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Absorption of VFA

Absorption of VFA. 70% of VFA absorbed from rumen-reticulum 60 to 70% of remainder absorbed from omasum Papillae are important – provide surface area Absorption from rumen is by passive diffusion Concentration in portal vein less than rumen VFA concentrations Rumen 50 - 150 mM

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Absorption of VFA

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  1. Absorption of VFA • 70% of VFA absorbed from rumen-reticulum • 60 to 70% of remainder absorbed from omasum • Papillae are important – provide surface area • Absorption from rumen is by passive diffusion • Concentration in portal vein less than rumen • VFA concentrations • Rumen 50 - 150 mM • Portal blood 1 - 2 mM • Peripheral blood 0.5 - 1 mM • Absorption increases at lower pH • H+ + Ac- HAc • Undissociated acids diffuse more readily • At pH 5.7 to 6.7 both forms are present, however most is dissociated • At higher pH, 1 equiv of HCO3 enters the rumen with absorption of • 2 equiv of VFA

  2. VFA AbsorptionAbsorption of Ac- Rumen Ac- Ac-Portal HAc blood H+Metabolism HCO3- H2O H2CO3 + CO2 CO2 Carbonic Metabolismanhydrase HAc HAc

  3. VFA Absorption • Rate of absorption: • Butyrate > Propionate > Acetate • Absorption greater with increasing concentrations • of acids in the rumen • Absorption increases at lower rumen pH • Absorption greater in grain fed animals • Faster fermentation – More VFA produced • Lower pH • Growth of papillae

  4. Metabolism of VFA by GIT Half or more of butyrate converted to - hydroxybutyric acid in rumen epithelium. 5% of propionate converted to lactic acid by rumen epithelium. Some acetate is used as energy by tissues of gut. VFA and metabolites carried by portal vein to liver.

  5. Tissue MetabolismVFA • VFA GIT tissues Liver • Body tissues • Use of VFA • Energy • Carbon for synthesis • Long-chain fatty acids • Glucose • Amino acids • Other

  6. Utilization of Acetate in Metabolism 1. Acetate (As energy) Energy Acetate Acetyl CoA Krebs cycle 2 CO2 2 carbons (10 ATP/mole) 2. Acetate (Carbon for synthesis of fatty acids – in adipose) Acetate Acetyl CoA Fatty acids Lipids H+NADPH NADP+ Glycerol Pentose PO4 CO2 shunt Glucose

  7. Utilization of Butryate in Metabolism Butyrate (As energy) Butyrate Butyrl CoA B-hydroxybutyrate Acetyl CoA Krebs cycle 2 CO2 Energy (27 ATP/mole) Some butyrate also used as a primer for short-chain fatty acids

  8. Utilization of Propionate in Metabolism Propionate Propionate Propionyl CoA Methylmalonyl CoA CO2 Succinyl CoA Vit B12 Glucose Krebs cycle 2 CO2 Energy (18 ATP/mole)

  9. Utilization of VFA in MetabolismSummary Acetate Energy Carbon source for fatty acids Adipose Mammary gland Not used for net synthesis of glucose Propionate Energy Precursor of glucose Butyrate Energy Carbon source for fatty acids - mammary

  10. Effect of VFA on Endocrine System • Propionate • Increases blood glucose • Stimulates release of insulin • Butryate • Not used for synthesis of glucose • Stimulates release of insulin • Stimulates release of glucagon • Increases blood glucose • Acetate • Not used for synthesis of glucose • Does not stimulate release of insulin • Glucose • Stimulates release of insulin

  11. Energetic Efficiency of VFA in Metabolism ATP/mole Energy in ATP % Heat of (kcal/mole) combustion Acetate 10 76.0 36.3 Propionate 18 136.8 37.2 Butyrate 27 205.2 39.1 Glucose 38 288.8 42.9

  12. Energetic Efficiency of VFAFermentation and Metabolism Cellulose 10 Glucose VFA ATP (6730 kcal) (5240 kcal (1946 kcal) 60A 28.9% Starch 30P 10B Absorbed as glucose ATP (6730 kcal) (2888 kcal) 42.9%

  13. Lower Energy Value of Roughage Compared with Grain • Less digested • Lignin limits digestibility of digestible fiber • - Greater energy lost from fermentation • CH4 Heat • - Increased rumination • Rumen contractions • Chewing • - More bulk in digestive tract

  14. Comparative Prices of Corn and Alfalfa Hay

  15. Requirements for GlucoseRuminants • 1. Nervous system • Energy and source of carbon • 2. Fat synthesis • NADPH • Glycerol • 3.Pregnancy • Fetal energy requirement • 4. Lactation • Milk sugar - lactose

  16. Sources of Glucose CarbonRuminants • Ruminants dependent on gluconeogenesis • for major portion of glucose • Sources of glucose in metabolism • 1. Propionate • 2. Amino acids • 3. Lactic acid • 4. Glycerol • 5. Carbohydrate digestion in intestine • Absorption of glucose from intestine

  17. Glucose Synthesis Acetate Amino acids Ketone Acetyl CoA Bodies Fatty Butyrate acids Citrate Glycerol Acetyl CoA Lactate CO2 2 CO2 Pyruvate Oxaloacetate PEP Glucose Succinate Proteins Amino acids Propionate

  18. Conservation of GlucoseRuminants • 1. Glucose not extensively used for synthesis • of long-chain fatty acids in adipose of ruminants • - Not clear why glucose carbon is not used • Glycerol is needed for synthesis of triglycerides • - Comes from glucose • Acetate supplies carbon for fatty acid synthesis • 2. Low hexokinase activity in the liver • 3. Ruminants have low blood glucose concentrations • - Low concentrations of glucose in RBC

  19. Consequences of Inadequate Glucose in Metabolism 1. Low blood glucose 2. High blood ketones 3. High blood concentrations of long-chain fatty acids (NEFA) Causes fatty liver and/or ketosis in lactating cows and pregnancy toxemia in pregnant ewes

  20. Pregnancy ToxemiaPregnant Ewes • During the last month of pregnancy • Ewes with multiple fetuses • Inadequate nutrition of ewe • High demands for glucose by fetuses • Low blood glucose and insulin • Mobilization of body fat • Increase in nonesterified fatty acids in blood • Increased ketone production by liver

  21. Fatty Acid MetabolismRelation to Glucose Diet fat Adipose Diet CHOH CO2 Acetate Malonyl CoA LCFA NEFA Acetate CO2 Glycerol LCFA acyl CoA 2 CO2 Triglycerides Carnitine FA acyl carnitine Malonyl CoA inhibits CO2(Mitochondria) Ketones

  22. Low Blood Glucose and Insulin • Increased release of nonesterified fatty acids • from adipose. • Less synthesis of fatty acids • Reduced malonyl CoA • Reduced sensitivity of carnitine palmitoyl- • transferase-1 to malonyl CoA • Increased transfer of fatty acids into • mitochondria for oxidation • Increased ketone production

  23. Fatty Acid Oxidation FA acyl carnitine Carnitine CoA FA acyl CoA Acetyl CoA CO2 Acetoacetyl CoA Acetoacetate (Mitochondria) 3-OH butyrate

  24. Low Milk Fat • Cows fed high grain diets: • Reduced milk fat percentage • Early theory • Low rumen pH • Shift from acetate to propionate production • Increased blood insulin • Decrease in blood growth hormone • More recent theory • Increased production of trans fatty acids in • the rumen • Trans fatty acids reduce milk fat synthesis

  25. Long-Chain Fatty Acid SynthesisRuminants Synthesis is primarily in adipose or mammary gland – Limited synthesis in the liver Ruminants conserve glucose supply – Glucose not extensively used for long chain fatty acid synthesis Most of carbon is supplied by acetate Some butyrate used in mammary gland Glucose metabolism supplies some of NADPH needed for fatty acid synthesis

  26. Long-Chain Fatty Acid Synthesis Lactic acid, Propionate, Amino acids Glucose Ruminants limit use of glucose Acetyl-CoA carboxylase Acetyl CoA Fatty acids Triglycerides NADPH NADP Acetate Glycerol-3-P Glu-6-P dehydrogenaseGly-3-P dehydrogenase Glucose

  27. Long-Chain Fatty Acid Synthesis Glucose NADPH NADP Pyruvate Malate Fatty acids Malate dehydrogenaseNADP Pyruvate Oxaloacetate NADPH Acetyl CoA Acetyl CoA Oxaloacetate Citrate lyase Citrate Citrate Acetate Mitocondria Cytosol

  28. Long-Chain Fatty Acid Synthesis Citrate Citrate Isocitrate NADP Isocitrate NADPHdehydrogenase a-Ketoglutarate MitochondriaCytosol Supplies about half of NADPH for fatty acid synthesis

  29. Long-Chain Fatty Acid Synthesis • Butyrate • Can be used in mammary gland as primer for • synthesis of fatty acids • Shorter chain acids • Methylmalonyl (propionate) • Is used as primer for synthesis of fatty acids • in sheep fed high-grain diets • Branched-chain acids

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