Effect Of Niacin On Lipid Metabolism In Heat-Stressed Beef Cattle And Its Mechanism

Heat stress is an important source of stress in beef cattle production,which has adverse effects on the health,productivity and product quality of beef cattle,and seriously damages the economic benefits of beef cattle production.Heat stress can not only lead to physiological and metabolic disorders in beef cattle,but also may affect meat quality by regulating muscle fiber types,and may also affect the growth of beef cattle by affecting fat metabolism.However,the mechanism of its action is currently unclear.Niacin（NA）has various biological functions such as anti-inflammatory,antioxidation,improving meat quality,regulation of lipid metabolism and relieving stress,but there are few studies on lipid metabolism and meat quality in beef cattle under heat stress,and its mechanism of action is still unclear.Therefore,this experiment took Jinjiang cattle as the research object,and based on the previous research,studied and discussed the effect of heat stress on blood indexes and meat quality of beef cattle,and the mechanism of AMPK signaling pathway in lipid metabolism of beef cattle under heat stress,as well as the effects of niacin on meat quality and lipid metabolism of beef cattle under heat stress and its mechanism.In this experiment,thirty-six healthy Jinjiang cattle with similar age and body weight were selected and randomly divided into 3 groups,each with 4 replicates and 3 cattle in each replicate.The normal temperature group（TN）and the heat stress group（HS）were fed the basal diet,and the heat stress+niacin group（HN）was fed the basal diet supplemented with 800 mg/kg niacin.The TN group adopted a cooling system to ensure that the experimental cattle were under suitable temperature conditions,while the HS group and the HN group were raised in a high temperature and humidity environment in summer.The pre-test period was 10 days and the pilot period was 60days.The study results were as follows:1.The effects of niacin on blood indexes of beef cattle under heat stress1)Serum enzyme activity:compared with HS group,the serum AST and ALT activities of beef cattle in HN group on day 30 were significantly decreased（P<0.05）;compared with TN group,serum ALP activity in HS group at day 30 and 60 was significantly decreased（P<0.05）,serum LDH,CK and ALT activities at day 30 were significantly increased（P<0.05）,serum CK activity at day 60 was inclined to increase（P=0.083）,AST activity was significantly increased（P<0.05）.2)Other serum biochemical indices:compared with HS group,the serum ALB concentration of beef cattle in HN group on day 30 was significantly increased（P<0.05）,and BUN concentration was significantly decreased（P<0.05）;compared with TN group,the concentrations of TP,ALB and GLU in serum of beef cattle in HS group on days 30 and 60 were significantly decreased（P<0.05）,and the concentration of BUN in serum was significantly increased（P<0.05）.3)Blood lipid content:compared with HS group,the serum TG content in HN group on day60 was significantly decreased（P<0.05）;compared with TN,serum TG content of HS group on day 30 tended to increase（P=0.060）.4)Serum hormones:compared with HS group,the contents of COR and INS in serum of beef cattle in HN group were significantly decreased on day 30 and day 60（P<0.05）,and the contents of GH were significantly increased on day 30（P<0.05）;compared with TN group,COR content in serum of beef cattle in HS group was significantly increased on day 60（P<0.05）,T₄and GH contents were significantly decreased on day 30 and day 60（P<0.05）,INS content was significantly increased on day 30 and day 60（P<0.05）.5)Immune indices:compared with HS group,the contents of Ig A and Ig G in serum of beef cattle in HN group were significantly increased on day 30（P<0.05）,and the contents of Ig M were significantly increased on day 30 and day 60（P<0.05）;compared with TN group,serum Ig A content of beef cattle in HS group was significantly decreased on day 60（P<0.05）,and serum Ig G and Ig M contents were significantly decreased on day 30 and day 60（P<0.05）.6)Antioxidant indices:compared with HS group,SOD activity and T-AOC content in serum of beef cattle in HN group were significantly increased on day 60（P<0.05）,GSH-Px activity was significantly increased on day 30 and day 60（P<0.05）,and MDA content was significantly decreased on day 30 and day 60（P<0.05）;compared with TN group,SOD activity in serum of beef cattle in HS group was significantly decreased on days 30 and 60（P<0.05）,GSH-Px activity and T-AOC content were significantly decreased on day 30（P<0.05）,and MDA content was significantly increased on days 30 and 60（P<0.05）.Compared with HS group,the activities of SOD and GSH-Px in liver of beef cattle in HN group were significantly increased（P<0.05）,while the content of MDA was significantly decreased（P<0.05）;compared with TN group,SOD activity in liver of beef cattle in HS group was significantly decreased（P<0.05）,and MDAcontent was significantly increased（P<0.05）.The results showed that heat stress could change the serum biochemical index of beef cattle,cause endocrine disorders,and reduce immune and antioxidant functions.However,niacin can relieve heat stress by affecting the serum biochemical index of beef cattle and improving the body’s immunity and antioxidant abilities.2.The effects of niacin on slaughtering performance,meat quality and muscle fiber type of beef cattle under heat stress1)Slaughter performance:compared with HS group,there were no significant differences in all indexes in HN group（P>0.05）;compared with TN group,the carcass yield of beef cattle in HS group was significantly decreased（P<0.05）,and the abdominal fat percentage and backfat thickness were significantly increased（P<0.05）.2)Meat quality:compared with HS group,p H_45minand p H_{24 h}of muscle in HN group were significantly increased（P<0.05）,CP content had a trend of increasing（P=0.057）,water loss rate and shear force were significantly decreased（P<0.05）;compared with TN group,p H_45min,p H_24hand CP content in muscle of HS group were significantly decreased（P<0.05）,and the water loss rate and shear force were significantly increased（P<0.05）.3)Muscle fiber types:compared with HS group,the m RNAexpression levels of My HCⅠin longissimus dorsi muscle of beef cattle in HN group were significantly increased（P<0.05）,while the m RNA expressions of My HCⅡx and My HCⅡb were significantly decreased（P<0.05）;compared with TN group,the m RNAexpression levels of My HCⅠin longissimus dorsi muscle of beef cattle in HS group were significantly decreased（P<0.05）,and the m RNA expressions of My HCⅡx and My HCⅡb were significantly increased（P<0.05）.The results showed that heat stress could reduce slaughter performance and meat quality in beef cattle,while niacin could improve meat quality by inducing the transformation of muscle fibers from typeⅡx andⅡb to type I.3.The effects of niacin on AMPK signaling pathway of beef cattle under heat stress1)The activities of fat metabolizing enzymes:compared with HS group,the activities of ACC and FAS in liver of beef cattle in HN group were significantly decreased（P<0.05）;compared with TN group,the activities of ACC and FAS in liver of beef cattle in HS group were significantly increased（P<0.05）,while the activities of LPL were significantly decreased（P<0.05）.2)AMPK content:compared with HS group,the content of AMPK in liver and longissimus dorsi muscle of beef cattle in HN group was not significantly different（P>0.05）;compared with TN group,AMPK content in liver and longissimus dorsi muscle of beef cattle in HS group was significantly decreased（P<0.05）.3)AMPK signaling pathway:compared with HS group,HN group significantly increased the m RNAexpressions of AMPKα,SIRT1,HSL and CPT2 in liver and muscle,ADPN,ATGL,LPL,CPT1,ACO in muscle and LKB1,PPARα,PGC-1αin liver（P<0.05）,SREBP-1c and PPARγin liver and muscle were significantly decreased,and the m RNA expressions of Ch REBP,ACC,SCD1 and FATP in liver were significantly decreased（P<0.05）.Compared with TN group,m RNA expression levels of AMPKα,LKB1,SIRT1,ADPN,HSL,LPL,CPT1,CPT2,ACO,PPARαin liver and muscle,ATGL in muscle and PGC-1αin liver in HS group were significantly decreased（P<0.05）,the levels of SREBP-1c,PPARγ,ACC,FAS,HMGR,SCD1,GPAT and C/EBPαin liver and muscle were significantly increased,and the m RNAexpressions of Ch REBP in liver and FATP in muscle were significantly increased（P<0.05）.The results showed that heat stress promoted fat deposition by inhibiting AMPK signaling pathway,while niacin could downregulate the expression level of fat synthesis genes and upregulate the expression level of adipose decomposition and oxidation genes in beef cattle tissues by activating AMPK signaling pathway,so as to inhibit the fat deposition caused by heat stress.4.The effect of niacin on serum lipidomics of beef cattle under heat stress1)87（30 up-regulated,57 down-regulated）and 93 differential metabolites（40 up-regulated,53 down-regulated）were screened in the two contrast combinations of HN-HS group and HS-TN group.2)KEGG enrichment analysis found that 9 and 24 metabolic pathways were enriched for differential metabolites between HN-HS and HS-TN groups,respectively.3)KEGG topological analysis found that niacin can regulate fat metabolism in heat-stressed beef cattle through two metabolic pathways:glycerolipid metabolism and glycerophospholipid metabolism.Heat stress can affect lipid metabolism pathways in beef cattle,including glycerolipid metabolism,glycerophospholipid metabolism,sphingolipid metabolism,ether lipid metabolism,and phosphatidylinositol signaling system.The results showed that heat stress could change lipid metabolites and metabolic pathways in the blood of beef cattle,while niacin could regulate fat metabolism through glycerolipid and glycerophospholipid metabolic pathways.The summary is as follows:1)Heat stress affects the serum biochemical indexes of beef cattle,and reduces the immune and antioxidant functions,while the dietary addition of niacin can enhance the immune and antioxidant capacity,and effectively relieve heat stress.2)Heat stress reduces slaughter performance and meat quality,while niacin can induce the transformation of muscle fibers from typeⅡx andⅡb to type I to improve meat quality.3)Heat stress promotes fat deposition by inhibiting AMPK signaling i pathway and affecting serum lipid metabolism pathway,while niacin regulates fat metabolism by activating AMPK signaling pathway and improving serum metabolite levels,thus alleviating heat stress.