Hankard et al. Am J Physiol, 1996.
Fourteen subjects received intravenous infusions of leucine to determine whether glutamine infusion inhibits protein breakdown and/or increase the rate of protein synthesis. Seven of these subjects received 800 micromol/kg/hr of glutamine via enteral infusion every other day. Seven other subjects received a similar amount of glycine also via enteral infusion (7).
During the infusion of glutamine, the rate of leucine appearance in the plasma remained unchanged indicating that glutamine supplementation inhibited the breakdown of muscle protein. In addition, the oxidation of leucine decreased and nonoxidative leucine disposal increased indicating an increase in protein synthesis. During glycine infusion, protein breakdown was also inhibited but did not result in an increase in protein synthesis (7).
MacLennan et al. FEBS Lett, 1988.
Glutamine significantly reduced protein breakdown and net protein loss in isolated perfused rat hindlimb skeletal muscle. Protein breakdown was inhibited in the soluble proteins though not in the myofibrillar proteins. Insulin also had a protein sparing effect on the muscle, but did not have as great an effect as glutamine. The combination of insulin and glutamine did not have an additive effect (8).
MacLennan et al. FEBS Lett, 1987.
Protein synthesis in isolated perfused rat skeletal muscle (rat hindquarter) increased by 66% when the concentration of intracellular glutamine was increased. This increase in protein synthesis occurred with or without the presence of insulin. Thus it was concluded that increased intramuscular glutamine induced protein synthesis independently of insulin (9).
Hickson et al. Am J Physiol, 1996.
To investigate the effect of glutamine supplementation on glucocorticoid metabolism. Rats were treated with hydrocortisone 21-acetate. An infusion of either alanine or alanyl-glutamine was given to the rats. In an increase in glutamine synthetase activity (and glutamine synthetase mRNA expression) was increased in the plantaris, superficial quadriceps, and deep quadriceps muscles. Compared to the rats treated with alanine, glutamine synthetase activity was 42-65% less in the rats treated with alanyl-glutamine, and glutamine synthetase mRNA expression was 31-37% less in the alanyl-glutamine treated rats as well. Alanyl-glutamine treatment prevented loss in overall body mass and fast-twitch muscle mass by over 70% as compared to alanine treatment. Thus it was concluded that glutamine infusions may have inhibited the catabolic effects of glucocorticoid hormones (10).