Gastrointestinal Physiology- Ruminent Flashcards
Ruminants
A diverse group of mammals that regurgitate and re-masticate their food
Suborders:
ruminantia- deer, elk, goat, cow, giraffe
Tylopoda- camel, llama
Fermentative Digestion
Occurs in specialized compartments localized before the stomach (forestomach in ruminants) or after stomach and small intestine (cecum and colon in horses)
Microbes in fermentative digestion
Bacteria, fungi, and protozoa
Enzymes for digestions are of microbial origin (not produced by host)
Regurgitation and re-mastication
provides more finely divided material and thereby a greater surface area for microbial digestion
nonglandular forestomachs
Rumen, reticulum, omasum
fermentative digestion
Terminal glandular
True stomach
Abomasum
Ruminent forestomachs
The forestomachs are lines with stratifies squamous epithelium
no villi or microvilli
Abomasum is mostly on the right side of the animal
Rumen development
The abomasum is the largest compartment of the newborns stomach
Enlargement of the forestomach occurs rapidly after birth but the rate depends on diet type (solid feeds and concentrate accelerate development) and contact with adult ruminants (inoculation of microorganisms)
Non-ruminant period: birth to 3 weeks
Transitional period: 3 to 8 weeks
Ruminent Stomach Protozoa
Most protozoa are ciliated and belong to the genus Isotricha or entodinium
Grouping of rumen protozoa according to morphology and size
Size: big, small, medium
Systematically: flagelates, ciliates
Ruminal environment
Substrate availability: food intake regulated by volume, structure, energy, palatability
Temperature: about 0.5-1 degree C above body temp
Fluids: drinking water and saliva
pH: 5.5-7 (acid synthesis and acid reabsorption, buffer substances coming from the saliva and rumen epithelium)
Ruminal ecosystem
Protozoa ingest large numbers of bacteria and hold bacterial numbers in check
Protozoa may also play a role in starch and protein digestion -> they prolong the digestion of these substances (ingest them and protect them from bacterial action)
Symbiosis
Waste products produced by only species serve as substrate for another
Generation and fate of VFA in the ruminant
Carbohydrates, proteins, and lipids
Digested by ruman microbes to VFA.
To propionate to the liver-glucose
To acetate butyrate to all tissues- energy
To acetate to adipose tissues- fatty acids
Fate of carbohydrates
The cell wall of plants has a large portion of carbohydrates whihc are important for stability and rigidity of the growing plant
Plant cell walls are important substrates for fermentative digestion (and significant nutrient source of many microorganism species)
Cellulose, hemicellulose, pectin- will be hydrolyzed by the enzyme cellulase. After hydrolysis, monosaccharides are not released from the polysaccharide. These monosaccharides are not available for absorption by the animal; further metabolized by the microbes
Lignin
Not a carbohydrate. Is essentially indigestible (although some fungi can digest lignin)
It increases with the age of a plant and ambiental temperature
Fate of carbs, proteins, and lipids
Essentially all dietary proteins and carbohydrates are subjected to fermentative digestion in the forestomachs
Products are glucose, other monosacch, and short chain polysacch. that are released into the fluid phase
These glucose and other sugars do not become available to the host animal; they are absorbed into the cell bodies of microbes
Within the microbial cell, glucose enters the glycolytic pathway
To produce 2 pyruvate from one glucose molecule (plus 2 NADH and 2 ATPs which is used by the microbes)
Fermentative digestion is an anaerobic and the products are volatile fatty acids (VFA aka short chain fatty acids SCFA)
Substrates and products of fermentative digestion
The primary VFAs are acetic acid (acetate), propionic acid (propionate), and butyric acid (butyrate).
Glucose –> pyruvate
Pyruvate to ACoA to acetate and butyrate OR Pyruvate to propionate OR Pyruvate (lack of O2) to lactate and to pripionate and acetate
Define Fermentative digestion
Enzymatic decomposition and utilization of foodstuffs by bacteria, fungi, and protozoa in forestomach of ruminants and cecum/colon in horses
What describes the rumen
Largest (by volume) compartment of ruminant forestomach
Seves as storach compartment to facilitate microbial fermentation of ingesta
Relevant ruminal microorganisms
Bacteria grouped according to food/ nutrients they ferment
Fungi assist with fermentative digestion
Protozoa are grouped according to their size and whether they have flagelates or cilates
Rumen environment
The time spent ruminating depends on type of feed ingested
Magic bus encounters glucose after eaten by cow
Glucose will leave the fluid phase and enter a microba when it will enter the glycolic pathway
Which do amino acids contribute to
Metabolized to VFA and ammonia
Synthesis of microbial protein
Protein and 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
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 and metabolized to VFA and ammonia
Amino acids are also synthesized intracellularly from NH3 and VFA
Deamination
Amino Acid to carbon skeleton and NH3
Normally the carbon structure of many animo acids can directly be used for VFA synthesis. An exception to this rule are the so called branch-chain amino acids (BCAAs)
Substrates and products of fermentative digestion
Because almost all dietary protein is fermented in the rumen, the ruminant depends on microbial protien to meet its own needs
Microbes washed out of the rumen -> microbial protein reached the abomasum and small intestine
Protein can be produced in the rumen from protein and non-protein sources such as ammonia, nitrates, urea
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 aa needs of the host animal -> under conditions of low dietary protein, ruminants are efficient conservers of nitrogen
Microorganisms produce the necessary enzymes for lipids digestion
Lipases
phospholipases
Fats and Lipids
Fats are rare in plans (<5% of dry mass); exception are oil seeds
Microorgansims 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
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 animal cells, and the surface of milk fat globules. Important in fat digestion inn the small intestine of cows
Free fatty acids
Minor component of dairy feeds, but major component of certain fat supplements
Fats
Hydrolyzed by microbial lipases
Results in glycerol, sugars, and free fatty acids.
Glycerol and sugars to VFA
Fatty acids synthesized in the rumen will pass to the abomasum and small intestine where absorption takes place
Ratio
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
Besides creating VFAs what else might the microbiome create that’s useful to the ruminant
Fat soluble vitamins and water soluble vitamins
Microbial Vitamin Synthesis
Microbes synthesize several vitamins: C, K and B
B1 (thiamin) deficiency is observed after a sudden change of feed from roughage to concentrate (grain)
B12 (cobalamin) deficiency is observed in cobalt poor soils or using diets with to 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 diet
Absorption of VFA in the Rumen
There are two main suggested mechaniams, depending on the ionization state of VFA
Ionized VFA: these cannot diffuse and need a carrier (HCO3-/Ac- antiport)
Non-ionized: these are lipophilic and can just diffuse through the apical membrane
Rumen acidosis
Fast-fermentable carbohydrates (starch-rich diet) lead to an increase in VFA production -> pH in the rumen gets more acidic
Ionization grade of VFA depends on the pH of the rumen
VFA have a pH of 4.8 (recall acid-base homeostasis -> pK indicate the pH at which a substance is 50% ionized and 50% non ionized)
The acidic pH in the rumen stimulates proliferation of lactate-producing bacteria -> exacerbation of the acidosis
Rumen acidosis- flow chart
fast-fermentable carbohydrates
- generation of high amounts of VFA
- Reduced mastication (low HCO3-)
Lead to pH < 4.8
more VFA will be in the non-ionized form and can easily diffuse into the cell
Absorption of Sodium in the rumen
Electrogenic transport: Na+ channel
Electroneutral transport: NHE (Na+/H+ exchanger)
Basolateral Na+/K+ ATPase
Absorption of Chloride in rumen
Cl=/HCO3- exchanger Basolateral channel (not fully identified)
Absorption of Potassium in rumen
Apical and basolateral channels
High luminal K+ concentration (transepithelial potential difference)
Absorption of Magnesium in rumen
Electrogenic transport: Mg2+ channel (dependent on potential difference between apical/basolateral side)
Affected in the presence of high K+ concentrations (young pastures or potassium-fertilized pastures) -> pastures grass tetany
Absorption of Calcium in the rumen
Reabsorption not fully understood but: Probably electroneutral (Ca/H exchanger; not fully understood) Basolateral Na/Ca exchanger and Ca ATPase
Ruminant Stomach
The omasum is composed of muscular folds (or leaves) that project from the greater curvature into the lumen. The canal connects reticulum with the abomasum
Omasum functions
Concentration of the ingest (absorption of water)
SCFA absorption (diffusion more relevant here)
Na+ and Cl- absorption
HCO3- absorption
Rumen Motility
Recall the reticular folds and rumen pillars are motile -> elevate and relax accentuating or reducing the divisions within the rumen
The walls of forestomach’s are motile too and possess an enteric nervous system -> motility patterns
There are two kinds of motility patterns:
Mixing (primary contractions)
Eructation (secondary contractions)
Primary Contractions
A. The bolus enters the rumen and remains suspended in the area near the cardia (it contains air bubbles)
B. Biphasic (double) contraction of the reticulum
- first contraction is weak
- second is forceful nearly obliterating the lumen of the reticulum -> bigger particles will be pushed into the dorsal sac
C. Caudal-moving contraction of the dorsal sac moves ingesta further back into the dorsal sac
D. Cranial-moving contraction of the dorsal sac mixes the ingesta. This ingesta is now under bacterial fermentation which produces gas. This gas (co2 and methane) accumulates in the dorsal sac
E. The smaller particles decant into the ventral sac
F. Contraction of the ventral sac separates big and small material; the small material goes over the cranial pillar into the cranial sac
G. Contraction of the cranial sac which further separates material into big and small
H. The reticulum contracts, the reticulo-omasal orifice relaxes and small particles (dense material) are forced through the opening into the omasum
A new cycle starts
Function of the primary contraction
Reduce particle size of forage (from 1-2cm to 2-3mm)
Digestibility and physical characteristic of feed have important influences on both the rate of particle passage from the rumen and the rate of feed intake
Poorly digestible fiber needs longer to be broken down compared to good quality ones
Contractions- rate
1-3 reticulo-rumen contractions occur per minute
More frequent during eating and disappear during sleep
Rate and strength of the contractions depend on the structure of the diet
Coarse, fibrous feeds stimulate the most frequent and strongest contractions
Secondary contractions
Function is to force gas toward the cranial portion of the rumen
They occur at the end of a primary contraction cycle
1: cranial-moving contraction starting in the caudo-dorsal blind sac
2: Forward-moving contraction of the dorsal sac that moves gas toward the cardia. Gas will enter the esophagus and can then be eructated
Regurgitation Reflex
Function is to bring large particles from the rumen back to the mouth so that it can be chewed to reduce particle size
- it begins with a contraction at the mid-dorsal rumen-> this pushes the gas cap caudally and the big particles toward cardia
- The lower esophageal sphincter reflexes and the bolus enters the esophagus and propelled to the mouth by antiperistalsis
Eructation
The gases produced during rumination are mainly carbon dioxide and methane
Eructation frequency is about 1/min
Eructation center is localized in the medulla and recieves afferent fibers from mechanoreceptors places in the dorsal sac of the rumen (where gas accumulates)
Tympanism
(Bloating) occurs in cattle when the eructation mechanism fails. Causes include:
- blockage of esophagus
- impaired vagal nerve function
- Rabies
- more typical due to the ingestion of legumes
Legume Bloat
When cattle feed on lush, rapidly growin galfalfa or clover pastures (waxy saponins). Gas becomes trapped in tiny bubbles and the normal free gas bubble cannot accumulate on top of the dorsal sac of the rumen
-the presence of gas is not detectable by the mechanoreceptors of the dorsal sac
Control of reticulorumen motility
ENS and Vagus nerve
Stretch receptors, chemoreceptors -> monitor distention, consistency of the ingesta, pH, VFA concentration, ions
Control center for reticulorumen motility is located in the brainstem (dorsal vagal nucleus) -> efferent fibers go to the rumen with the vagus nerve
Esophageal groove
Is a gutter like invagination traversing the wall of the reticulum from the cardia to the reticulo-omasal orifice
-it diverts milk away from the developing rumen and pass it directly to the abomasum
Reticular groove closure is a reflex action (brainstem impulses arrive through the vagus nerve)
Afferent stimuli arise from centrally (anticipation of suckling -> cephalic phase) and from the pharynx (suckling)
When stimulated, muscles of the groove contract cause it to twist -> lips of the groove close together forming a tube from the cardia to the omasal canal
Ruminant Ketosis
Ruminant ketosis (and associated hypoglycemia) occur most frequently in high-producing dairy cows (usually within 6 weeks after calving or in late gestation)
Acetate and butyrate enter the krebs cycle as follows: acetate, butyrate –> ACoA –> citrate
(provided there is enough oxaloacetate)
If oxaloacetate is not enough or if the amount of ACoA is excessive (excess oxidation of fat), ACoA accumulates as acetoacetyl CoA which is subsequently degradated to
- acetoacetate
- B-hydroxybutyrate
- acetone
Hindgut Fermentation
Horses and rabbits (huge colonic development) rely on fermentation in colon to cover their energy needs
Substrates for hindgut fermentation are structural and nonstructural carbohydrates as well as proteins
Fermentation products are VFA and absorption mechanisms are similar to that observed in ruminants
Hindgut Fermentation: special considerations
Ingesta passage through the stomach and small intestine before its arrival to the large intestine –> exposure to gastric acids and digestive enzymes may increase the digestion rate in the hindgut
Some proteins escape small intestine digestion and absorption and arrive the hindgut
There is an extensive urea recycling into the colon and cecum (similar to that in the rumen)
In contrast to the ruminant, horses do not have a mechanism to recover and utilize microbial proteins and most of them passes out in the feces (cecotrophy in rabbits helps recover microbial proteins)
Conditions for fermentation
Substrate supply Control of pH (buffers) Osmolality Anaerobiosis Retention of fermenting material Removal of wast product and residue
Rumen motility: colon
The predominant motility pattern in the cecum is of mixing nature with low-amplitude contraction that move the ingesta from haustra to haustra
The predominant motility patterns in the ventral colon are haustral segmentations propulsive peristalsis, retropulsive peristalsis
Small particles flow to distal leaving the ventral colon
Big particles retained
Glucose homeostasis in ruminants
Most carbohydrate digestion in ruminants occurs in the forestomach through fermentative digestion –> almost no digestible carbohydrate enters the intestine
All the glucose available to ruminants originates from gluconeogenesis. The most important precursor of glucose in ruminants is the VFA propionate
Propionate enters the Krebs cycle at the level of succinate
Succinate is a 4 carbon intermediate that can lead to the formation of ocalocacetate which is the entry metabolite for gluconeogenesis.
Rumen motility special considerations
Ruminants exist in a constant state of potential glucose deficiency
Gluconeogenesis covers 85-100% of glucose needs
Insulin levels are regulated by the concentration of VFA
All the glucose available to ruminants originates from gluconeogenesis. The most important precursor of glucose in ruminants is the VFA propionate
Glucose homeostasis
Almost all propionate absorbed from the rumen is extracted from the portal blood by the liver and never enters the systemic circulation
Ruminants also efficiently conserve glucose. In ruminants, fatty acids are synthesized only in the adipose tissues using acetate as a precursor molecules and never glucose
In high producing dairy cows nearly all glucose they produce goes to lactose (milk sugar); the remaining tissues function on alternative fuels
