L5: Comparative Digestive physiology

What is cellulose

• unbranched polymer of glucose

• residues are joined by beta-1,4 linkage

  • forms microfibrils which are hard to digest

what can break down cellulose?

• No vertebreates

  • do not synthesize cellulase

• rely on cellulase-secreting microbes in some part of the digestive tract to achieve this

What is this process called

anerobic→ fermentation

Types of fermenters

1. Foregut

2. Hind gut

Foregut fermenters examples

• Artiodactyl

  • suborder of Ruminantia

    • cow and sheep

• kangeroos

• sloths

• certain monkeys

• rodents

• Whales

  • use this for chitin

• Hoatzin

  • bird

  • has enlarged stomach

  • cannot fly?

What animals Ruminantia includes

• cows

• sheep

Stomach of cow

Four chambers→ takes up the entire left half of the abdomen

Ingest passes from the oesophagus into both…

1. Reticulum→ does not secrete acid or proteases

   • buffers pH (to help fermentation)

2. rumen

   • incompletely divided and often referred to reticulorumen

   • acts as fermenting vat

3. Omasum→ 100 internal foldings increase the surface area for absorption

   • some absorption here

4. Abomasum→ resembles true stomach of other mammals like humans

   • secretes acid and kills and begins digestion of the microbes

   • acid secretion

   • pepsin secretion

What are the rumen ,reticulum and omasum all lined with

• stratified, squamous epithelia

• similar to skin

Rumen: Characteristics (with some comparisons to humans)

• 60-300 litres in volume 85% of stomach volume (vs 1.5 litres in cow)

• Anaerobic

• 37-39 degrees

• pH 5.5 to 6.8

• 10^10 bacteria

  • human→ 10^12 intestines so cow is more dilute than humans

  • include: Fibrobacter and Ruminococcus species

• 10^6 ciliated protozoa

• 10^4 anearobic fungi /ml

• some archae too

note: relative proportions of these microbes vary with diet

Why is the reticulorumen slightly acidic if it doesn’t secrete acid

• due to fermentation products of the bacteria

• but cow continuously tries to buffer this so does not kill the micrboes

Layers of the reticulorumen

Layers separate due to density

1. Gas bubble

2. coarse, fibrous material floating on top of the liquid contents

3. Slurry → suspended particule

4. liquid virtually free of particles

What provides the liquid medium for these layesr

• ruminants secrete copious saliva

• 250L of saliva vs 1L in humans

What does the saliva provide

1. Liquid medium

2. Bicarbonate→ buffering

3. Urea→ to fertilise the microbes

Is the reitculorumen passive?

No

• active cycles of contractions that pass continuously

• to circulate and mix the contents

Two types of contractions and their use

1. Primary contractions→ mixing sequence

2. Secondary contractions→ movement of ruminal gas to the oesophagus

   • typically follows every second primary contraction

   • leads to eructation→ belching

1. Primary contractions what augmented by and how they work

Augmentation

• vagovagal reflexes

  • triggered by stretch, touch or chemosensory afferents

How they work

1. Wave starts with double reticular contraction (1, 1’)

2. then sequence of contractions around rumen (2,3, 4,5)

3. mixes the contents in a circular motion

   • Similar to normal mammalian stomach (humans)

see the data that has been recorded from the different pressure in sifferent parts

How does digesta move to the next chamber (omasum)

1. Reticular contractions, small amounts of digesta forced into omazum

2. via reticulo-omasal orifice

3. sucking action when the muscular omasum relaxes contributes to this

Ruminal half-life

Depends on the coarseness of food

1. Solid→ 30-50 hours

2. 15-20 hours for liquid contnets

Are movements of the abomasum tied to the cycles of the other stomachs?

No

What is rumination

Chewing the cud

• regurigitation of the cud

• and chewing and digesting this again

How long does this last for

• up to 10 hours per day

• depending on the nature of the food

How does rumination happen

1. Thoracic cavity is expanded with glottis closed

2. decreasing the thoracic pressure while the reticulum contracts

3. lower oesphageal sphincter is relaxed

4. some of the ruminal contents are sucked into the oesophagus

5. reverse peristalsis takes over→ delivers a bolus to the mouth

6. Remastication

similar to vomiting

How is rumination different to vomiting

• effortless

• non-painful

• involuntary

• not really digested yet

Regulation in rumiannts vs monoastric species

Monogastric

• modulatory role on the stomach

Ruminant

• ruminant cyclic movements, rumination and eructation

  • completely abolished by vagotomy

  • some, erratic, instricically driven motility reappears later

Cellulose fermentation by bacteria: why extracellular?

• too big to be taken up into the cell

• bacteria cannot do endocytosis

How do bacteria digest and metabolise cellulse (celluloase fermentation)

1. extracellular breakdown by secreting cellulase

2. microbes rapidly absorb the monosaccharides

3. break them down to pyruvate (glycosis)

4. further reduce pyruvate anearbocially

   • WHY: regenerate NAD+ to allow glycolysis to continue to make more ATP

     • similar to how we use lactate

5. RESULT: makes volatile fatty acids (VFAs, aka short chain fatty acids) as waste

   1. acetate

   2. propionate

   3. butyrate

6. MICROBES CANNOT USE THESE but can be taken up and used by the cow (Symbiosis)

One way NAD+ can be regenerated

Via formation of hydrogen:

• NADH + H+ → NAD+ + H2

What is this H2 then used for

• 4H2 + CO2 → CH4 + 2H2O

by methanogenic archaeons

Methanobrevibacter ruminantium

Reduce CO2 to methane

Ruminal gas composition and how much per day

Reactions make a considerable amount of ruminal gas

• 150-200 L per day→ average cow

• 500 L per day→ lactating

1. 50% CO2→ from the bicarbonate buffering of VFA

2. 35% CH4

3. 7% N2

4. 5% H2

5. Trace O2

What are the N2 and O2 from

• swallowed air

Issue with making methane

• greenhouse gas

Way to decrease greenhouse gas

• fed a methanogenic archae inhibitor

• stops methan production

Why else is methane production a disadvanatge (to the cow)

• represents significant loss of energy

• 10% of energy loss of what is ingested

What can happen if too much gas and waht caued by

Bloat

Caused by

• plants that cause foaming (clover)

• Rumen becoming too acidic→ stops motility→ no belching→ bloat

VFA produced in what proportions

1. Acetate→ 65%

2. Propionate→ 25%

3. Butyrate→ 10%

Where are VFAs absorbed

• wall of rumen and reticulum

note: no absorption of stuff in the human stomach

ruminant host is then able to metabolise these aerobically so can use them for energy

What challenges are faced when VTA are being absorbed

1. Low SA of the rumen/reticulum

2. Stratified squamous epithelium is like skin (unlike what is seen in the rumen)→ difficult to get through

1. Solution

Increasing SA

• Reticulum→ hexagonal ridges

• Rumen→ protrusions

2. Solution

• transporters responsible are localised on the different cell layers

• cells connected by gap junctions to form syncytium

• rich subepithelial capillary plexus to facilitate removal of absorbed substances

not really an optimal strategy

Fats of absorbed products: what generally used for

• supply 60-80% of the daily energy requirements of the ruminant

1. Acetate

• Respired directly by many tissues

• also converted into fatty acids by mammary or adipose tissue

2. Propionate

• passes in the portal blood to the liver

  • major substrate for gluconeogenesis

• note: short-chain fatty acids can be made into glucose (unlike in long chain)

• or made into fat

3. Butyrate

• converted to ketone body beta-hydroxybutyrate

  • in its passage across the rumen wall

• Used as a source of energy by many tissues

How is this VFA use similar to humans?

Similar to how humans get energy in starvation

• humans→ up to 10% of energy like this from starvation→ ketosis

  1. Fat oxidation for energy

  2. Gluconeogenesis

  3. Ketone body production

human fasting metbolism

• but note the cow gets 80% of energy from this

Blood glucose level in ruminants vs humans

• only half that in humans

What is glucose still needed for in cows

e.g

• high-producing dairy cows suspectible to lactation ketosis

Protein metabolism in cows

1. Urea enters the rumen both directly across wall and in saliva

2. microbs in rumen hydrolyze it to produce ammonia

3. incorporate into their own protein

4. ammonia not used reabsorbed by host

5. reconverted to urea in the liver

6. The host then digests the microbes when get to abomasum

7. amino acids are absorbed in the small intestine

What does this nitrogen cycle allow

• less nitrogen is lost in the urine as urea

Why are these micrboes good for protein metabolism

• can synthesise all the amino acids needed by the host

  • so not really an ‘essential’ amino acids

note: but they still may not be made at a high enough rate

• so dietary supplements may need to be required

• e.g in high-yielding dairy cows

Fat and vitamins: What happens as fat is eaten

1. Eaten: only polyunsaturated fats

2. Hydrolysed in rumen by microbial lipases

3. glycerol product is fermented to propionate

4. fatty acids produced are hydrogenated by bacteria

5. so become saturated

6. These are long chain fatty acids→ (so unlike STFA)

7. go on to be emulsified

8. absorbed in the small intestine

How this has been used to make spreadable butter

• fed pellet with oil

• this is protected from micrboes

• released into the milk

• unsaturated to makes the butter less solid

Where do the vitamins come from

• syntehesize K and B-group vitamins

• animals themselves can synthesise their own C and D

Hind gut fermenters: what animals are included

1. Perissodactyl

   • odd toed undulates

2. Pigs

3. elephants

4. sea-cows

5. hyraxes

6. Lagomorphs

   1. rabbits

What parts of the hind gut fementers are enlarged

• Caecum

• Ascending colon (proximal)

note: this embryologically is the midgut

In horses and rabbits

• provides a substantial contribution to daily energy intake→ 60-80%

• other animals→ provide a modest amount of energy

  • also seen in cows

  • generally just everywhere

  • human→ 10%

Horse digestive system features

Most fermentaion in

1. Enlarged caecum→ 30 L capacity

2. Expanded proximal colon→ ‘large colon’→ >60L

Smaller parts

• distal ‘small colon’ is narrower→ used for absorption of e.g VFAs

Digestive tract comparison with cow and human

Rabbit: what else do they do to increase absorption

Coprophagy→ eat feaces (but not really feaces) ceacal pellets

1. urea enters the large intestine from blood

2. can be convreted into microbial protein

3. BUT has no means to break this down so often lost in faces

4. THEREFORE reingest microbial protein (that would otherwise be lost because oly found in the hind gut (cannot be digested by the stomach because already passed)

5. remain as nodules in the stomach for a while

6. have a phosphate buffered, strong mucus envelope

   • protects the bacteria from stomach acid

7. allows to continue to ferment for 6 hours in fundus

8. eventually broken down

9. broken down the small intestine

10. improves nitrogen use efficiency

mainly in small animals

What happens to coarser fibres

coarser fibres are lost in hard pellets which are not eaten

• how this fibre separation is achieved is unclear

Koala digestive system

• 2.5 m caecum→ very long

How to kaola acquire mirboes for special eucalptus tree break down

• eat mother ‘pap’ feaces

What kind of digestive system should you have?

• Comparing sheep, cows and horse

• should be able to extrapolate these data from foregut and hindgut to see which kind we have:

1. Passage of dried straw markers through the gut

2. Digestion of crude fibre in various foodstuffs

1. Passage of dried straw markers through the gut

note: straw is low nutrition while Hay is high nutriion and green

Result: Hind gut passes it out quicker

2. Digestion of crude fibre in various food stuffs

Result: rumen does better job of breaking fibre down

Advatnage of foregut

1. more complete digestion of fibre

2. Food can be bolted and stored for later mastication

3. toxins can be dealt with by microbes rather than the host

4. Nitrogen recycling ore efificent

   • because micrboical protein can be digested throughly in the small intestine

Advantages of hindgut fermentation

1. Easily digestible food processed directly

   • good if you want the energy now

   • more active species?

     • compared to foregt fermenters

       • have to first be processed by micrboes and then micrboes digested

       • so more stages for loss of energy at each stage

       • esp loss from methane

2. transit time quicker: more food can be ingested

3. (Coprophagy) gets over the nitrogen recyling inefficiency of uding the hindgut

What did Alexnader conclude from mathematical models of the digestive tract (ie. what fermentr for what food eaten)

1. If veggie with high digestible content can be process fast enough→ no fermentation chamber is necessary

2. If food quality is intermediate→ hind gut fermentation

3. If food quality poor→ large foregut fermentation chamber is needed

4. Coprophagy is much more advantageous in hindgut than foregut

   • particulalry if the fermentation chamber is small and the animal feed on low-quality, fibrous food

     • which is incompletely fermented first time around

Wouldn’t it ben better to eat fresh food than caecal pellets to get more out of it??

• idk maybe coz you get more micrboes that have grow ndue to the aas they made for themselves?

Which mammal adopts this strategy?

Panda

• carnivore GI tract

• but eat high quality bamboo

Essay questions

1. Discuss the relative merits of foregut and hind-gut fermentation.

2. Compare and contrast carbohydrate digestion in a cow and in a human.

3. Compare and contrast the metabolic strategies of a cow and a human.