Effects Of FGF21 On Skeletal Muscle Fiber Types And Insulin Sensitivity And The Regulation Of Low-protein Diet

The purpose of this study was to investigate the effect of fibroblast growth factor21(FGF21)on skeletal muscle fiber types and insulin sensitivity,and to explore the effect of low protein diets on pork quality and muscle fiber types,and the role of FGF21 in it.Exp.1: Effect of FGF21 on skeletal muscle fiber types and insulin sensitivity in miceThe experiment adopts a 2×2 test design.Twenty-six 6-week-old healthy C57BL/6 male mice with similar body weight were divided into four treatments,with 6replicates for each treatment and 1 mouse for each replicate.At the beginning of the experiment,treatment 1 and 2 were subcutaneously implanted with a micro-osmotic pump containing saline,and treatments 3 and 4 were subcutaneously implanted with a micro-osmotic pump containing 0.5 mg/kg/d recombinant FGF21 protein.After subcutaneous implantation,treatment 1 and treatment 3 were fed with regular chow(RC),and treatment 2 and treatment 4 were fed with high-fat diet(HFD).The glucose tolerance test(GTT)was performed on d11,and the insulin tolerance test(ITT)was performed on d14.Tissue and blood samples were collected after 14 days experiment.Serum FGF21 levels,FGF21 expression in different skeletal muscles,muscle fiber types and insulin sensitivity-related gene expression were explored.The results showed that HFD significantly increased the final weight and body weight gain of mice(P<0.05),and FGF21 administration significantly reduced the final weight and body weight gain of mice(P<0.05).FGF21 significantly increased serum FGF21 levels and hepatic FGF21 expression(P<0.05);FGF21 treatment significantly increased the expression of My HCI,My HCIIa,and My HCIIx m RNA in SOL and EDL of mice(P<0.01),and significantly reduced the expression of My HCIIb in GAS of basic diet fed mice(P<0.05);FGF21 administration significantly increased the m RNA expression of GLUT4,IRS1,AKT and PI3 K in skeletal muscle of mice(P<0.05).FGF21 significantly improved the GTT(glucose tolerance)of mice fed high fat diet(P<0.05).The results showed that FGF21 treatment promoted the formation of oxidative muscle fibers of SOL and EDL and improved insulin sensitivity in mice.Exp.2: Effects of low-protein diet on pork quality,muscle fiber types and FGF21 expression in fattening pigsTwenty-four healthy DLY fattening barrows with initial body weight at 66.9±1.2kg were divided into three groups according to the body weight.Treatment 1 is the control group(CP15%),treatment 2 is the low protein(CP12%)unbalanced amino acids group(LP1),treatment 3 is the low protein(CP 12%)and balances lysine,methionine,tryptophan,threonine,valine and isoleucine group(LP2).The effects of low-protein diets on meat quality and muscle fiber types and the role of FGF21 in it were determined by recording growth performance,carcass characteristics,meat quality,serum FGF21 levels,liver FGF21 expression,and expression of different muscle fiber types of the longissimus dorsi muscle.The results showed that the serum FGF21 level and hepatic FGF21 m RNA expression in pigs fed low protein diet were significantly higher than that in control group(P<0.01).In longissimus dorsi muscle,the My HCI m RNA expression in the LP2 group was significantly higher than that in the control group(P<0.05).The My HCIIa m RNA expressions in the LP1 and LP2 groups were significantly higher than that in the control group(P<0.01).There was no significant difference in My HCIIx and My HCIIb expression among the three group.Low-protein diets with different amino acid balance patterns have no significant effect on the growth performance of fattening pigs,including average daily feed intake,average daily weight gain,and feed-to-meat ratio.The heart index of LP2 group was significantly higher than that of LP1 group(P<0.05).The liver index of the LP1 group was significantly lower than that of the control group and the LP2 group(P<0.01).Abdominal fat weight and average backfat thickness in LP1 group tended to be higher than those in control group and LP2 group(0.05<P<0.1).The measurement results of meat quality characteristics showed that the cooking loss of the LP1 group was significantly lower than that of the control and LP2 group(P<0.01).The shear force of LP1 group was significantly lower than that of control and LP2 group(P<0.01).The intramuscular fat content of the LP1 group was significantly higher than that of control group(P<0.05),and the intramuscular fat content of LP2 group also tended to be higher than that of the control group(0.05<P<0.1).The redness of LP1 group was significantly higher than that of control group(P<0.05).The results showed that low-protein diet treatment can increase the proportion of oxidative muscle fibers,which may be related to the increased FGF21 expression.The low-protein diet with unbalanced amino acids significantly increased the fat content and muscle tenderness of the longissimus dorsi muscle in the fattening pigs.In summary,FGF21 promotes the formation of oxidative muscle fibers in soleus and extensor digitorum longus muscles,and improves insulin sensitivity in mice.Reduced crude protein level of the diet can promote the formation of oxidative muscle fibers.Feeding low-protein diets with unbalanced essential amino acids can increase the intramuscular fat content and improve muscle tenderness of the longissimus dorsi muscle in fattening pigs.The reason may be related to the low protein diet-induced FGF21 expression.