The effect of condensed tannins on the nutrition and health of ruminants fed fresh temperate forages: a review

This study was designed to evaluate the effects of enriching an essential amino acid (EAA) mixture with leucine on muscle protein metabolism in elderly and young individuals. Four (2 elderly and 2 ...

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

Essential amino acids are primarily responsible for the amino acid stimulation of muscle protein anabolism in healthy elderly adults

BCAAs stimulate protein synthesis in in vitro preparations of skeletal muscle. Likewise, the stimulation of protein synthesis in skeletal muscle produced by intake of a mixed meal is due largely to BCAAs. Of the three BCAAs, leucine is the one primarily responsible for the stimulation of protein synthesis under these circumstances. The stimulatory effect of leucine on protein synthesis is mediated through upregulation of the initiation of mRNA translation. A number of mechanisms, including phosphorylation of ribosomal protein S6 Kinase, eukaryotic initiation factor (eIF)4E binding protein-1, and eIF4G, contribute to the effect of leucine on translation initiation. These mechanisms not only promote global translation of mRNA but also contribute to processes that mediate discrimination in the selection of mRNA for translation. A key component in a signaling pathway controlling these phosphorylation-induced mechanisms is the protein kinase, termed the mammalian target of rapamycin (mTOR). The activity of mTOR toward downstream targets is controlled in part through its interaction with the regulatory-associated protein of mTOR (known as raptor) and the G protein beta-subunit-like protein. Signaling through mTOR is also controlled by upstream members of the pathway such as the Ras homolog enriched in brain (Rheb), a GTPase that activates mTOR, and tuberin (also known as TSC2), a GTPase-activating protein, which, with its binding partner hamartin (also known as TSC1), acts to repress mTOR. Candidates for mediating the action of leucine to stimulate signaling through the mTOR pathway include TSC2, Rheb, and raptor. The current state of our understanding of how leucine acts on these signaling pathways and molecular mechanisms to stimulate protein synthesis in skeletal muscle is summarized in this article.

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Signaling Pathways and Molecular Mechanisms through which Branched-Chain Amino Acids Mediate Translational Control of Protein Synthesis

Amino acids have been shown to stimulate protein synthesis, inhibit proteolysis, and decrease whole-body and forearm glucose disposal. Using cultured hepatoma and myotube cells, we demonstrate that amino acids act as novel signaling elements in insulin target tissues. Exposure of cells to high physiologic concentrations of amino acids activates intermediates important in the initiation of protein synthesis, including p70 S6 kinase and PHAS-I, in synergy with insulin. This stimulatory effect is largely due to branched chain amino acids, particularly leucine, and can be reproduced by its transamination product, ketoisocaproic acid. Concurrently, amino acids inhibit early steps in insulin action critical for glucose transport and inhibition of gluconeogenesis, including decreased insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2, decreased binding of grb 2 and the p85 subunit of phosphatidylinositol 3-kinase to IRS-1 and IRS-2, and a marked inhibition of insulin-stimulated phosphatidylinositol 3-kinase. Taken together, these data support the hypothesis that amino acids act as specific positive signals for maintenance of protein stores, while inhibiting other actions of insulin at multiple levels. This bidirectional modulation of insulin action indicates crosstalk between hormonal and nutritional signals and demonstrates a novel mechanism by which nutritional factors contribute to insulin resistance.

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Bidirectional modulation of insulin action by amino acids.

Rates of muscle protein synthesis were measured in vivo in tissues of post-absorptive young rats that were given intravenous infusions of various combinations of insulin and amino acids. In the absence of amino acid infusion, there was a steady rise in muscle protein synthesis with plasma insulin concentration up to 158 mu units/ml, but when a complete amino acids mixtures was included maximal rates were obtained at 20 mu units/ml. The effect of the complete mixture could be reproduced by a mixture of essential amino acids or of branched-chain amino acids, but not by a non-essential mixture, alanine, methionine or glutamine. It is concluded that amino acids, particularly the branched-chain ones, increase the sensitivity of muscle protein synthesis to insulin.

Amino acid infusion increases the sensitivity of muscle protein synthesis in vivo to insulin. Effect of branched-chain amino acids.

The effect of corticosterone treatment on the sensitivity of muscle protein synthesis to insulin infusion was assessed in post-absorptive young rats. To select the optimal time period for corticosterone treatment, protein synthesis was measured by injection of L-[2,6-3H]phenylalanine (1.5 mmol/kg body weight) 1, 4, 12 or 24 h after injection of corticosterone (5 mg/kg body wt.). Muscle protein synthesis was significantly decreased at 4 h and the effect was maximal by 12 h; liver protein synthesis was elevated at 12 h and 24 h. The dose-response of muscle protein synthesis to a 30 min infusion with 0-150 munits of insulin/h was then compared in rats pretreated with corticosterone (10 mg/100 g body wt.) or vehicle alone. When no insulin was infused, corticosterone inhibited protein synthesis in gastrocnemius muscle. High doses of insulin stimulated protein synthesis, but the inhibition by corticosterone was similar to that in the absence of insulin. At intermediate doses of insulin there was an increased requirement for insulin to elicit an equivalent response in muscle protein synthesis. Plantaris muscle responded in a manner similar to that of gastrocnemius, but neither soleus muscle nor liver responded significantly to insulin. These data suggest that corticosterone has two modes of action; one which is independent from and opposite to that of insulin, and a second which causes insulin-resistance through a decrease in sensitivity rather than a change in responsiveness.

Acute effects of corticosterone on tissue protein synthesis and insulin-sensitivity in rats in vivo.

1. Rates of protein synthesis in liver and muscle of 100 g male rats were measured in vivo at 1 h or 4 h after injection of 2.5 mg of corticosterone and compared with those from animals given carrier medium alone. 2. In post-absorptive rats, corticosterone for 1 h had no effect on either muscle or liver protein synthesis. After 4 h there was a decrease in both tissues, but this was only statistically significant in muscle. 3. In fed rats, rates of protein synthesis were higher than those in post-absorptive animals, but the effects of corticosterone injection were similar. 4. Re-feeding of post-absorptive rats led to an increase in muscle protein synthesis after 1 h and 4 h. At 1 h this increase was not inhibited when plasma corticosterone concentrations were maintained high by injection of the hormone immediately before feeding commenced, but at 4 h there was a small inhibition. 5. It is concluded that the action of corticosterone in depressing muscle protein synthesis is time-dependent and requires longer than 1 h to develop. The failure of the hormone to alter the response to re-feeding for 1 h in post-absorptive rats suggest that corticosteroids are not important mediators of the acute stimulation of muscle protein synthesis by food intake.

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Short-term effects of corticosterone treatment on muscle protein synthesis in relation to the response to feeding.

The effect of glucagon on the rate of muscle protein synthesis was examined in vivo and in the isolated perfused rat hemicorpus. An inhibition of protein synthesis in skeletal muscles from overnight-fasted rats at various plasma concentrations of glucagon was demonstrated in vivo. The plantaris muscle (Type II, fibre-rich) was more sensitive than the soleus (Type I, fibre-rich). Myofibrillar and sarcoplasmic proteins were equally sensitive in vivo. However, protein synthesis in mixed protein and in sarcoplasmic and myofibrillar fractions of the heart was unresponsive to glucagon in vivo. In isolated perfused muscle preparations from fed animals, the addition of glucagon also decreased the synthesis of mixed muscle proteins in gastrocnemius (Type I and II fibres) and plantaris, but not in the soleus. The sarcoplasmic and myofibrillar fractions of the plantaris were also equally affected in vitro. Similar results were observed in vitro with 1-day-starved rats, but the changes were less marked.

Inhibition of protein synthesis by glucagon in different rat muscles and protein fractions in vivo and in the perfused rat hemicorpus.

1. Male rats (110-140 g body wt.) were restrained by a standard laboratory technique, by wrapping in a linen towel, and subjected to a constant intravenous infusion of saline (0.15 M-NaCl) for periods of 1 or 6 h. Fractional rates of protein synthesis (ks, %/day) were estimated at the start and at the end of the infusion period, by injection of a large concentration of [3H]phenylalanine. 2. In fed and overnight-fasted rats, restraint and infusion of saline for 1 and 6 h decreased ks in skeletal muscle by 15-20% and 30-35% respectively. Plasma glucose, insulin, glucagon and corticosterone concentrations in restrained and infused rats were not characteristic of immobilization stress. 3. Restrained rats responded to nutrient administration; ks in skeletal muscle increased by 35-40% after infusion of a mixture of amino acids and glucose for 1 or 6 h, as compared with saline-infused rats. 4. Restraint and infusion for 1 or 6 h did not overtly decrease ks and kRNA (protein synthesis per unit of RNA) in hypoxaemia-sensitive tissues, such as heart and liver. Restraint and infusion in an open cage, or in a cloth of open weave, did not decrease ks in muscle after 1 h. Blood gas measurements showed that rats restrained in a linen cloth were hypercapnic and acidotic compared with rats in an open cage. 5. It was concluded that respiratory acidosis, rather than hypoxia, resulting from restraint in a linen cloth decreases muscle protein synthesis.

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P J Garlick

Papers

Amino acid infusion increases the sensitivity of muscle protein synthesis in vivo to insulin. Effect of branched-chain amino acids.

Acute effects of corticosterone on tissue protein synthesis and insulin-sensitivity in rats in vivo.

Short-term effects of corticosterone treatment on muscle protein synthesis in relation to the response to feeding.

Inhibition of protein synthesis by glucagon in different rat muscles and protein fractions in vivo and in the perfused rat hemicorpus.

The influence of restraint and infusion on rates of muscle protein synthesis in the rat. Effect of altered respiratory function.