In the present study, we show that short-term (4 h) fasting significantly decreased the levels of protein synthesis-related factors such as the plasma insulin concentration, skeletal muscle pAkt, and pS6 levels in 2-wk-old chickens (P < 0.05). An intravenous injection of insulin significantly elevated the contents of pAkt and p-S6 in the skeletal muscle (P < 0.01). These findings suggest that decreasing the plasma insulin causes the downregulation of the Akt/S6 pathway in chicken skeletal muscle under short-term fasting conditions. However, protein synthesis was not significantly affected by short-term fasting. In addition, no significant change was observed in the levels of proteolysis-related factors such as plasma N-methylhistidine, phosphorylated forkhead box class O, and muscle ring finger-1 during 4-h fasting, indicating that short-term fasting does not induce skeletal muscle proteolysis in chickens. Interestingly, atrogin-1 expression significantly increased after 2-h fasting (P < 0.05), and insulin injection significantly reversed the fasting-induced atrogin-1 expression in chicken skeletal muscle (P < 0.01). Collectively, these findings suggest that short-term fasting downregulates the insulin-stimulated Akt/S6 pathway but does not significantly affect protein synthesis and proteolysis in chicken skeletal muscle, and that atrogin-1 expression is upregulated in a FOXO1-independent manners.

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Skeletal muscle mass is an important trait in the animal industry. We previously reported an age-dependent downregulation of the insulin-like growth factor 1 (IGF-1)/Akt/S6 pathway, major protein synthesis pathway, in chicken breast muscle after 1 week of age, despite a continuous increase of breast muscle weight. Myosin heavy chain (HC), a major protein in muscle fiber, has several isoforms depending on chicken skeletal muscle types. HC I (fast-twitch glycolytic type) is known to be expressed in adult chicken breast muscle. However, little is known about the changes in the expression levels of protein synthesis-related factors and HC isoforms in perihatching chicken muscle. In the present study, protein synthesis-related factors, such as IGF-1 messenger RNA (mRNA) levels, phosphorylation of Akt, and phosphorylated S6 content, increased in an age-dependent manner after post-hatch day (D) 0. The mRNA levels of HC I, III and V (fast-twitch glycolytic type) dramatically increased after D0. The increase ratio of breast muscle weight was approximately 1100% from D0 to D7. To our knowledge, these findings provide the first evidence that upregulation of protein synthesis pathway and transcription of fast twitch glycolytic HC isoforms play critical roles in the increase of chicken breast muscle weight during the first week after hatching.