Effects of the SLC38A2–mTOR Pathway Involved in Regulating the Different Compositions of Dietary Essential Amino Acids–Lysine and Methionine on Growth and Muscle Quality in Rabbits

SIMPLE SUMMARY: China is not only a huge meat rabbit consumer but also the largest meat rabbit producer in the world, contributing a large amount of rabbit meat products to the domestic and foreign markets every year. Therefore, it is important for the domestic and international rabbit meat market to improve rabbit breeding production efficiency and rabbit meat quality based on the good use of domestic feed resources in China. It is well known that dietary amino acid nutrition is of great importance to animal growth. Lysine and methionine are limited in the common domestic rabbit feed sources in China, but they play an important role in rabbit growth. Moreover, different lysine and methionine compositions of the diets respond differently to rabbit growth. Consequently, the search for a better composition of dietary lysine and methionine is the main objective of this study. ABSTRACT: In recent years, ensuring food security has been an important challenge for the world. It is important to make good use of China’s domestic local feed resources to provide safe, stable, efficient, and high-quality rabbit meat products for China and the world. Lysine and methionine are the two most limiting essential amino acids in the rabbit diet. However, little is known about the rational composition of lysine and methionine in rabbit diets and the mechanisms that affect growth and development. Accordingly, in this study, we sought to address this knowledge gap by examining the effects of different compositions of lysine and methionine in rabbit diets. Subsequently, the growth status, nitrogen metabolism, blood biochemical indexes, muscle development, muscle quality, and the growth of satellite cells were evaluated in the animals. The results showed that diets containing 0.80% Lys and 0.40% Met improved average daily weight gain, feed conversion, nitrogen use efficiency, and muscle quality in the rabbits (p < 0.05). Additionally, it altered the amino acid transport potential in muscle by upregulating the expression of the SLC7A10 gene (p < 0.05). Meanwhile, the cell viability and the rate of division and migration of SCs in the 0.80% Lys/0.40 % Met composition group were increased (p < 0.05). SLC38A2 and P–mTOR protein expression was upregulated in the 0.80% lysine/0.40% methionine composition group (p < 0.05). In conclusion, 0.80% Lys/0.40% Met was the most suitable lysine and methionine composition in all tested diets. SLC38A2 acted as an amino acid sensor upstream of mTOR and was involved in the 0.80% Lys/0.40% Met regulation of muscle growth and development, thus implicating the mTOR signaling pathway in these processes.