The effect of severe dietary restriction on intramuscular glutamine concentrations and protein synthetic rate

Biochemical approaches for nutritional support of skeletal muscle protein metabolism during sepsis

Sepsis initiates a unique series of modifications in the homeostasis of N metabolism and profoundly alters the integration of inter-organ cooperatively in the overall N and energy economy of the host. The net effect of these alterations is an overall N catabolic state, which seriously compromises recovery and is semi-refractory to treatment with current therapies. These alterations lead to a functional redistribution of N (amino acids and proteins) and substrate metabolism among injured tissues and major body organs. The redistribution of amino acids and proteins results in a quantitative reordering of the usual pathways of C and N flow within and among regions of the body with a resultant depletion of the required substrates and cofactors in important organs. The metabolic response to sepsis is a highly integrated, complex series of reactions. To understand the regulation of the response to sepsis, a comprehensive, integrated analysis of the fundamental physiological relationships of key metabolic pathways and mechanisms in sepsis is essential. The catabolism of skeletal muscles, which is manifested by an increase in protein degradation and a decrease in synthesis, persists despite state-of-the-art nutritional care. Much effort has focused on the modulation of the overall amount of nutrients given to septic patients in a hope to improve efficiencies in utilisation and N economies, rather than the support of specific end-organ targets. The present review examines current understanding of the processes affected by sepsis and testable means to circumvent the sepsis-induced defects in protein synthesis in skeletal muscle through increasing provision of amino acids (leucine, glutamine, or arginine) that in turn act as nutrient signals to regulate a number of cellular processes.

Is muscle protein turnover regulated by intracellular glutamine during sepsis?

BACKGROUND

Low muscle glutamine levels during sepsis are associated with reduced protein synthesis and elevated protein breakdown, in particular myofibrillar protein breakdown. It is not known if this is a causal or coincidental relationship. We tested the hypothesis that muscle protein turnover rates are directly regulated by glutamine.

METHODS

Paired extensor digitorum longus muscles from nonseptic (sham-operated) and septic rats (16 hours after cecal ligation and puncture) were incubated in the absence or presence of 15 mmol glutamine/L. The effect of glutamine was tested in unsupplemented medium or in medium containing 1 mU/mL of insulin or a mixture of amino acids at normal plasma concentrations. Protein synthesis was measured as incorporation of 14C-phenylalanine into protein; total and myofibrillar protein breakdown was determined by measuring tyrosine and 3-methylhistidine, respectively.

RESULTS

Muscles accumulated intracellular glutamine well above normal concentrations in the presence of 15 mmol glutamine/L. In spite of this, protein synthesis was not affected by glutamine, neither when muscles were incubated in unsupplemented medium nor in medium containing insulin or amino acid mixture. Total protein breakdown was not influenced by glutamine when muscles were incubated in unsupplemented medium or with insulin but was reduced by glutamine in the presence of an amino acid mixture. Myofibrillar protein breakdown was unaffected by glutamine in unsupplemented medium and in medium containing insulin but was increased by glutamine in the presence of amino acid mixture.

CONCLUSION

Reduced muscle protein synthesis and increased myofibrillar protein breakdown during sepsis are probably not caused by the low intracellular glutamine levels noticed in this condition.

Ornithine alpha-ketoglutarate limits muscle protein breakdown without stimulating tumor growth in rats bearing Yoshida ascites hepatoma

The growth of the Yoshida ascites hepatoma AH130 (YAH) is associated with early wasting, depletion of intracellular amino acid pools, and a pronounced activation of protein degradation in skeletal muscle of the host animal. Ornithine alpha-ketoglutarate (OKG) is used in the treatment of hypercatabolic states, and it has been suggested that it may improve nitrogen balance through repletion of free amino acid pools and suppression of protein catabolism. In cancer, OKG might similarly improve host nutritional status or stimulate tumor growth if its metabolites are limiting for tumor growth. Enteral supplementation with OKG was investigated in Sprague-Dawley rats bearing YAH. Tumor-bearing rats were compared with ad libitum- and pair-fed controls. Rats received OKG (3.4 to 4.0 g/kg body weight/d) or an equal amount of nitrogen as glycine (n = 8 in each group) for 5 days. Tumor implantation decreased cumulative food intake (-40%), host weight (-6%), skeletal muscle weight, and free amino acid levels in muscle and plasma. Muscle protein balance was estimated in vitro; decreased protein synthesis (-30%) and increased proteolysis (+113%) were observed in epitrochlearis muscles (EPI) of YAH-bearing rats compared with control groups. OKG had no effect on the wet weight (10 +/- 1 g) and nitrogen content of the tumor, or on free amino acid levels in the tumor. In tumor-bearing rats, OKG improved muscle protein balance by reducing breakdown by 33% and overall amino acid release of incubated EPI by 46%.

Effect of glutamine infusions on glutamine concentration and protein synthetic rate in rat muscle

Studies were undertaken in sedated and unsedated rats to raise the depleted intramuscular glutamine concentrations produced by aseptic abscesses, and to assess the effect of this change on muscle protein fractional synthetic rate. Age- and weight-matched control animals were also included in the study. The rats were infused for up to 5 hours via the lateral tail vein with 1 mL/100 g of body weight per hour of either saline or 0.22 M glutamine. The intramuscular concentration of glutamine (mmol/L of intracellular water), which was reduced by 45% after turpentine in the sedated animals, was restored to within 79% of control values, but the muscle fractional protein synthetic rate, which was also reduced by 41% in these animals, was not improved by the glutamine infusions. Glutamine administration also failed to increase muscle protein synthesis in unsedated rats and in those supplemented with a liquid meal. It is concluded that acute elevations in muscle glutamine concentrations do not increase protein synthesis in this tissue and that therefore glutamine is unlikely to be a mediator in the control of muscle protein synthesis under these circumstances.