Enhanced Glutamine Availability Exerts Different Effects on Protein and Amino Acid Metabolism in Skeletal Muscle From Healthy and Septic Rats

Background: Enhanced glutamine (GLN) intake may affect the catabolism of branched-chain amino acids (BCAAs; valine, leucine, and isoleucine), which play a regulatory role in protein turnover. We examined the effects of enhanced GLN availability on leucine oxidation, amino acid concentrations, and protein metabolism in muscles from healthy and septic rats. Methods: Cecal ligation and puncture were used as a model of sepsis. Twenty-four hours after surgery, the soleus (SOL, red muscle) and the extensor digitorum longus (EDL, white muscle) were incubated in medium containing 0.5 or 2.0 mM GLN. Protein breakdown, protein synthesis, and leucine oxidation were determined via 3-methylhistidine release, muscle L-[1-¹⁴C]leucine radioactivity, and the radioactivity of released ¹⁴CO₂, respectively. Results: In muscles from septic animals, increased proteolysis and leucine oxidation and decreased protein synthesis were detected. These effects were more pronounced in the EDL. In septic muscles, the addition of GLN decreased leucine oxidation in both muscles and increased protein synthesis in the EDL. In muscles from untreated animals, decreased leucine oxidation after the addition of GLN to the medium was associated with decreased protein synthesis in the SOL and decreased concentrations of serine, glycine, histidine, alanine, arginine, proline, and lysine in both muscles. Conclusions: White muscle fibers are more sensitive to septic stimuli than red fibers are. In sepsis, enhanced GLN intake may ameliorate GLN deficiency, inhibit BCAA catabolism, and stimulate protein synthesis. In the healthy state, surplus of GLN may lead to severe alterations in the intramuscular concentration of several amino acids and impair protein synthesis.