Vitamin A injection at birth improves muscle growth in lambs

Vitamin A and its metabolite,retinoic acid(RA)play important roles in regulating skeletal muscle development.This study was conducted to investigate the effects of early intramuscular vitamin A injection on the muscle growth of lambs.A total of 16 newborn lambs were given weekly intramuscular injections of corn oil(control group,n=8)or 7,500 IU vitamin A palmitate(vitamin A group,n=8)from birth to 3 wk of age(4 shots in total).At 3 wk of age and weaning,biceps femoris muscle samples were taken to analyze the effects of vitamin A on the myogenic capacity of skeletal muscle cells.All lambs were slaughtered at 8 months of age.The results suggest that vitamin A treatment accelerated the growth rate of lambs and increased the loin eye area(P<0.05).Consistently,vitamin A increased the diameter of myofibers in longissimus thoracis muscle(P<0.01)and increased the final body weight of lambs(P<0.05).Vitamin A injection did not change the protein kinase B/mammalian target of rapamycin and myostatin signaling(P>0.05).Moreover,vitamin A upregulated the expression of PAX7(P<0.05)and the myogenic marker genes including MYOD and MYOG(P<0.01).The skeletal muscle-derived mononuclear cells from vitamin A-treated lambs showed higher expression of myogenic genes(P<0.05)and formed more myotubes(P<0.01)when myogenic differentiation was induced in vitro.In addition,in vitro analysis showed that RA promoted myogenic differentiation of the skeletal muscle-derived mononuclear cells in the first 3 d(P<0.05)but not at the later stage(P>0.05)as evidenced by myogenic gene expression and fusion index.Taken together,neonatal intramuscular vitamin A injection promotes lamb muscle growth by promoting the myogenic potential of satellite cells.