| It was well established that effects of essential amino acid supply on milk protein synthesis in lactating ruminants could not be described by the single limiting amino acid theory,which justified the development of a new theory to direct researches on the balancing technologies of dietary amino acid in order to promote milk protein synthesis and to elevated the conversion efficiency of absorbed amino acid into milk protein in lactating ruminants.For lactating ruminants,absorbed amino acids are mainly metabolized in the digestive tracts,the liver,and the mammary glands.Previous studies revealed that metabolic losses of individual essential amino acids in the digestive tract might relate only to its supply;the liver might have active cross-talk mechanisms in amino acid metabolism with the peripheral tissues;the mass action might not be the main way for a limiting amino acid to depress milk protein synthesis.These are three key questions related to the basis of the new regulating theory of essential amino acid on milk protein synthesis in lactation ruminants.In this study,non-pregnant lactating goats were used as experimental animals,combined with vascular intubation and blood flow measurement,tries to clarify the ways of essential amino acid supply to regulate amino acid metabolism in three tissues and organs through the changes of metabolic flow of amino acids.Results of the present study will contribute the development of the new regulation theory of amino acid in lactating ruminants.1.Effects of graded removal of MP on lactation performance and splanchnic net fluxes of amino acids in lactating dairy goatsIn this experiment,4 non-pregnant Saanen dairy goats（BW 50±5 kg,DIM 60±10 d,second-lactation）,with similar milk yield were selected as experimental animals.One month in advance,chronic vascular catheters were installed in portal vein,hepatic vein and mesenteric vein.The right carotid artery was also raised to a subcutaneous position and the T-type Tygon tube was surgically implanted to abomasum.Using a 4 × 4 Latin design,the abomasum was infused with a mixture of glucose and amino acids,and the experimental treatment was a mixture of graded removal of amino acids to simulate the graded removal of metabolizable protein（MP）.The infused treatment was 100%,60%,30% and 0% of the requirement,respectively.Goats were allowed free access to water throughout the experiment.The results showed that with the graded removal of MP,milk yield and milk protein yield decreased significantly,while portal vein blood flow and mammary blood flow increased significantly.Except for Glu and Tyr,the concentration of amino acids（AA）in plasma decreased linearly with the graded removal of MP.The removal amount of Leu,Tyr and branched chain amino acids（BCAA）from liver did not change,the peripheral tissue flow behind liver was not affected,the uptake amount of Phe on mammary gland showed a quadratic decrease,and the uptake amount of Glu and Cit on mammary gland did not change.In addition,the flow of AA in Portal Drained Viscera（PDV）,liver,splanchnic tissue and mammary gland decreased linearly with the graded removal of MP.However,when MP decreased to 60%,the peripheral tissue supply of Arg was less than that of mammary gland uptake,and when MP decreased to 30%,the peripheral tissue supply of Met,His,Thr and Arg was less than mammary gland uptake.The loss rates of Phe,Met and His in PDV did not change significantly,while the loss rates of other EAA decreased linearly.The removal ratio of Met,His,Phe,Thr and Arg decreased linearly in the liver,but there was no significant change in the removal ratio of BCAA and Lys.The decomposition and utilization of BCAA and Lys in peripheral tissue decreased,and for most EAA,the clearance rate of mammary gland decreased.2.Effects of graded removal of single EAA on lactation performance and splanchnic net fluxes of amino acids in lactating dairy goatsIn this experiment,4 non-pregnant Saanen dairy goats（BW 50±5 kg,DIM 60±10 d,second-lactation）,with similar milk yield were selected as experimental animals.One month in advance,chronic vascular catheters were installed in portal vein,hepatic vein and mesenteric vein.The right carotid artery was also raised to a subcutaneous position and the T-type Tygon tube was surgically implanted to abomasum.Using a 4 × 4 Latin design,the abomasum was infused with a mixture of glucose and amino acids,and the experimental treatment was a mixture of graded removal of single EAA that is the Met,His,Lys,Leu.The infused treatment was 100%,60%,30% and 0% of the requirement,respectively.Goats were allowed free access to water throughout the experiment.The results showed that the removal of four kinds of EAA resulted in the decrease of milk protein production.The plasma AA concentrations of Met,His and Lys all decreased linearly,and the plasma concentrations of Leu decreased quadratically with the graded removal of Leu.The removal of the four EAA all increased the mammary blood flow linearly,but there were differences on the effect of splanchnic blood flow.Graded removal of Met resulted in linear decrease of hepatic blood flow,and linear increase of portal vein blood flow after graded removal of His.Graded removal of Lys and Leu had no significant effect on portal vein and hepatic vein.At the same time,the removal of Met significantly increased the concentration of insulin and decreased the concentration of prolactin,the removal of His caused the linear increase of prolactin,and the removal of Lys and Leu increased the concentration of glucagon.In the four experiments,the flows of Met,Lys and Leu in PDV decreased linearly with the graded removal of Met,Lys and Leu,while the His flows decreased quadratically.The AA balance analysis of PDV showed that the loss ratio of Met,His,Lys and Leu decreased linearly with the removal of them respectively.The clearance rates of Met,His,Lys and Leu in PDV decreased significantly.The removal rates of Met and His in the liver decreased linearly with the decrease of total hepatic influx,but there was no significant change of Lys and Leu in the liver.In contrast,the clearance rates of Met and His in liver decreased linearly,but there was no significant change on the clearance rates of Lys and Leu.The supply of peripheral tissue of Met,His,Lys and Leu decreased linearly.The uptake of His,Leu and Lys in mammary gland decreased linearly with the removal of AA,but there was no significant change in the uptake of Met.Except for the mammary UO ratio of His,the UO ratio of Met,Lys and Leu decreased linearly with the removal of AA.3.Effects of different infused sites on lactation performance and splanchnic net fluxes of amino acids in lactating dairy goatsIn this experiment,4 non-pregnant Saanen dairy goats（BW 50±5 kg,DIM 60±10 d,second-lactation）,with similar milk yield were selected as experimental animals.One month in advance,chronic vascular catheters were installed in portal vein,hepatic vein and mesenteric vein.The right carotid artery was also raised to a subcutaneous position and the T-type Tygon tube was surgically implanted to abomasum.Using a 2 × 2 cross-design,the abomasum and jugular vein were infused with a mixture of glucose and amino acids to meet with the requirement of lactating goats.The results showed that compared with abomasal infusion,jugular infusion had a tendency to increase milk protein and milk protein production.Although only the concentration of Ile in arterial plasma increased significantly,the concentration of other EAA also increased numerically.Compared with abomasal infusion,jugular infusion increased the portal blood flow,hepatic blood flow and mammary blood flow.Compared with abomasal infusion,the flow rate of 10 EAA in PDV increased significantly under jugular infusion,and there were significant differences in Lys,Ile,Thr,BCAA,EAA,nonessential amino acid（NEAA）and total amino acid（TAA）.The analysis of AA balance showed that during jugular infusion,except for Arg,the loss ratio of the other 9 EAA in PDV decreased significantly.In the liver,the removal amount of Met,Lys,His,Phe,Thr,Trp and TAA increased significantly under the jugular infusion,and the removal amount of TAA in the liver also increased significantly.Through AA balance analysis,it was found that the liver removal ratio of Met,His,Phe and Thr increased significantly or tended to increase,but the liver removal ratio of BCAA and Lys were not affected.For the AA supply of peripheral tissue,there was no significant difference in Met,His,Phe,Thr and Arg,but the peripheral supply of Lys,Trp and BCAA increased significantly under jugular infusion.At the same time,for Met,His,Phe,Thr and Arg,there was no significant change in mammary gland uptake and milk secretion,but the mammary gland uptake and milk secretion of Lys and BCAA were significantly increased.Except for Glu,the UO ratio of mammary gland had no significant changes.The results suggested that:（1）The conversion efficiency of the absorbed AA to milk protein increased with the decrease of MP supply,but would reduce milk protein production.（2）The average proportion of total absorbed EAA in PDV,liver and mammary was 31%,15%and 38%,leaving only a limited proportion of metabolizable EAA in the remaining peripheral tissues.（3）The metabolic loss in PDV was different with the graded removal of EAA,so the metabolic loss of digestive tract can be regulated nutritionally.（4）PDV,liver,peripheral tissues and mammary gland are involved in AA regulation with the decrease of MP supply.However,the response of single EAA is different in different tissues and organs.（5）Liver played an active regulatory role in EAA which can actively control the removal rate of AA according to different types of EAA and the needs of peripheral tissues.（6）In the absence of overall AA,the affinity of most EAA in the mammary gland decreased,which was completely opposite to the removal of single EAA.（7）The removal of single EAA reduced milk protein production,which was affected by the quantitative effect of AA,the regulation of tissues and organs and the endocrine status of the body. |
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