GH) subjects. Each received continuous iv infusions of somatostatin, L-[13-C]leucine, and L-[2H5]phenylalanine throughout the study. Femoral arterial and venous sampling allowed for simultaneous measurements across the leg and in the whole body. C-Peptide levels were suppressed throughout the infusion; insulin, glucagon, insulin-like growth factor I, cortisol, epinephrine, norepinephrine, and glucose concentrations were not different between groups. Glycerol concentrations increased 3-fold in GH subjects during the final 3.5-h period (P = 0.04). Concentrations of several amino acids declined through the study, but no differences were observed between treatment groups. Leucine oxidation was reduced in GH compared to control subjects (P = 0.04). No changes in CO2 production or whole body leucine or phenylalanine flux were observed, whereas nonoxidative disposal of leucine was marginally higher in GH compared to control subjects (P = 0.07). By contrast, rates of appearance and disappearance of both leucine and phenylalanine across the leg all were relatively lower in GH compared to control subjects; leucine balance across the leg was reduced by GH (P = 0.03), whereas phenylalanine balance was not influenced by GH. Our data thus demonstrate an acute stimulatory effect of GH on lipolysis, a decrease in leucine oxidation, and no stimulation of muscle protein synthesis in spite of enhanced protein synthesis in nonmuscle tissue.     

Defective nonoxidative leucine degradation and endogenous leucine flux in cirrhosis during an amino acid infusion

The metabolic fate of leucine's first and second carbon may be different depending on the tissue in which leucine is metabolized, as well as the prevailing hormonal milieu of that tissue. However, previous studies of leucine kinetics in humans have used only leucine labeled (as tracer) at the first carbon position. Because cirrhosis is associated with factors (such as insulin resistance and altered fuel substrate utilization) that may influence how leucine is degraded, the kinetics of leucine's first and second carbon using a simultaneous infusion of [1-14C] leucine and [2-13C] leucine were studied in the postabsorptive state and during an amino acid infusion in 6 stable cirrhotic patients and 6 matched controls. The data were normalized for different body compartments that were quantified from the dilution of H2 [180] and bromide. The body cell mass, but not body weight or fat-free body mass, was decreased in cirrhosis (P < .001). In response to the amino acid infusion, total leucine appearance from proteolysis and leucine's incorporation into protein increased significantly in both groups, but were higher in cirrhotic patients. Endogenous protein breakdown decreased in normals but remained unchanged in cirrhosis. These alterations in leucine metabolism became more prominent when data were expressed based on the body cell mass rather than on body weight. The oxidation of leucine's first carbon (C1) was decreased in cirrhosis, but the oxidation of leucine's second carbon (C2) did not differ between groups during both the postabsorptive period and the amino acid infusion, while nonoxidative leucine degradation [the difference between the oxidation of leucine's (C1) and (C2)] was also decreased in cirrhosis. In addition, there was a positive correlation between nonoxidative leucine degradation (which represents leucine incorporation into fat), and the respiratory quotient obtained from indirect calorimetry (r = .87; P < .001). These data suggest that the extent of leucine carbon oxidation is dependent on whether fat or carbohydrate is the prevailing fuel substrate. In addition, cirrhotic patients have decreased nonoxidative leucine degradation and are unable to suppress endogenous protein breakdown normally in response to amino acid administration. These abnormalities may contribute to the diminished fat stores and body cell mass commonly observed in cirrhosis.     

Differences in the stress response of Wistar-Kyoto (WKY) rats from different vendors

This study tested the hypothesis that during treatment of kwashiorkor (including marasmic kwashiorkor) with infection there is a lower rate of amino acid oxidation when the dietary intake of amino acids resembles the amino acid composition of acute phase proteins (APPs). Twenty-two children in Blantyre, Malawi, with kwashiorkor and acute infection were fed an isoenergetic, isonitrogenous diet with either egg white or milk as a protein source. The whole-body amino acid oxidation rate was measured after 24 h by determining the plasma urea rate of appearance, and whole-body protein breakdown and synthesis rates were determined from the plasma leucine rate of appearance. Plasma concentrations of C-reactive protein, alpha1-antitrypsin, tumor necrosis factor alpha (TNF-alpha), and interleukin 6 (IL-6) were determined on admission and at 24 and 48 h. The 11 children who received milk had a lower rate of amino acid oxidation than the children who received egg white (x +/- SD: 137 +/- 65 compared with 195 +/- 66 micromol urea x kg body wt(-1) x h(-1), P < 0.05). No significant differences were found between the two groups in the rate of whole-body protein breakdown or protein synthesis. The TNF-alpha concentration correlated inversely with whole-body protein breakdown and synthesis rates, and the IL-6 concentration correlated directly with C-reactive protein. We conclude that by making the amino acid composition of the diet resemble that of APPs in the treatment of acute kwashiorkor, the rate of amino acid oxidation can be decreased.     

Skeletal muscle myosin heavy chain synthesis in type 1 diabetes

Although insulin's anticatabolic effect on protein metabolism in type 1 diabetes has been clearly shown to be related to the inhibition of protein breakdown, insulin's effect on muscle protein synthesis remains controversial. Cross-limb studies and measurements of synthesis rates of mixed muscle protein have yielded conflicting results. These measurements represent the mean synthesis of several muscle proteins and may miss changes in the synthesis rates of individual muscle proteins. We measured the fractional synthesis rates of myosin heavy chain (MHC), the principal muscle contractile protein, and mixed muscle protein (MMP) in six type 1 diabetic patients during insulin deprivation and insulin treatment. Comparisons were made with six healthy control subjects. Muscle biopsies were taken at 2 h and 8 h during a primed continuous infusion of L-[1-13C]leucine. MHC was purified by a preparative continuous elution gel electrophoresis, and fractional synthesis rates were calculated. We found that in type 1 diabetic subjects, the fractional synthesis rates of MHC and MMP during insulin treatment are similar to those of control subjects. Acute insulin deprivation did not affect either the synthesis rate or the ratio of MHC to MMP in type 1 diabetic subjects. In the postabsorptive state, acute insulin deprivation has no effect on MHC or MMP synthesis in type 1 diabetic patients.     

Evidence for a catabolic role of glucagon during an amino acid load

Despite the strong association between protein catabolic conditions and hyperglucagonemia, and enhanced glucagon secretion by amino acids (AA), glucagon's effects on protein metabolism remain less clear than on glucose metabolism. To clearly define glucagon's catabolic effect on protein metabolism during AA load, we studied the effects of glucagon on circulating AA and protein dynamics in six healthy subjects. Five protocols were performed in each subject using somatostatin to inhibit the secretion of insulin, glucagon, and growth hormone (GH) and selectively replacing these hormones in different protocols. Total AA concentration was the highest when glucagon, insulin, and GH were low. Selective increase of glucagon levels prevented this increment in AA. Addition of high levels of insulin and GH to high glucagon had no effect on total AA levels, although branched chain AA levels declined. Glucagon mostly decreased glucogenic AA and enhanced glucose production. Endogenous leucine flux, reflecting proteolysis, decreased while leucine oxidation increased in protocols where AA were infused and these changes were unaffected by the hormones. Nonoxidative leucine flux reflecting protein synthesis was stimulated by AA, but high glucagon attenuated this effect. Addition of GH and insulin partially reversed the inhibitory effect of glucagon on protein synthesis. We conclude that glucagon is the pivotal hormone in amino acid disposal during an AA load and, by reducing the availability of AA, glucagon inhibits protein synthesis stimulated by AA. These data provide further support for a catabolic role of glucagon at physiological concentrations.     

Leucine metabolism during chronic ethanol consumption

Chronic ethanol consumption is known to increase plasma concentrations of branched-chain amino acids (BCAA) in rats and man, but the mechanisms of this effect are not known. Chronic ethanol consumption may increase levels of BCAA by altering protein turnover and/or by affecting the oxidation of BCAA. These possibilities were investigated in rats pair-fed liquid diets containing either 0% or 36% of total calories as ethanol for 21 days. In the fed state, ethanol-treated rats had a plasma ethanol level of 20 +/- 5 mmol/L and twofold increases in BCAA concentrations in plasma. There were also significant increases (37% to 63%) in muscle, liver, and jejunal mucosa BCAA concentrations. Chronic ethanol consumption significantly increased whole-body rates (mumol/100 g/h) of leucine turnover (73.8 +/- 7.5 v 104 +/- 5.6, P < .01) and oxidation (12.0 +/- 1.7 v 17.7 +/- 1.1, P < .05). In contrast, it significantly decreased leucine incorporation (nmol/mg protein/240 min) into both muscle (0.61 +/- 0.07 v 0.35 +/- 0.05, P < .01) and liver (13.25 +/- 1.40 v 6.78 +/- 0.98, P < .01) proteins. Incorporation of leucine into the mucosal proteins of jejunum (17.42 +/- 1.42 v 15.85 +/- 1.90, P = NS) was not significantly altered by ethanol. These results suggest that reduced protein synthesis and/or increased protein breakdown may account for the elevated tissue BCAA concentrations in chronic ethanol consumption. The consequences of these increased tissue concentrations are increases in tissue oxidation and plasma concentrations of BCAA.     

Ethanol impairs post-prandial hepatic protein metabolism

The effects of acute ethanol ingestion on whole body and hepatic protein metabolism in humans are not known. To simulate social drinking, we compared the effects of the association of a mixed meal (632 kcal, 17% amino acids, 50% glucose, 33% lipids) with a bottle of either table wine (ethanol content 71 g) or water on the estimates ([1-14C]-leucine infusion) of whole body protein breakdown, oxidation, and synthesis, and on the intravascular fractional secretory rates (FSR) of hepatically (albumin, fibrinogen) and extrahepatically (IgG) synthesized plasma proteins in two randomized groups (ethanol n = 7, water n = 7) of healthy nonalcoholic volunteers. Each study was carried out for 8 h. Protein kinetics were measured in the overnight post-absorptive state, over the first 4 h, and during a meal infusion (via a nasogastric feeding tube at constant rate) combined with the oral ingestion of wine or water, over the last 4 h. When compared with water, wine ingestion during the meal reduced (P < 0.03) by 24% the rate of leucine oxidation, did not modify the estimates of whole body protein breakdown and synthesis, reduced (P < 0.01) by approximately 30% the FSR of albumin and fibrinogen, but did not affect IgG FSR. In conclusion, 70 g of ethanol, an amount usual among social drinkers, impairs hepatic protein metabolism. The habitual consumption of such amounts by reducing the synthesis and/or secretion of hepatic proteins might lead to the progressive development of liver injury and to hypoalbuminemia also in the absence of protein malnutrition.     

Changes in hepatic nitrogen balance in plasma concentrations of amino acids and hormones and in cell volume after overnight fasting in perinatal and adult rat

Important regulatory factors of intrahepatic protein synthesis and proteolysis are amino acids, glucagon, insulin, and cell volume. We have investigated the changes in these factors with development and after an overnight fast and evaluated their contribution to changes in the hepatic nitrogen balance in vivo. In the fed state, glucagon levels were highest in suckling animals and gradually declined in older rats, whereas the concentration of insulin increased during development. The amino acid concentrations in liver and plasma declined during the suckling period to levels that in vitro are highly permissive for induction of autophagic proteolysis. In all age groups investigated, fasting was associated with a drop in hepatic protein content, together with a marked decrease in hepatocellular volume and insulin concentrations. On the other hand, glucagon concentrations and the concentration of many amino acids in plasma and liver responded to fasting with a pronounced decrease in perinatal and suckling animals, but this response had become blunted at weaning and had disappeared in adult animals. These findings suggest that insulin and/or hepatocellular volume are more likely candidates as short-term physiologic regulators of the hepatic nitrogen balance than are glucagon or amino acids. In glucose-supplemented fetuses, high levels of insulin could not compensate for a decreased hepatocellular volume in averting a catabolic state, suggesting that cell volume is the more important factor. Although our study cannot discriminate between the effects of fasting on protein synthesis and degradation, our findings show unequivocally that, for a rapid growth of the liver, suckling animals have to be fed around-the-clock.     

Effects of stress on amino acids and related compounds in various tissues of fasted rats

The purpose of this study was to examine the effect of stress on the free amino acid pattern of plasma and various organs. Two groups of rats were deprived of food, for 24 hrs. One group was sacrificed after this time (fasting control representing mostly free endogenous amino acids) and the second group was first restrained in wire cages for 120 min before being sacrificed (fasting stress representing mostly the effects of stress on endogenous free amino acids). A third group had free access to food and was sacrificed at the same time (fed control representing mostly free amino acids absorbed from the gut and endogenous free amino acid metabolism). Fasting (as compared to fed controls) reduced alanine and arginine but increased ethanolamine, glutamic acid and glutamine in the plasma; increased ethanolamine, phosphoethanolamine and glutamic acid in the liver; increased carnosine, glutamic acid, phosphoethanolamine and glutamine in the ventricle; increased oxidized glutathione in the aorta; decreased alanine, aspartic acid, glutamic acid, leucine and methionine and increased glutamine in the pancreas; and decreased arginine in skeletal muscle. Fasting plus stress (as compared to fasting controls) reduced alanine and glutamine in the plasma; increased methionine in the liver; increased ethanolamine, GABA, and glutamic acid in the aorta; reduced arginine, glutamic acid, glutamine, leucine and methionine but increased ethanolamine in the ventricle; reduced ammonia and ethanolamine but increased histidine, isoleucine, leucine, lysine, phenylalanine, tyrosine and valine in the pancreas; and reduced ammonia in skeletal muscle. Fasting plus stress affects the amino acid composition of plasma and various of tissues but effects seen were individually different and strongly substance and tissue specific. Plasma changes did not coincide with tissue changes. Changes in the endogenous pattern of amino acids and related compounds in response to stress could be first indications of stress induced organ pathology.     

Molecular cloning of Saccharomyces cerevisiae MLF4/SSH4 gene which confers the immunosuppressant leflunomide resistance

Otsuka Long-Evans Tokushima Fatty (OLETF) rats showed that the distribution of plasma membrane content of insulin-regulated glucose transporter in skeletal muscle was reminiscent of that in human non-insulin-dependent diabetes mellitus (NIDDM). To obtain more information on the cellular mechanisms of muscle insulin resistance, hexokinase activities were measured in the skeletal muscle of OLETF rats. The results showed that the activity of the type II enzyme in the diabetic rats was significantly decreased (P < 0.05) compared with Long-Evans Tokushima Otsuka (LETO) control rats. No significant differences in the activity of the type I hexokinase were observed between these rats. Western blot analysis showed that the protein content of the type II in OLETF rats was also significantly lower than that in LETO rats (P < 0.05). After insulin stimulation, the intramuscular content of glucose 6-phosphate, which regulates glycogen synthesis in skeletal muscle, was significantly decreased in OLETF rats (P < 0.01). However, glycogen synthase activity in vitro and intramuscular lactate concentration in these rats did not show significant differences. These results suggest that the G6P content of the diabetic rats is decreased as a result of an impaired early event of glucose metabolism, indicating that the molecular defects of skeletal muscle in OLETF rats are similar to those in NIDDM patients.     

Diurnal changes in plasma amino acids in maple syrup urine disease

Protein turnover is a cyclic process with a net loss of protein in the (catabolic) fasted state and a net gain in the (anabolic) fed state. In maple syrup urine disease (MSUD) the early block of degradation of the branched-chain amino acids (BCAA) brings about the opportunity for evaluation of the diurnal variation in net protein anabolism and catabolism by studying cyclic changes in the plasma concentrations of BCAA. The alterations in plasma BCAA in a 3-y-old boy with classical MSUD were studied in the fed and fasted state over a period of 19 months. For each amino acid a total of 34 data pairs was calculated. The plasma concentrations of the BCAA leucine, valine and isoleucine were constantly higher in the fasted than in the fed state. Plasma concentrations of alloisoleucine, being a non-protein amino acid, did not participate in cyclic changes. In contrast, the essential amino acid pair tyrosine and phenylalanine increased after meals. The fasting concentration of alanine increased after feeding, while glycine did not change significantly. Healthy subjects show a decrease in all amino acids in the fasted (mild catabolic) state and an increase in the fed state. These findings in MSUD suggest a net decrease in non-BCAA as result of a greater rate of amino acid oxidation rate than of protein breakdown and a net entry of BCAA into plasma in the fasted state due to the specific metabolic block. Such changes in amino acid plasma pools have to be taken into account during monitoring of treatment and especially when in vivo leucine oxidation is assessed.     

Exogenous insulin reduces proteolysis and protein synthesis in extremely low birth weight infants

OBJECTIVE: To determine the effect of a continuous insulin infusion on protein and glucose metabolism in extremely low birth weight (ELBW) infants. STUDY DESIGN: We measured the rate of appearance (Ra) of the essential amino acids leucine and phenylalanine (reflecting proteolysis), utilization of phenylalanine for protein synthesis, and glucose Ra using stable isotope tracers during a basal infusion of glucose (6 mg/kg/min) and in response to a continuous infusion of insulin (0.05 U/kg/hr) by means of the euglycemic hyperinsulinemic clamp technique. Four clinically stable, euglycemic ELBW infants (26 +/- 0 weeks' gestation, 894 +/- 44 gm birth weight, 2.8 +/- 0.8 days of age) were studied. RESULTS: In response to a greater than tenfold increase in insulin concentration (from 7 +/- 2 to 79 +/- 13 microU/ml), there was a 20% decrease in leucine Ra (Basal: 272 +/- 27 mumol/kg/hr; Insulin: 226 +/- 29 mumol/kg/hr; p < 0.01) and in phenylalanine Ra (Basal: 91 +/- 5 mumol/kg/hr; Insulin: 72 +/- 2 mumol/kg/hr; p < 0.05). Use of phenylalanine for protein synthesis also decreased by a similar magnitude (Basal: 77 +/- 4 mumol/kg/hr; Insulin: 62 +/- 1 mumol/kg/hr; p < 0.05). Glucose utilization doubled (from 8 +/- 0.9 to 15.7 +/- 1.1 mg/kg/min; p = 0.0003) and plasma lactate concentrations tripled (from 2.1 +/- 0.5 to 5.7 +/- 1.0 mmol/L; p < 0.05) during the insulin infusion. CONCLUSIONS: During an infusion of glucose alone, pharmacologic concentrations of insulin in ELBW infants produced no net protein anabolic effect. Furthermore, euglycemic hyperinsulinemia was accompanied by development of significant metabolic acidosis.     

Catabolic hormones alone fail to reproduce the stress-induced efflux of amino acids

OBJECTIVE: To determine the impact of catabolic hormones on the pattern of amino acid efflux from human skeletal muscle during stress. DESIGN: Cohort analytical study. SETTING: Burn intensive care unit and clinical research facility at a university hospital. PATIENTS: Five patients with severe burns and five healthy volunteers of similar size and age. INTERVENTIONS AND MEASUREMENTS: The net balance of amino acids across the leg was determined in five healthy volunteers prior to and following a 2-hour infusion of the catabolic hormones epinephrine, cortisol, and glucagon into the femoral artery. These results were compared with amino acid net balance measurements in five severely burned patients. RESULTS: Hormonal simulation of stress in the normal volunteers increased glutamine efflux from the leg to an extent similar to that of the burn patients. Alanine efflux, however, was not affected by the hormonal infusion. Because alanine efflux constituted a major proportion of the total peripheral amino acid catabolism in the burn patients, there was significantly less total amino acid nitrogen loss from the healthy volunteers receiving the stress hormones. CONCLUSIONS: Catabolic hormones alone fail to reproduce the stress-induced pattern and quantity of amino acid efflux from human skeletal muscle. This discrepancy is largely due to an unresponsiveness of alanine to hormonally induced muscle protein catabolism.     

Mechanisms of liver and muscle insulin resistance induced by chronic high-fat feeding

To elucidate cellular mechanisms of insulin resistance induced by excess dietary fat, we studied conscious chronically high-fat-fed (HFF) and control chow diet-fed rats during euglycemic-hyperinsulinemic (560 pmol/l plasma insulin) clamps. Compared with chow diet feeding, fat feeding significantly impaired insulin action (reduced whole body glucose disposal rate, reduced skeletal muscle glucose metabolism, and decreased insulin suppressibility of hepatic glucose production [HGP]). In HFF rats, hyperinsulinemia significantly suppressed circulating free fatty acids but not the intracellular availability of fatty acid in skeletal muscle (long chain fatty acyl-CoA esters remained at 230% above control levels). In HFF animals, acute blockade of beta-oxidation using etomoxir increased insulin-stimulated muscle glucose uptake, via a selective increase in the component directed to glycolysis, but did not reverse the defect in net glycogen synthesis or glycogen synthase. In clamp HFF animals, etomoxir did not significantly alter the reduced ability of insulin to suppress HGP, but induced substantial depletion of hepatic glycogen content. This implied that gluconeogenesis was reduced by inhibition of hepatic fatty acid oxidation and that an alternative mechanism was involved in the elevated HGP in HFF rats. Evidence was then obtained suggesting that this involves a reduction in hepatic glucokinase (GK) activity and an inability of insulin to acutely lower glucose-6-phosphatase (G-6-Pase) activity. Overall, a 76% increase in the activity ratio G-6-Pase/GK was observed, which would favor net hepatic glucose release and elevated HGP in HFF rats. Thus in the insulin-resistant HFF rat 1) acute hyperinsulinemia fails to quench elevated muscle and liver lipid availability, 2) elevated lipid oxidation opposes insulin stimulation of muscle glucose oxidation (perhaps via the glucose-fatty acid cycle) and suppression of hepatic gluconeogenesis, and 3) mechanisms of impaired insulin-stimulated glucose storage and HGP suppressibility are not dependent on concomitant lipid oxidation; in the case of HGP we provide evidence for pivotal involvement of G-6-Pase and GK in the regulation of HGP by insulin, independent of the glucose source.     

Protein synthesis and degradation change rapidly in response to food intake in muscle of food-deprived mice

The short-term changes in muscle protein synthesis and degradation after food intake are unclear. We investigated muscle protein metabolism after food intake in mice that were starved for 18 h and refed for 1 h. Protein synthesis activity was estimated by the polysome profiles, and protein degradation was estimated by plasma N tau-methylhistidine (MeHis) concentration, reflecting translational activity and myofibrillar protein degradation, respectively. MeHis is an index of myofibrillar protein degradation because it is not reused for protein synthesis and it is not metabolized. Stimulation of protein synthesis (polysome profile) and the reduction of protein degradation (plasma N tau-methylhistidine concentration) were observed immediately after feeding began. Protein synthesis returned to the prefeeding level by 6 h after refeeding, whereas protein degradation remained at a low level. The decreased plasma MeHis concentration after refeeding was not due to a decrease in MeHis release from muscle cells and an increase in the free MeHis pool size, because the changes in free MeHis concentration in muscle were similar to that of plasma. Plasma insulin concentration immediately rose with feeding and it returned to the prefeeding level by 3 h after refeeding. These results suggest that responses of postprandial protein metabolism are very rapid and that protein synthesis is regulated by insulin, whereas degradation is regulated by insulin and other dietary factors. Thus the ability of skeletal muscle to use nutrients more effectively by stimulating protein synthesis and reducing protein degradation may cause the accelerated rate of protein accretion in skeletal muscle during the short postprandial period.