The Protein and Energy Metabolic Response of Skeletal Muscle to the Low-Protein Diets in Growing Pigs

This study was conducted to determine the effect of low-protein diets on protein and energy metabolism in skeletal muscle, and to elucidate the underlying mechanism. A total of 18 growing pigs (average initial body weight = 36.47 kg) were individually penned and assigned to three treatments; each treatment was fed one of three diets containing either 18%, 15%, or 12% CP. The results showed that reducing dietary CP contents decreased (P < 0.05) the weight of half Longissimus dorsi (LD) muscle and serum concentration of insulin-like growth factor 1 (IGF-1). Compared with the 18% and 15% CP treatments, the 12% CP treatment suppressed (P < 0.05) the components of mammalian target of rapamycin complex 1 (mTORC1) pathway, but upregulated (P < 0.05) the mRNA levels for proteolysis-related genes, and concomitantly caused an increase (P < 0.05) in the percentage of apoptotic cells in LD muscle. Along with lower (P < 0.05) AMP/ATP ratio and greater (P < 0.05) energy charge value in LD muscle of the 12% CP treatment, there was a concurrent decrease (P < 0.05) in the proteins for AMP-activated protein kinase α (AMPKα) pathway. Likewise, these results were also observed in the Biceps femoris muscle with slightly different degree of impacts. These results indicate that the retardation effect of low-protein supply on muscle growth of growing pigs could be likely regulated by inhibiting IGF-1/mTORC1 protein synthesis cascade, along with strong alterations in energy status and AMPKα pathway.