| An 80-day feeding trial was conducted to assess the growth,quality and protein metabolism of triploid rainbow trout(initial average weight: 233 g),six isolipidic and isoenengetic diets were formulated to contain increasing levels of protein(31.6,35.4,40.7,43.3,45.8 and 52.4 %).Each diet was fed to quadruplicate groups of fish cages(100 fish per each group).Weighed and took samples of blood,stomach and intestine after the end of the culture experiment,and measured the growth and health,quality and protein metabolism of triploid rainbow trout.The objective was to determine the optimum protein requirement of triploid rainbow trout,analyze the effect of dietary protein level on the quality of triploid rainbow trout,and infer the protein metabolism characteristics of triploid rainbow trout in the special habitat of Qinghai-Tibet Plateau,provide theoretical basis for healthy breeding and development of high quality domestic feed for triploid rainbow trout.The main results are as follows:1.Growth and health.Through the analysis of growth performance,feed utilization,tissue and organ development,growth related gene expression,fish composition,intestinal health,antioxidant indicators,the effects of feed protein levels on the growth and health of triploid rainbow trout were explored.(1)Growth performance: With the increase of dietary protein level up to 45.8 %,the final weight and specific growth rate(SGR)of triploid rainbow trout were significantly increased(P<0.05)and then plateaued(P>0.05).No significant change in survival rate(P>0.05).Based on the SGR,the optimum protein requirement of triploid rainbow trout was 45.8 % dry matter.(2)Feed utilization: Feed conversion ratio(FCR)of 45.8 % and 52.4 % dietary protein group were significantly lower than other groups(P<0.05).The feed intake(FI)was increased with the increase of dietary protein level up to 40.7 %(P<0.05)and then plateaued(P>0.05);With the increase of dietary protein levels,protein intake(PI)and protein gain(PG)were significantly increased(P<0.05);Protein retention(PR)and protein efficiency ratio(PER)were significantly decreased(P < 0.05).(3)Tissue and organ development: When the dietary protein level increased from 31.6 % to 40.7 %,the condition factor(CF)of triploid rainbow trout was significantly increased and the Hepatosomatic index(HSI)was significantly decreased(P<0.05),and then plateaued(P>0.05);The liver red value of the 31.6 % dietary protein group was the lowest,significantly lower than 40.7 %,43.3 % and 45.8 % dietary protein group(P<0.05).The relative gut length(RGL)and Zihler index(ZI)were significantly decreased with the increase of dietary protein level(P< 0.05);There was no significant change in the Viscerosomatic index(VSI)(P>0.05).(4)The mRNA expression of growth-related genes: With the increase of dietary protein levels,mRNA expression of insulin-like growth factor 1(IGFⅠ)increased(P<0.05)and then plateaued(P>0.05).The mRNA expression of insulin-like growth factor receptor(IGFRⅠ)gene was did not change when the dietary protein level increased from 35.4 % to 45.8 %(P>0.05),and then significantly increased(P<0.05).(5)Body composition: When the dietary protein level increased from 31.6 % to 43.3 %,the moisture and ash content of the fish were significantly decreased(P<0.05),and continued increase had no significant effect on water and ash(P>0.05);With the increase of dietary protein level,the crude protein content of fish was significantly increased(P<0.05);The crude lipid content of the fish body increased and then decreased(P<0.05).(6)Intestinal health: With the increase of dietary protein level: the intestinal fold height and epithelial cell thickness of triploid rainbow trout increased(P<0.05);The intestinal microvilli height decreased(P<0.05);The muscle thickness of the 43.3 % protein level group was significantly lower than the 35.4 % protein level group(P<0.05);There was no significant change in the number of cup cells in the intestinal tissue(P>0.05).(7)Antioxidation: With the increase of dietary protein level,the content of malondialdehyde(MDA)in plasma and intestine was significantly decreased(P<0.05);The content of MDA e in liver was the highest in 31.6 % of dietary protein group(P<0.05);The total antioxidant capacity(T-AOC)in plasma increased and then decreased(P<0.05);When the protein level of dietary increased from 31.6 % to 40.7 %,the T-AOC in the intestine was significantly increased(P<0.05)in the liver.The T-AOC was significantly decreased(P<0.05);the T-AOC of muscle in 52.4 % proteome was significantly lower than that in 31.6 % proteome(P<0.05);There was no significant change in muscle MDA content(P>0.05).2.Quality.Through the analysis of meat color,flesh quality and muscle nutrient index,the effect of feed protein level on the quality of triploid rainbow trout meat was explored.(1)Flesh color: As the protein level of feed increased,the brightness value of triploid rainbow trout muscle(L*)was significantly decreased(P<0.05);The yellow value of muscle showed a trend of increasing firstly and then decreasing,of which 40.7 % of the feed protein group had the highest yellow value(P<0.05);When the dietary protein level increased from 31.6 % to 40.7 %,the muscle red value(a*)was significantly increased(P<0.05);and continued increase had no significant effect on red values in muscle(P>0.05).(2)Flesh quality: With the increase of dietary protein level,muscle thickness,elasticity value,chewiness value,lactate dehydrogenase activity,alkali soluble HYP,alkali insoluble HYP,total HYP and PYD showed a significant decrease trend(P<0.05);Muscle pH,water-soluble protein content,salt-soluble protein content was significantly increased(P<0.05);40.7 %,43.3 %,45.8 %,and 52.4 % of the dietary proteome hardness was significantly lower than 31.6 % and 35.4 % of the dietary proteome(P<0.05).Muscle adhesion value decreased and then increased(P<0.05);Muscle Water holding capacity(WHC)decreased(P<0.05)and then plateaued(P>0.05);There was no significant change in muscle rupture force value,cohesive value,the adhesiveness value and muscle glycogen value(P>0.05).(3)Muscle nutrients: With the increase of dietary protein levels,muscle crude protein,crude lipid content,total fatty acids,saturated fatty acids(SFA),monounsaturated fatty acids(MUFA),n-3,n-6,The contents of ARA(C20:4n-6),EPA(C20:5n-3)and DHA(C22:6n-3)showed an increasing trend(P<0.05);Muscle water and n-3/n-6 showed a downward trend(P<0.05).Linoleic acid(C18:2n-6)and linolenic acid(C18:3n-3)showed a trend of stationary(P>0.05)and then rising(P<0.05);AI and TI showed a trend of increasing and then decreasing(P<0.05);There was no significant change in muscle ash(P>0.05).3.Protein decomposition metabolism.Through the analysis of protein digestive enzyme activity,small peptide absorption of key gene mRNA expression,metabolism-related enzyme activity and gene mRNA expression,the effects of feed protein levels on protein decomposition metabolism of triploid rainbow trout were explored.(1)Protein digestive enzyme activity: With the increase of dietary protein level,the activity of pepsin was significantly increased(P<0.05),the activity of trypsin and chymotrypsin in the intestine increased(P<0.05)and then plateaued(P>0.05).The activity of lipase and amylase in the pylorus and the activity of amylase in the intestine were significantly decreased(P<0.05);The activity of lipase in the intestine was not significantly changed(P>0.05);There was no significant change in intestinal lipase activity(P>0.05).(2)Key gene expression of protein absorption: The mRNA expression of the small peptide transporter T1(PepT1)gene in the 45.8 % protein level group was the highest,which was significantly higher than the 35.4 %,43.3 % and 52.4 % protein level group(P<0.05).(3)Metabolism-related enzyme activity and gene mRNA expression: Low dietary protein level The experimental group(31.6 %)had higher alanine aminotransferase(ALT)and aspartate aminotransferase(AST)activity,but the lowest ALT activity in the liver(P<0.05).With the increase of dietary protein level,the activity of glutamate dehydrogenase(GDH)in liver decreased and then increased(P<0.05).There was no significantly change in the activity of aspartate aminotransferase in the liver and the expression of rapamycin target protein(mTOR),4E binding protein 1(4EBP1)and S6 protein kinase 2(S6K2)gene in the liver.In summary,under the conditions of this study:(1)The optimal protein requirement for triploid rainbow trout(initial weight 233.30 ± 0.39 g)was 45.8 % dry matter(feed fat 20 %,feed energy 22 KJ / g).(2)Compared with the low dietary protein level group(31.6 %-35.4 %),the triploid rainbow trout muscles in the high dietary protein level group(45.8 %-52.4 %)were softer in texture and lower in collagen content;However,they had better flesh color and increased nutrient content,which improved the nutritional value.(3)The triploid rainbow trout in the optimal protein level group(45.8 %)has the strongest digestion and absorption capacity for protein;when the dietary protein level is low(31.6 %-35.4 %),the oxidative damage of the fish liver will occur,but the protein utilization ability will be enhanced by adaptation to meet physiological needs;When the feed protein level is high(52.4 %),triploid rainbow trout will enhance protein absorption and promote growth. |
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