L30: Intestinal Transport of Electrolytes & water

Question 1

where is water lost

Answer 1

• skin (sweat)
• lungs
• GI tract (urine, feces)
• kidneys
• milk

Question 2

what part of the GI tract does a majority of water reabsorption

Answer 2

MOSTLY the small and large intestine, followed by the colon which has a high absorptive capacity; provides “reserve capacity”

Question 3

what is the site for control of enterosystemic fluid balance

Answer 3

epithelium, where abs and secretion occur simultaneously
secretion occurs at the crypts of the small & large intestine
absorption occurs in epithelial cells of villi in the SI

Question 4

the secretory mechanisms for fluid in the epithelium are activated in response to…?

Answer 4

a meal
they change in response to changes of diet

the submucosal plexus is involved in mediating the secretory response

Question 5

why is water so important in the kidneys

Answer 5

needed to be able to eliminate urea which is water soluble

Question 6

what are the sources of fluids

Answer 6

• oral intake
• saliva
• secretions from stomach, pancreas, liver, small and large intestine

Question 7

where is fluid absorption isomolar

Answer 7

SI

Question 8

what plexus is involved in mediating the secretory response of the luminal surface in response to the passage of food

Answer 8

Submucosal plexus

Question 9

what is the effect of mechanical stimulation of the luminal surface

Answer 9

• release of prostaglandins
• Cl- secretion by acting on secretomotor neurons that release VIP
• release of serotonin (5HT) from enterochromaffin cells

Question 10

where are chloride secretion mechanisms located along the GI tract

Answer 10

the entire way
cystic fibrosis transmembrane conductance regulator or CTFR

Question 11

Calcium and cAMP both activate what ion channel

Answer 11

CFTR

Question 12

how do increasing cAMP levels increase Cl- secretion

Answer 12

cAMP activates PKA which phosphorylates CFTR causing Cl- secretion down electrochemical gradient

Question 13

what agonists activate the basolateral K+ and CFTR channels?

Answer 13

ACh, histamine, bile acids

Question 14

what molecules can stimulate PKA

Answer 14

VIP or PGE2

Question 15

what transporter supplies the cell with Cl-

Answer 15

Na-K-Cl- channel on basolateral membrane

Question 16

how does cholera toxin activate CFTR chloride channel

Answer 16

toxin activates intracellular Gprotein, active Gprotein activates adenylyl cyclase which increases cAMP levels; high cAMP levels open CFTR channel: draws Na+H2O into lumen –> Diarrhea

can do this even in the absence of food!

Question 17

water diffuses across epithelium via ?

Answer 17

tight junctions; water follows absorbed substances into blood

Question 18

most important mechanism for fluid reuptake in the GI tract

Answer 18

Na/glucose (SGLT-1) & Na/amino acid co-transporters
(secondary active)

Question 19

how do rehydration salts work

Answer 19

make use of secondary active Na/Glucose transport (SGLT-1) in small intestine

Question 20

what channel is responsible for Na uptake from luminal fluid

Answer 20

ENaC; modulated by Aldosterone

Question 21

aldosterone antagonist that blocks ENaC & inhibits Na absorption in the distal colon & kidney

Answer 21

Amiloride; promotes loss of Na and water

Question 22

where is K+ absorption passive, where is it active?

Answer 22

passive: in SI by solvent drag
active: in distal colon by K/H+ ATPase

Question 23

where is the Cl-/HCO3 exchanger present (and coupled with a Na/H exchanger)

Answer 23

epithelial cells of villi & surface cells of ileum and proximal colon

Question 24

how is Cl- mostly absorbed

Answer 24

paracellularly
1. voltage dependent Cl- absorption driven by electrochemical gradient
2. Cl-/HCO3 exchange (esp in LI)

Question 25

what triggers the Na/H exchanger

Answer 25

high luminal pH caused by secretin