L29: Hindgut Fermentative Digestion Flashcards
almost all vertebrates facilitate bacterial digestion ______ auto-enzymatic digestion, whereas ruminants digest fiber _____ to glandular/autoenzymatic digestion
after
before
what is the purpose of foregut fermenters digesting fiber prior to auto-enzymatic digestion
- allows for use of bacterial biomass
- detox bacteria (ex: E.coli)
- upgrade NPN - but downgrade of high quality protein
what are the nutritional advantages of foregut fermentation
- bacterial products are formed prior to enzymatic digestion + absorption so bacterial protein and products (Bvits) can be fully used
- bacteria helps w/ detoxification
- endogenous nitrogen products (urea) can be recycled
how is urea recycled in foregut fermentation
introduction into fermentation chamber where microbes use them to re-digest N as bacterial amino acids
what is the downside of foregut fermentation
- loss of easily digestible substrates to bacterial modification
- possibiliy for mal-fermentation
what is the benefit of hindgut fermentation
allows for more efficient use of substrates that can be digested auto-enzymatically
nutritional advantage of hindgut fermentation
- use of easily digestible substrates
- diet shifts are less of an issue
how can the loss of bacterial protein & products (B vitamins) be avoided in hindgut fermentation
by coprophagy (eating their feces)
what are the 2 classes of hindgut fermenters
cecal & colon fermenters; depends on if they dependent on cecum or colon primarily for microbial digestion
what type of fermenters are rabbits, guinea pigs, chinchillas, rats
cecal
what type of fermenters are large nonruminant herbivores (horses, rhinos, gorillas, elephants)
colon fermenters
the digestive function of the stomach and SI of horses is similar to that of ?
monogastric animals
what happens in the stomach and SI of the horse
- dietary protein is digested and absorbed as amino acids and di/tripeptides
- soluble CHOs get hydrolyzed and absorbed as monosaccharides
- lipids digest into micelles
hindgut fermentation requires what 2 main functions
- water flow - water must be recovered from gut content along w/ electrolytes and buffers (HCO3 & P)
- optimal conditions needed for microbial growth / VFA production + abs
where is most water absorbed in hindgut fermenters and how does it happen
cecum; water is reabsorbed into ECF & blood by following VFA absorption
how is water secreted in the LI
Cl- moves into the cell via the Na/K/Cl- cotransporter
Na and water passively follow Cl- secretion into the lumen by the apical chloride channel (CFTR)
Na/K pump maintains the electrochemical gradient
what is the function of CFTR
- transports Cl- and other negatively charged ions from inside the cell to outside the cell
- this builds a negative charge outside of the cell so that positively charged Na ions follow the Cl- out by passive diffusion
(the presence of food activates this signaling pathway)
what is the major anion in the large intestine
VFAs; water follows VFA absorption
how is water absorbed from LI
- water follows VFAs
- Na/H exchanger and Cl-/HCO3 exchanger - water follows NaCl absorption
- water is absorbed by the paracellular pathway
- water also abs by apical and basolateral AQ channels
what are the optimal conditions for microbial growth / VFA production + abs
- substrate (CHOs, protein (and NPN)
- pH, osmolality, anaerobiosis
- removal of waste
- space and time (retention of fermenting material, mixing)
how is pH maintained
secretions of HCO3 and phosphate in the ileum & by removal of waste/abs of VFAs and lactate
what is the problem with large amounts of lactic acid
can cause intraluminal acidosis & increased intraluminal osmolality
what is the are main ‘waste’ pdts of fermentation and where are they absorbed or excreted
- VFAs (acetic, propionic, butyric acid); absorbed in cecum and colon
- non-digestible material/waste is removed in feces
in what form can VFAs be absorbed in
protonated (ionized) or dissociated
ionized VFAs: absorbed by VFA-HCO3 antiporter
undissociated VFAs: lipophilic diffusion
VFA and Na absorption leads to _____ absorption of water
osmotic
why is there 2 methods of VFA absorption
b/c the colon has variable pH
pH > pKa favors ionized VFAs
ph = pKa favors protonated VFAs
(pKa of VFAs = 4.8)
what is selective retention and where does it occur
selective retention is the separation of fermenting material based on particle size (separation of fully fermented material (smaller) from fresh material (larger)
occurs in cecum and colon
what are the 3 locations where selective retention occurs
- cecal constriction that forces ingesta into the ventral colon
- flexura pelvina
- right dorsal colon
major site of Large Intestinal fermentation in ruminants, rodents and rabbits; is partially filled w/ watery ingesta and has a gas filled dorsal portion
cecum
cecal motility / constriction
contractions begin in cecal body near the apex & move towards the base creating a pressure gradient that forms a consitrction before at the opening of the cecocoli orific
constriction of cecal base forces materal through the cecocolic orifice into the colon
flexure that facilitates selective retention of larger particles
flexura pelvina between left ventral and left dorsal colon
location of selective retention where sand colic can occur
right dorsal colon; selective retention of fine particles and fluid
what section of the colon undergoes antiperistalsis
left ventral colon
peristalsis in the ventral colon
colon diameter changes from wide (left ventral) to narrow (left dorsal), causing a squeezing action at the pelvic flexure so that only small particles and liquids can pass
what is responsible for the mixing of ingesta in the ventral colon
- haustral segmentation mixes ingesta, increasing retention time which increases time for fermentation and VFA absorption
- antiperistalsis of left ventral colon
- propulsive peristalsis
what determines retention time of food particles
their size; larger particles take longer than smaller; fluids pass much faster than food
why is grain overfeeding an issue in horses
the digestive + absorptive capacity of the stomach and SI is exceeded so the material doesn’t get broken down thoroughly before reaching the LI
fermentation produces gases that can’t be eliminated by eructation, large amounts of gas interrupt motility and distend the GI tract = tympany, causing bloat and colic
tympany (distension of GI tract) of the LI causes what symptoms
- abdominal discomfort and colic (excess gas)
- GI hypoperfusion (blood vessels get compressed)
- respiratory distress (tympany pushes against diaphragm, reducing respiratory movements)
why does excess grain consumption in equine cause lowered pH and what is the consequence
excess VFAs cause low pH which promote lactic acid production but an increase in lactate-utilizing microbes takes time
lower pH destroys mucosal barrier of cecum/colon causing damage and ulcerations, allowing for the passage of endotoxins
what type of bacteria release endotoxins
gram - bacteria release endotoxins such as lipopolysaccharides which enter circulation and trigger an inflammatory response
