Glutamine supplementation stimulates protein-synthetic and inhibits protein-degradative signaling pathways in skeletal muscle of diabetic rats.

In this study, we investigated the effect of glutamine (Gln) supplementation on the signaling pathways regulating protein synthesis and protein degradation in the skeletal muscle of rats with streptozotocin (STZ)-induced diabetes. The expression levels of key regulatory proteins in the synthetic pathways (Akt, mTOR, GSK3 and 4E-BP1) and the degradation pathways (MuRF-1 and MAFbx) were determined using real-time PCR and Western blotting in four groups of male Wistar rats; 1) control, non-supplemented with glutamine; 2) control, supplemented with glutamine; 3) diabetic, non-supplemented with glutamine; and 4) diabetic, supplemented with glutamine. Diabetes was induced by the intravenous injection of 65 mg/kg bw STZ in citrate buffer (pH 4.2); the non-diabetic controls received only citrate buffer. After 48 hours, diabetes was confirmed in the STZ-treated animals by the determination of blood glucose levels above 200 mg/dL. Starting on that day, a solution of 1 g/kg bw Gln in phosphate buffered saline (PBS) was administered daily via gavage for 15 days to groups 2 and 4. Groups 1 and 3 received only PBS for the same duration. The rats were euthanized, and the soleus muscles were removed and homogenized in extraction buffer for the subsequent measurement of protein and mRNA levels. The results demonstrated a significant decrease in the muscle Gln content in the diabetic rats, and this level increased toward the control value in the diabetic rats receiving Gln. In addition, the diabetic rats exhibited a reduced mRNA expression of regulatory proteins in the protein synthesis pathway and increased expression of those associated with protein degradation. A reduction in the skeletal muscle mass in the diabetic rats was observed and was alleviated partially with Gln supplementation. The data suggest that glutamine supplementation is potentially useful for slowing the progression of muscle atrophy in patients with diabetes

Soleus cross-sectional area (µm²) analysis.

<p>The results are expressed as the means ± SEM. The values represent 6 animals/group. * <i>p</i><0.05, as indicated by the Anderson–Darling Normality Test. C = control rats; CS = control rats supplemented with glutamine; D = diabetic rats; DS = diabetic rats supplemented with glutamine.</p

Representative Western blots for total mTOR protein content (A); mTOR mRNA expression (B).

<p>The results are expressed as the means ± SEM. The values represent 6 animals/group. * <i>p</i><0.05, as indicated by ANOVA and the Bonferroni post-hoc test. C = control rats; CS = control rats supplemented with glutamine; D = diabetic rats; DS = diabetic rats supplemented with glutamine.</p

Plasma glutamine concentration (mM).

<p>The results are expressed as the means ± SEM. The values represent 6 animals/group. * <i>p</i><0.05, as indicated by ANOVA and Bonferroni post-hoc test. C = control rats; CS = control rats supplemented with glutamine; D = diabetic rats; DS = diabetic rats supplemented with glutamine.</p

Sequences of the primers, and annealing temperatures for the Real Time PCR of the genes studied.

<p>Sequences of the primers, and annealing temperatures for the Real Time PCR of the genes studied.</p

Representative Western blots for total MuRF-1 protein content (A); MuRF-1 mRNA expression (B).

<p>The results are expressed as the means ± SEM. The values represent 6 animals/group. * <i>p</i><0.05, as indicated by ANOVA and the Bonferroni post-hoc test. C = control rats; CS = control rats supplemented with glutamine; D = diabetic rats; DS = diabetic rats supplemented with glutamine.</p

Soleus muscle glutamine concentration (nmols/mg protein) (A); soleus muscle glutamate concentration (nmols/mg protein) (B); glutamine/glutamate ratio (C).

<p>The results are expressed as the means ± SEM. The values represent 6 animals/group. * <i>p</i><0.05, as indicated by ANOVA and the Bonferroni post-hoc test. C = control rats; CS = control rats supplemented with glutamine; D = diabetic rats; DS = diabetic rats supplemented with glutamine.</p

Representative Western blots for phosphorylated and total Akt (A); pAkt/tAKT ratio (B); Akt mRNA expression (C).

<p>The results are expressed as the means ± SEM. The values represent 6 animals/group. * <i>p</i><0.05, as indicated by ANOVA and the Bonferroni post-hoc test. C = control rats; CS = control rats supplemented with glutamine; D = diabetic rats; DS = diabetic rats supplemented with glutamine.</p

Representative Western blots for total 4E-BP1 protein content (A), 4E-BP1 mRNA expression (B).

<p>The results are expressed as the means ± SEM. The values represent 6 animals/group. * <i>p</i><0.05, as indicated by ANOVA and the Bonferroni post-hoc test. C = control rats; CS = control rats supplemented with glutamine; D = diabetic rats; DS = diabetic rats supplemented with glutamine.</p