Effects Of Oxidative Stress On Tryptophan Metabolism And Requirement For Piglets And The Underlying Mechanisms

It is well known that oxidative stress decreases growth performance and health of piglets,but little is known about the nutrient metabolism and requirement for piglets under oxidative stress.This study was conducted based on the reliable oxidative stress model of weaned pigs to investigate the effects and mechanisms of oxidative stress on tryptophan metabolism and tryptophan requirement for piglets.Results of the study may enrich the nutritional principles of tryptophan in piglets,and provide information to alleviate the damage by oxidative stress and use tryptophan additives properly.The main research contents and results were listed as following.Exp.1 Effects of oxidative stress on tryptophan metabolism in weaned pigsThis study was designed to evaluate the effects of Diquat-induced oxidative stress on tryptophan metabolism in weaned pigs.According to a single factorial arrangement,a total of 24 Landrance X Yorkshire weaned pigs were assigned to one of three treatments of 8 animals each,consisting of pigs given ad libitum access to feed(control),challenged with Diquat(10 mg/kg BW)and given ad libitum access to feed(oxidative stress),or pair-fed to receive the same amount of feed as the Diquat-challenged pigs(pair-fed)without stress.The trial lasted for 7 days.Results showed that:Compared to control pigs,the growth performance and activities of SOD(P<0.01)and GSH-PX(P<0.01)significantly declined in Diquat-challenged pigs,which followed by increased serum malondialdehyde(MDA)(P<0.01);Diquat-induced oxidative stress decreased free Trp content and Trp/LNAA in serum and 5-HT content in hypothalamus,augmented serum Kyn(P<0.05),Kyn/Trp(P<0.01)and 5-HT(P<0.05);After Diquat injection,the liver TDO activities including total activity(P<0.05),holoenzyme(P<0.05)and apoenzyme(P<0.05)were enhanced,the TDO mRNA expression was upregulated.Effect of oxidative stress on IDO was no significant difference.The ?-GT activity was increased by oxidative stress.Effects of oxidative stress on amino acid transporter B0AT mRNA level was no significant difference.Compared to pair-fed pigs,oxidative stress decreased ADG and G/F,decreased SOD,GSH-PX,Trp/LNAA in serum and 5-HT in hypothalamus.Injection of Diquat increased MDA,Kyn,Kyn/Trp and 5-HT in serum,upregulated TDO mRNA expression and TDO activity.The activity of ?-GT was enhanced.These results suggested that Diquat-induced oxidative stress depressed growth performance and increased catabolism of tryptophan by augmenting TDO expression and activity in weaned pigs.Exp.2 Effects of oxidative stress on tryptophan requirement for weaned pigsBased on the reliable oxidative stress model established in Exp.1,this study was conducted to evaluate the effects of oxidative stress on tryptophan requirement for piglets.A total of 36 piglets were assigned to one of three treatments of 12 animals each,piglets received diets containing 0.18%Trp,0.30%Trp,and 0.45%Trp respectively.The trial lasted for 14d.On 7d,six piglets(three female and three male)of each group received Diquat at dose of 10 mg/kg BW,the others received sterile normal saline at the same dose.Results showed that:Under normal circumstances,piglets received 0.30%Trp and 0.45%Trp of diets had higher ADG and ADFI compared to piglets fed with 0.18%Trp in diets,followed with lower F/G.Higher Trp in diets resulted in lower plasma urea nitrogen.The concentration of Met,Trp,and Trp/LNAA in plasma were increased significantly for piglets received 0.30%and 0.45%Trp than those fed with 0.18%Trp,however,the concentration of Thr and Val were reduced.Diquat challenge decreased the growth performance,anti-oxidative stress capability,free Trp in plasma,5-HT in hypothalamus,and ratio of intestinal villus height to crypt depth,followed by increased MDA,urea nitrogen in plasma and villus width in duodenum.Tryptophan supplementation could alleviate the impacts of oxidative stress,increase the growth performance,anti-oxidative stress capability,free Trp and Trp/LNAA in plasma,5-HT in hypothalamus,villus height,and villus height/crypt.depth;followed by decreased MDA and urea nitrogen in plasma and crypt depth.There was no significant difference between 0.30%Trp and 0.45%Trp.These results suggested that dietary supplementation of Trp to stressed piglets increased growth performance,anti-stress capability and hypothalamic serotonergic activity,improved intestinal morphology.Exp.3 The underlying mechanism of tryptophan for anti-oxidative stressThe purpose of this experiment was to study the mechanism of tryptophan for anti-oxidative stress by detecting the protein secretion and mRNA expression of related genes in neuro-endocrine system.Experimental design was the same as Exp.2.The results showed that oxidative stress increased cortisol in plasma and free heme in liver,upregulated Ghrelin mRNA abundance in gastric mucosa and intestinal mucosa,and PPAR-? and IL-6 mRNA expression in liver and intestinal mucosa,decreased Ghrelin and IGF-1 concentration in plasma,and Ghrelin,NPY,AgRP mRNA expression in hypothalamus.Also,oxidative stress reduced IGF-1 mRNA abundance in liver as well as IGF-1R and IGFBP-3 mRNA in muscle.Supplementation of tryptophan to piglets increased Ghrelin and IGF-1 secretion,upreguated Ghrelin,NPY,AgRP mRNA expression in hypothalamus,and increased IGF-1 and IGFBP-3 mRNA abundance in liver,and followed by reduced the cortisol concentration and IGF-1,IGF-1R,IGFBP-3 mRNA abundance in muscle,decreased PPAR-?,IL-6,TNF-? mRNA expression in liver and intestinal mucosa.The results indicated that dietary supplementation of tryptophan to oxidative stressed pigs could play an important role in anti-oxidative stress by influencing the neuro-endocrine system and regulating the secretion of Ghrelin,IGF-1 and cortisol,and gene expression related to growth,appetite,and inflammation..In conclusion,Oue results showed that oxidative stress increased tryptophan catabolism via kynurenine pathway mainly by upregulating liver TDO mRNA gene expression and TDO activity.0.30%L-tryptophan in diets accelerated the recovery from damages induced by oxidative stress.The mechanism of anti-oxidative stress effects by tryptophan was associated with its influence on neuro-endocrine system and regulations on secretion of Ghrelin,IGF-1 and cortisol,and gene expressions related to growth,appetite,and inflammation.