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VFA Absorption

VFA Absorption. References Church 176-177 Sjersen 173-195 Significance of VFA absorption VFA production = 5 moles/kg DM 95% of the VFA are absorbed before the abomasum 15% of the VFA (primarily butyrate) do not enter portal circulation

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VFA Absorption

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  1. VFA Absorption • References • Church 176-177 • Sjersen 173-195 • Significance of VFA absorption • VFA production = 5 moles/kg DM • 95% of the VFA are absorbed before the abomasum • 15% of the VFA (primarily butyrate) do not enter portal circulation • VFA absorption is through diffusion and facilitated transport modified by metabolism • No active transport

  2. Rumen HAc pH dependent Ac HCO3- CO2 H+ Na+ Epithelium HAc Ac- Metabolism (30% Ac, 50% Prop, 90% But) Aerobic Anerobic CO2 Ketones Carbonic (Acetoacetate, anhydrase B-(OH)-butyrate) H2CO3 HCO3- Lactate H+ Na+ Portal blood HAc To liver & peripheral tissue H+ H+ Na+ Transporters: Anion exchange, Putative anion transport, Downregulated In adenoma Proton-linked monocarboxylate transporter Na+/H+ exchange Na+/H+ exchange

  3. Results of VFA absorption and metabolism • 90% of the circulating VFA is acetate • 80% of the circulating ketones is B-OH-butyrate • Some produced from metabolism of acetoacetate in the liver • Factors affecting VFA absorption • VFA concentration in rumen • 3-fold increase (no change in pH) • Increases acetate absorption 2-fold • Increases butyrate absorption 4-fold with no increase in metabolism • Chain length • At pH 7.4, Ac > Prop > But • pH • Decreased pH increases VFA absorption • From pH 7.4 to 6.4 • No effect on acetate • 2-fold increase in propionate • 4-fold increase in butyrate • Result: But > Prop > Ac

  4. Advantages of epithelial metabolism of VFAs • Provides energy to epithelial cells • Total viscera requires 25% of the total energy requirement • Use of VFAs as an energy source spares glucose • Relatively minor use • Aerobic metabolism yields CO2 used for production of HCO3 needed for acid-base balance • Reduces concentration gradient to allow more VFA absorption • Ketone bodies can bypass liver metabolism and, thereby, provide energy to peripheral tissues and C for fatty acid synthesis • Detoxifies n-butyrate

  5. Effects of diet on VFA absorption • Increased proportion of grain in diet • Increased VFA production in rumen • Decreased rumen pH • Greater proportion of VFA in undissociated form • Greater size of papillae, number of epithelial cells, and blood flow • Upregulation of transport proteins • Increased VFA metabolism in epithelium • Only because of increased number of epithelial cells • Increased VFA absorption by 4-fold

  6. VFA Metabolism • References • Church pp 279-290; 286-289 • Ruckebusch pp. 485-500 • Sjersen pp. 249-265; 389-409

  7. Post-absorption • Uptake by the liver • Acetate • Very little removed by liver • Most transported to peripheral tissues for • Oxidation • Long chain fatty acid synthesis • Propionate • 94% of propionate entering liver is metabolized • Use • Gluconeogenisis • Butyrate • Liver has low affinity for B-OH-butyrate • Approx 20% of butyrate in rumen is metabolized in the liver • Acetoacetate > B-OH-butyrate • Uses • Oxidation • Long chain fatty acid synthesis

  8. Control of location of VFA metabolism • Location of specific acyl-CoA synthetases • Acetate • Acetyl CoA synthetase High in peripheral tissues Low in rumen epithelium and liver • Propionate • Propionyl CoA synthetase Low in rumen epithelium High in liver • Butyrate • Butyryl CoA synthetase High in rumen epithelium Present in heart muscle May also be activitated by medium chain fatty acid-CoA synthetase in peripheral tissues

  9. Uses of VFAs • Maintenance energy ATP CoA Net/mole Acetate Acetyl CoA TCA cycle 12 ATP + 2CO2 10 2ATP CoA CO2 Propionate Succinyl CoA TCA cycle 20 ATP + 4CO2 18 2ATP CoA Butyrate Acetyl CoA TCA cycle 24 ATP + 4CO2 27 5 ATP

  10. Efficiency of VFAs for energy metabolism Mole ATP Heat of Mole acid Efficiency /mole Efficiency combustion produced of acid of VFAkcal/mole/glucoseFermentationoxidizedcombustion Acetate 209.4 2 62.2 10 34.8 (209.4 x 2 (7.3 x 10 /673) /209.4) Propionate 367.2 2 109.1 18 35.8 Butyrate 524.3 1 77.9 27 37.6 Glucose 673 - - 38 41.2 ____________________________________________________________ • Implications • Little difference in efficiency of use of Acetate, Propionate and Butyrate over a wide range of concentrations • Balance is required between VFAs for efficient use • Difference in total efficiency between different fermentation types is associated with the efficiency of fermentation

  11. Gluconeogenesis • Glucose requirements • Central nervous system • 15 – 20% of glucose utilization • Pregnancy • For fetus • Lactation • Lactose synthesis • Lipid synthesis • NADPH for fatty acid synthesis • Glycerol

  12. Precursors for gluconeogenesis % of Glucose from: PrecursorFedFasted Propionate 40 – 60 0 Amino acids 15 – 30 35 (Primarily Alanine, Glutamine, Glutamate) Lactate 15 40 Glycerol 5 25

  13. Mechanism of gluconeogenesis • Controlling enzymes • Pyruvate carboxylase (Pyr > OAA) • NAD-malate dehydrogenase (Mal > OAA) • PEP carboxykinase (OAA > PEP) • Fructose-1,6- diphosphatase (Fru-1,6-P > Fru-6-P) • Glucose-6-phosphatase (Glu-6-P > Glu) • Hormones • Glucagon and Glucorticoids • Insulin

  14. Glucose conservation • Low blood glucose • Low hexokinase activity in the liver • Little glucose used for long chain fatty acid synthesis in ruminants

  15. Fatty acid synthesis • Locations • Nonlactating animals • 92% of fatty acid synthesis is in adipose tissue and 6% is in the liver • Lactating animals • 40% of fatty acids in milk fat are synthesized in mammary gland

  16. Why glucose is not a C-source for fatty acid synthesis • Limiting enzymes • Bauman Citrate lyase Malate dehydrogenase • Baldwin Pyruvate kinase Pyruvate dehydrogenase • Use of glucose for fat synthesis • Supply NADPH • Synthesis of glycerol

  17. Precursors for fatty acid synthesis in ruminants • Acetate • 75 – 90% of C in C4 – C14 fatty acids • 20% of C in palmitate (C16) • 0% of C in C18 • Butyrate • Acetate and B(OH)butyrate contribute equally to the first 4 carbons • Must be converted to acetyl CoA for additional C • Lactate • 5 – 10% of the fatty acids in milk • Inversely related to the amount of acetate available • Controlled by pyruvate dehydrogenase • Additional uses of lactate • Glycerol • NADPH from Isocitrate cycle • Propionate • Precursor for odd and branched chain fatty acids • Increased by increased concentration of methylmalonyl CoA from vitamin B12 deficiency

  18. Energy partitioning between adipose and milk fat • High grain diets with deficient eNDF will result in reduction in milk fat synthesis and increase adipose tissue • Insulin-glucogenic theory • Increases propionate and reduces acetate production • Increases glucose synthesis • Increases insulin secretion • Increases glucose uptake by adipose tissue, but not mammary gland • Increases NADPH synthesis in adipose tissue • Increases fatty acid synthesis in adipose tissue, making less acetate available for mammary gland • Now believed that insulin plays a minor role in milk fat depression

  19. Biohydrogenation theory • High grain diets, diets with deficient eNDF, or diets high in polyunsaturated fatty acids • Increase production of trans-10, cis-12 conjugated linoleic acid (CLA) Linoleic acid (cis-9, cis-12 C18:2) High forage High grain Conjugated linoleic acid Conjugated linoleic acid (cis-9, trans-11 CLA) (trans-10, cis-12 CLA) Vaccenic acid trans-10 C18:1 (trans-11 C18:1) Stearic acid Stearic acid C18:0 C18:0

  20. Even at low doses (<5 g/d), trans-10, cis-12 CLA inhibits fat synthesis in mammary gland • Mechanism • trans-10, cis-12 CLA inhibits migration of Sterol Response Element-Binding Protein (SREBP) transcriptional factor to the nucleus of mammary cells • Results in reduction in mRNA for genes involved in: • Fatty acid uptake Lipoprotein lipase • Fatty acid transport Fatty acid binding protein • Fatty acid synthesis Acetyl CoA carboxylase Fatty acid synthase • Fatty acid desaturation Stearoyl-CoA desaturase • Triglyceride synthesis Acylglycerol phosphate acyl transferase Glycerol phosphate acyl transferase

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