It is economically important to increase the rate and speed of skeletal muscle growth in animals raised for meat including pigs. The growth of animals is a net result of complicated metabolic and physiological network including synthesis and utilization of amino acids (AA), intracellular protein turnover and deposition, as well as their regulation by nutrients, age, endocrine and exocrine secretion and other factors.

Genetic background can influence growth and nutrient requirements in animals, including pigs. According to the previous study of Ph.D candidate LIU Yingying (Institute of Subtropical Agriculture, the CAS), Landrace pigs showed faster growth rate and better muscle growth than Bama mini-pig. In contrast, Bama mini-pigs presented higher quality meat traits but more fat deposition capacity together with lower growth rate than Landrace pigs. How to find the optimal balance between economic aspect of pig production and meat nutritional quality is the goal of many researchers.

Currently, researchers in Prof. YIN Yulong's group from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) speculated that the difference between these two genotypes of pigs in their muscle growth, meat quality, and intermuscular adipose deposition may lead to dietary protein-dependent differences in protein deposition and related signaling pathways in different types of skeletal muscles.

In this study, 48 Landrace pigs and 48 pure-bred Bama mini-pigs were assigned to each of 2 dietary treatments: lower/GB (Chinese conventional diet)- or higher/NRC (National Research Council)-protein diet, which were fed from 5 weeks of age to respective market weights of each genotype.

The data showed that the concentrations of most AAs in longissimus dorsi muscle and biceps femoris muscle of pigs increased gradually with increasing age. Bama mini-pigs had generally higher muscle concentrations of flavor-related AA, including Met, Phe, Tyr, Pro, and Ser, compared with Landrace pigs. The mRNA levels for myogenic determining factor, myogenin, myocyte-specific enhancer binding factor 2 A, and myostatin of Bama mini-pigs were higher than those of Landrace pigs, while total and phosphorylated protein levels for protein kinase B, mechanistic target of rapamycin (mTOR), and p70 ribosomal protein S6 kinases (p70S6K), and ratios of p-mTOR/mTOR, p-AKT/AKT, and p-p70S6K/p70S6K were lower. There was a significant pig genotype-dependent effect of dietary protein on the levels for mTOR and p70S6K. When compared with the higher protein-NRC diet, the lower protein-GB diet increased the levels for mTOR and p70S6K in Bama mini-pigs, but repressed the level for p70S6K in Landrace pigs. The higher protein-NRC diet increased ratio of p-mTOR/mTOR in Landrace pigs.

These findings suggest that variations in AA deposition and protein synthesis are greatly regulated by dietary protein level, being different according to pig genotype, developmental stage, and muscle type. Their study not only provides an important basis for further studies aiming at deciphering the molecular mechanisms responsible for differences in growth rate and meat quality between different pig genotypes, but also contributes to the optimization of animal feeding.

The article has been published on PLoS One (DOI:10.1371/journal.pone.0138277). More details can be found at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4578863/.