| A 4-week growth trial was conducted to investigate the effect of low-protein level diets on the growth and metabolism enzymes of grass carp(Ctenopharyngodon idellus)and black carp(Mylopharngodon piceus),and subsequent recovery when the fish were then switched back to the control diet for a further 4 weeks.In the restricted phase(1-4 week),grass carp were fed 3 kinds of isolipidic and isocaloric diets containing 35%(Control),27%(D27),20%(D20)protein individually,and black carp were fed 3 kinds isolipidic and isocaloric of diets containing 37%(Control),31%(T31),25%(T25)protein individually.In the realimentation phase(5-8 week),all grass carp were fed 35%protein diet and black carp were fed 37%protein diet.Activities of several key metabolic enzymes including pyruvate kinase(PK),lactate dehydrogenase(LDH),succinate dehydrogenase(SDH),glutamic oxaloacetic transaminase(GOT)and glutamic pyruvic transaminase(GPT)in the grass carp and black carp liver were investigated after each phase.(1)The grass carp trial results showed that during the protein restriction period,body weight,specific growth rate(SGR),feed conversion efficiency(FCE)of treatment groups fish was significantly reduced compared to the control fish(P<0.05),and the fish in group D20 was significant decreased in the above indexes compared to group D27(P<0.05).In the realimentation phase,the above indexes of treatment group fish were still lower than the control fish(P<0.05),while,there were no significant difference in the above indexes between group D20 and group D27(P>0.05).However,in the whole cultured period,there was no significant difference in the SGR between group D27 and control group(P>0.05).Still,there were no significant differences in feeding rate(FR),protein efficiency ratio(PER)and protein retention efficiencies(PRE)among all the groups throughout the experiment(P>0.05),except that the fish in the group D20 showed higher protein retention efficiencies(PRE)than those in group D27 during the protein restriction and the whole cultured period(P<0.05).At the end of the protein restriction period,the fish in the control group showed lower viscera index(VSI),intraperitoneal fat ratio(IPR)and whole body lipid content than those in treatment groups(P<0.05).However,there were no significant difference in the above indexes between treatment groups and control group after realimentation(P>0.05).Throughout the experiment,there were no significant differences in condition factor(CF),hepatopancreas index(HIS),whole body moisture,protein,ash content among all the groups(P>0.05).After protein restriction,the activity of LDH and SDH were gradually increased following the decrease of dietary protein level,the activity of GPT in group D20 was significantly reduced compared to the control group(P<0.05),the activity of PK and GOT were no significant difference between treatment groups and control group(P>0.05).After realimentation,the activity of LDH in group D20 was still higher than control group(P<0.05);while SDH activity of treatment groups were significantly decreased compared to the control group(P<0.05);the activity of GPT in group D27 was higher than control group(P<0.05),however,there was no significant difference between D20 group and control group(P>0.05);GOT activity of treatment groups were significantly increased compared to the control group(P<0.05);there was still no significant difference on the PK activity between treatment groups and control group(P＞0.05).The results indicate that grass carp failed to obtain compensation growth,but achieved body composition compensation.(2)The black carp trial results showed that during the protein restriction period,body weight,specific growth rate(SGR),feed conversion efficiency(FCE)of treatment groups fish were significantly reduced compared to the control fish(P<0.05),while significant improvements in feeding rate(FR)were observed in fish fed with the T25 diet(P<0.05).In the realimentation phase,however,there were no significant difference in the above indexes between treatment groups and control group(P>0.05),except that the fish in group T25 was still significantly decreased in the body weight,increased in the feed conversion efficiency FCE(P<0.05).However,the SGR of T25 group fish was still significantly reduced compared to the control fish(P<0.05),and there was no significant difference in FCE among all the groups during the whole cultured period(P>0.05).Throughout the experiment,significant improvements in protein efficiency ratio(PER)and protein retention efficiencies(PRE)were observed in fish fed in fish fed with the T25 diet(P<0.05).Still,there were no significant differences in the biometric indices,whole body,muscle and liver composition between the group T31 and the control group throughout the experiment(P>0.05),except that the fish in the control group showed higher intraperitoneal fat ratio(IPR)than those in group T31 at the end of the protein restriction period(P<0.05).The T25-treated fish had significantly lower intraperitoneal fat ratio,whole body moisture,protein,lipid content and liver protein content compared to the control fish at the end of the protein restriction period(P<0.05),however,there were no significant difference in the above biometric indices,whole body,muscle and liver composition between group T25 and control group after realimentation(P＞0.05).After protein restriction,PK and GOT activities of treatment groups were significantly reduced compared to the control group(P<0.05),while the activity of SDH was significantly increased compared to control group(P<0.05).PK activity of treatment groups were still higher than control group(P<0.05),the activity of GOT was no significant difference between treatment groups and control group(P>0.05),while SDH activity of treatment groups were lower than control group after realimentation(P<0.05).Still,the activities of LDH and GPT were no significant differences between treatment groups and the control group throughout the experiment(P＞0.05).The results indicate that black carp in T31 group has got completed compensation growth and body composition after protein restriction.In summary,the trial showed black carp had better compensatory growth than grass carp.The stress of protein restriction promoted TCA cycle,inhibited protein degradation in the liver of grass carp and black carp,and the grass carp and black carp have different metabolic pathways in response to stress protein restriction. |
