Leucine: a key amino acid in ageing-associated sarcopenia?

During ageing, a progressive loss of muscle mass has been well described in both man and rodents. This loss of proteins results from an imbalance between protein synthesis and degradation rates. Although some authors have shown a decrease of myofibrillar protein synthesis rates in human volunteers, this imbalance is not clearly apparent when basal rates of protein turnover are measured. A decrease in muscle protein synthesis stimulation was detected nevertheless in ageing rats during the postprandial period, suggesting that the 'meal signal' was altered during ageing. Many results now suggest that aged muscle is less sensitive to the stimulatory effect of amino acids at physiological concentrations but is still able to respond if the increase in aminoacidaemia is sufficiently large. Indeed amino acids play an important role in regulating muscle protein turnover both in vitro and in vivo. At the molecular level, amino acids modulate gene expression. Amino acid response elements have been characterised in the promoter of transcriptional factor CCAAT-enhancer binding protein homologous protein and asparagine synthetase genes. Among amino acids, leucine seems to play the major role in regulating the metabolic function. It inhibits proteolysis and stimulates muscle protein synthesis independently of insulin. Leucine has been shown to act as a real mediator by modulating specifically the activities of intracellular kinases linked to the translation of proteins such as phosphatidylinosinol 3' kinase and mammalian target of rapamycin-70 kDa ribosomal protein S6 (p70S6K) kinases. We recently demonstrated in vitro that protein synthesis of ageing rat muscles becomes resistant to the stimulatory effect of leucine in its physiological concentration range. However, when leucine concentration was increased greatly above its postprandial level, protein synthesis was stimulated normally. Moreover, we studied the effect of meal leucine supplementation on in vivo protein synthesis in adult and ageing rats. Leucine supplementation had no additional effect on muscle protein synthesis in adults but totally restored its stimulation in ageing rats. Whether chronic oral leucine supplementation would be beneficial for maintaining muscle protein mass in elderly men and women remains to be studied.

Regulation of protein metabolism by insulin: value of different approaches and animal models

Insulin induces protein accretion by stimulating protein synthesis and inhibiting proteolysis. However, the mechanisms of regulation of protein metabolism by insulin are complex and still not completely understood. The use of approaches combining hyperinsulinemic clamp and isotopic methods, or measurement of the activation of intracellular kinases involved in insulin signaling, in addition to the use of different animal models in a comparative physiology process, provide better understanding of the potential regulation of protein metabolism by insulin. Studies using the clamp technique in lactating goats have shown a clear inhibitory effect of insulin on proteolysis, with an interaction between the effects of insulin and amino acids. Such studies revealed that the insulin-inhibited proteolysis is improved in lactating goats, this adaptative process limiting the mobilization of body protein under the conditions of amino acid deficit which occurs during early lactation. Insulin signaling studies in growing chickens have also provided some interesting features of insulin regulation compared to mammals. Refeeding or insulin injection leads to the activation of the early steps of insulin receptor signaling in the liver but not in the muscle. Muscle p70 S6 kinase, a kinase involved in the insulin activation of protein synthesis, was found to be markedly activated in response to insulin and to refeeding, suggesting that other signaling pathways than those classically described in mammalian muscles may be involved in signal transduction. Finally, although the role of insulin has been doubtful and has long been considered to be minor in ruminants and in avian species, this hormone clearly regulates protein metabolism in both species.

Amino acid supplementation prevents the loss of appetite for casein in old Lou/Cjall rats

A deleterious reduction of casein intake occurring earlier in males than in females had been previously observed in old Lou/Cjall rats. On the contrary, protein intake was observed to be maintained in old males when they were offered whey protein. Present studies were designed to investigate the effect of dietary casein modification on protein decrease. In two lifelong studies, male and female Lou/Cjall rats were tested every four months in order to study protein intake depending on the protein available: casein, whey protein or casein supplemented with an amino acid mixture (SC). In subsequent cross-sectional studies, young, adult, middle-aged and old rats were successively fed with casein, casein supplemented either with leucine or with alanine or with glycine. Supplementing casein with an amino acid mixture both globally increased protein intake and allowed old males to maintain a high rate of protein intake. In cross-sectional experiments, no effect of supplementation was seen in the young group. In older animals, the greatest effect was seen when casein was supplemented with alanine or glycine, independently of sex and age. We therefore, concluded that supplemented casein is more beneficial for old rats than casein alone, probably by increasing amino acid availability. We hypothesize that alanine could act through its effect on gluconeogenesis.

Increased availability of leucine with leucine-rich whey proteins improves postprandial muscle protein synthesis in aging rats

OBJECTIVE

We previously found that aging was characterized by a decreased sensitivity of muscle protein synthesis to leucine and that a free leucine-supplemented diet corrected this defect in old rats and elderly humans. The present experiment was undertaken to evaluate the efficiency of selected leucine-rich proteins to stimulate postprandial muscle protein synthesis in old rats to optimize nutritional protein support in the elderly.

METHODS

Sixty rats (22 mo old) received an experimental meal for the first hour of feeding and a standard diet for the rest of the day for 30 d. Experimental meals contained milk proteins that differed in leucine content: beta-lactoglobulin (14.5% leucine), Prolacta (13.4%), alpha-lactalbumin (10.9%), and casein (10%). As a control, a fifth group was added that received herring flour protein (7.3% leucine). Muscle protein synthesis was determined in vivo in the postprandial state at the end of the 30-d nutritional period using the flooding dose method (1-(13)C phenylalanine).

RESULTS

Leucine intake and plasma leucine concentrations were significantly increased in rats fed meals containing the leucine-rich proteins (i.e., beta-lactoglobulin and Prolacta). As previously observed with free leucine-supplemented meals, postprandial muscle protein synthesis was significantly improved in rats fed the meals containing the leucine-rich proteins. Interestingly, the beneficial effect was maintained after the 30-d supplementation.

CONCLUSION

The results indicated that leucine-rich proteins were efficient in improving muscle protein synthesis in old rats. Thus, nutritional supplements containing such proteins may be efficient in preventing sarcopenia in the elderly and would represent a safe and optimized nutritional strategy. However, further experiments are necessary to determine the duration of such nutritional support to obtain a significant protein gain in muscle.

Postprandial leucine deficiency failed to alter muscle protein synthesis in growing and adult rats

OBJECTIVE

This study examined the effect of a specific acute postprandial leucine deficiency on skeletal muscle protein synthesis in growing and adult rats. Because the anabolic action of dietary leucine supplementation is controversial, except during aging, we hypothesized that the maximum leucine effect might be already achieved for a normal postprandial rise of leucine. Preventing this rise during the 1- to 3-h period after feeding may reveal the leucine regulation.

METHODS

On the day of the experiment, rats were fasted (postabsorptive, PA group) or fed for 1 h a control meal (postprandial, control, PP group) or a leucine-poor meal (postprandial, PP-Leu group). Muscle protein synthesis was assessed in vivo, over the 1- to 3-h period after meal distribution, using the flooding dose method (L-1-(13)C phenylalanine).

RESULTS

As expected, the postprandial increase in plasma free leucine was specifically abolished after feeding the leucine-poor meal, whereas all the other plasma free amino acids were roughly at normal postprandial levels. Plasma insulin increased after feeding in young rats but was constant in adult rats. Plasma insulin was similar whatever dietary leucine levels. Rates of muscle protein synthesis were stimulated by feeding in gastrocnemius and soleus muscles from young rats but only in gastrocnemius muscles from adult rats. The PP-Leu group did not differ from the control PP group regarding muscle protein synthesis.

CONCLUSION

The rise in plasma free leucine is not required for the stimulation of muscle protein synthesis during the 1- to 3-h period after feeding young and adult rats, as previously observed in old rats.

Leucine supplementation improves muscle protein synthesis in elderly men independently of hyperaminoacidaemia

The present study was designed to assess the effects of dietary leucine supplementation on muscle protein synthesis and whole body protein kinetics in elderly individuals. Twenty healthy male subjects (70 +/- 1 years) were studied before and after continuous ingestion of a complete balanced diet supplemented or not with leucine. A primed (3.6 micromol kg(-1)) constant infusion (0.06 micromol kg(-1) min(-1)) of L-[1-13C]phenylalanine was used to determine whole body phenylalanine kinetics as well as fractional synthesis rate (FSR) in the myofibrillar fraction of muscle proteins from vastus lateralis biopsies. Whole body protein kinetics were not affected by leucine supplementation. In contrast, muscle FSR, measured over the 5-h period of feeding, was significantly greater in the volunteers given the leucine-supplemented meals compared with the control group (0.083 +/- 0.008 versus 0.053 +/- 0.009% h(-1), respectively, P < 0.05). This effect was due only to increased leucine availability because only plasma free leucine concentration significantly differed between the control and leucine-supplemented groups. We conclude that leucine supplementation during feeding improves muscle protein synthesis in the elderly independently of an overall increase of other amino acids. Whether increasing leucine intake in old people may limit muscle protein loss during ageing remains to be determined.

Whole body leucine flux in HIV-infected patients treated with or without protease inhibitors

The present study was carried out to assess the effects of protease inhibitor (PI) therapy on basal whole body protein metabolism and its response to acute amino acid-glucose infusion in 14 human immunodeficiency virus (HIV)-infected patients. Patients treated with PIs (PI+, 7 patients) or without PIs (PI-, 7 patients) were studied after an overnight fast during a 180-min basal period followed by a 140-min period of amino acid-glucose infusion. Protein metabolism was investigated by a primed constant infusion of l-[1-(13)C]leucine. Dual-energy X-ray absorptiometry for determination of fat-free mass (FFM) and body fat mass measured body composition. In the postabsorptive state, whole body leucine balance was 2.5 times (P < 0.05) less negative in the PI+ than in the PI- group. In HIV-infected patients treated with PIs, the oxidative leucine disposal during an acute amino acid-glucose infusion was lower (0.58 +/- 0.09 vs. 0.81 +/- 0.07 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]leucine enrichment, P = 0.06; or 0.70 +/- 0.10 vs. 0.99 +/- 0.08 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]ketoisocaproic acid enrichment, P = 0.04 in PI+ and PI- groups, respectively) than in patients treated without PIs. Consequently, whole body nonoxidative leucine disposal (an index of protein synthesis) and leucine balance (0.50 +/- 0.10 vs. 0.18 +/- 0.06 micromol x kg FFM x (-1) x min(-1) in PI+ and PI- groups respectively, P < 0.05) were significantly improved during amino acid-glucose infusion in patients treated with PIs. However, whereas the response of whole body protein anabolism to an amino acid-glucose infusion was increased in HIV-infected patients treated with PIs, any improvement in lean body mass was detected.

A leucine-supplemented diet restores the defective postprandial inhibition of proteasome-dependent proteolysis in aged rat skeletal muscle

We tested the hypothesis that skeletal muscle ubiquitin-proteasome-dependent proteolysis is dysregulated in ageing in response to feeding. In Experiment 1 we measured rates of proteasome-dependent proteolysis in incubated muscles from 8- and 22-month-old rats, proteasome activities, and rates of ubiquitination, in the postprandial and postabsorptive states. Peptidase activities of the proteasome decreased in the postabsorptive state in 22-month-old rats compared with 8-month-old animals, while the rate of ubiquitination was not altered. Furthermore, the down-regulation of in vitro proteasome-dependent proteolysis that prevailed in the postprandial state in 8-month-old rats was defective in 22-month-old rats. Next, we tested the hypothesis that the ingestion of a 5% leucine-supplemented diet may correct this defect. Leucine supplementation restored the postprandial inhibition of in vitro proteasome-dependent proteolysis in 22-month-old animals, by down-regulating both rates of ubiquitination and proteasome activities. In Experiment 2, we verified that dietary leucine supplementation had long-lasting effects by comparing 8- and 22-month-old rats that were fed either a leucine-supplemented diet or an alanine-supplemented diet for 10 days. The inhibited in vitro proteolysis was maintained in the postprandial state in the 22-month-old rats fed the leucine-supplemented diet. Moreover, elevated mRNA levels for ubiquitin, 14-kDa ubiquitin-conjugating enzyme E2, and C2 and X subunits of the 20S proteasome that were characteristic of aged muscle were totally suppressed in 22-month-old animals chronically fed the leucine-supplemented diet, demonstrating an in vivo effect. Thus the defective postprandial down-regulation of in vitro proteasome-dependent proteolysis in 22-month-old rats was restored in animals chronically fed a leucine-supplemented diet.

Altered responses in skeletal muscle protein turnover during aging in anabolic and catabolic periods

One of the most important effects of aging is sarcopenia, which is associated with impaired locomotion and general weakness. In addition, there is increased susceptibility to illness in aging, which often results in muscle wasting episodes. In such instances, the mobilization of muscle proteins provides free amino acids that are used for energetic purpose, the synthesis of acute phase proteins, and the immune response. However, since muscle protein mass is already depleted, the ability of the aged organism to recover from stress is impaired. Therefore, elucidating the mechanisms that result in sarcopenia is of obvious importance. Age-related changes in protein synthesis and proteolysis are rather small and our current methodology does not enable one to establish unequivocally whether sarcopenia results from depressed protein synthesis, increased proteolysis or both. By contrast, in anabolic and catabolic periods, a number of dysregulations in muscle protein turnover became clearly apparent. The aim of this review is to provide an overview of such altered responses to nutrients and catabolic treatments, which may ultimately contribute to explain sarcopenia. This includes impaired recovery in catabolic states, impaired anabolic effects of nutrients, in particular leucine, and a lack of regulation of the ubiquitin-proteasome proteolytic system. These alterations are discussed with respect to modifications in the insulin/IGF-1 axis and glucocorticoid related effects.

Insulin-dependent glycogen synthesis is delayed in onset in the skeletal muscle of food-deprived aged rats

Insulin resistance with aging may be responsible for impaired glycogen synthesis in the skeletal muscle of aged rats and contribute to the well-known decreased ability to respond to stress with aging. For this reason, to assess the ability of the skeletal muscle to utilize glucose for glycogen synthesis during aging, the time course of glycogen synthesis was continuously monitored by 13C nuclear magnetic resonance for 2 h in isolated [13C] glucose-perfused gastrocnemius-plantaris muscles of 5-day food-deprived adult (6-8 months; n=10) or 5-day food-deprived aged (22 months; n=8) rats. [13C] glucose (10 mmol/L) perfusion was carried out in the presence or absence of an excess of insulin (1 micromol/L). Food deprivation only decreased glycogen level in adult rats (8.9+/-2.4 micromol/g in adults vs. 35.6+/-2.4 micromol/g in aged rats; P<.05). In the presence of an excess of insulin, muscle glycogen synthesis was stimulated in both adult and aged muscles, but the onset was delayed with aging (40 min later). In conclusion, this study highlights the important role of glycogen depletion in stimulating glycogen synthesis in muscles. Consequently, the absence of glycogen depletion in response to starvation in aged rats may be the origin of the delay in insulin-stimulated glycogen synthesis in the skeletal muscle. Glycogen synthesis clearly was not impaired with aging.

Effect of treatment with dexamethasone on protein intake in adult and old Lou/c/jall rats

A deleterious decrease of protein intake had been evidenced in Lou/c/jall rats during ageing. This result could be induced by an impaired regulation of feeding behaviour. Glucocorticoids inducing specific amino-acid needs for gluconeogenesis and for the synthesis of inflammatory proteins by the liver, we investigated the age-related effect of a 4-days treatment with dexamethasone (DEX) on caloric and protein intake. Males and females aged 7, 19, 25 and 31 months received 573.6 +/- 65.6 microg/(kg day) of dexamethasone via the drinking water. Body weight (BW), caloric and macronutrients intakes were monitored during treatment and during 10 days after the treatment. A strong hypophagia was seen during treatment in all groups, which was mainly due to a decrease in fat intake. In the same time, rats maintained their protein intake so that protein became the main macronutrient of the diet in most of the groups. However, older males showed a lesser efficiency in adjusting their diet. These results are in agreement with previous data obtained in a protein deprivation study. They lead to the conclusion that the loss of appetite for protein in old age probably does not reflect a loss of ability to choose the needed amount of protein. We can hypothesise that the decrease of protein intake in old rats could be due to some inadequacy of casein to the metabolic requirement of aged animals.

Glucocorticoid excess induces a prolonged leucine resistance on muscle protein synthesis in old rats

This experiment was undertaken to examine leucine responsiveness of muscle protein synthesis during dexamethasone treatment and the subsequent recovery in young (4-5 weeks), adult (10-11 months) and old rats (21-22 months). Rats received dexamethasone in their drinking water. The dose and length of the treatment was adapted in order to generate the same muscle atrophy. Protein synthesis was assessed in vitro by incorporation of radiolabelled phenylalanine into proteins at the end of the treatment and after 3 or 7-day recovery. Results showed that dexamethasone did not alter muscle protein synthesis stimulation by leucine in young rats. In contrast, muscles from adult and old rats became totally resistant to leucine. Furthermore, the recovery of leucine responsiveness after dexamethasone withdrawal was slowed down in old rats when compared to younger rats. We concluded that glucocorticoids exert their catabolic action in adult and old rats partly through antagonising the stimulatory effect of leucine and may contribute to sarcopenia in old rats.

Differential effects of insulin and dietary amino acids on muscle protein synthesis in adult and old rats

The potential roles of insulin and dietary amino acids in the regulation of skeletal muscle protein synthesis were examined in adult and old rats. Animals were fed over 1 h with either a 25% or a 0% amino acid/protein meal. In each nutritional condition, postprandial insulin secretion was either maintained or blocked with diazoxide injections. Protein synthesis in gastrocnemius and soleus muscles was assessed in vivo using the flooding dose method. Insulin suppression decreased protein synthesis in both muscles irrespective of the nutritional condition and age of the rats. Moreover, reduced insulinaemia was associated with 4E-BP1 dephosphorylation, enhanced assembly of the 4E-BP1-eIF4E inactive complex and hypophosphorylation of eIF4E, p70S6k and protein kinase B, key intermediates in the regulation of translation initiation and protein synthesis. Old rats did not differ from adult rats. The lack of amino acids in the meal of insulin-suppressed rats did not result in any additional decrease in protein synthesis. In the presence of insulin secretion, dietary amino acid suppression significantly decreased gastrocnemius protein synthesis in adult but not in old rats. Amino acid suppression was associated with reduced phosphorylation of 4E-BP1 and p70S6k in adults. Along with protein synthesis, only the inhibition of p70S6k phosphorylation was abolished in old rats. We concluded that insulin is required for the regulation of muscle protein synthesis irrespective of age and that the effect of dietary amino acids is blunted in old rats.

The reduction of protein intake observed in old rats depends on the type of protein

Previous experiments have shown in Lou/c/jall rats growing old a deleterious reduction of protein intake, which occurs earlier in males than in females. We previously showed that this decrease could not be attributed to a loss of regulation of protein intake with age. Present studies were designed to investigate if the age-related decrease of protein intake was dependent on the type of protein used. In a first sectional study, adult, middle and old-aged Lou/c/jall rats were submitted to a self-selection procedure. They were fed successively with casein, whey protein and fish flour as protein. In a second longitudinal study, self-selected males and females were tested each 4 months (at 3, 7, 11, 15, 19, 23 and 27 months of age) with only casein and whey protein as protein. In the two experiments, the type of dietary protein had an influence on the protein intake: when casein is offered, the well-established decrease in protein consumption was seen after 15 months of age in male groups. The introduction of whey protein induced maintenance of protein intake in old male groups at the level of female's protein intake. Moreover, young females showed an obvious preference for casein versus whey protein. This preference disappeared in old ages. These data showed that casein, even if it was an appropriate protein for young animals, could become inadequate for old animals and could result in a protein aversion in old rats. On the contrary, whey protein seemed to be a more appropriate protein than casein for old rats.

Impaired anabolic response of muscle protein synthesis is associated with S6K1 dysregulation in elderly humans

Age-related loss of muscle protein may involve a decreased response to anabolic factors of muscle protein synthesis through dysregulation of translation factors. To verify this hypothesis, we simultaneously investigated muscle protein synthesis and expression of some factors implicated in insulin signal transduction during hyperinsulinemia and hyperaminoacidemia in 6 young (25+/-1 year; mean+/-sem) and 8 elderly subjects (72+/-2 year). Incorporation of L-[1-13C] leucine in muscle proteins (fractional synthesis rate, FSR) was measured in vastus lateralis, before and during a euglycemic hyperinsulinemic hyperaminoacidemic clamp, together with Western blot analysis of protein kinase B (PKB), mTOR, 4E-BP1, and S6K1 phosphorylation. In basal state, muscle protein FSR was reduced in elderly in comparison with young subjects (0.061+/-0.004% per hour) vs 0.082+/-0.010% per hour, elderly vs. young, P<0.05). During clamp, muscle protein FSR was stimulated in young (0.119+/-0.006% per hour; P<0.05), but this response was significantly lower in elderly subjects (0.084+/-0.005% per hour, P<0.05 vs young subjects). Phosphorylation of PKB, mTOR, and 4E-BP1 were similarly increased by insulin and amino acid in both groups, except for S6K1 phosphorylation, which was not stimulated in elderly subjects. In conclusion, 1) response of muscle protein synthesis to insulin and amino acid is impaired in elderly humans; 2) a defect in S6K1 pathway activation may be responsible for this alteration. This modification is a mechanistic basis of sarcopenia development during aging.

Glucocorticoids regulate mRNA levels for subunits of the 19 S regulatory complex of the 26 S proteasome in fast-twitch skeletal muscles

Circulating levels of glucocorticoids are increased in many traumatic and muscle-wasting conditions that include insulin-dependent diabetes, acidosis, infection, and starvation. On the basis of indirect findings, it appeared that these catabolic hormones are required to stimulate Ub (ubiquitin)-proteasome-dependent proteolysis in skeletal muscles in such conditions. The present studies were performed to provide conclusive evidence for an activation of Ub-proteasome-dependent proteolysis after glucocorticoid treatment. In atrophying fast-twitch muscles from rats treated with dexamethasone for 6 days, compared with pair-fed controls, we found (i) increased MG132-inhibitable proteasome-dependent proteolysis, (ii) an enhanced rate of substrate ubiquitination, (iii) increased chymotrypsin-like proteasomal activity of the proteasome, and (iv) a co-ordinate increase in the mRNA expression of several ATPase (S4, S6, S7 and S8) and non-ATPase (S1, S5a and S14) subunits of the 19 S regulatory complex, which regulates the peptidase and the proteolytic activities of the 26 S proteasome. These studies provide conclusive evidence that glucocorticoids activate Ub-proteasome-dependent proteolysis and the first in vivo evidence for a hormonal regulation of the expression of subunits of the 19 S complex. The results suggest that adaptations in gene expression of regulatory subunits of the 19 S complex by glucocorticoids are crucial in the regulation of the 26 S muscle proteasome.

Insulin and amino acids both strongly participate to the regulation of protein metabolism

PURPOSE OF REVIEW

The application of tracer kinetic methods, combined with measurements of the activity of components of the cellular signaling pathways involved in protein synthesis and degradation, affords new insights into the regulation of skeletal muscle protein metabolism in vivo in humans. Feeding is associated with an increase in protein synthesis and a decrease in proteolysis. These changes are mediated by feeding-induced increases in plasma concentrations of both nutrients and hormones.

RECENT FINDINGS

Recent studies definitely demonstrated that insulin and amino acids directly interacted in promoting postprandial anabolism. However, the contribution of amino acids was abolished in old individuals in whom only insulin action persisted. There was a line of evidence that the effect of amino acids originates from leucine, which should not be viewed simply as a building block for protein synthesis, but as a signal in the regulation of cell functions. Although their cellular signaling pathways do not completely overlap, insulin and amino acids both activate the translation initiation of protein synthesis. Insulin presumably inhibits skeletal muscle protein degradation through a decrease in the activity of the ubiquitin proteasome-dependent pathway.

SUMMARY

Whether or not amino acids modify insulin action and have specific effects on proteolysis has not yet been documented. At the molecular level, amino acids such as insulin modulate gene expression. Such studies are needed to gain a better understanding of the interactions between insulin and amino acids in the regulation of skeletal muscle protein anabolism.

Leucine-supplemented meal feeding for ten days beneficially affects postprandial muscle protein synthesis in old rats

Acute leucine supplementation of the diet has been shown to blunt defects in postprandial muscle protein metabolism in old rats. This study was undertaken to determine whether the effect of leucine persists in a 10-d experiment. For this purpose, adult (9 mo) and old (21 mo) rats were fed a semiliquid 18.2 g/100 g protein standard diet during the 8-h dark period for 1 mo. Then, each group was given either a leucine-supplemented meal or an alanine-supplemented meal (as the control meal) for 1 h and the standard diet the rest of the feeding period. On d 10, rats were fed either no food (postabsorptive group) or the supplemented meal for 1 h. Muscle protein synthesis was assessed in vivo 90-120 min after meal distribution using the flooding dose method (1-(13)C phenylalanine). Leucinemia was similar in rats of both ages in the postabsorptive state. Postprandial plasma leucine concentrations were one- to twofold greater after the leucine meal than after the control meal. In the postabsorptive state, leucine supplementation did not modify the muscle protein synthesis rate in old rats but enhanced it to the postprandial rate in adult rats. As expected, muscle protein synthesis was stimulated by the control meal in adult rats but not in old rats. The leucine meal restored this stimulation in old rats but did not further stimulate muscle protein synthesis in adult rats. In conclusion, the beneficial effect of leucine supplementation on postprandial muscle protein anabolism persists for at least 10 d. The long-term utilization of leucine-rich diets may therefore limit muscle protein wasting during aging.

Effect of medroxyprogesterone acetate on the efficiency of an oral protein-rich nutritional support in HIV-infected patients

We have examined the effect of a medroxyprogesterone therapy in HIV-infected patients under appropriate nutrition for anabolism. The experiments were performed on 12 men (mean age 40 y), HIV seropositive but free of any clinically active opportunistic infection for at least one month. The patients underwent a 2-week baseline diet period (1.2 g protein x kg(-1) body weight (BW) x d(-1)) and then a 5-week experimental period with again the baseline diet in conjunction with supplements including Tonexis HP (0.7 g protein x kg(-1) BW) x d(-1)), L-threonine (0.018 g x kg(-1) BW x d(-1)) and L-methionine (0.013 g x kg(-1) BW x d(-1)). Indeed HIV-infected patients showed deficiencies in these amino acids. They were randomly divided into groups I and II under double-blinded condition. Group II was given medroxyprogesterone acetate (0.4 g x d(-1)) during the last 3 weeks whereas group I received a placebo. All the patients significantly increased their body weight (P < 0.05) during the experimental periods. Those under medroxyprogesterone tended to show a higher but not significant weight gain (+3.1 +/- 1.0 kg in group II and +1.9 +/- 0.3 kg in group I). Blood free amino acids were used as rough indicators of amino acid utilization and were analyzed prior and during acute 150 min intravenous infusion of a complete glucose-amino acid mixture. This test was done before and at the end of the experimental periods. Basal essential blood free amino acids were similar in the two groups and did not change during the experimental period. Most essential amino acids increased following glucose-amino acid infusions. The incremental increase was of less magnitude after the experimental period than before when medroxyprogesterone was present (P < 0.05 for valine, leucine, lysine, threonine and methionine). This was not the case in the absence of the hormone. We concluded that medroxyprogesterone might improve the efficacy of an oral protein-rich nutritional support in HIV-infected patients.

Dexamethasone-mediated regulation of death and differentiation of muscle cells. Is hydrogen peroxide involved in the process?

The hypothetical involvement of H2O2 in dexamethasone-mediated regulation of muscle cell differentiation and elimination was studied. Rat L6 myoblasts and mouse C2C12 satellite cells were chosen for acute (24 h) and chronic (5 or 10 day) experiments. Mitogenicity and anabolism were both affected by H2O2. Micromolar concentrations of H2O2 inhibited DNA while stimulating protein synthesis. At the millimolar level, H2O2 led to cell death by apoptosis. Synthetic glucocorticoi - dexamethasone (Dex) was shown to effect muscle cell fate similarly to H2O2. Chronic treatment with H2O2 or Dex dose-dependently accelerated either the formation of myotubes or cell elimination. Dex-induced cell death slightly differed from classical apoptosis and was featured by the symptoms of cell senescence such as extensive cytoplasm vacuolisation, accumulation of inclusion-bodies and lack of low molecular weight oligonucleosomal DNA fragmentation but chromatin condensation. Antioxidants (sodium ascorbate, N-acetyl-L-cysteine, catalase) abrogated Dex-dependent cell death. We conclude that H2O2 directly influences myogenesis and muscle cell elimination. Moreover, H2O2 can be considered as the potent mediator of glucocorticoid-dependent effects on muscle cells.

Short-term mild hyperglycemia enhances insulin-stimulated glucose disposal in lactating goats

This work was designed to study the effect of a 3-day mild hyperglycemia (5.3 vs. 3.3 mM) on the regulation of glucose metabolism in lactating goats. Glucose was intravenously infused at variable rates simultaneously with a constant potassium-amino acid infusion. Diet plus substrate infusion maintained net energy but not protein supply. Milk yield did not change. Skeletal muscle glucose transporter (GLUT-4) was analyzed before and after hyperglycemia. In addition, the acute effect of medium and high insulin doses on glucose turnover was measured in vivo during euglycemic and hyperglycemic hyperinsulinemic clamps under potassium and amino acid replacement. Hyperglycemia reduced the endogenous glucose appearance but increased glucose disposal. It decreased the total membrane-associated GLUT-4 protein in skeletal muscle. In contrast, it improved the acute insulin-stimulated glucose disposal. Both the level and duration (3 days) of hyperglycemia contributed to this improvement. We conclude that short-term mild hyperglycemia has similar effects in lactating goats as those already observed in nonlactating rodents or humans.

Postprandial stimulation of muscle protein synthesis in old rats can be restored by a leucine-supplemented meal

Aging is characterized by a progressive loss of muscle mass. A decrease of muscle protein synthesis stimulation has been detected in the postprandial state and correlated to a decrease of muscle protein synthesis sensitivity to leucine in vitro. This study was undertaken to examine the effect of a leucine-supplemented meal on postprandial (PP) muscle protein synthesis during aging. Adult (8 mo old) and old (22 mo old) rats were fed a semiliquid 18.2% protein control diet for 1 mo. The day of the experiment, rats received no food (postabsorptive group) or either an alanine or leucine-supplemented meal for 1 h (postprandial groups: PP and PP + Leu groups, respectively). Muscle protein synthesis was assessed in vivo 90-120 min after the meal distribution using the flooding dose method (1-(13)C phenylalanine). Plasma leucine concentrations were significantly greater in the PP + Leu group compared with the PP group at both ages. Muscle protein synthesis was significantly greater in the adult PP group, whereas it was not stimulated in the old PP group. When supplemented with leucine, muscle protein synthesis in old rats was stimulated and similar to that observed in adults. We conclude that acute meal supplementation with leucine is sufficient to restore postprandial stimulation of muscle protein synthesis in old rats. Whether chronic leucine meal supplementation may limit muscle protein wasting during aging remains to be verified.

Differential regulation of the lysosomal, Ca2+-dependent and ubiquitin/proteasome-dependent proteolytic pathways in fast-twitch and slow-twitch rat muscle following hyperinsulinaemia

In order to characterize the poorly defined mechanisms that account for the anti-proteolytic effects of insulin in skeletal muscle, we investigated in rats the effects of a 3 h systemic euglycaemic hyperinsulinaemic clamp on lysosomal, Ca(2+)-dependent proteolysis, and on ubiquitin/proteasome-dependent proteolysis. Proteolysis was measured in incubated fast-twitch mixed-fibre extensor digitorum longus (EDL) and slow-twitch red-fibre soleus muscles harvested at the end of insulin infusion. Insulin inhibited proteolysis (P<0.05) in both muscles. This anti-proteolytic effect disappeared in the presence of inhibitors of the lysosomal/Ca(2+)-dependent proteolytic pathways in the soleus, but not in the EDL, where only the proteasome inhibitor MG 132 (benzyloxycarbonyl-leucyl-leucyl-leucinal) was effective. Furthermore, insulin depressed ubiquitin mRNA levels in the mixed-fibre tibialis anterior, but not in the red-fibre diaphragm muscle, suggesting that insulin inhibits ubiquitin/proteasome-dependent proteolysis in mixed-fibre muscles only. However, depressed ubiquitin mRNA levels in such muscles were not associated with significant decreases in the amount of ubiquitin conjugates, or in mRNA levels or protein content for the 14 kDa ubiquitin-conjugating enzyme E2 and 20 S proteasome subunits. Thus alternative, as yet unidentified, mechanisms are likely to contribute to inhibit the ubiquitin/proteasome system in mixed-fibre muscles.

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.

Evidence for an alteration of plasma and liver proteins response to dexamethasone in aging rats

The aim of this study was carried out to analyse the liver and plasma proteins response to dexamethasone in adult (6-8 months) and old (24 months) rats in order to ascertain the involvement of glucocorticoids in the aging process. The animals received dexamethasone (Dex) for 5 or 6 days. As Dex decreased food intake, all groups were pair fed to dexamethasone-treated old rats. The synthesis of mixed plasma and liver proteins (assessed by a flooding dose of [13C] valine) was similarly greatly improved in adult and old rats after Dex treatment. However, the level of mixed plasma proteins was only slightly increased. When specific plasma proteins were assessed, a similar increase in the concentration of albumin and alpha1 acid glycoprotein was observed in adult and old rats. By contrast, fibrinogen decreased to a greater extend in old rats and alpha2 macroglobulin became undetectable in old animals. It was concluded that the response of plasma and liver proteins to Dex was altered in old rats and may contribute to the pathogenesis of several diseases which occur during aging.

Stimulation of in vitro rat muscle protein synthesis by leucine decreases with age

Aging is characterized by a decrease of muscle mass associated with a decrease in postprandial anabolism. This study was performed to gain a better understanding of the intracellular mechanisms involved in the stimulation of muscle protein synthesis by amino acids and their role in the decrease of muscle sensitivity to food intake during aging. The effects of amino acids or leucine alone were assessed in vitro on epitrochlearis muscle from young, adult and old rats. Protein synthesis was assessed by incorporation of radiolabeled phenylalanine into protein and p70 S6 kinase activity by incorporation of (32)P into a synthetic substrate. Amino acids, at physiologic concentrations, stimulated muscle protein synthesis (P < 0.05) and leucine reproduced this effect. The intracellular targets of amino acids were phosphatidylinositol 3' kinase and the rapamycin-sensitive pathways mammalian target of rapamycin (mTOR)/p70 S6 kinase. In old rats, the sensitivity of muscle protein synthesis to leucine was lower than in adults (P < 0.05) and this paralleled the lesser ability of leucine to stimulate the rapamycin-sensitive pathways (P < 0.05). We demonstrated that amino acids and leucine stimulate muscle protein synthesis and that aging is associated with a decrease in this effect. However, because aged rats are still able to respond normally to high leucine concentrations, we hypothesize that a nutritional manipulation increasing the availability of this amino acid to muscle could be beneficial in maintaining the postprandial stimulation of protein synthesis.

Muscle protein turnover during early development in chickens divergently selected for growth rate

To explore the mechanisms involved in the genetic control of muscle growth and protein gain, protein metabolism was assessed in the pectoralis major muscle of two chicken lines selected for either fast or slow growth. Protein synthesis was measured in vivo at various ages from 1 to 4 wk, using a flooding dose of L-[4-3H] phenylalanine. Protein degradation was estimated as the difference between synthesis and deposition. Over the experimental period, BW were about 2-fold greater (P < 0.001), and pectoralis major muscle weights were 2.4- to 3.6-fold higher (P < 0.001), in chicks from the fast-growing line (FGL) than those from the slow-growing line (SGL). Independent of age, absolute rates of protein deposition, synthesis, and breakdown were higher in FGL than in SGL chickens. Fractional rates of muscle protein synthesis clearly decreased with age. When comparing birds of the same age, fractional rates of muscle protein synthesis tended to be lower in the FGL. Fractional degradation rates (KD) were significantly lower in FGL chickens during the first 2 wk of post-natal growth, whereas KD were similar between lines in older chickens. In this experimental model of chicken lines divergently selected for BW, the greatest line-related difference in muscle protein metabolism was in KD, and was observed in the early growth phases.

Protein turnover modifications induced by the protein feeding pattern still persist after the end of the diets

This study was undertaken to determine whether the protein feeding pattern could induce chronic adaptation of protein turnover. After a 15-day adaptive period, elderly (68 yr) and young (26 yr) women received, for 14 days, a diet providing 200 KJ x kg fat-free mass (FFM)(-1) x day(-1), where the daily protein intake (1.7 g protein x kg FFM(-1) x day(-1)) was either spread over 4 meals in the spread pattern or mainly (80%) consumed at noon in the pulse pattern. One day after the end of the dietary treatment, whole body leucine kinetics were measured by use of a continuous [(13)C]leucine infusion, both in the postabsorptive state and in the same fed state. The pulse pattern was able to induce, in young as in elderly women, a lower postabsorptive leucine oxidation and endogenous leucine flux than the spread pattern and improved the responsiveness of nonoxidative leucine disposal during 4-h oral feeding. Thus the pulse pattern was able to induce chronic regulation of protein metabolism in young as in elderly women.

Lack of effect of ageing on acetate oxidation in rat skeletal muscle during starvation: a (13)C NMR study

Acetate oxidation was examined by (13)C nuclear magnetic resonance in skeletal muscle from adult and old rats. Rats fasted for 5 days were perfused with [2-(13)C]acetate over 2 h, and muscle extracts were analyzed for [(13)C]glutamate isotopomers. This study shows that approximately 80 % of acetyl-coenzyme A entering the tricarboxylic cycle was derived from substrate infusion in both adult and old rats, and that the flux through anaplerotic pathways was approximately 21 % of the flux through citrate synthase. These data demonstrate that skeletal muscle from adult and old rats oxidizes the same proportion of exogenous acetate.

Response of leucine metabolism to hyperinsulinemia in hypothyroid patients before and after thyroxine replacement

We have investigated the effect of hypothyroidism and insulin on protein metabolism in humans. Six hypothyroid patients were studied in a postabsorptive state before and after 5 months of regular treatment for hypothyroidism (153 +/- 17 microg/day of L-T4). The effect of insulin was assessed under hyperinsulinemic euglycemic and eukalemic conditions. Insulin was infused for 140 min at 0.0063 +/- 0.0002 nmol/kg x min. An amino acid infusion was used to blunt insulin-induced hypoaminoacidemia. Whole body protein turnover was measured using L-[1-13C] leucine. When compared to L-T4-induced subclinical thyrotoxic state, hypothyroidism induced a significant decrease (P < 0.05) in leucine endogenous appearance rate (a reflection of proteolysis; 0.89 +/- 0.09 vs. 1.33 +/- 0.05 micromol/kg x min), oxidation (0.19 +/- 0.02 vs. 0.25 +/- 0.03 micromol/kg x min), and nonoxidative disposal (a reflection of protein synthesis; 0.87 +/- 0.11 vs. 1.30 +/- 0.05 micromol/ kg x min). Insulin lowered proteolysis during both the subclinical thyrotoxic and hypothyroid states. Hypothyroidism impaired the antiproteolytic effects of insulin. Thyroid hormones are, therefore, essential for the normal antiproteolytic action of insulin.

Pentoxifylline improves insulin action limiting skeletal muscle catabolism after infection

We investigated the ability of pentoxifylline (PTX) to modulate protein synthesis and degradation in the presence and absence of insulin during incubation of epitrochlearis muscle, 2 or 6 days after injection of Escherichia coli. On days 2 and 6 after infection, protein synthesis was inhibited by 25%, whereas proteolysis was enhanced by 75%. Insulin (2 nM) in vitro stimulated protein synthesis in muscles from infected rats to the same extent as in controls. The ability of insulin to limit protein degradation was severely blunted 48 h after infection. On day 6 after infection, insulin inhibited proteolysis to a greater extent than on day 2. PTX suppressed the increase in plasma concentrations of tumor necrosis factor more than 600-fold after injection of bacteria, and partially prevented the inhibition of protein synthesis and stimulation of protein degradation during sepsis. Moreover, PTX administration maintained the responsiveness of protein degradation to insulin during sepsis. Thus cytokines may influence skeletal muscle protein metabolism during sepsis, both indirectly through inhibition of the effects of insulin on proteolysis, and directly on the protein synthesis and degradation machinery.

Glucocorticoid-induced insulin resistance of protein synthesis is independent of the rapamycin-sensitive pathways in rat skeletal muscle

This study was designed to evaluate the role of p70 S6 kinase (p70(S6K) ), p90 S6 kinase (p90(RSK)) and mitogen-activated protein (MAP) kinase pathways in the insulin resistance of muscle protein synthesis observed during glucocorticoid treatment. Dexamethasone treatment decreased the effect of insulin on protein synthesis (-35. 2%) in epitrochlearis muscle incubated in vitro. This resistance is associated with a total blockage of the stimulation of p70(S6K) by insulin without any significant decrease in the amount of the kinase. However, the effect of rapamycin (inhibitor of several intracellular pathways including p70(S6K) pathways) on muscle protein synthesis was not modified by dexamethasone in rat muscles. This suggested that 'rapamycin-sensitive pathways' associated with the insulin stimulation of protein synthesis were not altered by glucocorticoids and thus are not responsible for the insulin resistance observed. As incubation of muscles with a MAP kinase inhibitor (PD98059) did not modify the stimulation of protein synthesis by insulin and as glucocorticoids did not alter the effect of insulin on p90(RSK )activity, our results provide evidence that glucocorticoid-induced alterations in muscle protein synthesis regulation by insulin do not involve factors or kinases that are dependent on MAP kinase and/or p90(RSK).

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.

Involvement of the rapamycin-sensitive pathway in the insulin regulation of muscle protein synthesis in streptozotocin-diabetic rats

Insulin resistance in 3-day streptozotocin (STZ)-treated rats was manifested by the lack of antiproteolytic action of insulin as well as by a reduction of its stimulatory effect on protein synthesis (-60% compared with the control group) in epitrochlearis muscle incubated in vitro. In the present study, we have investigated the diabetes-associated alterations in the insulin signalling cascade, especially the phosphatidylinositol-3 kinase (PI-3 kinase)/p70 S6 kinase (p70(S6K)) pathway, in rat skeletal muscle. LY 294002, a specific inhibitor of PI-3 kinase, markedly decreased the basal rate of protein synthesis and completely prevented insulin-mediated stimulation of this process both in control and diabetic rats. Thus, PI-3 kinase is required for insulin-stimulated muscle protein synthesis in diabetic rats as in the controls. Rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), had no effect on the basal rate of protein synthesis in either of the experimental groups. In control rats, the stimulatory action of insulin on muscle protein synthesis was diminished by 36% in the presence of rapamycin, whereas in diabetic muscles this reduction amounted to 68%. The rapamycin-sensitive pathway makes a relatively greater contribution to the stimulatory effect of insulin on muscle protein synthesis in diabetic rats compared with the controls, due presumably to the preferential decrease in the rapamycin-insensitive component of protein synthesis. Neither basal nor insulin-stimulated p70(S6K) activity, a signalling element lying downstream of mTOR, were modified by STZ-diabetes.

Insulin action on skeletal muscle protein metabolism during catabolic states

Insulin plays a major role in the regulation of skeletal muscle protein turnover but its mechanism of action is not fully understood, especially in vivo during catabolic states. These aspects are presently reviewed. Insulin inhibits the ATP-ubiquitin proteasome proteolytic pathway which is presumably the predominant pathway involved in the breakdown of muscle protein. Evidence of the ability of insulin to stimulate muscle protein synthesis in vivo was also presented. Many catabolic states in rats, e.g. streptozotocin diabetes, glucocorticoid excess or sepsis-induced cytokines, resulted in a decrease in insulin action on protein synthesis or degradation. The effect of catabolic factors would therefore be facilitated. In contrast, the antiproteolytic action of insulin was improved during hyperthyroidism in man and early lactation in goats. Excessive muscle protein breakdown should therefore be prevented. In other words, the anabolic hormone insulin partly controlled the 'catabolic drive'. Advances in the understanding of insulin signalling pathways and targets should provide information on the interactions between insulin action, muscle protein turnover and catabolic factors.

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.

Threonine and methionine are limiting amino acids for protein synthesis in patients with AIDS

This study was conducted to identify the most rate-limiting amino acids for whole-body protein synthesis in acquired immunodeficiency syndrome (AIDS) patients. We postulated that an essential amino acid that would be rate limiting in AIDS should have a low basal plasma concentration and should remain at a low level during amino acid infusion. Seven male AIDS patients (median age 37 y, CD4 cell count: 76 mm-3) without any clinically active opportunistic infection during the month before the experiment were infused intravenously with a complete amino acid-glucose mixture for 2.5 h. Eight healthy volunteers were used as controls. Before the infusion, the concentrations of most free essential amino acids (methionine, threonine, histidine, isoleucine, leucine and tryptophan) were significantly lower (P < 0.05) in AIDS patients than in controls. Most plasma free essential amino acids increased significantly during infusion. However, the absolute increase above basal levels for threonine, valine, lysine, (P < 0.05) and methionine (P < 0.073) was smaller in AIDS patients than in control subjects. Thus, threonine and possibly methionine may be rate limiting for whole-body protein synthesis in AIDS patients, suggesting that there are selective amino acid requirements in patients with AIDS.

Acute hyperinsulinemia fails to change GLUT-4 content in crude membranes from goat skeletal muscles and adipose tissue

The effect of insulin on GLUT-4 protein level in samples of adipose tissue and skeletal muscles from goats was studied in vivo using an euglycemic hyperinsulinemic clamp. The clamp was maintained in conscious goats for 6 h in the presence of amino acids to prevent insulin-induced hypoaminoacidemia. GLUT-4 protein was assessed in crude membrane preparations from adipose tissue and four skeletal muscles (longissimus dorsi, tensor fasciae latae, anconeus and diaphragm) by Western blot analysis. No changes of GLUT-4 protein content were detected after 6 h of hyperinsulinemia in either adipose tissue or skeletal muscles from goats. These results suggest that insulin is not the prime factor involved in the short-term regulation of GLUT-4 protein transporter content in insulin-sensitive tissues from goats.

Effect of glucocorticoid excess on skeletal muscle and heart protein synthesis in adult and old rats

This study was carried out to analyse glucocorticoid-induced muscle wasting and subsequent recovery in adult (6-8 months) and old (18-24 months) rats because the increased incidence of various disease states results in hypersecretion of glucocorticoids in ageing. Adult and old rats received dexamethasone in their drinking water for 5 or 6 d and were then allowed to recover for 3 or 7 d. As dexamethasone decreased food intake, all groups were pair-fed to dexamethasone-treated old rats (i.e. the group that had the lowest food intake). At the end of the treatment, adult and old rats showed significant increases in blood glucose and plasma insulin concentrations. This increase disappeared during the recovery period. Protein synthesis of different muscles was assessed in vivo by a flooding dose of [13C]valine injected subcutaneously 50 min before slaughter. Dexamethasone induced a significant decrease in protein synthesis in fast-twitch glycolytic and oxidative glycolytic muscles (gastrocnemius, tibialis anterior, extensor digitorum longus). The treatment affected mostly ribosomal efficiency. Adult dexamethasone-treated rats showed an increase in protein synthesis compared with their pair-fed controls during the recovery period whereas old rats did not. Dexamethasone also significantly decreased protein synthesis in the predominantly oxidative soleus muscle but only in old rats, and increased protein synthesis in the heart of adult but not of old rats. Thus, in skeletal muscle, the catabolic effect of dexamethasone is maintained or amplified during ageing whereas the anabolic effect in heart is depressed. These results are consistent with muscle atrophy occurring with ageing.

Glucocorticoid effects on insulin- and IGF-I-regulated muscle protein metabolism during aging

This study was performed to assess the effect of glucocorticoids (dexamethasone) on insulin- and IGF-I-regulated muscle protein metabolism in adult and old rats. Muscle atrophy occurred more rapidly in old rats, and recovery of muscle mass was impaired when compared with adults. Muscle wasting resulted mainly from increased protein breakdown in adult rat but from depressed protein synthesis in the aged animal. Glucocorticoid treatment significantly decreased the stimulatory effect of insulin and IGF-I on muscle protein synthesis in adult rats by 25.9 and 58.1% respectively. In old rats, this effect was even greater, being 49.3 and 100% respectively. With regard to muscle proteolysis, glucocorticoids blunted the anti-proteolytic action of insulin and IGF-I in both age groups. During the recovery period, adult rats reversed the glucocorticoid-induced resistance of muscle protein metabolism within 3 days, at which time old rats still exhibited the decrease in insulin-regulated proteolysis. In conclusion, the higher sensitivity of old rat muscle to glucocorticoids may in part result from the greater modification of the effects of insulin and IGF-I on muscle protein metabolism. These responses to glucocorticoids in old rats may be associated with the emergence of muscle atrophy with advancing age.

pH is regulated differently by glucose in skeletal muscle from fed and starved rats: a study using 31P-NMR spectroscopy

The aim of this study was to determine whether exogenous glucose metabolism influences the pH in superfused EDL muscle from growing rats fed or starved for 48 h (body weight 55 and 45 g, respectively). Energy state and intracellular pH of muscle were repeatedly monitored by 31P-nuclear magnetic resonance spectroscopy (31P-NMRS); glycogen and other energy metabolites were assayed enzymatically in muscle extracts at the end of the experiment. In EDL muscles from starved rats superfused with glucose for 4 h, intracellular pH was elevated (7-7.3), lactate concentration low, glycogen repletion very intense and citrate synthase activity high. We conclude that glucose was routed mainly toward both oxidative phosphorylation and glycogen synthesis in EDL muscles after food deprivation of rats. In contrast, the major pathway in muscles from fed rats may be glycolysis because the glycogen pool remained constant throughout the experiment. The additional and minor pH component (in the range of 6.5 to 6.8) seen in muscles from fed rats, even in the presence of exogenous glucose, might be due to impaired glucose utilization because this component appears also in muscles from starved rats superfused without glucose or with a nonmetabolizable analog of glucose. Consequently, direct pH measurement by 31P-NMR may be considered to be a precise criterion for evaluation of differences in metabolic potentialities of muscle studied ex vivo in relation to the nutritional state of rats.

Decrease in insulin and insulin-like growth factor I (IGF-I) binding to erythrocytes from patients with cystic fibrosis

Cystic fibrosis, an autosomal recessive disease affecting exocrine glands, is associated in many cases with a severe undernutritional state, growth retardation and glucose intolerance. To obtain a better definition of the possible defects of insulin and insulin-like growth factor I (IGF-I) receptors, we investigated 125I-insulin and 125I-IGF-I binding to erythrocytes from patients with cystic fibrosis (n = 23) and controls (n = 13). Erythrocytes were isolated by Ficoll-Hypaque gradient centrifugation, and hormone binding was performed in cell suspensions of 3 x 10(9) cells/ml. Cystic fibrosis patients displayed a statistically significant 33% and 40% (p < 0.05) decrease of insulin and IGF-I binding, respectively, compared to controls. These alterations were due to an almost 50% reduction in the binding capacity of the high-affinity receptor compartment. Affinity constants were modified to a lesser extent, except for a two-fold decrease in K1 of the high-affinity compartment of insulin receptors. Interestingly, the decrease in insulin binding was proportional to the degree of growth failure. The statistical significance of hormone binding alterations was assessed in terms of the graphic distribution of individual affinity constants and binding capacity values. Although variable, 50 to 60% of cystic fibrosis patients displayed alterations in stoichiometric binding parameters located outside the area described by the 95% tolerance interval of controls. A major reduction in insulin and IGF-I binding in conditions of low and normal insulin and IGF-I plasma levels, respectively, as well as the correlation with the degree of growth failure in patients with cystic fibrosis, may contribute to an understanding of the pathogenesis of insulin resistance and glucose abnormalities in undernutritional states.

The effect of a high dose of 3-hydroxy-3-methylbutyrate on protein metabolism in growing lambs

The effect of a high dose of 3-hydroxy-3-methylbutyrate (HMB, a leucine catabolite) on protein metabolism was investigated in growing male lambs fed on hay and concentrate. Concentrate was supplemented with either Ca(HMB)2 (4 g/kg) or Ca(CO3)2 in experimental (HMB) and control groups respectively. Both groups consisted of six 2-month old lambs. Three complementary methods to study protein metabolism were carried out consecutively 2.5 months after beginning the dietary treatment: whole body phenylalanine fluxes, postprandial plasma free amino acid time course and fractional rates of protein synthesis in skeletal muscles. Feeding a high dose of HMB led to a significant increase in some plasma free amino acids compared with controls. Total, oxidative and non-oxidative phenylalanine fluxes were not modified by dietary HMB supplementation. Similarly, an acute infusion of HMB, in the control group, did not change these fluxes. In skeletal muscles, fractional rates of protein synthesis were not affected by long-term dietary supplementation with HMB. Taken together our results showed that administration of a high dose of HMB to lambs was able to modify plasma free amino acid pattern without any effect on whole-body protein turnover and skeletal muscle protein synthesis.

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.

Effect of short-term starvation on Leydig cell function in adult rats

An experiment was carried out to analyze the effect of 3 days of starvation on the Leydig cell function in adult rats. Starvation markedly decreased plasma insulin and testosterone levels (p < .05). The weight of testes was maintained, whereas the testicular interstitial fluid volume decreased (p < .05). The level of testosterone decreased in this fluid (60%), whereas insulin levels showed no significant change. Purified Leydig cells showed normal LH/hCG binding in fasted rats but low insulin binding. The ability of these cells to produce testosterone in vitro was normal under both basal conditions and hCG stimulation in the absence and presence of insulin in the incubation medium. These data suggest that there is no gross impediment in the Leydig cell capacity to produce testosterone that might explain the starvation-associated decrease in plasma testosterone.

Glutamine synthetase induction by glucocorticoids is preserved in skeletal muscle of aged rats

Glutamine synthetase (GS) is a glucocorticoid-inducible enzyme that has a key role for glutamine synthesis in muscle. We hypothesized that the glucocorticoid induction of GS could be altered in aged rats, because alterations in the responsiveness of some genes to glucocorticoids were reported in aging. We compared the glucocorticoid-induced GS in fast-twitch and slow-twitch skeletal muscles (tibialis anterior and soleus, respectively) and heart from adult (age 6-8 mo) and aged (age 22 mo) female rats. All animals received dexamethasone (Dex) in their drinking water (0.77 +/- 0.10 and 0.80 +/- 0.08 mg/day per adult and aged rat, respectively) for 5 days. Dex caused an increase in both GS activity and GS mRNA in fast-twitch and slow-twitch skeletal muscles from adult and aged rats. In contrast, Dex increased GS activity in heart of adult rats, without any concomitant change in GS mRNA levels. Furthermore, Dex did not affect GS activity in aged heart. Thus the responsiveness of GS to an excess of glucocorticoids is preserved in skeletal muscle but not in heart from aged animals.

Phosphatidylinositol 3-kinase and p70 s6 kinase participate in the regulation of protein turnover in skeletal muscle by insulin and insulin-like growth factor I

This study was designed to evaluate the role of phosphatidylinositol (PI3) kinase, p70 S6 kinase (p70S6K), and mitogen-activated protein (MAP) kinase in the regulation of muscle protein metabolism by insulin and insulin-like growth factor I (IGF-I). Wortmannin and LY294002 (inhibitors of P13 kinase) both abolished the stimulation of protein synthesis by insulin or IGF-I in epitrochlearis muscle incubated in vitro. LY294002 also totally reversed the antiproteolytic action of these hormones. Although p70S6K activation by insulin and IGF-I may be mediated by PI3 kinase in epitrochlearis muscle, the specific inhibition of this kinase by rapamycin caused only partial (25%) inhibition of the stimulation of protein synthesis by these two hormones. Rapamycin had no effect on proteolysis. Finally, insulin or IGF-I did not stimulate MAP kinase activity at any of the times tested (2-25 min), suggesting that this protein kinase was not directly involved in the regulation of muscle protein metabolism. These observations provide evidence that PI3 kinase and p70S6K, but not MAP kinase, play a role in the regulation of muscle protein turnover by insulin or IGF-I.

Euglycemic hyperinsulinemia and hyperaminoacidemia decrease skeletal muscle ubiquitin mRNA in goats

Insulin inhibits protein breakdown at the whole body level, but neither the tissues nor the proteolytic pathways on which insulin exerts its antiproteolytic effect are well characterized. We measured the effects of insulin on mRNA levels for cathepsin D and m-calpain (a lysosomal and Ca2(+)-dependent proteinase, respectively) and ubiquitin (a component of ubiquitin-dependent proteolysis) in skeletal muscle, skin, liver, and intestine. We used a 6-h hyperinsulinemic, euglycemic, and hyperaminoacidemic clamp in goats, a species in which insulin markedly inhibited whole body protein breakdown under similar conditions [S. Tesseraud, J. Grizard, E. Debras, I. Papet, Y. Bonnet, G. Bayle, and C. Champredon. Am. J. Physiol. 265 (Endocrinol. Metab. 28): E402-E413, 1993]. Hyperinsulinemia and hyperaminoacidemia had no effect on cathepsin D, m-calpain, and ubiquitin mRNA levels in liver, skin, and jejunum. In contrast, depressed ubiquitin mRNA levels were seen in skeletal muscle without any concomitant reduction in mRNA levels for cathepsin D, m-calpain, and other components of the ubiquitin-dependent proteolytic pathway. The reduced ubiquitin mRNA levels in skeletal muscle may represent a possible mechanism explaining the antiproteolytic effect of insulin in vivo.