A High Protein Diet Stimulates Myostatin mRNA Expression in Rat Skeletal Muscles

Abstract

1369 Although a high protein (HP) diet is expected to provide a favorable condition for tissue construction, more than 15% of protein has reportedly not led to significant muscle hypertrophy in normal growing rats. Molecular events in such situation are still uncertain. A negative regulatory element of muscle hypertrophy is expected to influence this phenomenon. PURPOSE: The aim of this study is to test whether a HP diet alters expressions of negative regulatory element of muscle hypertrophy, myostatin (MSTN), in a growing rat gastrocnemius muscle. Expression level of myogenin mRNA was also examined. METHODS: Twenty-four male Wistar rats (4-week-old) were divided into three groups: 1) Control diet based on AIN93-M (15% protein, N = 8), 2) the moderate HP (mHP) diet group (25% protein, N = 8), and 3) the HP diet group (35% protein, N = 8). After three weeks of iso-calorimetric feedings, the rats were killed with anesthesia. Soleus and gastrocnemius muscles were excised. After measuring the tissue masses, these specimens were quickly frozen in liquid N2 for extraction of total RNA. Expression levels of mRNAs in skeletal muscles were determined using northern hybridization technique. Digoxigenin-labeled cRNA probes were used to detect target mRNAs. Relative expression ratio against 18S ribosomal RNA was used for the evaluation of expression level. Statistical analysis was performed with one-way ANOVA and a post hoc test. Significance was accepted at a 5% level. RESULTS: Tissue weights of the HP group were not significantly higher than those of the other two groups. MSTN mRNA of the HP diet group was significantly higher than that of the control group. Although statistical significance was not obtained, myogenin mRNA of the HP diet group was also higher than the other two groups. CONCLUSION: Obtained results suggest that although the HP diet leads to a high expression of myogenin to promote muscle hypertrophy, simultaneously up-regulated myostatin prevents it. Down-regulation of MSTN and/or up-regulation of myogenic factors such as IGF (insulin-like growth factor) −1 should promote muscle hypertrophy under the HP diet.