L29: Hindgut Fermentative Digestion

Question 1

almost all vertebrates facilitate bacterial digestion ______ auto-enzymatic digestion, whereas ruminants digest fiber _____ to glandular/autoenzymatic digestion

Answer 1

after
before

Question 2

what is the purpose of foregut fermenters digesting fiber prior to auto-enzymatic digestion

Answer 2

• allows for use of bacterial biomass
• detox bacteria (ex: E.coli)
• upgrade NPN - but downgrade of high quality protein

Question 3

what are the nutritional advantages of foregut fermentation

Answer 3

• 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

Question 4

how is urea recycled in foregut fermentation

Answer 4

introduction into fermentation chamber where microbes use them to re-digest N as bacterial amino acids

Question 5

what is the downside of foregut fermentation

Answer 5

• loss of easily digestible substrates to bacterial modification
• possibiliy for mal-fermentation

Question 6

what is the benefit of hindgut fermentation

Answer 6

allows for more efficient use of substrates that can be digested auto-enzymatically

Question 7

nutritional advantage of hindgut fermentation

Answer 7

• use of easily digestible substrates
• diet shifts are less of an issue

Question 8

how can the loss of bacterial protein & products (B vitamins) be avoided in hindgut fermentation

Answer 8

by coprophagy (eating their feces)

Question 9

what are the 2 classes of hindgut fermenters

Answer 9

cecal & colon fermenters; depends on if they dependent on cecum or colon primarily for microbial digestion

Question 10

what type of fermenters are rabbits, guinea pigs, chinchillas, rats

Answer 10

cecal

Question 11

what type of fermenters are large nonruminant herbivores (horses, rhinos, gorillas, elephants)

Answer 11

colon fermenters

Question 12

the digestive function of the stomach and SI of horses is similar to that of ?

Answer 12

monogastric animals

Question 13

what happens in the stomach and SI of the horse

Answer 13

• dietary protein is digested and absorbed as amino acids and di/tripeptides
• soluble CHOs get hydrolyzed and absorbed as monosaccharides
• lipids digest into micelles

Question 14

hindgut fermentation requires what 2 main functions

Answer 14

• 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

Question 15

where is most water absorbed in hindgut fermenters and how does it happen

Answer 15

cecum; water is reabsorbed into ECF & blood by following VFA absorption

Question 16

how is water secreted in the LI

Answer 16

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

Question 17

what is the function of CFTR

Answer 17

• 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)

Question 18

what is the major anion in the large intestine

Answer 18

VFAs; water follows VFA absorption

Question 19

how is water absorbed from LI

Answer 19

• 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

Question 20

what are the optimal conditions for microbial growth / VFA production + abs

Answer 20

• substrate (CHOs, protein (and NPN)
• pH, osmolality, anaerobiosis
• removal of waste
• space and time (retention of fermenting material, mixing)

Question 21

how is pH maintained

Answer 21

secretions of HCO3 and phosphate in the ileum & by removal of waste/abs of VFAs and lactate

Question 22

what is the problem with large amounts of lactic acid

Answer 22

can cause intraluminal acidosis & increased intraluminal osmolality

Question 23

what is the are main ‘waste’ pdts of fermentation and where are they absorbed or excreted

Answer 23

• VFAs (acetic, propionic, butyric acid); absorbed in cecum and colon
• non-digestible material/waste is removed in feces

Question 24

in what form can VFAs be absorbed in

Answer 24

protonated (ionized) or dissociated
ionized VFAs: absorbed by VFA-HCO3 antiporter
undissociated VFAs: lipophilic diffusion

Question 25

VFA and Na absorption leads to _____ absorption of water

Answer 25

osmotic

Question 26

why is there 2 methods of VFA absorption

Answer 26

b/c the colon has variable pH pH > pKa favors ionized VFAs ph = pKa favors protonated VFAs (pKa of VFAs = 4.8)

Question 27

what is selective retention and where does it occur

Answer 27

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

Question 28

what are the 3 locations where selective retention occurs

Answer 28

* cecal constriction that forces ingesta into the ventral colon * flexura pelvina * right dorsal colon

Question 29

major site of Large Intestinal fermentation in ruminants, rodents and rabbits; is partially filled w/ watery ingesta and has a gas filled dorsal portion

Answer 29

cecum

Question 30

cecal motility / constriction

Answer 30

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

Question 31

flexure that facilitates selective retention of larger particles

Answer 31

flexura pelvina between left ventral and left dorsal colon

Question 32

location of selective retention where sand colic can occur

Answer 32

right dorsal colon; selective retention of fine particles and fluid

Question 33

what section of the colon undergoes antiperistalsis

Answer 33

left ventral colon

Question 34

peristalsis in the ventral colon

Answer 34

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

Question 35

what is responsible for the mixing of ingesta in the ventral colon

Answer 35

* **haustral segmentation** mixes ingesta, increasing retention time which increases time for fermentation and VFA absorption * **antiperistalsis of left ventral colon** * **propulsive peristalsis**

Question 36

what determines retention time of food particles

Answer 36

their size; larger particles take longer than smaller; fluids pass much faster than food

Question 37

why is grain overfeeding an issue in horses

Answer 37

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**

Question 38

tympany (distension of GI tract) of the LI causes what symptoms

Answer 38

* abdominal discomfort and colic (excess gas) * GI hypoperfusion (blood vessels get compressed) * respiratory distress (tympany pushes against diaphragm, reducing respiratory movements)

Question 39

why does excess grain consumption in equine cause lowered pH and what is the consequence

Answer 39

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

Question 40

what type of bacteria release endotoxins

Answer 40

gram - bacteria release endotoxins such as lipopolysaccharides which enter circulation and trigger an inflammatory response