Role of encapsulated amino acids on post-natal skeletal muscle growth and development in feedlot steers
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Post-natal skeletal muscle growth and development is impacted by many factors. The purpose of experiment 1 was to investigate the effect of encapsulated amino acids on post-natal skeletal muscle growth and development in feedlot steers during the finishing period. Continental crossbred steers (n = 20; 457 ± 58 kg) were blocked by weight and sorted randomly to treatment with 5 head per pen and 2 pens per treatment. Treatments were 1) 0 grams of encapsulated methionine and 4 grams of encapsulated lysine (0Met), and 2) 8 grams of encapsulated methionine and 4 grams of encapsulated lysine (8Met). Longissimus muscle biopsies were performed on d 0, 14, 28, 42, and 56. Muscle samples were analyzed for muscle growth and mammalian target of rapamycin (mTOR) pathway signaling via PCR, Western blotting, SDS-PAGE, and immunohistochemistry. Factors evaluated included AKT, AMP-0Met cattle had a tendency for increased abundance of eIF4EBP1 mRNA (P = 0.053). There was a day effect on mRNA abundance of AKT, AMPKα, IGF-I, MHC-I, MHC-IIA, MHC-IIX, and RAGA (P < 0.05). Expression of AKT protein decreased in 8Met compared with 0Met (P < 0.05). 8Met tended to decrease abundance of eIF4EBP1 (P = 0.06) and Raptor (P = 0.07). Abundance of AMPKα, eIF4EBP1, MHC-I, MHC-II, pAMPKα, RAGA, and Raptor protein responded according to day of biopsy (P < 0.001). Treatment by day interactions were found for muscle fiber type distribution and cross-sectional area, and Myf5 positive satellite cell density (P < 0.001). There was a tendency for interaction of treatment and day (P 0 0.08) for PAX7/Myf5 positive satellite cells, however both treatments had a decrease in density for d 42 and 56. For both treatments, cross sectional area increased from d 0 to 56. The 8Met treatment had a greater (P < 0.05) density of PAX7 positive satellite cells. There was an interaction of day and treatment for both density of total nuclei and myofiber nuclei (P < 0.05); there was no difference in total nuclei on d 0 and 56 for both treatments, and nuclei associated with muscle fibers increased for both treatments from d 0 to 56. Supplementation of encapsulated methionine impacted mTOR signaling and skeletal muscle growth potential. Experiment 2 was performed to evaluate the change in skeletal muscle and satellite cell populations in living steers during 56 d during the finishing phase. Ten feedlot steers were biopsied over 56 d as in experiment 1. The longissimus sample collected via biopsy were evaluated for myosin heavy chain isoforms and satellite cell populations using PCR, SDS-PAGE, and immunohistochemical staining. The abundance of MHC-I, IIA, and IIX mRNA increased (P < 0.05) on d 42 and 56. There was an increase (P < 0.05) of MHC-I and II protein from d 1 to 56. Cross-sectional area increased (P < 0.05) from d 1 to 56. There was a decrease (P < 0.05) in the abundance of type I skeletal muscle fibers from d 1 to 56. As days on feed increased, the density of PAX7-positive satellite cells increased (P < 0.05). The density of PAX7/Myf5 and Myf5 positive satellite cells decreased (P < 0.05) on d 42 and 56 compared with the earlier biopsy days. There was no difference in the density of total nuclei (P > 0.05) on d 1 and 56, and the density of myofiber nuclei increased (P < 0.05) from d 1 to 56. These data show that during the 56 d period of the finishing phase, satellite cells fused into existing skeletal muscle fibers to support the post-natal skeletal muscle hypertrophy that occurred as days on feed increased.