6.1b - Absorptive Mechanisms, Electrolyte and Water Absorption II

Question 1

How does SGLT 1 expression change in the SI?

Answer 1

-decreases with aboral direction

Question 2

SGLT 1 affinity and capacity:

Answer 2

-high affinity
-low capacity

Question 3

What is the role of the distal SI absorptive cell?

Answer 3

-SGLT 1 expression is decreased
-isosmotic absorption becomes important
-slightly tighter junctions than proximal SI
*minimal amount of energy

Question 4

Colon fluid and electrolyte absorption:

Answer 4

-strong Na+ absorption
-‘tighter’ tight junctions
-K+ secretion lowers membrane potential
-K+/H+ ATPase apical
>some K+ absorption in distal colon
-short chain FAs

Question 5

Where are the absorptive cells found in the colon?

Answer 5

-apical portion

Question 6

Role of colon absorptive cell:

Answer 6

-involved in absorption of Na, Cl- and volatile FA/short chain FAs
>with absorption of Na, water follows
*VERY tight junctions
-allows the development of a large osmotic gradient to pull water into the body

Question 7

What drives the absorptive process in the colon?

Answer 7

-3Na/2K ATPase

Question 8

How does sodium enter a colonocyte?

Answer 8

-through epithelial sodium channel
>pumped out the basolateral side creating a large gradient
>pulls water in through aquaporin
*same channel is in distal tubule to pull water

Question 9

What else does the colonocyte use to pull in sodium and water?

Answer 9

-anion exchanger
-sodium hydrogen exchanger
*more important than in the SI

Question 10

Where does the energy, HCO3- and H+ that drive the processes in the colon come from?

Answer 10

-metabolism of short chain FAs produced by digestion/fermentation of fibre

Question 11

What do short chain FAs stimulate in the colonocyte?

Answer 11

-electroneutral uptake of Na+
>by acidification of colonocytes and activation of Na/H exchangers

Question 12

What stimulates Cl- absorption in the colonocyte?

Answer 12

-increased HCO3- produced during short chain FAs metabolism and stimulation of Cl/HCO3 exchanger

Question 13

What is another of Cl- absorption into the colonocyte that involves the monocarboxylate transporter?

Answer 13

-butyrate is taken up via non-ionic diffusion (exchange SCFA/HCO3-)
-NaCl absorption is activated by parallel Cl-/butyrate and Na/H exchange

Question 14

Why is resistant starch added to oral rehydration?

Answer 14

-colonic bacteria use it to produce SCFA=increases fluid absorption

Question 15

What are the main transporters present in duodenum and upper jejunum? (summary)

Answer 15

-SGLT1
-Na+/AA cotransporter’
-paracellular Cl- absorption (not in upper jejunum)
-water

Question 16

What transporters are present in colon? (summary)

Answer 16

-Na/Cl cotransporter
-Cl-/HCO3- exchange
-paracellular K+ absorption
-slight water

Question 17

Electrolyte movements in GIT of weaning swine:

Answer 17

-used PEG (indigestible/not absorbed) to determine absorption of other nutrients
*as PEG increases, the other nutrients have been absorbed/decreased
*occurs in the colon!
>more gradual in humans, dogs, horses, and cattle

Question 18

Osmolarity in the GIT of weaning swine:

Answer 18

*stomach is hypotonic and then becomes hyperosmotic
-aboral pH increased and then a dip in colon (due to short chain FAs being synthesized)

Question 19

What would presence of an endotoxin which enhances secretion of more fluid do to absorption in the GIT of weaning swine?

Answer 19

-add an excessive quantity of fluid to the distal small and large intestine
*if absorptive capacity of colon is exceed=get diarrhea

Question 20

Water balance of a dog vs. horse?

Answer 20

-dog: largely absorbed from SI and less in colon
-horse: more saliva, more in SI, colon more important
*dog retained 83%
*horse retained 95%

Question 21

What 2 forces are utilized for absorption of water?

Answer 21

1. Osmotic pressure
2. Hydrostatic pressure (from increase in pressure due to water in a confined area

Question 22

How does absorption of water occur in the villous tip?

Answer 22

-countercurrent flow of arteriole and venule
>solute in descending venule diffuses into ascending arteriole to be carried up to the tip

Question 23

Within individual enterocytes: water absorption

Answer 23

-use aquaporin to follow osmotic gradient created by absorption of Na+ and AA into extracellular fluid
*hydrostatic pressure drives water into capillaries which have lower resistance than tight junctions

Question 24

What causes thirst?

Answer 24

-excessive water excretion without intact
*extracellular fluid hyperosmolality

Question 25

Hyperosmolality causes:

Answer 25

-increase in secretion of ADH
-sensation of thirst
*regions of brain have been identified

Question 26

Oral rehydration therapy:

Answer 26

-used extensively in human and vet med
-first introduced in 1960s
>used to treat fluid loss as a result of acute diarrhea
>increase in absorption of Na following glucose administration
*diarrhea-associated hypovolemia second leading cause of death among children below 5 years

Question 27

What are other molecules/solutions that have been investigated for oral rehydration solutions?

Answer 27

-zinc
-probiotics (source of short chain FAs)
-prebiotics
-glucose polymers
-AAs

Question 28

Hyper-osmotic/hypertonic glucose oral rehydration solution: (1980s)

Answer 28

-importance of SGLT 1 glucose mediated active absorption
>usually not significantly affected during diarrheal states

Question 29

Hypo-osmolar/hypotonic oral rehydration solutions (currently):

Answer 29

-reduced fecal fluid loss
-reduced need for IV therapy
-reduced duration of illness

Question 30

Oral rehydration therapy in vet med contains:

Answer 30

-Na+
-K+
-Cl-
-NaHCO3 to help combat metabolic acidosis with diarrhea
-glucose and AA to use Na/glucose and Na/AA transport mechanisms and provide energy
-SCFA and/or probiotics: increase water reabsorption in colon
*typically isotonic (hypertonic if can’t give multiple times a day)

Question 31

Pig vs. calf:

Answer 31

-pig: hypoosmotic as you can give it more times a day
-calf: hyperosmotic as it might only get it 1-2 times a day