Significance of 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

Portal blood To liver & Na+ Rumen Epithelium HAc peripheral tissue H+ 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+ Transporters: Anion exchange, Putative anion transport, Downregulated In adenoma Proton-linked monocarboxylate transporter Na+/H+ exchange Na+/H+ exchange

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

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

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

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

Post-absorption Uptake by the liver Butyrate 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

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

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

Efficiency of VFAs for energy metabolism Mole ATP Heat of Mole acid Efficiency /mole Efficiency combustion produced of acid of VFA kcal/mole /glucose Fermentation oxidized combustion 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

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

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

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

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

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

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

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

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

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

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