Insulin fails to stimulate muscle protein synthesis in sepsis despite unimpaired signaling to 4E-BP1 and S6K1

Induction of sepsis in rats causes an inhibition of protein synthesis in skeletal muscle that is resistant to the stimulatory actions of insulin. To gain a better understanding of the underlying reason for this lack of response, the present study was undertaken to investigate sepsis-induced alterations in insulin signaling to regulatory components of mRNA translation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Sepsis resulted in a 50% reduction in protein synthesis in the gastrocnemius. Protein synthesis in muscles from septic rats, but not controls, was unresponsive to stimulation by insulin. The insulin-induced hyperphosphorylation response of the translation repressor protein 4E-binding protein 1 (4E-BP1) and of the 70-kDa S6 kinase (S6K1) (1), two targets of insulin action on mRNA translation, was unimpaired in gastrocnemius of septic rats. Hyperphosphorylation of 4E-BP1 in response to insulin resulted in its dissociation from the inactive eukaryotic initiation factor (eIF)4E. 4E-BP1 complex in both control and septic rats. However, assembly of the active eIF4F complex as assessed by the association of eIF4E with eIF4G did not follow the pattern predicted by the increased availability of eIF4E resulting from changes in the phosphorylation of 4E-BP1. Indeed, sepsis caused a dramatic reduction in the amount of eIF4G associated with eIF4E in the presence or absence of insulin. Thus the inability of insulin to stimulate protein synthesis during sepsis may be related to a defect in signaling to a step in translation initiation involved in assembly of an active eIF4F complex.

Effects of chronic alcohol consumption on regulation of myocardial protein synthesis

Heart disease represents an important etiology of mortality in chronic alcoholics. The purpose of the present study was to examine potential mechanisms for the inhibitory effect of chronic alcohol exposure (16 wk) on the regulation of myocardial protein metabolism. Chronic alcohol feeding resulted in a lower heart weight and 25% loss of cardiac protein per heart compared with pair-fed controls. The loss of protein mass resulted in part from a diminished (30%) rate of protein synthesis. Ethanol exerted its inhibition of protein synthesis through diminished translational efficiency rather than lower RNA content. Chronic ethanol administration decreased the abundance of eukaryotic initiation factor (eIF)4G associated with eIF4E in the myocardium by 36% and increased the abundance of the translation response protein (4E-BP1) associated with eIF4E. In addition, chronic alcohol feeding significantly reduced the extent of p70S6 kinase (p70(S6K)) phosphorylation. The decreases in the phosphorylation of 4E-BP1 and p70(S6K) did not result from a reduced abundance of mammalian target of rapamycin (mTOR). These data suggest that a chronic alcohol-induced impairment in myocardial protein synthesis results in part from inhibition in peptide chain initiation secondary to marked changes in eIF4E availability and p70(S6K) phosphorylation.

Amino acids and insulin are both required to regulate assembly of the eIF4E. eIF4G complex in rat skeletal muscle

The respective roles of insulin and amino acids in regulation of skeletal muscle protein synthesis and degradation after feeding were examined in rats fasted for 17 h and refed over 1 h with either a 25 or a 0% amino acid/protein meal. In each nutritional condition, postprandial insulin secretion was either maintained (control groups: C(25) and C(0)) or blocked with diazoxide injections (diazoxide groups: DZ(25) and DZ(0)). Muscle protein metabolism was examined in vitro in epitrochlearis muscles. Only feeding the 25% amino acid/protein meal in the presence of increased plasma insulin concentration (C(25) group) stimulated protein synthesis and inhibited proteolysis in skeletal muscle compared with the postabsorptive state. The stimulation of protein synthesis was associated with increased phosphorylation of eukaryotic initiation factor (eIF)4E binding protein-1 (4E-BP1), reduced binding of eIF4E to 4E-BP1, and increased assembly of the active eIF4E. eIF4G complex. The p70 S6 kinase (p70(S6k)) was also hyperphosphorylated in response to the 25% amino acid/protein meal. Acute postprandial insulin deficiency induced by diazoxide injections totally abolished these effects. Feeding the 0% amino acid/protein meal with or without postprandial insulin deficiency did not stimulate muscle protein synthesis, reduce proteolysis, or regulate initiation factors and p70(S6k) compared with fasted rats. Taken together, our results suggest that both insulin and amino acids are required to stimulate protein synthesis, inhibit protein degradation, and regulate the interactions between eIF4E and 4E-BP1 or eIF4G in response to feeding.

Alcohol myopathy: impairment of protein synthesis and translation initiation

Alcohol consumption leads to numerous morphological, biochemical and functional changes in skeletal and cardiac muscle. One such change observed in both tissues after either acute alcohol intoxication or chronic alcohol consumption is a characteristic decrease in the rate of protein synthesis. A decrease in translation efficiency appears to be responsible for at least part of the reduction. This review highlights advances in determining the molecular mechanisms by which alcohol impairs protein synthesis and places these observations in context of earlier studies on alcoholic myopathy. Both acute and chronic alcohol administration impairs translational control by modulating various aspects of peptide-chain initiation. Moreover, this alcohol-induced impairment in initiation is associated with a decreased availability of eukaryotic initiation factor (eIF) 4E in striated muscle, as evidenced by an increase in the amount of the inactive eIF4E.4E-BP1 complex and decrease in the active eIF4E.eIF4G complex. In contrast, alcohol does not produce consistent alterations in the control of translation initiation by the eIF2 system. The etiology of these changes remain unresolved. However, defects in the availability or effectiveness of various anabolic hormones, particularly insulin-like growth factor-I, are consistent with the alcohol-induced decrease in protein synthesis and translation initiation.

Selected Contribution: IGF-I antibody prevents increases in protein synthesis in epitrochlearis muscles from refed, diabetic rats

The purpose of this study was to examine whether immune neutralization of muscle-produced insulin-like growth factor I (IGF-I) would prevent an appropriate anabolic response to refeeding in diabetic rats. Male Sprague-Dawley rats were made diabetic by partial pancreatectomy and were randomly assigned to be either control-fed, fasted, or fasted-refed (n = 7-8 per group). Diabetes decreased rates of protein synthesis and increased rates of protein degradation in incubated epitrochlearis muscles (P < 0.05). In both groups of rats, fasting lowered protein synthesis and increased proteolysis and subsequent refeeding returned both parameters to near basal values (P < 0.05). Neutralization of muscle IGF-I by the addition of IGF-I antibody to the incubation medium reduced protein synthesis an average of 22% for all groups (P < 0.05). However, rates of protein degradation were not affected. In nondiabetic rats, refeeding increased protein synthesis in both control and antibody-treated muscles (P < 0.05). Refeeding also increased protein synthesis in the control muscles from diabetic rats (P < 0.01). In contrast, muscles from diabetic rats that were incubated with anti-IGF-I did not increase protein synthesis in response to refeeding. These data suggest that immune neutralization of muscle IGF-I in hypoinsulinemic rats negated the ability of endogenous IGF-I to promote protein synthesis and thereby prevented an appropriate anabolic response.

Impaired myocardial protein synthesis induced by acute alcohol intoxication is associated with changes in eIF4F

The purpose of the present study was to examine potential mechanisms for the known inhibitory effect of acute alcohol exposure on myocardial protein synthesis. Rats were injected intraperitoneally with either ethanol (75 mmol/kg) or saline, and protein synthesis was measured in vivo 2.5 h thereafter by use of the flooding-dose L-[(3)H]phenylalanine technique. Rates of myocardial protein synthesis and translational efficiency in alcohol-treated rats were decreased compared with control values. Free (nonpolysome bound) 40S and 60S ribosomal subunits were increased 50% after alcohol treatment, indicating an impaired peptide-chain initiation. To identify mechanisms responsible for this impairment, several eukaryotic initiation factors (eIF) were analyzed. Acute alcohol intoxication did not significantly alter the myocardial content of eIF2 alpha or eIF2B epsilon, the extent of eIF2 alpha phosphorylation, or the activity of eIF2B. Acute alcohol exposure increased the binding of 4E-binding protein 1 (4E-BP1) to eIF4E (55%), diminished the amount of eIF4E bound to eIF4G (70%), reduced the amount of 4E-BP1 in the phosphorylated gamma-form (40%), and decreased the phosphorylation of p70S6 kinase and the ribosomal protein S6. There was no significant difference in either the plasma insulin-like growth factor (IGF) I concentration (total or free) or expression of IGF-I or IGF-II mRNA in heart between the two groups. These data suggest that the acute alcohol-induced impairment in myocardial protein synthesis results, in part, from an inhibition in peptide-chain initiation, which is associated with marked changes in eIF4E availability and p70S6 kinase phosphorylation but is independent of changes in the eIF2/2B system and IGFs.

IGF-I/IGFBP-3 binary complex modulates sepsis-induced inhibition of protein synthesis in skeletal muscle

The present study evaluated the ability of insulin-like growth factor I (IGF-I) complexed with IGF binding protein-3 (IGFBP-3) to modulate the sepsis-induced inhibition of protein synthesis in gastrocnemius. Beginning 16 h after the induction of sepsis, either the binary complex or saline was injected twice daily via a tail vein, with measurements made 3 and 5 days later. By day 3, sepsis had reduced plasma IGF-I concentrations approximately 50% in saline-treated rats. Administration of the binary complex provided exogenous IGF-I to compensate for the sepsis-induced diminished plasma IGF-I. Sepsis decreased rates of protein synthesis in gastrocnemius relative to controls by limiting translational efficiency. Treatment of septic rats with the binary complex for 5 days attenuated the sepsis-induced inhibition of protein synthesis and restored translational efficiency to control values. Assessment of potential mechanisms regulating translational efficiency showed that neither the sepsis-induced change in gastrocnemius content of eukaryotic initiation factor 2B (eIF2B), the amount of eIF4E associated with 4E binding protein-1 (4E-BP1), nor the phosphorylation state of 4E-BP1 or eIF4E were altered by the binary complex. Overall, the results are consistent with the hypothesis that decreases in plasma IGF-I are partially responsible for enhanced muscle catabolism during sepsis.

Effect of sepsis on eIE4E availability in skeletal muscle

Chronic septic abscess formation causes an inhibition of protein synthesis in gastrocnemius that is not observed in rats with a sterile abscess. The inhibition is associated with an impaired translation initiation. The present study was designed to investigate the effects of sepsis on phosphorylation and availability of eukaryotic initiation factor (eIF)4E in gastrocnemius 5 days after induction of a sterile or septic abscess. Neither sepsis nor sterile inflammation altered the extent of eIF4E phosphorylation. Moreover, no changes in the amount of the binding protein 4E-BP1 associated with eIF4E or in the phosphorylation of 4E-BP1 were observed during sepsis or sterile inflammation. In contrast, sepsis and sterile inflammation caused a reduction in the relative amount of eIF4G bound to eIF4E compared with controls. The diminished amount of eIF4G bound to eIF4E was not the result of a reduced abundance of eIF4E. Sepsis, but not sterile inflammation, caused an increase in the cellular abundance of eIF4E. The results provide evidence that alterations in the eIF4E system are probably not rate controlling for the synthesis of total, mixed proteins in gastrocnemius during sepsis. Instead, on the basis of our previous studies, changes in eIF2B appear to be responsible for limiting protein synthesis in skeletal muscle during sepsis.

Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway

The objectives of the present study were twofold: 1) to determine whether leucine is unique among the branched-chain amino acids (BCAA) in its ability to stimulate protein synthesis in skeletal muscle of food-deprived rats; and 2) to investigate whether changes in muscle protein synthesis after leucine administration involve a signaling pathway that includes the protein kinase mammalian target of rapamycin (mTOR). In the first set of experiments, food-deprived (18 h) male rats (200 g) were orally administered saline or 270 mg valine, isoleucine or leucine. In the second set of experiments, food-deprived rats were injected intravenously with rapamycin (0.75 mg/kg), a specific inhibitor of mTOR, before leucine administration. Only leucine stimulated protein synthesis in skeletal muscle above saline-treated controls (P: < 0.05). Furthermore, leucine was most effective among the BCAA at enhancing phosphorylation of eukaryotic initiation factor (eIF), 4E binding protein 1 (4E-BP1) and the 70-kDa ribosomal protein S6 kinase (S6K1). Leucine-dependent hyperphosphorylation of 4E-BP1 increased the availability of eIF4E to form the active eIF4G.eIF4E complex. To a lesser extent, isoleucine also enhanced phosphorylation of 4E-BP1 and S6K1. Rapamycin inhibited protein synthesis in both leucine-treated and food-deprived rats. Additionally, rapamycin prevented the stimulatory effects of leucine on eIF4E availability for binding eIF4G and inhibited leucine-dependent phosphorylation of S6K1. The data demonstrate that leucine is unique among the BCAA in its ability to stimulate protein synthesis in muscle of food-deprived rats. We show for the first time that leucine-dependent stimulation of translation initiation in vivo occurs via a rapamycin-sensitive pathway.

Effects of tumor necrosis factor-binding protein on hepatic protein synthesis during chronic sepsis

BACKGROUND

Cytokines are thought to play a role in the stimulation of protein synthesis in liver during inflammation and sepsis. We previously showed that administration of tumor necrosis factor-binding protein (TNFbp) prevents the sepsis-induced inhibition of protein synthesis in skeletal muscle. The purpose of the present set of experiments was to investigate the effect of TNFbp on hepatic protein synthesis and its ability to modulate the mechanisms responsible for increased hepatic protein synthesis during chronic (5-day) intraabdominal sepsis.

MATERIALS AND METHODS

We examined the effects of TNFbp on hepatic protein synthesis during sepsis in four groups of rats: control, control + TNFbp, septic, and septic + TNFbp. Saline (1.0 ml) or TNFbp (1 mg/kg, 1.0 ml) was injected daily starting 4 h prior to the induction of sepsis. The effect of sepsis and TNFbp administration on hepatic protein synthesis in vivo was examined 5 days later.

RESULTS

Sepsis increased the rate of protein synthesis by 35% relative to controls. Accelerated rates of protein synthesis were accompanied by increased total RNA content, eukaryotic initiation factor (eIF) 2alpha content, and phosphorylation of p70S6 kinase. Injection of TNFbp into septic rats for 5 days did not diminish the sepsis-induced stimulation of hepatic protein synthesis. Compared with controls, septic rats treated with TNFbp also showed elevated total RNA content, elF2alpha content, and phosphorylation of p70S6 kinase. No significant differences in any of the parameters measured were observed between untreated and TNFbp-treated septic rats. Treatment of control animals with TNFbp for 5 days was without effect on any of the parameters examined.

CONCLUSIONS

TNFbp did not prevent the sepsis-induced stimulation of hepatic protein metabolism or modulate the septic-induced changes in factors regulating protein synthesis. Global rates of protein synthesis in livers from septic rats are accelerated by increases in the abundance or activity of components of translational apparatus.

Endotoxin-induced decrease in muscle protein synthesis is associated with changes in eIF2B, eIF4E, and IGF-I

The present study examined potential mechanisms contributing to the inhibition of protein synthesis in skeletal muscle after administration of endotoxin (LPS). Rats implanted with vascular catheters were injected intravenously with a nonlethal dose of Escherichia coli LPS, and samples were collected at 4 and 24 h thereafter; pair-fed control animals were also included. The rate of muscle (gastrocnemius) protein synthesis in vivo was reduced at both time points after LPS administration. LPS did not alter tissue RNA content, but the translational efficiency was consistently reduced at both time points. To identify mechanisms responsible for regulating translation, we examined several eukaryotic initiation factors (eIFs). The content of eIF2alpha or the amount of eIF2alpha in the phosphorylated form did not change in response to LPS. eIF2B activity was decreased in muscle 4 h post-LPS but activity returned to control values by 24 h. A decrease in the relative amount of eIF2Balpha protein was not responsible for the LPS-induced reduction in eIF2B activity. LPS also markedly altered the distribution of eIF4E in muscle. Compared with control values, LPS-treated rats demonstrated 1) a transient increase in binding of the translation repressor 4E-binding protein-1 (4E-BP1) with eIF4E, 2) a transient decrease in the phosphorylated gamma-form of 4E-BP1, and 3) a sustained decrease in the amount of eIF4G associated with eIF4E. LPS also decreased insulin-like growth factor (IGF) I protein and mRNA expression in muscle at both times. A significant linear relationship existed between muscle IGF-I and the rate of protein synthesis or the amount of eIF4E bound to eIF4G. In summary, these data suggest that LPS impairs muscle protein synthesis, at least in part, by decreasing translational efficiency, resulting from an impairment in translation initiation associated with alterations in both eIF2B activity and eIF4E availability.

Growth hormone does not attenuate the inhibitory effects of sepsis on wound healing

Chronic abdominal sepsis is associated with impaired tissue repair. Treatment of burn patients with growth hormone results in improved healing of skin graft donor sites. The goal of this study was to determine whether administration of growth hormone could attenuate the inhibitory effects of sepsis on cutaneous wound healing. Four groups of male Sprague Dawley rats were studied: control, control + growth hormone, sepsis, and sepsis + growth hormone. Sepsis was caused by implantation of a bacterial focus in the peritoneal cavity. Control animals underwent sham laparotomy, and polyvinyl alcohol sponge implants were placed in subdermal pockets in all animals. Saline or growth hormone (400 microg) was injected subcutaneously every 12 hours. On day 5, the incisional wounds and polyvinyl alcohol sponge implants were harvested. The breaking strength of abdominal incisions was measured. Granulation tissue penetration and quality were determined by scoring polyvinyl alcohol sponge implant histology from 1 to 4 in a blinded fashion. Collagen deposition in polyvinyl alcohol sponge implants was quantitated by hydroxyproline assay. Septic mortality was not altered by growth hormone administration. Septic animals showed a reduction in food consumption for 2 days after surgery (p < 0.05 vs. controls), which was not affected by growth hormone administration. The breaking strength of incisional wounds and hydroxyproline content of polyvinyl alcohol sponge implants was reduced in septic rats (p < 0.001 vs. controls) but administration of growth hormone for 5 days did not improve breaking strength or collagen deposition in either group. We conclude that the administration of growth hormone for 5 days did not improve collagen deposition or breaking strength in cutaneous wounds from control or septic animals. The results suggest that growth hormone treatment is unlikely to improve tissue repair in sepsis-induced catabolic illness.

Impaired protein synthesis induced by acute alcohol intoxication is associated with changes in eIF4E in muscle and eIF2B in liver

BACKGROUND

Acute alcohol intoxication in rats decreases protein synthesis in skeletal muscle and, to a lesser extent, in liver. The purpose of the present study was to examine potential mechanisms for the inhibitory effect of acute ethanol exposure.

METHODS

Rats were injected intraperitoneally with either ethanol (75 mmol/kg) or saline, and tissues were examined 2.5 hr later. Rates of protein synthesis in vivo were determined by [3H]phenylalanine incorporation into protein, and various eukaryotic initiation factors (eIFs) were quantitated by Western blot analysis to identify possible mechanisms for regulating translation.

RESULTS

Protein synthesis in gastrocnemius and liver was decreased (39% and 21%, respectively) after alcohol administration, compared with saline-injected control animals. Alcohol administration did not alter tissue RNA content but diminished translational efficiency in muscle (43%) and liver (24%). Hepatic eIF2B activity was decreased 24% in alcohol-treated rats, and this was associated with a 95% increase in eIF2alpha phosphorylation. However, alcohol did not alter the amount of 4E-binding protein 1 (4E-BP1) bound to eIF4E, cIF4E bound to eIF4G, or the phosphorylation state of either 4E-BP1 or eIF4E. In contrast to liver, neither eIF2B activity nor the phosphorylation of eIF2alpha was affected in muscle of alcohol-treated rats. However, acute alcohol intoxication increased binding of 4E-BP1 to eIF4E (113%), decreased the amount of cIF4E bound to cIF4G (81%), and decreased the amount of 4E-BP1 in the phosphorylated gamma-form (77%). The plasma concentrations of insulin and insulin-like growth factor-I were unchanged by alcohol, but muscle insulin-like growth factor-I messenger ribonucleic acid abundance was decreased 35%.

CONCLUSIONS

These data suggest that acute alcohol intoxication decreases translation initiation and protein synthesis in liver and muscle via different mechanisms. Changes in eIF2B appear to predominate in liver, whereas alterations in eIF4E availability appear more critical in skeletal muscle for controlling translation initiation.

Regulation of amino acid-sensitive TOR signaling by leucine analogues in adipocytes

In adipocytes, amino acids stimulate the target of rapamycin (TOR) signaling pathway leading to phosphorylation of the translational repressor, eIF-4E binding protein-I (4E-BP1), and ribosomal protein S6. L-leucine is the primary mediator of these effects. The structure-activity relationships of a putative L-leucine recognition site in adipocytes (LeuR(A)) that regulates TOR activity were analyzed by examining the effects of leucine analogues on the rapamycin-sensitive phosphorylation of the translational repressor, eIF-4E binding protein-I (4E-BP1), an index of TOR activity. Several amino acids that are structurally related to leucine strongly stimulated 4E-BP1 phosphorylation at concentrations greater than the EC(50) value for leucine. The order of potency was leucine > norleucine > threo-L-beta-hydroxyleucine approximately Ile > Met approximately Val. Other structural analogues of leucine, such as H-alpha-methyl-D/L-leucine, S-(-)-2-amino-4-pentenoic acid, and 3-amino-4-methylpentanoic acid, possessed only weak agonist activity. However, other leucine-related compounds that are known agonists, antagonists, or ligands of other leucine binding/recognition sites did not affect 4E-BP1 phosphorylation. We conclude from the data that small lipophilic modifications of the leucine R group and alpha-hydrogen may be tolerated for agonist activity; however, leucine analogues with a modified amino group, a modified carboxylic group, charged R groups, or bulkier aliphatic R groups do not seem to possess significant agonist activity. Furthermore, the leucine recognition site that regulates TOR signaling in adipocytes appears to be different from the following: (1) a leucine receptor that regulates macroautophagy in liver, (2) a leucine recognition site that regulates TOR signaling in H4IIE hepatocytes, (3) leucyl tRNA or leucyl tRNA synthetase, (4) the gabapentin-sensitive leucine transaminase, or (5) the system L-amino acid transporter.

Eukaryotic initiation factors and protein synthesis after resistance exercise in rats

Translational control of protein synthesis depends on numerous eukaryotic initiation factors (eIFs) and we have previously shown (Am. J. Physiol. Endocrinol. Metab. 276: E721-E727, 1999) that increases in one factor, eIF2B, are associated with increases in rates of protein synthesis after resistance exercise in rats. In the present study we investigated whether the eIF4E family of initiation factors is also involved with an anabolic response to exercise. Male Sprague-Dawley rats either remained sedentary (n = 6) or performed acute resistance exercise (n = 6), and rates of protein synthesis were assessed in vivo 16 h after the last session of resistance exercise. eIF4E complexed to eIF4G (eIF4E x eIF4G), eIF4E binding protein 1 (4E-BP1) complexed to eIF4E, and phosphorylation state of eIF4E and 4E-BP1 (gamma-form) were assessed in gastrocnemius. Rates of protein synthesis were higher in exercised rats compared with sedentary rats [205 +/- 8 (SE) vs. 164 +/- 5.5 nmol phenylalanine incorporated x g muscle(-1) x h(-1), respectively; P < 0.05]. Arterial plasma insulin concentrations were not different between the two groups. A trend (P = 0.09) for an increase in eIF4E x eIF4G with exercise was noted; however, no statistically significant differences were observed in any of the components of the eIF4E family in response to resistance exercise. These new data, along with our previous report on eIF2B, suggest that the regulation of peptide chain initiation after exercise is more dependent on eIF2B than on the eIF4E system.

Orally administered leucine stimulates protein synthesis in skeletal muscle of postabsorptive rats in association with increased eIF4F formation

We investigated the protein synthetic response of skeletal muscle to an orally administered dose of leucine given alone or in combination with carbohydrate. Male rats were freely fed (F) or food deprived for 18 h; food-deprived rats were then administered saline (S), carbohydrate (CHO), leucine (L) or a combination of carbohydrate plus leucine (CL). CHO and CL meals were isocaloric and provided 15% of daily energy requirements. L and CL meals each delivered 270 mg leucine. Muscle protein synthesis in S was 65% of F (P<0.01) 1 h after meal administration. Concomitant with lower rates of protein synthesis, phosphorylation of the translational repressor, eukaryotic initiation factor (eIF)4E-binding protein 1 (4E-BP1), was less in S, leading to greater association of 4E-BP1.eIF4E, and reduced formation of the active eIF4G.eIF4E complex compared with F (P<0.01). Oral administration of leucine (L or CL), but not CHO, restored protein synthesis equal to that in F and resulted in 4E-BP1 phosphorylation that was threefold greater than that of S (P<0.01). Consequently, formation of 4E-BP1.eIF4E was inhibited and eIF4G.eIF4E was not different from F. The amount of eIF4E in the phosphorylated form was greater in S and CHO (P<0.01) than in all other groups. In contrast, no differences in the phosphorylation state of eIF2alpha or the activity of eIF2B were noted among treatment groups. Serum insulin was elevated 2.6- and 3.7-fold in CHO and CL, respectively, but was not different in L, compared with S (P<0.05). These results suggest that leucine stimulates protein synthesis in skeletal muscle by enhancing eIF4F formation independently of increases in serum insulin.

Severe diabetes prohibits elevations in muscle protein synthesis after acute resistance exercise in rats

This study determined whether rates of protein synthesis increase after acute resistance exercise in skeletal muscle from severely diabetic rats. Previous studies consistently show that postexercise rates of protein synthesis are elevated in nondiabetic and moderately diabetic rats. Severely diabetic rats performed acute resistance exercise (n = 8) or remained sedentary (n = 8). A group of nondiabetic age-matched rats served as controls (n = 9). Rates of protein synthesis were measured 16 h after exercise. Plasma glucose concentrations were >500 mg/dl in the diabetic rats. Rates of protein synthesis (nmol phenylalanine incorporated. g muscle(-1). h(-1), means +/- SE) were not different between exercised (117 +/- 7) and sedentary (106 +/- 9) diabetic rats but were significantly (P < 0.05) lower than in sedentary nondiabetic rats (162 +/- 9) and in exercised nondiabetic rats (197 +/- 7). Circulating insulin concentrations were 442 +/- 65 pM in nondiabetic rats and 53 +/- 11 and 72 +/- 19 pM in sedentary and exercised diabetic rats, respectively. Plasma insulin-like growth factor I concentrations were reduced by 33% in diabetic rats compared with nondiabetic rats, and there was no difference between exercised and sedentary diabetic rats. Muscle insulin-like growth factor I was not affected by resistance exercise in diabetic rats. The results show that there is a critical concentration of insulin below which rates of protein synthesis begin to decline in vivo. In contrast to previous studies using less diabetic rats, severely diabetic rats cannot increase rates of protein synthesis after acute resistance exercise.

Role of eIF4E in stimulation of protein synthesis by IGF-I in perfused rat skeletal muscle

Insulin-like growth factor I (IGF-I) promotes anabolism by stimulating protein synthesis in skeletal muscle. In the present study, we have examined mechanisms by which IGF-I stimulates protein synthesis in skeletal muscle with a perfused rat hindlimb preparation. IGF-I (10 nM) stimulated protein synthesis over 2.7-fold. Total RNA content was unaffected, but translational efficiency was increased by IGF-I. We next examined the effect of IGF-I on eukaryotic initiation factor (eIF) 4E as a mechanism regulating translation initiation. IGF-I did not alter either the amount of eIF4E associated with the eIF4E binding protein 4E-BP1 or the phosphorylation state of 4E-BP1. Likewise, the phosphorylation state of eIF4E was unaltered by IGF-I. In contrast, the amount of eIF4E bound to eIF4G was increased threefold by IGF-I. We conclude that IGF-I regulates protein synthesis in skeletal muscle by enhancing formation of the active eIF4E x eIF4G complex.

Amino acid-induced stimulation of translation initiation in rat skeletal muscle

Amino acids stimulate protein synthesis in skeletal muscle by accelerating translation initiation. In the two studies described herein, we examined mechanisms by which amino acids regulate translation initiation in perfused skeletal muscle hindlimb preparation of rats. In the first study, the effects of supraphysiological amino acid concentrations on eukaryotic initiation factors (eIF) 2B and 4E were compared with physiological concentrations of amino acids. Amino acid supplementation stimulated protein synthesis twofold. No changes were observed in eIF2B activity, in the amount of eIF4E associated with the eIF4E-binding protein (4E-BP1), or in the phosphorylation of 4E-BP1. The abundance of eIF4E bound to eIF4G and the extent of phosphorylation of eIF4E were increased by 800 and 20%, respectively. In the second study, we examined the effect of removing leucine on translation initiation when all other amino acids were maintained at supraphysiological concentrations. Removal of leucine from the perfusate decreased the rate of protein synthesis by 40%. The inhibition of protein synthesis was associated with a 40% decrease in eIF2B activity and an 80% fall in the abundance of eIF4E. eIF4G complex. The fall in eIF4G binding to eIF4E was associated with increased 4E-BP1 bound to eIF4E and a reduced phosphorylation of 4E-BP1. In contrast, the extent of phosphorylation of eIF4E was unaffected. We conclude that formation of the active eIF4E. eIF4G complex controls protein synthesis in skeletal muscle when the amino acid concentration is above the physiological range, whereas removal of leucine reduces protein synthesis through changes in both eIF2B and eIF4E.

Chronic alcohol feeding impairs hepatic translation initiation by modulating eIF2 and eIF4E

The present study examined potential cellular mechanisms responsible for the inhibition of protein synthesis in liver after chronic alcohol consumption. Rats were maintained on an alcohol-containing diet for 14 wk; control animals were fed isocalorically. Hepatic ATP content was not different in alcohol-fed and control animals. No alcohol-induced reduction in total hepatic RNA content (an estimate of ribosomal RNA) was detected, suggesting that alcohol decreased translational efficiency. Alcohol feeding increased the proportion of 40S and 60S ribosomal subunits in the nonpolysome-associated fraction by 30%. To identify mechanisms responsible for the impairment in initiation, several eukaryotic initiation factors (eIF) were analyzed. Alcohol feeding decreased hepatic eIF2B activity by 36%. This reduction was associated with a 20% decrease in eIF2Bepsilon content and a 90% increase in eIF2alpha phosphorylation. Alcohol also dramatically influenced the distribution of eIF4E. Compared with pair-fed control values, alcohol feeding increased the amount of eIF4E present in the inactive 4E-binding protein 1 (4E-BP1). eIF4E complex by 80% and decreased binding of eIF4G to eIF4E by 70%. However, the phosphorylation status of 4E-BP1 and eIF4E was not altered by alcohol. Although the plasma concentrations of threonine, proline, and citrulline were mildly decreased, the circulating amount of total amino acids was not altered by alcohol feeding. In summary, these data suggest that chronic alcohol consumption impairs translation initiation in liver by altering eIF2B activity as well as eIF4F function via changes in eIF4E availability.

Interleukin-1 receptor antagonist attenuates tumor necrosis factor-induced alterations in wound breaking strength

BACKGROUND

Tumor necrosis factor (TNF) is an important mediator of impaired wound healing during sepsis. To determine whether the inhibitory effects of systemic TNF on wound healing are mediated directly by TNF or by means of the induction of interleukin-1 (IL-1), we investigated the effects of TNF and interleukin- receptor antagonist (IL-1ra) on wound healing in healthy rats.

METHODS

Male Sprague-Dawley rats were anesthetized, and jugular catheters were placed. After recovery of 48 hours, osmotic minipumps were inserted into the peritoneal cavity and polyvinyl alcohol implants were placed subcutaneously. Control rats were infused with saline (24 microL/day, i.p., and 15 mL/day, i.v.). TNF rats received TNF i.p. (100 microg/kg per day) and saline i.v. (15 mL/day). TNF+IL-1ra rats received TNF i.p. (100 microg/kg per day) and IL-1ra i.v. (2 mg/kg per day;15 mL/day). All animals were pair fed to the TNF group. On day 6, the wounds were harvested. The breaking strength of the abdominal incision was measured. Granulation tissue penetration and quality were determined by scoring polyvinyl alcohol implant histology from 1 to 4 in a blinded manner. Collagen deposition in polyvinyl alcohol implants was quantified by hydroxyproline assay.

RESULTS

TNF decreased the breaking strength of incisional wounds to 40% of control levels (p < 0.001). IL-1ra restored the breaking strength of incisions from TNF-infused animals to 80% of control levels. Similar reductions in granulation tissue penetration, quality, and hydroxyproline content were observed in TNF-treated animals and were partially ameliorated by IL-1ra.

CONCLUSION

IL-1ra significantly attenuates the inhibitory effects of systemic TNF on wound healing. These results suggest that the inhibitory effects of TNF on cutaneous tissue repair are mediated in part by IL-1.

Hypertrophy of skeletal muscle in diabetic rats in response to chronic resistance exercise

This study had the following objectives: 1) to determine whether diabetic rats could increase muscle mass due to a physiological manipulation (chronic resistance exercise), 2) to determine whether exercise training status modifies the effect of the last bout of exercise on elevations in rates of protein synthesis, and 3) to determine whether chronic resistance exercise alters basal glycemia. Groups consisted of diabetic or nondiabetic rats that performed progressive resistance exercise for 8 wk, performed acute resistance exercise, or remained sedentary. Arterial plasma insulin in diabetic groups was reduced by about one-half (P < 0.05) compared with nondiabetic groups. Soleus and gastrocnemius-plantaris complex muscle wet weights were lower because of diabetes, but in response to chronic exercise these muscles hypertrophied in diabetic (0.028 +/- 0.003 vs. 0.032 +/- 0.0015 g/cm for sedentary vs. exercised soleus and 0.42 +/- 0.068 vs. 0.53 +/- 0.041 g/cm for sedentary vs. exercised gastrocnemius-plantaris, both P < 0.05) but not in nondiabetic (0.041 +/- 0.0026 vs. 0.042 +/- 0.003 g/cm for sedentary vs. exercised soleus and 0.72 +/- 0.015 vs. 0.69 +/- 0.013 g/cm for sedentary vs. exercised gastrocnemius-plantaris) rats when muscle weight was expressed relative to tibial length or body weight (data not shown). Another group of diabetic rats that lifted heavier weights showed muscle hypertrophy. Rates of protein synthesis were higher in red gastrocnemius in chronically exercised than in sedentary rats: 155 +/- 11 and 170 +/- 7 nmol phenylalanine incorporated x g muscle(-1) x h(-1) in exercised diabetic and nondiabetic rats vs. 110 +/- 14 and 143 +/- 7 nmol phenylalanine incorporated x g muscle(-1) x h(-1) in sedentary diabetic and nondiabetic rats. These elevations, however, were lower than in acutely exercised (but untrained) rats: 176 +/- 15 and 193 +/- 8 nmol phenylalanine incorporated x g muscle(-1) x h(-1) in diabetic and nondiabetic rats. Finally, chronic exercise training in diabetic rats was associated with reductions in basal glycemia, and such reductions did not occur in sedentary diabetic groups. These data demonstrate that, despite lower circulating insulin concentrations, diabetic rats can increase muscle mass in response to a physiological stimulus.

Sepsis alters pyruvate dehydrogenase kinase activity in skeletal muscle

Chronic sepsis promotes a stable increase in pyruvate dehydrogenase kinase (PDHK) activity in skeletal muscle. PDHK is found tightly bound to the pyruvate dehydrogenase (PDH) complex and as free kinase. We investigated the ability of sepsis to modify the activity of the PDHK intrinsic to the PDH and free PDHK. Sepsis was induced by the intraabdominal introduction of a fecal-agar pellet infected with E. coli and B. fragilis. Five days later, mitochondria were isolated from skeletal muscle and PDHK measured in mitochondrial extracts. Sepsis caused an approximate 2-fold stimulation of PDHK. The mitochondrial extracts from control and septic rats were fractionated by gel chromatography on Sephacryl S-300 to separate PDHK intrinsic to PDH complex and free PDHK. PDH complex eluted at void volume and was assayed for PDHK intrinsic to the complex. The activity of PDHK intrinsic to PDH complex was a significantly increased 3 fold during sepsis. Free PDHK activity eluted after the PDH complex and its activity was enhanced by 70% during sepsis. Incubation of PDHK intrinsic to PDH with dichloroactate, an uncompetitive inhibitor of PDHK, showed the PDHK from septic rats relatively less sensitive to inhibition than controls. These results indicate that sepsis induces stable changes in PDHK in skeletal muscle.

Inhibition of muscle protein synthesis by alcohol is associated with modulation of eIF2B and eIF4E

The present study examined potential mechanisms for the inhibition of protein synthesis in skeletal muscle after chronic alcohol consumption. Rats were maintained on an alcohol-containing diet for 14 wk; control animals were pair fed. Alcohol-induced myopathy was confirmed by a reduction in lean body mass as well as a decrease in the weight of the gastrocnemius and psoas muscles normalized for tibial length. No alcohol-induced decrease in total RNA content (an estimate of ribosomal RNA) was detected in any muscle examined, suggesting that alcohol reduced translational efficiency but not the capacity for protein synthesis. To identify mechanisms responsible for regulating translational efficiency, we analyzed several eukaryotic initiation factors (eIF). There was no difference in the muscle content of either total eIF2alpha or the amount of eIF2alpha in the phosphorylated form between alcohol-fed and control rats. Similarly, the relative amount of eIF2Bepsilon in muscle was also not different. In contrast, alcohol decreased eIF2B activity in psoas (fast-twitch) but not in soleus or heart (slow-twitch) muscles. Alcohol feeding also dramatically influenced the distribution of eIF4E in the gastrocnemius (fast-twitch) muscle. Compared with control values, muscle from alcohol-fed rats demonstrated 1) an increased binding of the translational repressor 4E-binding protein 1 (4E-BP1) with eIF4E, 2) a decrease in the phosphorylated gamma-form of 4E-BP1, and 3) a decrease in eIF4G associated with eIF4E. In summary, these data suggest that chronic alcohol consumption impairs translation initiation in muscle by altering multiple regulatory sites, including eIF2B activity and eIF4E availability.

Chronic infusion of interleukin 1 induces hyperlactatemia and altered regulation of lactate metabolism in skeletal muscle

BACKGROUND

Hyperlactatemia is observed commonly in patients with severe inflammation syndrome or sepsis. Elevated plasma lactate concentrations may be caused by cytokine-mediated alterations in specific organ systems responsible for lactate homeostasis. The role of interleukin 1 (IL-1) in inducing hyperlactatemia and derangements in skeletal muscle and hepatic lactate metabolism was investigated by examining the consequences of infusing IL-1 continuously into normal rats.

METHODS

Male Sprague-Dawley rats were anesthetized, and catheters were placed in the jugular vein. Rats were allowed to recover for 48 hours and were infused subsequently with either saline (control) or human recombinant IL-1alpha (20 microg/kg/d) for 6 days. On day 6, plasma, liver, and muscle samples were extracted and assayed for lactate and pyruvate dehydrogenase (PDH) activity.

RESULTS

Plasma glucose concentrations were not different in the two groups. IL-1 infusion resulted in a twofold (p < .05) increase in the plasma lactate concentration compared with controls. IL-1 infusion also resulted in an elevated lactate content in skeletal muscle (p < .05) but not in liver. The proportion of PDH in the active form (PDHa) was reduced significantly (p < .05) in the skeletal muscle of animals infused with IL-1 compared with controls. In contrast to muscle, hepatic PDHa did not differ between the two groups. Total PDH complex activity was not affected in either liver or skeletal muscle.

CONCLUSIONS

IL-1 infusion results in hyperlactatemia, increased skeletal muscle lactate, and a reduced PDHa in skeletal muscle. We conclude that IL-1 is a potential mediator of the derangements in lactate metabolism in skeletal muscle but not in liver.

Inter-organ protein and carbohydrate metabolic relationships during sepsis: necessary evils or uncanny coincidences?

Sepsis alters the dynamic flux of metabolic substrates between skeletal muscle and liver. Derangements in skeletal muscle glucose metabolism evoked by sepsis to a certain extent determine the rate of gluconeogenesis in the liver. In contrast, accelerated rates of gluconeogenesis do not drive net catabolism in skeletal muscle, nor does the upregulation of hepatic protein metabolism in sepsis or inflammation appear to be contingent or dependent upon the catabolism of muscle proteins during sepsis.

TNF-binding protein ameliorates inhibition of skeletal muscle protein synthesis during sepsis

We examined the effects of TNF-binding protein (TNFBP) on regulatory mechanisms of muscle protein synthesis during sepsis in four groups of rats: Control; Control+TNFBP; Septic; and Septic+TNFBP. Saline (1. 0 ml) or TNFBP (1 mg/kg, 1.0 ml) was injected daily starting 4 h before the induction of sepsis. The effect of TNFBP on gastrocnemius weight, protein content, and the rate of protein synthesis was examined 5 days later. Sepsis reduced the rate of protein synthesis by 35% relative to controls by depressing translational efficiency. Decreases in protein synthesis were accompanied by similar reductions in protein content and muscle weight. Treatment of septic animals with TNFBP for 5 days prevented the sepsis-induced inhibition of protein synthesis and restored translational efficiency to control values. TNFBP treatment of Control rats for 5 days was without effect on muscle protein content or protein synthesis. We also assessed potential mechanisms regulating translational efficiency. The phosphorylation state of p70(S6) kinase was not altered by sepsis. Sepsis reduced the gastrocnemius content of eukaryotic initiation factor 2Bepsilon (eIF2Bepsilon), but not eIF2alpha. The decrease in eIF2Bepsilon content was prevented by treatment of septic rats with TNFBP. TNFBP ameliorates the sepsis-induced changes in protein metabolism in gastrocnemius, indicating a role for TNF in the septic process. The data suggest that TNF may impair muscle protein synthesis by reducing expression of specific initiation factors during sepsis.

Regulation of protein synthesis after acute resistance exercise in diabetic rats

These studies determined whether insulin-like growth factor-I (IGF-I) involvement in exercise-stimulated anabolic processes becomes more evident during hypoinsulinemia. Male Sprague-Dawley rats (n = 6-12/group) were made diabetic (blood glucose congruent with 300 mg/dl) by partial pancreatectomy (PPX) or remained nondiabetic (glucose congruent with 144 mg/dl). Rats performed acute resistance exercise by repetitive standing on the hindlimbs with weighted backpacks (ex), or they remained sedentary (sed). Resistance exercise caused increases in rates of protein synthesis (nmol Phe incorporated. g muscle-1. h-1, measured for gastrocnemius muscle in vivo 16 h after exercise) for both nondiabetic [sed = 154 +/- 6 (SE) vs. ex = 189 +/- 7] and diabetic rats (PPXsed = 152 +/- 11 vs. PPXex = 202 +/- 14, P < 0.05). Arterial plasma insulin concentrations in diabetic rats, congruent with180 pM, were less than one-half those found in nondiabetic rats, congruent with444 pM, (P < 0.05). The activity of eukaryotic initiation factor 2B (eIF2B; pmol GDP exchanged/min) was higher (P < 0.05) in ex rats (sed = 0.028 +/- 0.006 vs. ex = 0.053 +/- 0.015; PPXsed = 0.033 +/- 0.013 vs. PPXex = 0.047 +/- 0.009) regardless of diabetic status. Plasma IGF-I concentrations were higher in ex compared with sed diabetic rats (P < 0.05). In contrast, plasma IGF-I was not different in nondiabetic ex or sed rats. Muscle IGF-I (ng/g wet wt) was similar in ex and sed nondiabetic rats, but in diabetic rats was 2- to 3-fold higher in ex (P < 0.05) than in sed rats. In conclusion, moderate hypoinsulinemia that is sufficient to alter glucose homeostasis does not inhibit an increase in rates of protein synthesis after acute moderate-intensity resistance exercise. This preserved response may be due to a compensatory increase in muscle IGF-I content and a maintained ability to activate eIF2B.

Mechanism of IL-1 induced inhibition of protein synthesis in skeletal muscle

Chronic interleukin (IL)-1 administration is associated with negative nitrogen balance and the loss of lean body mass. To elucidate the molecular mechanism(s) by which IL-1 modulates protein metabolism in muscle, we investigated the effects of chronic (6 day) IL-1alpha infusion on protein synthesis in Individual muscles (gastrocnemius, soleus, heart) compared with saline-infused control rats. IL-1 significantly decreased muscle weight, protein content, and the rate of protein synthesis in gastrocnemius (fast-twitch muscle). IL-1 had no effect on these parameters in the heart, whereas only the rate of protein synthesis was reduced in soleus (slow-twitch muscle). The reduction in gastrocnemius protein synthesis was not the result of a decrease in total RNA content, but was associated with a diminished translational efficiency. The diminished translational efficiency correlated with a 40% reduction in the epsilon-subunit of eukaryotic initiation factor 2B (elF2Bepsilon) in gastrocnemius from IL-1 -treated animals. However, the content of the alpha-subunit of elF2 (elF2alpha) was unaffected. In contrast, the elF2alpha content in heart was increased by IL-1, although elF2Bepsilon levels were unchanged. Reductions in skeletal muscle protein synthesis were not associated with a concomitant reduction in circulating or tissue content of insulin-like growth factor I. In summary, the IL-1-induced decrease in gastrocnemius protein synthesis appears to be regulated at the level of RNA translation via a reduction in elF2Bepsilon. These findings support a regulatory role for IL-1 as a mediator of muscle protein synthesis and the alterations in body composition observed in catabolic states where this cytokine is overexpressed.

Diazoxide-induced insulin deficiency greatly reduced muscle protein synthesis in rats: involvement of eIF4E

We have investigated the effect of a postprandial acute insulin deficiency induced by diazoxide injection on rat skeletal muscle protein synthesis. Diazoxide administration lowered plasma insulin >85% within 3 h after injection, whereas other hormones (insulin-like growth factor I, glucagon, corticosterone) involved in the regulation of muscle protein synthesis were not altered significantly compared with control animals. The fractional rate of muscle protein synthesis, measured in vivo, was reduced significantly (P < 0.05) in epitrochlearis (-46%), gastrocnemius (-41%), and soleus (-35%). The reduction in protein synthesis did not result from a reduced total RNA content but was associated with diminished translation efficiency. Analysis of ribosomal subunits revealed that the decreased translation efficiency resulted from an impairment in the initiation phase of protein synthesis. Diazoxide-induced insulin deficiency was associated with a dramatic decrease in eukaryotic initiation factor (eIF) 4G bound to eIF4E and a 2.5-fold increase in the amount of the eIF4E. 4E-binding protein 1 (BP1) complex. In contrast, diazoxide injection did not change either the relative amount of eIF4E present in gastrocnemius or its phosphorylation state. These results indicate that an acute insulin deficiency significantly decreases postprandial muscle protein synthesis by modulating the interaction between 4E-BP1, eIF4G, and eIF4E to control translation initiation.

Effects of intensity of acute-resistance exercise on rates of protein synthesis in moderately diabetic rats

These studies determined whether increases in rates of protein synthesis observed in skeletal muscle after moderate or severe acute-resistance exercise were blunted by insulinopenia. Rats (n = 6-9 per group) were made insulin deficient by partial pancreatectomy or remained nondiabetic. Groups either remained sedentary or performed acute-resistance exercise 16 h before rates of protein synthesis were measured in vivo. Exercise required 50 repetitions of standing on the hindlimbs with either 0.6 g backpack wt/g body wt (moderate exercise) or 1.0 g backpack wt/g body wt (severe exercise). Insulin-deficient rats had a mean blood glucose concentration >15 mM and reduced insulin concentrations in the plasma. Rates of protein synthesis in gastrocnemius muscle were not different in all sedentary groups. The moderate-exercised nondiabetic group (192 +/- 12 nmol phenylalanine incorporated. g muscle-1. h-1) and moderate-exercised diabetic group (215 +/- 18) had significantly (P < 0.05, ANOVA) higher rates of protein synthesis than did respective sedentary groups. In contrast, diabetic rats that performed severe-resistance exercise had rates of protein synthesis (176 +/- 12) that were not different (P > 0.05) from diabetic sedentary rats (170 +/- 9), whereas nondiabetic rats that performed severe exercise had higher (212 +/- 24) rates compared with nondiabetic sedentary rats (178 +/- 10) P < 0.05. The present data in combination with previous studies [J. D. Fluckey, T. C. Vary, L. S. Jefferson, and P. A. Farrell. Am. J. Physiol. 270 (Endocrinol. Metab. 33): E313-E319, 1996] show that the amount of insulin required for an in vivo permissive effect of insulin on rates of protein synthesis can be quite low after moderate-intensity resistance exercise. However, severe exercise in combination with low insulin concentrations can ablate an anabolic response.

Effect of dietary protein on translation initiation in rat skeletal muscle and liver

The effect of dietary protein on the initiation of mRNA translation was examined in rats starved for 18 h and then fed isocaloric diets containing either 20% protein (20P) or no added protein (0P). Feeding the 20P diet, but not the 0P diet, stimulated protein synthesis in skeletal muscle and liver by 38 and 41%, respectively. The stimulation was associated with reduced binding of eukaryotic initiation factor (eIF) 4E to the translational repressor 4E-BP1, increased formation of the active eIF4E-eIF4G complex, and increased phosphorylation of 4E-BP1. In contrast, feeding a 0P diet had no effect on any of these parameters. Feeding a 20P diet resulted in partial dephosphorylation of eIF4E in both tissues. In liver, refeeding a 0P diet also resulted in partial eIF4E dephosphorylation, suggesting that the phosphorylation state of eIF4E is not important in the stimulation of protein synthesis under these conditions. Finally, plasma insulin concentrations were the same in rats fed either diet (14.8 +/- 4.9 vs. 15.5 +/- 4.5 microU/ml for 20P and 0P groups, respectively), suggesting that feeding-induced changes in plasma insulin are not sufficient to stimulate protein synthesis. Instead, a combination of dietary protein and insulin may be required to stimulate translation initiation.

TNF binding protein prevents hyperlactatemia and inactivation of PDH complex in skeletal muscle during sepsis

BACKGROUND

Hyperlactatemia is a metabolic complication of hypermetabolic, hyperdynamic sepsis. An important mechanism responsible for elevating plasma lactate concentrations in sepsis is altered regulation of the pyruvate dehydrogenase complex (PDH) in skeletal muscle. We investigated the ability of a specific tumor necrosis factor binding protein, TNFbp, to modulate lactate concentrations and skeletal muscle PDH activity in a rodent model of chronic abdominal sepsis.

MATERIALS AND METHODS

We examined the regulation of lactate metabolism in four groups of animals: Control, Control + TNFbp, Septic, and Septic + TNFbp. Chronic (5 days) sepsis was induced by the creation of a stable intraabdominal abscess using a sterile fecal-agar pellet inoculated with E. coli plus B. Fragilis as the foreign body nidus. TNFbp (1 mg/kg/day) was injected subcutaneously daily.

RESULTS

Sepsis increased plasma and skeletal muscle lactate concentrations 2-fold compared with control. In septic rats treated with TNFbp, plasma and skeletal muscle lactate concentrations were significantly decreased compared with untreated septic rats. In skeletal muscle, sepsis resulted in a 70% decrease in the proportion of the PDH in the active form compared with controls. The sepsis-induced inhibition in the PDH complex activity was prevented by TNFbp. PDH kinase was enhanced 1.8-fold in sepsis, and the increase in PDH kinase activity was prevented by treatment with TNFbp. TNFbp treatment did not have any effects on plasma lactate or the proportion of active skeletal muscle PDH activity in control animals.

CONCLUSIONS

TNFbp prevents the sepsis-induced hyperlactatemia and derangements in skeletal muscle lactate concentrations and PDH activity. These observations suggest that TNF is an important mediator responsible for lactate dyshomeostasis during sepsis.

Differential regulation of skeletal muscle protein turnover by insulin and IGF-I after bacteremia

Skeletal muscle catabolism is a characteristic metabolic response to sepsis. We investigated the ability of physiological insulin (2 nM) or insulin-like growth factor I (IGF-I, 10 nM) concentrations to modify protein metabolism during incubation of epitrochlearis 2, 6, or 15 days after injection of live Escherichia coli. On days 2 and 6 postinfection, skeletal muscle exhibited an exacerbated negative protein balance resulting from both an inhibition in protein synthesis (25%) and an enhanced proteolysis (90%) compared with controls. By day 15 postinfection, protein balance in infected rats was significantly improved compared with either day 2 or 6. At this time, protein synthesis was augmented and protein degradation was decreased in infected rats relative to day 6. Insulin or IGF-I stimulated protein synthesis in muscles from septic and control rats in vitro to the same extent at each time point examined. The ability of insulin or IGF-I to limit protein degradation was severely blunted 48 h after infection. On day 6 postinfection, the effect of insulin or IGF-I to inhibit proteolysis was more pronounced than on day 2. Incubation with IGF-I limited proteolysis to a greater extent than insulin on both days in infected but not control rats. By day 15, insulin diminished proteolysis to the same extent as in controls. The results suggest that injection of bacteria causes fundamental derangements in protein metabolism that persist for days after infection.

Role of central IL-1 in regulating peripheral IGF-I during endotoxemia and sepsis

Inflammatory cytokines may mediate the host response to infection via central nervous system, endocrine, and/or paracrine/autocrine signaling mechanisms. Previous studies have shown that intravenous administration of interleukin (IL)-1 beta alters the concentration of the anabolic hormone insulin-like growth factor (IGF)-I in plasma and various tissues. The purpose of the present study was to determine 1) whether the intracerebroventricular injection of IL-1 beta can influence peripheral IGF-I levels in control animals and 2) whether the central administration of a IL-1 receptor antagonist (IL-1ra) can prevent the changes in peripheral IGF-I induced by endotoxin [lipopolysaccharide (LPS)] or sepsis produced by cecal ligation and puncture. In the first experiment, injection of IL-1 beta (100 ng/rat) decreased IGF-I levels in plasma, liver, and gastrocnemius muscle 28-36% by 1.5 h in conscious fasted rats. IGF-I levels remained reduced at 3 h, but returned to baseline by 6 h. IGF-I content was not altered in soleus, kidney, spleen, intestine, or whole brain after IL-1 beta. In the second series of experiments, LPS injected intravenously decreased IGF-I levels in plasma, liver, and gastrocnemius at 1.5 h, and levels were even further reduced at 3 and 6 h in these tissues (59, 57, and 48%, respectively). Moreover, the IGF-I content was also decreased in soleus (30-35%) and increased in kidney (2- to 3-fold) after LPS. In the third experiment, changes in IGF-I levels in plasma and tissues, similar to those seen in LPS-treated rats, were detected 24 h after induction of peritonitis. Intracerebroventricular infusion of IL-1ra did not alter any of the changes in IGF-I produced by either LPS or sepsis, although it did attenuate the concomitant changes in growth hormone levels. These data suggest that, although central IL-1 beta is capable of modulating peripheral IGF-I levels, central administration of IL-1ra was unable to modulate the changes in peripheral IGF-I in blood and tissues produced by either endotoxemia or peritonitis.

Regulation of skeletal muscle protein turnover during sepsis

Wasting of skeletal muscle protein is a prominent feature of the metabolic response to sepsis. Persistent protein wasting leads to muscle dysfunction and prolongs recovery from the septic insult. Unfortunately, conventional nutritional support alone does not prevent the sepsis-induced weight loss and catabolism of muscle. Hence, mechanisms other than substrate deficiency appear to be involved in the derangements in protein metabolism during sepsis. The catabolism of muscle during sepsis results from a stimulation of proteolysis and an inhibition of protein synthesis. Despite the importance of these pathways in maintaining muscle mass, the regulation of protein synthesis and proteolysis during sepsis remains poorly understood. This review summarizes the mechanisms responsible for alterations in protein synthesis and degradation in muscle during sepsis at the biochemical level. The ability of hormones (insulin, insulin-like growth factor I, glucocorticoids) or cytokines (tumor necrosis factor, interleukin-1) to act as mediators or modulators of protein catabolism is also examined. A picture is emerging suggesting that cytokines may influence skeletal muscle protein metabolism during sepsis both indirectly, through inhibition of the regulatory actions of anabolic hormones on protein turnover, and directly, through modulation of the protein synthesis and degradation enzymatic machinery.

Reduced 40S initiation complex formation in skeletal muscle during sepsis

Severe muscle wasting is a characteristic feature of sepsis. We have previously established that the rate of protein synthesis in muscles composed of fast-twitch fibers is severely diminished in response to sepsis. The present studies investigate the biochemical reactions responsible for the decreased rate of protein synthesis using gastrocnemius from control and septic rats perfused in situ. Analysis of free ribosomal subunits indicated peptide-chain initiation was impaired by infection. To characterize biochemical reactions in the pathway of peptide-chain initiation affected, the effect of sepsis on the incorporation of initiator [35S]methionyl-tRNA (met-tRNA(imet)) into the 40S initiation complex was examined. Sepsis caused a 65% decrease in the binding of radiolabelled met-tRNA(imet) to the 40S initiation complex compared with controls. The binding of met-tRNA(met) to the 40S ribosome is regulated by eukaryotic initiation factor eIF-2B, whose activity can be modulated in part by the redox state of pyridine dinucleotides. The mean cytoplasmic NADH/NAD+ ratio was increased 2 fold in sepsis, while the NADPH/NADP+ ratio was unchanged. These findings identify the formation of the 40S initiation complex as a defect in the protein synthesis machinery during sepsis. The decreased formation of the 40S initiation complex in muscle could not be explained by changes in the cytoplasmic redox state.

Activation of pyruvate dehydrogenase improves heart function and metabolism after hemorrhagic shock

This study was designed to test the hypothesis that activation of myocardial pyruvate dehydrogenase (PDH) would improve recovery of heart function after brief, severe hemorrhagic shock. Pentobarbital-anesthetized rats were instrumented to monitor arterial blood pressure and right ventricular pressures. Rats were hemorrhaged via femoral artery to 25-30 mmHg mean arterial pressure (MAP) for 60 min, followed by retransfusion of shed blood with either 1.0 cc saline with no dichloroacetate (-DCA) or 1.0 cc saline containing 150 mg/kg sodium dichloroacetate (+DCA). Rats were observed for 3 h after retransfusion. Hearts were freeze-clamped in situ for analysis of adenosine triphosphate (ATP), creatine phosphate (CrP), lactate and pyruvate content as well as PDH activity (PDHa) and total PDH activity (PDHt). Three h after retransfusion, the rate pressure product (RPP=HRxPSP) was 23 000+/-2733 with no DCA treatment v 36 2769 mmHg/min with DCA treatment (P<0.05, ANOVA). Treatment with DCA also increased myocardial tissue content of high energy phosphates (ATP=10.1+/-1.1 and CrP=5.8+/-1.0 micromol/g weight-DCA, v 15.1+/-0.9 and 14.7+/-1.0 micromol/g dry weight+DCA, P<0.05, both measurements). DCA administration also significantly reduced myocardial lactate contents (14.6+/-2.7 micromol/g dry weight-DCA v 5.9+/-1.0+DCA). Hemorrhagic shock did not change PDHa or PDHt compared to hearts obtained during the pre-hemorrhage period. Retransfusion with DCA significantly increased PDHa activity (6.8+/-1.1 micromol/g dry weight/min-DCA v 29.7+/-2.0 micromol/g dry weight/min+DCA). PDHt was not different between controls and DCA-treated groups. These data indicate that activation of myocardial PDH by adding DCA to retransfused blood improved heart function and metabolism after severe hemorrhagic shock.

Modulation of skeletal muscle lactate metabolism following bacteremia by insulin or insulin-like growth factor-I: effects of pentoxifylline

Hyperlactatemia is a frequent complication of sepsis. We investigated the effect of pentoxifylline on plasma lactate concentrations and lactate release by epitrochlearis incubated in vitro following intravenous injection of Escherichia coli. Plasma lactate concentrations were elevated on day 2 postinfection and remained elevated for at least another 4 days. Lactate production by incubated epitrochlearis was not increased in septic rats on day 2 postinfection, and lactate production from muscles incubated with insulin (2 nM) or insulin-like growth factor-I, (10 nM) was similar in control and septic rats. On day 6 postinfection, lactate production was augmented 1.8-fold in muscles from septic rats and both insulin and IGF-I caused an exaggerated stimulation of lactate production compared with control. Pentoxifylline decreased plasma TNF concentrations 100-fold following injection of bacteria and prevented the sepsis-induced hyperlactatemia and increase in lactate production by incubated muscles in presence or absence of insulin or IGF-I. Thus, pentoxifylline prevented the sepsis-induced abnormalities in skeletal muscle lactate production and plasma lactate concentrations.

Regulation of skeletal muscle protein turnover during sepsis: mechanisms and mediators

Skeletal muscle protein wasting is a prominent feature of the metabolic response to sepsis. Persistent protein wasting leads to muscle dysfunction and prolongs recovery from the septic insult. Unfortunately, conventional nutritional support alone does not prevent the sepsis-induced weight loss and catabolism of muscle. Hence, mechanisms other than substrate deficiency appear to be involved in the derangements in protein metabolism during sepsis. The catabolism of muscle during sepsis results from a stimulation of proteolysis and an inhibition of protein synthesis. This review summarizes the mechanisms responsible for alterations in protein synthesis and degradation in muscle during sepsis at the biochemical level. The ability of hormones (insulin, insulin-like growth factor I, glucocorticoids) or cytokines (tumor necrosis factor, interleukin-1) to act as mediators of protein catabolism is also examined. Finally, we discuss the potential role of anticytokine therapies in preventing derangements in protein metabolism during sepsis. A picture is emerging which suggests that cytokines may influence skeletal muscle protein metabolism during sepsis both indirectly through inhibition of the regulatory actions of anabolic hormones on protein turnover, and directly through modulation of the protein synthesis and degradation enzymatic machinery.

Insulin stimulation of protein synthesis in rat skeletal muscle following resistance exercise is maintained with advancing age

This study examined whether or not insulin elevates rates of protein synthesis in muscle following four days of resistance exercise in young (4-mo), middle-aged (12-mo), and old (32-mo) rats. Thirty-six male Fischer 344/BN F1 rats (n = 12 in each group) performed an operantly conditioned activity which required full extension of the hindlimbs with weights over the scapula (ACUTE; n = 6 for each age group) or with no additional weight (nonexercised; NONEX; n = 6 for each age group). Acutely exercised animals engaged in four distinct exercise sessions with each session separated by 48 h. Rates of protein synthesis were assessed in soleus, gastrocnemius (GAST), and extensor digitorum longus (EDL) muscles 16 h after the last exercise bout using a bilateral hindlimb perfusion to measure the incorporation of tritiated phenylalanine (F) into muscle protein. One limb of the bilateral hindlimb preparation received a medium that contained rat insulin at a physiological concentration (6.25 ng.ml-1), while the other limb did not. Rates of protein synthesis in soleus with insulin supplementation were significantly higher within all age groups following resistance exercise vs ACUTE without insulin and NONEX with or without insulin (p < .05). Rates of protein synthesis in soleus were not different within age groups for NONEX with or without insulin (p < .05), but rates of protein synthesis for young NONEX were significantly higher (p < .05) than middle-aged or old NONEX (204 +/- 9 vs 149 +/- 6 or 141 +/- 9 nmol F incorporated.g-1.h-1, respectively; means +/- SE). Rates of protein synthesis in GAST with insulin were also significantly higher within all age groups following resistance exercise than ACUTE without insulin or NONEX with or without insulin (p < .05). Unlike soleus, rates of protein synthesis in GAST were significantly higher for old NONEX vs young NONEX (68 +/- 6 vs 45 +/- 5 nmol F incorporated.g-1.h-1, respectively; P < .05), but not middle-aged NONEX (51 +/- 3 nmol F incorporated.g-1.h-1). Translational efficiency (rates of protein synthesis.unit of RNA-1.h-1) for GAST supplemented with insulin was significantly greater in ACUTE with insulin than ACUTE without insulin or NONEX with or without insulin (p < .05). There were no effect of age, insulin, or exercise on rates of protein synthesis in EDL (p > .05). These data suggest that following resistance exercise, insulin increased rates of protein synthesis in both soleus and GAST regardless of age, and it appeared that this insulin-mediated elevation may have occurred at the level of translation.

Regulation of peptide-chain initiation in muscle during sepsis by interleukin-1 receptor antagonist

The mechanism by which interleukin-1 (IL-1) regulates protein synthesis in skeletal muscle during hypermetabolic sepsis in rats was investigated. Treatment of septic rats with a specific interleukin-1 receptor antagonist (IL-1ra) prevented the sepsis-induced inhibition of protein synthesis and translational efficiency in gastrocnemius. Analysis of ribosomal subunits revealed that the increase in free 40S and 60S ribosomal subunits observed in septic rats was prevented by infusion of IL-1ra, indicating peptide-chain initiation was maintained at control values. The failure of sepsis to inhibit peptide-chain initiation after infusion of IL-1ra correlated with a maintenance of the epsilon-subunit of eukaryotic initiation factor (eIF) 2B (eIF-2B epsilon) protein at control values. The alterations in the eIF-2B epsilon protein content in gastrocnemius of septic rats treated with or without IL-1ra were associated with corresponding changes in the abundance of eIF 2B epsilon mRNA. The results provide evidence that infusion of IL-1ra attenuates the sepsis-induced inhibition of protein synthesis by preventing the inhibition of peptide-chain initiation and downregulation of eIF-2B expression during sepsis.

Sepsis-induced alterations in pyruvate dehydrogenase complex activity in rat skeletal muscle: effects on plasma lactate

The pyruvate dehydrogenase (PDH) complex undergoes reversible phosphorylation catalyzed by a PDH kinase (inactivating) and a PDH phosphatase (activating). In skeletal muscle, a decreased proportion of PDH complex in the active, nonphosphorylated form (PDHa) limits glucose oxidation and promotes the conversion of pyruvate to lactate. Increased lactate formation with the accompanying hyperlactatemia is a frequent metabolic complication of sepsis. The time course for inactivation of the PDH complex in skeletal muscle during sepsis was contrasted with changes in PDHa during sterile inflammation 3,7, or 14 days following the implantation of the foreign body nidus. Total PDH complex activity was not altered in any of the conditions examined. Sepsis, but not sterile inflammation, caused a reduction in the muscle PDHa measured 3 or 7 days following induction of sepsis. The inhibition of the muscle PDHa during sepsis was associated with a sustained hyperlactatemia. PDH kinase activity measured in extracts of mitochondria was enhanced twofold during this period. Fourteen days after induction of sepsis, there were no differences in the PDHa or plasma lactate concentrations in septic rats compared with either control or sterile inflammation. Furthermore, the PDH kinase activity was decreased to values observed in control values. The results are consistent with the hypothesis that a reduced PDHa in skeletal muscle during sepsis is responsible, in part, for the hyperlactatemia characteristic of septic hypermetabolism. Furthermore, the results provide evidence that the decrease in PDHa results from a stable stimulation of PDH kinase activity.

Central interleukin-1 partially mediates endotoxin-induced changes in glucose metabolism

The purpose of the present study was to determine whether intracerebral interleukin (IL)-1 mediates the endotoxin [lipopolysaccharide (LPS)]-induced increase in glucose flux. To accomplish this goal, a specific receptor antagonist for IL-1 (IL-1ra) or artificial cerebrospinal fluid was infused into the lateral ventricle via an intracerebroventricular cannula before, and for 4 h after, the intravenous injection of LPS. Whole body glucose flux was measured in conscious unrestrained rats using [3-3H]glucose. LPS increased both the plasma glucose concentration and the rate of glucose production (95 and 80%, respectively). In contrast, intracerebroventricular infusion of IL-1ra (2 mg/kg + 2 mg-kg-1.h-1) attenuated by approximately 50% the LPS-induced changes in glucose metabolism. IL-1ra also blunted the increase in plasma catecholamines, but not the elevation in glucagon and corticosterone concentrations, observed after LPS. Intracerebroventricular infusion of IL-1ra greatly reduced the LPS-induced hyperlactacidemia but did not alter the increase in muscle pyruvate dehydrogenase activity. An intravenous infusion of a 10-fold greater dose of IL-1ra, however, did not antagonize the LPS-induced increase in glucose flux. These data indicate that a major portion of the stimulation of glucose flux, as well as the increase in plasma catecholamines in response to LPS, is mediated by IL-1 within the central nervous system.

Sepsis inhibits synthesis of myofibrillar and sarcoplasmic proteins: modulation by interleukin-1 receptor antagonist

The breakdown of myofibrillar and sarcoplasmic (nonmyofibrillar) proteins are regulated independently in sepsis, however, the factors regulating their synthesis are unknown. In this study, we assessed the effects of sepsis and interleukin-1 receptor antagonist on sarcoplasmic and myofibrillar protein synthesis in gastrocnemius. The rate of sarcoplasmic protein synthesis was 3.5 times that of myofibrillar proteins in control and septic rats. The synthesis of both sarcoplasmic and myofibrillar proteins was diminished proportionately during sepsis (p < .05). Infusion of interleukin-1 receptor antagonist (2 mg.kg.-1.h.-1) prevented the sepsis-induced inhibition of total, sarcoplasmic, and myofibrillar protein synthesis. Changes in the abundance of messenger RNA could not account for the inhibition of protein synthesis observed in sepsis. Furthermore, in vitro translation of messenger RNA isolated from control and septic muscle revealed no major differences. These results suggest the following: 1) the inhibition of total mixed proteins during sepsis is a consequence of reduced synthesis of both myofibrillar and sarcoplasmic proteins; 2) IL-1ra maintains control values of protein synthesis by sparing the reduction in synthesis of both myofibrillar and sarcoplasmic proteins during sepsis; and 3) the abundance of messenger RNA is not a rate-limiting determinant of protein synthesis in muscle from septic rats. An alteration in the translational efficiency of existing mRNA appears to be the major mechanism responsible for the inhibition of protein synthesis during sepsis.

Prevention of skeletal muscle catabolism in sepsis does not impair visceral protein metabolism

We investigated whether the preservation of gastrocnemius proteins by interleukin-1 receptor antagonist (IL-1ra) during sepsis altered protein metabolism in visceral tissues. Sepsis was induced by creation of an abdominal abscess followed by infusion of saline of IL-1ra. Five days later, the tissue protein content and rate of protein synthesis were measured. IL-1ra did not significantly alter hepatic protein metabolism in septic or control animals. In kidney, the protein content and rate of protein synthesis were both decreased by sepsis and significantly ameliorated by the infusion of IL-1ra. Sepsis decreased the rate of protein synthesis in the small intestine. IL-1ra increased intestinal protein synthesis in both control and septic animals; however, the effects were localized to the seromuscular layer. The preservation of muscle protein by IL-1ra in sepsis did not adversely affect protein synthesis in any of the visceral tissues examined. IL-1 appears to mediate the sepsis-induced changes in protein synthesis in kidney and small intestine but not in liver or spleen. Protein synthesis in each visceral organ responds differently to the septic insult and modulation of IL-1 bioactivity.

IL-1 receptor antagonist attenuates sepsis-induced alterations in the IGF system and protein synthesis

The purpose of the present investigation was to determine whether endogenously produced interleukin (IL)-1 mediates the changes in insulin-like growth factor (IGF) I and IGF binding proteins (IGFBP) induced by chronic abdominal sepsis in rats and to correlate the changes in the IGF system with the alternations in protein synthesis. A constant infusion of IL-1 receptor antagonist (IL-1ra) was begun after the induction of sepsis and was continued for 5 days. Sepsis decreased IGF-I levels in the blood, liver, and gastrocnemius muscle, increased the content in the kidney, and did not alter IGF-I levels in heart, jejunum, and spleen. IL-1ra attenuated the sepsis-induced decrease in plasma IGF-I and completely prevented the changes in IGF-I observed in liver, kidney, and the gastrocnemius. IGFBP-1 was increased in the blood, liver, and muscle of septic rats. IL-1ra prevented this increase in IGFBP-1 in blood and liver but not in muscle. The rate of in vivo protein synthesis was decreased in the gastrocnemius and kidney and unaltered in the heart, liver, jejunum, and spleen. A strong linear correlation existed between levels of IGF-I and the rate of protein synthesis determined simultaneously in the gastrocnemius. These results provide evidence for the role of IL-1 as an endogenous mediator of the sepsis-induced changes in IGF-I and IGFBP-1 and suggest that the accompanying changes in muscle protein synthesis are partially mediated via changes in IGF-I.

Amrinone prevents the inhibition of muscle pyruvate dehydrogenase complex activity during sepsis

A decreased proportion of active pyruvate dehydrogenase complex (PDH) in skeletal muscle has been implicated as an important factor in elevating plasma lactate concentrations in hypermetabolic sepsis. The mediators of the septic process responsible for the inhibition of PDH complex in muscle are unknown. To assess the role of tumor necrosis factor in mediating the effects of sepsis, the effect of daily injections of amrinone (5 mg/kg/day), which inhibits the release of tumor necrosis factor during sepsis, on the proportion of PDH in the active form (PDHa) was investigated in a model of chronic hypermetabolic sepsis. In skeletal muscle from untreated septic rats, PDHa was decreased 50%. Treatment of septic rats with amrinone for 5 days prevented the sepsis-induced decrease in PDHa. Sepsis caused a 2.5-fold elevation in plasma lactate concentrations. The maintenance of the PDH complex activity at control values following injection of amrinone in septic rats was associated with reduced lactate concentrations in plasma. Thus, amrinone prevented the sepsis-induced abnormalities in skeletal muscle PDH activity and plasma lactate concentrations.

Augmented insulin action on rates of protein synthesis after resistance exercise in rats

This study investigated whether insulin has a modulatory effect on protein synthesis rates in skeletal muscle after four sessions of resistance exercise. Male rats engaged in resistance exercise (Acute) that required full extension of the hindlimbs with weights over the scapula or performed the standing movement with no additional weight (Nonex). Two separate studies were conducted. Rates of protein synthesis for study 1 (Acute, n = 6; Nonex, n = 6) were assessed 16 h postexercise by incorporation of [3H]phenylalanine ([3H]F) into muscle protein by use of an in vivo flooding dose protocol. Rates of protein synthesis in soleus of Acute (100 +/- 9 nmol F.g-1.h-1) were significantly higher than in Nonex (72 +/- 9 nmol F.g-1.h-1, P < 0.05). Rates of protein synthesis were significantly higher in gastrocnemius of Acute vs. Nonex (48 +/- 7 vs. 25 +/- 2 nmol F.g-1.h-1) but not in extensor digitorum longus (EDL). Assessment of protein synthesis rates for study 2 was conducted 16 h after resistance exercise with use of [3H]F incorporation into muscle protein during in situ bilateral hindlimb perfusion, with each leg perfused simultaneously but separately. Perfusion medium for one leg, but not the other, contained insulin (6.25 ng/ml). Soleus and gastrocnemius of Acute had higher protein synthesis rates than Nonex only in the leg that received insulin. For gastrocnemius, rates of protein synthesis in Acute without insulin were significantly lower than in Nonex with or without insulin. Insulin had no effect on protein synthesis rates for any muscle in Nonex rats. Neither exercise nor insulin affected protein synthesis rates in EDL. We conclude that insulin is a necessary component in elevated protein synthesis rates after resistance exercise in muscles composed of primarily slow-or fast-twitch fibers, and that a physiological perturbation (resistance exercise in this study) is required to observe such modulation, because rates of protein synthesis in Nonex muscles were not influenced by insulin.