| Dietary protein levels and cysteamine (CS) supplementation can affect growth performance and protein metabolism of pigs. Some researchers have demonstrated that feeding low-protein diets supplemented with essential amino acids is an effective strategy to reduce nitrogen excretion in pigs. The nitrogen excretion will be reduced by 8 percentage units if the dietary crude protein (CP) is reduced by 1 percentage unit. Cysteamine could improve the growth performance and affect nitrogen metabolism by decreasing the protein breakdown in finishing pigs. However, the influence of dietary protein intake on the growth response of CS-treated pigs is unclear, and the mechanisms involved in protein metabolism remain unknown. Hence, we investigated the interactions between dietary protein levels and CS supplementation and the effects of dietary crude protein levels and CS supplementation on protein metabolism of growing and finishing pigs, and furtherly elucidate its related mechanism.1. Effects of low protein level diets supplemented with enough essential amino acids and CS supplementation on growth performance, plasma biochemical parameters and the contents of nitrogenous compounds in feces of growing and finishing pigs(1) Experiment 1 (growing pigs): One hundred twenty barrow (Yorkshire × Landrace× Meishan) pigs with an average initial body weight of 42.18 ± 0.70 kg (mean ±SEM)were randomly allocated into four treatments using a 2 × 2 factorial arrangement with five replicates (pens) and six pigs each. The primary variations of this experiment were the dietary CP levels (16% or 12%) and CS supplemental levels (0 or 100 mg/kg). The pigs were fed with one of the four diets, as follows: [normal protein level diet (NP) (16% CP),NP supplemented with CS, low protein level diet (LP) (12% CP), and LP supplemented with CS]. The pure CS supplementation level was 100 mg/kg.The NP was supplemented with lysine, methionine, threonine, tryptophan and Valine. However, the LP was supplemented with enough EAA to meet the NRC AA requirements of pigs and maintain the balanced supply of eight EAA including lysine, methionine, threonine, tryptophan,valine, phenylalanine, isoleucine, and leucine. The duration of the experiment was 31 days.The results showed that LP supplemented with EAA resulted in decreased concentrations of plasma alanine aminotransferase (ALT), aspartate aminotransferase (AST), total protein(TP), albumin (Alb) and plasma urea nitrogen (PUN) (P<0.05), and decreased the contents of putrescine and tryptamine in feces (P<0.05), while dietary protein levels did not affect other traits (P>0.05); CS supplementation increased the average daily gain (ADG) (P<0.05),decreased the feed to gain (F/G)(P<0.05),but did not affect average daily feed intake(ADFI)(P>0.05). CS supplementation increased the lean percentage (P<0.05). CS supplementation increased the concentrations of plasma ALT,AST,and Alb (P<0.05),and decreased the PUN concentrations (P<0.05). CS supplementation increased the concentrations of plasma insulin-like growth factor 1 (IGF-1), insulin, and gastrin (P<0.05),and decreased the concentrations of leptin, glucagon, and SS (P<0.05). CS supplementation decreased the contents of ammonium, putrescine, cadaverine and histamine in feces(P<0.05); There were no interactions between dietary protein levels and CS supplementation for all variables (P>0.05).(2) Experiment 2 (finishing pigs): One hundred twenty barrow (Yorkshire x Landrace x Meishan) pigs with an average initial body weight of 65.84 kg ± 0.61 kg (mean ± SEM)were randomly allocated into four treatments using a 2 × 2 factorial arrangement with five replicates (pens) and six pigs each. The primary variations of this experiment were the dietary CP levels (14% or 10%) and CS supplemental levels (0 or 140 mg/kg). The pigs were fed with one of the four diets, as follows: [normal protein level diet (NP) (14% CP),NP supplemented with CS, low protein level diet (LP) (10% CP), and LP supplemented with CS]. The pure CS supplementation level was 140 mg/kg.The NP was supplemented with lysine, methionine, threonine, and tryptophan. However, the LP was supplemented with enough EAA to meet the NRC AA requirements of pigs and maintain the balanced supply of eight EAA including lysine, methionine, threonine, tryptophan, valine,phenylalanine, isoleucine, and leucine. The duration of the experiment was 41 days. The results showed that LP supplemented with EAA resulted in decreased concentrations of plasma FT3, FT4, SS and PUN (P<0.05), and decreased the contents of putrescine and cadaverine in feces (P<0.05), while dietary protein levels did not affect other traits(P>0.05); CS supplementation increased the ADG (P<0.05), decreased the F/G (P<0.05),but did not affect ADFI (P>0.05). CS supplementation increased the lean percentage(P<0.05),and decreased back fat (P<0.05). CS supplementation increased the concentrations of plasma ALT, AST, TP and Alb (P<0.05), and decreased the PUN concentrations (P<0.05). CS supplementation increased the concentrations of plasma FT3,FT4, T3, T4, IGF-1, and gastrin (P<0.05), and decreased the concentrations of leptin and SS (P<0.05). CS supplementation decreased the contents of tryptamine and histamine in feces (P<0.05); There were no interactions between dietary protein levels and CS supplementation for all variables (P>0.05).2. Effects of low protein level diets supplemented with enough essential amino acids and CS supplementation on nutrients apparent digestibility and intestinal expression of amino acid and peptide transporters of growing and finishing pigsThe experimental design was as same as experiment 1 and experiment 2.(1) (growing pigs): The results showed that LP supplemented with EAA resulted in increased apparent digestibility of CP and increased concentrations of plasma lysine,phenylalanine,valine, alanine and proline (P<0.05),and decreased the concentrations of plasma arginine and histidine (P<0.05), while dietary protein levels did not affect other traits (P>0.05); CS supplementation increased the apparent digestibility of CP, dry matter(DM), organic matter (OM), and ether extract (EE). CS supplementation increased the trypsin and chymotrypsin activity in jejunal digesta (P<0.05). CS supplementation increased the concentrations of plasma lysine, histidine, alanine, cysteine, proline and tyrosine (P<0.05). CS supplementation increased the mRNA abundance of solute carrier family 6 member 19, solute carrier family 7 member 9, and solute carrier family 15 member 1 in jejunal mucosa (P<0.05); There were no interactions between dietary protein levels and CS supplementation for all variables (P>0.05).(2) (finishing pigs): The results showed that LP supplemented with EAA resulted in increased apparent digestibility of CP,DM and OM and increased concentrations of plasma threonine, phenylalanine, isoleucine and alanine (P<0.05), and decreased the concentrations of plasma arginine (P<0.05), while dietary protein levels did not affect other traits (P>0.05);CS supplementation increased the apparent digestibility of CP, DM and OM (P<0.05). CS supplementation increased the trypsin and chymotrypsin activity in jejunal digesta (P<0.05).CS supplementation increased the concentrations of plasma arginine, histidine, proline and cysteine (P<0.05). CS supplementation increased the mRNA abundance of solute carrier family 7 member 7, solute carrier family 7 member 9, and solute carrier family 15 member 1 in jejunal mucosa (P<0.05). There were no interactions between dietary protein levels and CS supplementation for all variables except the apparent digestibility of DM.3. Effects of low protein level diets supplemented with enough essential amino acids and CS supplementation on liver protein synthesis of growing and finishing pigsThe experimental design was as same as experiment 1 and experiment 2.(1) (growing pigs): The results showed that LP supplemented with EAA resulted in decreased mRNA abundance of glutamine synthetase (GS), glutamate dehydrogenase(GDH),carbamyl phosphate synthetase 1 (CPS-1),sirtuin 5 (SIRT5) in liver (P<0.05) of growing pigs, while dietary protein levels did not affect other traits (P>0.05); CS supplementation increased the mRNA abundance of sirtuin 4 (SIRT4), decreased the mRNA abundance of GS, GDH, CPS-1 and SIRT5 in liver of growing pigs (P<0.05). CS supplementation increased the mRNA abundance of growth hormone receptor (GHR),IGF-1, IGF-1 receptor (IGF-1R), insulin receptor substrate 1 (IRS-1) and protein kinase B(Akt) (P<0.05), but did not affect the mRNA abundance of insulin receptor (IR) in liver(P>0.05). CS supplementation increased the protein abundance of phosphorylated mTOR,4E-BP1, and S6K1 in the liver (P<0.05). There were no interactions between dietary protein levels and CS supplementation for all variables (P>0.05).(2) (finishing pigs): The results showed that LP supplemented with EAA did not affect mTOR pathway in the liver of finishing pigs (P>0.05); CS supplementation increased the mRNA abundance of GHR、IGF-1、IGF-1R、IRS-1 and Akt in the liver of finishing pigs(P<0.05); CS supplementation increased the protein abundance of phosphorylated mTOR,4E-BP1,and S6K1 in the liver (P<0.05). There were no interactions between dietary protein levels and CS supplementation for all variables.4. Effects of low protein level diets supplemented with enough essential amino acids and CS supplementation on muscle protein synthesis and degradation and muscle development of growing and finishing pigsThe experimental design was as same as experiment 1 and experiment 2.(1) (growing pigs): The results showed that LP supplemented with EAA resulted in decreased mRNA abundance of GS in skeletal muscle of growing pigs (P<0.05), while dietary protein levels did not affect other traits (P>0.05); CS supplementation decreased the mRNA abundance of GS in skeletal muscle of growing pigs (P<0.05). CS supplementation increased the mRNA abundance of IR, IRS-1, and Akt, but did not affect the mRNA abundance of IGF-1, IGF-1R (P>0.05). CS supplementation decreased the mRNA abundance of Forkhead Box O 4 (FOXO4), muscle atrophy F-box (MAFbx), muscle Ring finger 1 (MuRF1) (P<0.05). CS supplementation increased the mRNA abundance of myogenic differentiation 1 (MyOD1), myocyte enhancer factor 2B (MEF2B) (P<0.05),decreased the mRNA abundance of myostatin (MSTN) (P<0.05).There were no interactions between dietary protein levels and CS supplementation for all variables (P>0.05).(2) (finishing pigs): The results showed that LP supplemented with EAA did not affect the mTOR or ubiquitin-proteasome pathway in skeletal muscle of finishing pigs (P>0.05);CS supplementation increased the mRNA abundance of IGF-1, IGF-1R, IRS-1, and Akt(P<0.05), but did not affect the mRNA abundance of IR in skeletal muscle of finishing pigs(P>0.05). CS supplementation increased the protein abundance of phosphorylated mTOR,4E-BP1, and S6K1 in skeletal muscle (P<0.05). CS supplementation decreased the mRNA abundance of FOXO4 and MAFbx (P<0.05). There were no interactions between dietary protein levels and CS supplementation for all variables (P>0.05).As mentioned above, the conclusions are as follows:(1) LP supplemented with enough EAA maintained the growth performance of growing and finishing pigs, increased the apparent digestibility of crude protein, and reduced the nitrogenous compounds excretion, which led to an increase in nitrogen utilization. However, LP supplemented with enough EAA did not affect the mTOR signaling in liver and skeletal muscle, and the UPP signaling in skeletal muscle.(2) CS supplementation increased the apparent digestibility of crude protein, and reduced the nitrogenous compounds excretion, which led to an increase in nitrogen utilization.(3) CS supplementation improved the growth performance of growing pigs, possibly by increasing plasma IGF-1 and insulin concentrations. IGF-1 could activate the mTOR signaling in the liver, and insulin could suppress the Akt/FOXO signaling in skeletal muscle,which led to an increase in growth performance. CS supplementation improved growth performance of finishing pigs, possibly by mediating plasma IGF-1 concentrations, thereby promoting mTOR signaling in liver and skeletal muscle and suppressing Akt/FOXO signaling in skeletal muscle of finishing pigs. In addition, CS could promote muscle development by enhancing the mRNA abundance of MyoD1 and MEF2B, and decreasing the mRNA abundance of MSTN in skeletal muscle of growing pigs.(4) LP supplemented with EAA and CS supplementation were effective strategies to reduce nitrogen excretion in pigs; There were no interactions between dietary protein levels and CS supplementation in terms of growth performance and protein metabolism in growing and finishing pigs. Dietary protein levels and CS supplementation influenced growth and protein metabolism through independent mechanisms in pigs. |
