Lecture 30: Intestinal Transport of Electrolytes and Water Flashcards
What are the 3 ways for water intake
Food, drink and metabolic water
What are the 5 ways to lose water
Skin, lungs, GI tract, kidneys and milk
What are the enterosystemic sources of fluid
- Oral intake
- Saliva
- Secretion form stomach, pancreas, liver, SI and LI
What is insensible evaporation
Humidification of breathing
What is a normal fluid balance important for intestinal function
Creates an appropriate environment for digestion, fermentation, and absorption, stabilizes pH
What % of fluid is recovered in small and large intestine
98%
In dogs and cats, what % of fluid is reabsorbed in small intestine
80%
In dogs and cats, what % of fluid is reabsorbed in large intestine
18%
Colonic fluid provides __
Reserve capacity
What portion of the LI has a high absorptive capacity
Colon
___is the site for control of enterosystemic fluid balance
Epithelium
At what area of the epithelium does secretion occur
Crypts of small and large intestine
At what area of the epithelium does absorption occur
Villi of small intestine
What activates secretory mechanisms
Meals/food
What compensatory mechanism occur in order to increase uptake or excrete solutes and water
Expression of intestinal/epithelial transporters which can match the need
What does the SI absorbed
Water, Na+, Cl-, K+
What does the SI secrete
HCO3-
What does the LI absorb
Water, Na+, Cl-
What does LI secrete
K+ and HCO3-
T or F: SI fluid absorption is isomolar
True
What causes an increase in water absorption in GI
Increase in luminal osmotic pressure resulting from an influx and digestion of food
What ion facilitates the absorption of Na+ and water in the GI
Cl-
__stimulation of the luminal surface triggers chloride secretion
Mechanical
___plexus is involved in mediating the secretory response
Submucosal
Stroking of the luminal surface releases ___ from unknown sources and causes __ secretion
Prostaglandins, and Cl-
Stroking of the musical surface also causes ___ release from enterochromaffin cells
Serotonin
Serotonin activates 5-HT1P/5-HT4 receptors containing substance P, ACh, glutamate. Release of these neurotransmitters activates release of ACh or VIP and cause __secretion
Cl-
What receptor is Cl- secretion linked to
CFTR
What are the two mechanisms in which CFTR can be activated
- Food binds receptors on apical membrane
- Receptors on basolateral membrane activated and increase Ca2+ and cAMP and pKA which phosphorylates CFTR channel and transport Cl- into intestinal lumen
What are some agonists that can activate basolateral K channel and CFTR
ACh, histamine, bile acids
What channel on basolateral membrane supplies cell with Cl-
NKCC1
How does V. Cholera toxin activate CFTR channel
- Toxin is released from bacteria
- Toxin activates G-protein and then AC
- CAMP increase and opens CFTR
How does V. Cholera cause diarrhea
When it activates CFTR channel and increase Cl- secretion into lumen it draws Na+ and water into lumen resulting in diarrhea
How does the chyme remain isosmotic throughout passage throughout SI and LI
Balance between water secretion and absorption
How does water cross epithelium in GI
Passes through tight junctions
What two secondary active co-transporters are most important for fluid reuptake in GI
- Na+/glucose
- Na+/Amino acids
How does oral rehydration therapy work
Utilizes SGLT1 channel to drive H20 reabsorption
How do you activate the colonic reserve capacity
Stimulate ENaC channel in CD
How is ENaC activated
- Aldosterone activates Na/K pump
- Triggers insertion of preformed Na+ channels
- Triggers expression of new channel proteins
__ is responsible for Na+ uptake from luminal fluid
ENaC
___ alters the rate of insertion, degradation and recycling of ENaC
Aldosterone
Describe the pathway of activating the RAAS system
- Decrease pressure/blood loss
- Angiotensinogen
- AI
- AII
- Aldosterone
- Kidney reabsorb Na+ and water
- Colon reabsorbs Na+ and Cl- via ENac
How does amiloride work
Potassium sparing diuretic that blocks ENaC and inhibits Na+ absorption in distal colon of GI, promotes loss of Na+ and H20
Describe K+ absorption in the small intestine
Passive via solvent drag, uptake H20 take K+ with it
Describe K+ absorption in distal colon
Active K+ absorption based on pH. Increase H+ ions, decrease pH causes active K+ uptake
What transporter recovers K+ in the distal colon if diet K+ is low
H/K+ ATPase
What happens to K+ absorption if water is lost (ex: diarrhea)
Can’t build up gradient and can lead to hypokalemia
what are the 2 mechanisms Cl- is absorbed in GI tract
- Voltage gated dependent- passive paracellular and transcellular Cl- absorption drive by SGLT or ENac
- Electroneutral Cl-HCO3- exchanger
Where is the Cl-HCO3- present in GI
Epithelial cells of villi and surface cells of ileum and proximal colon
Why is the Cl/HCO3- exchanger important for hindgut
Net movement of bicarbonate into the gut elevates the pH which is important for hindgut fermentation
Tightly linked to acid base homeostasis
What triggers Na+/H+ exchanger
High luminal pH
What causes the luminal pH to be high and then trigger Na/H+ exchanger
Secretin is stimulated in the SI from the arrival of acidic material from stomach, increase secretion of HCO3- in ductal cells, increasing pH and triggering Na/H+ exchanger
Describe the electroneutral absorption of Na and Cl-
The Cl-/HCO3- exchanger stimulated ton combat low pH from stomach acid by releasing bicarb into lumen and in response Na/H exchanger is activated to combat high luminal pH
Why can diarrhea occur after surgical resection of large part of the ileum
- Bile salts and acids spill over into the colon because transporters in ileum reduced and maxed out.
- Bile acid build up stimulate CFTR in colon which stimulates motility
- Isosmotic absorption of h20 is compromised and decreased NACl- is decreased so resulting in diarrhea
