Gastrointestinal Physiology Part 10 PDF

Physiology II – Gastrointestinal Physiology Part 10 Fermentative Digestion Sarah Hooper, DVM, MS, PhD Todo E-mail: [email protected]  Learning Objectives Explain microbial protein synthesis in the rumen and its utilization...

Physiology II – Gastrointestinal Physiology Part 10 Fermentative Digestion Sarah Hooper, DVM, MS, PhD Todo E-mail: [email protected]  Learning Objectives Explain microbial protein synthesis in the rumen and its utilization Explain urea recycling in the ruminant Describe the fate of fats in the ruminant animal List vitamins that are synthesized by microbes in the rumen Describe the composition of ruminal fluid in a healthy animal Describe the absorption of VFA in the rumen Explain the effects of the diet on ruminal pH; rumen acidosis; buffer substances in the rumen Describe how rumen acidosis occurs Describe the absorption of sodium, potassium, chloride, magnesium in the rumen Describe the functions of the omasum 2 3/4/2024 Gastrointestinal Physiology  Proteins & Ruminant Digestion Proteins are particularly vulnerable to fermentation because they are made of carbon compounds that can be further reduced to provide energy for anaerobic microbes Microbes do produce endopeptidases that form short-chain peptides as end products these peptides are then absorbed into the microbial cell bodies Once in the microbial cell, peptides can be used to form microbial protein or can be further degraded for the production of energy (VFA pathway) 3 3/4/2024 Gastrointestinal Physiology 4 3/4/2024 Add a footer  Protein Metabolism by Rumen Microbes Proteases on the microbe surfaces generate peptides Intracellularly, peptides are hydrolyzed to amino acids Amino acids contribute to: Synthesis of microbial protein Metabolized to VFA and ammonia Amino acids are also synthesized intracellularly from NH3 and VFA From: Klein. Cunningham‘s Textbook of Veterinary Physiology 2012 5 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion To enter the VFA pathway an amino acid must be first deaminated Deamination: Amino acid ---------------> Normally, the carbon structure of many amino acids can directly be used for VFA synthesis. An exception to this rule are the so called branch-chain amino acids (BCAAs) Valine + 2 H2O --------> Isobutyrate + NH3 + CO2 Leucine + 2 H2O -------> Isovalerate + NH3 + CO2 Isoleucine + 2 H2O ---------> 2-Methylbutyrate + NH3 + CO2 6 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion To enter the VFA pathway an amino acid must be first deaminated Deamination: Amino acid ---------------> NH3 + carbon skeleton Normally, the carbon structure of many amino acids can directly be used for VFA synthesis. An exception to this rule are the so called branch-chain amino acids (BCAAs) Valine + 2 H2O --------> Isobutyrate + NH3 + CO2 Leucine + 2 H2O -------> Isovalerate + NH3 + CO2 Isoleucine + 2 H2O ---------> 2-Methylbutyrate + NH3 + CO2 7 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion Because almost all dietary protein is fermented in the rumen, the ruminant depends on microbial protein to meet its own needs Microbes washed out of the rumen -----> microbial protein reaches the abomasum and small intestine Protein can be produced in the rumen from protein and non-protein sources such as ammonia, nitrates, urea 8 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion In monogastric animals, urea is excreted by the kidneys; in ruminants urea is also excreted into the rumen (through rumen epithelium and saliva). A portion of the urea reaching the rumen can be resynthesized into protein that will contribute to the amino acid needs of the host animal under conditions of low dietary protein, ruminants are efficient conservers of nitrogen 9 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion Urea recycling: Urea, the nitrogen waste product of protein catabolism, is synthesized in the liver of the ruminant from two sources: (1) Urea coming from deamination of endogenous amino acids (2) Nitrogen absorbed as ammonia from the rumen In monogastric animals, urea is excreted by the kidneys; in ruminants urea is also excreted into the rumen (through rumen epithelium and saliva). A portion of the urea reaching the rumen can be resynthesized into protein that will contribute to the amino acid needs of the host animal under conditions of low dietary protein, ruminants are efficient conservers of nitrogen 10 3/4/2024 Gastrointestinal Physiology 11 3/4/2024 Add a footer  Substrates and Products of Fermentative Digestion Fats and lipids: Fats are rare in plants (< 5% of dry mass); exception are oilseeds Microorganisms produce the necessary enzymes for lipids digestion, such as lipases and phospholipases Over the last 25 years, the use of supplemental fats and oils in dairy cow rations has become a standard practice in some production systems Fats are supplemented to increase ration energy density Most of the cattle diet contains polyunsaturated fatty acids (PUFA) 12 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion Fats and lipids: ____________: Major lipid type found in cereal grains, oilseeds, animal fats, and byproduct feeds. Also present in milk (milk fat) _____________: Major lipid type found in forages _____________: Minor component of most feeds. Form the cell membrane of all animal cells, and the surface of milk fat globules. Important in fat digestion in the small intestine of cows _____________: Minor component of dairy feeds, but major component of certain fat supplements 13 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion Fats and lipids: Triglycerides: Major lipid type found in cereal grains, oilseeds, animal fats, and byproduct feeds. Also present in milk (milk fat) Glycolipids: Major lipid type found in forages Phospholipids: Minor component of most feeds. Form the cell membrane of all animal cells, and the surface of milk fat globules. Important in fat digestion in the small intestine of cows Free fatty acids: Minor component of dairy feeds, but major component of certain fat supplements 14 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion Fats will be hydrolized by microbial lipases, e.g. Anaerovibrio lipolytica --------> Triglycerids Butyrivibrio fibrisolvens -------> Phospho- and glycolipids Results in: Glycerol, sugars, and free fatty acids Glycerol and sugars --------------------> VFA Fatty acids synthesized in the rumen will pass to the abomasum and small intestine where absorption takes place 15 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion The typical ruminal acetic/propionic/butyric acid concentration ratio in ruminants ranges from 70:20:10 for animals eating high- forage diets to 60:30:10 for animals eating high-grain diets Although the percentage of acetate is lower on the high- starch diet group, the total amount is considerable greater than in the high-fiber diet group From: Klein. Cunningham‘s Textbook of Veterinary Physiology 2012 16 3/4/2024 Gastrointestinal Physiology 17 3/4/2024 Add a footer  Microbial Vitamin Synthesis Microbes synthesize several vitamins Vitamin C Vitamin K Vitamin B - B1 (thiamin) deficiency is observed after a sudden change of feed from roughage to concentrate - B12 (cobalamin) deficiency is observed in cobalt poor soils or using diets with too much grain Recall: due to the relative small fermentative activity in young ruminants, calves and lambs are not able to synthesize vitamins and these must be supplied with the diet 18 3/4/2024 Gastrointestinal Physiology  Substrates and Products of Fermentative Digestion Concentration of common substances in the rumen Substance Concentration in ruminal fluid VFA ca. 100 mM Minerals - Na+ 20-110 mM - Cl- 10-20 mM - Mg++ 1-10 mM - Ca++ 1-10 mM H2PO4- 10-15 mM HCO3- 20-60 mM Glucose