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Absorption of VFA70% of VFA absorbed from rumen-reticulum60 to 70% of remainder absorbed from omasum
Papillae are important – provide surface area
Absorption from rumen is by passive diffusionConcentration in portal vein less than rumen
VFA concentrationsRumen 50 - 150 mMPortal blood 1 - 2 mMPeripheral blood 0.5 - 1 mM
Absorption increases at lower pHH+ + Ac- HAc
Undissociated acids diffuse more readily
At pH 5.7 to 6.7 both forms are present, however most is dissociatedAt higher pH, 1 equiv of HCO3 enters the rumen with absorption of2 equiv of VFA
VFA AbsorptionAbsorption of Ac-
Ac- Ac- PortalHAc blood
H+ Metabolism
HCO3-
H2O H2CO3
+CO2 CO2 Carbonic
Metabolism anhydraseHAc HAc
Rumen
VFA Absorption
Rate of absorption:
Butyrate > Propionate > Acetate
Absorption greater with increasing concentrationsof acids in the rumen
Absorption increases at lower rumen pH
Absorption greater in grain fed animalsFaster fermentation – More VFA producedLower pHGrowth of papillae
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.
Tissue MetabolismVFA
VFA GIT tissues Liver
Body tissuesUse of VFA
EnergyCarbon for synthesis
Long-chain fatty acidsGlucoseAmino acidsOther
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 CO2shunt Glucose
Utilization of Butryate in Metabolism
Butyrate (As energy)
Butyrate Butyrl CoA B-hydroxybutyrate Acetyl CoA
Krebscycle 2 CO2
Energy (27 ATP/mole)
Some butyrate also used as a primer for short-chain fatty acids
Utilization of Propionate in Metabolism
Propionate
Propionate Propionyl CoA Methylmalonyl CoA
CO2 Succinyl CoA Vit B12
Glucose Krebscycle 2 CO2
Energy(18 ATP/mole)
Utilization of VFA in MetabolismSummary
Acetate Energy Carbon source for fatty acids
AdiposeMammary gland
Not used for net synthesis of glucosePropionate Energy Precursor of glucoseButyrate Energy Carbon source for fatty acids - mammary
Effect of VFA on Endocrine System
PropionateIncreases blood glucoseStimulates release of insulin
ButryateNot used for synthesis of glucoseStimulates release of insulinStimulates release of glucagon
Increases blood glucoseAcetate
Not used for synthesis of glucoseDoes not stimulate release of insulin
GlucoseStimulates release of insulin
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
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%
Lower Energy Value of Roughage Compared with Grain
- Less digested- Lignin limits digestibility of digestible fiber
- Greater energy lost from fermentationCH4 Heat
- Increased rumination Rumen contractions Chewing- More bulk in digestive tract
Comparative Prices of Corn and Alfalfa Hay
NEgMcal/kg
$/ton DM
$/McalNEg
Corn 1.55 121.75 0.0864
Alfalfa hay 0.68 75.00 0.1213
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
Sources of Glucose CarbonRuminants
Ruminants dependent on gluconeogenesisfor major portion of glucose
Sources of glucose in metabolism1. Propionate2. Amino acids3. Lactic acid4. Glycerol5. Carbohydrate digestion in intestine
Absorption of glucose from intestine
Glucose Synthesis
Acetate Amino acidsKetone Acetyl CoA Bodies
FattyButyrate acids
CitrateGlycerol
Acetyl CoALactate CO2 2 CO2
Pyruvate OxaloacetatePEP
Glucose SuccinateProteins Amino acids
Propionate
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 liver3. Ruminants have low blood glucose concentrations
- Low concentrations of glucose in RBC
Consequences of Inadequate Glucose in Metabolism
1. Low blood glucose2. High blood ketones3. High blood concentrations of long-chain fatty acids (NEFA)
Causes fatty liver and/or ketosis inlactating cows and pregnancy toxemiain pregnant ewes
Pregnancy Toxemia Pregnant 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
Fatty Acid MetabolismRelation to Glucose
Diet fat Adipose Diet CHOH CO2 Acetate Malonyl CoA
LCFA NEFA AcetateCO2
Glycerol LCFA acyl CoA 2 CO2
Triglycerides
CarnitineFA acyl carnitine
Malonyl CoA inhibits CO2 (Mitochondria)
Ketones
Low Blood Glucose and Insulin
• Increased release of nonesterified fatty acids from adipose.
• Less synthesis of fatty acidsReduced malonyl CoA
• Reduced sensitivity of carnitine palmitoyl- transferase-1 to malonyl CoA
Increased transfer of fatty acids intomitochondria for oxidation
• Increased ketone production
Fatty Acid Oxidation
FA acyl CoAAcetyl CoA
CO2
Acetoacetyl CoA
Acetoacetate (Mitochondria)
3-OH butyrate
FA acyl carnitine
Carnitine CoA
Low Milk Fat
Cows fed high grain diets:Reduced milk fat percentage
Early theoryLow rumen pHShift from acetate to propionate productionIncreased blood insulinDecrease in blood growth hormone
More recent theoryIncreased production of trans fatty acids inthe rumenTrans fatty acids reduce milk fat synthesis
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
Long-Chain Fatty Acid Synthesis
Lactic acid, Propionate, Amino acidsGlucose Ruminants
limit use of glucoseAcetyl-CoA carboxylase
Acetyl CoA Fatty acids Triglycerides
NADPH NADPAcetate Glycerol-3-PGlu-6-P dehydrogenase Gly-3-P dehydrogenase
Glucose
Long-Chain Fatty Acid Synthesis
Glucose NADPH NADP
Pyruvate Malate Fatty acidsMalate dehydrogenase NADP
Pyruvate OxaloacetateNADPH
Acetyl CoA Acetyl CoAOxaloacetate Citrate lyase
Citrate Citrate Acetate
Mitocondria Cytosol
Long-Chain Fatty Acid Synthesis
Citrate Citrate
Isocitrate NADP Isocitrate NADPH dehydrogenase
a-Ketoglutarate
Mitochondria Cytosol
Supplies about half of NADPH for fatty acid synthesis
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