l9: GI salt and water transport

Question 1

what does the jejunum reabsorb

Answer 1

Na+ cl- H20 & K+

Question 2

What does the ileum reabsorb and secrete

Answer 2

Na H20 cl k

Secretes- bicarbonate

Question 3

What in total does the colon secrete and absorb

Answer 3

Secretes- k+ and bicarbonate

Absorbs- Na cl and H20

Question 4

Describe enterocytes

Answer 4

columnar epithelial cells in small intestine
Polarised
Apical side- faces lumen; has microfolds
Basal side: Communicates with bloodstream & lymphatic lacteals
Seperated by tight junctions

Question 5

function of tight junctions

Answer 5

Allow passage of water &

Restrict passive flow of solutes after secretion or absorption

Question 6

What are the 2 types of epithelial transport & describe each

Answer 6

Paracellular: Forms regulated and restrictive Permeation in both directions through tight junctions: uses electrochemical gradients to move water & solute

Transcellular: Employs membrane transporters to move H20 & molecules through cell-active process
Sets up gradient, via solute movement, for passive transport of H20 & solutes.

Question 7

What are the 3 types of transepithelial channels

Answer 7

Channels- gated and ion specific, via electrochemical gradient

Carrier -

pumps

Question 8

What are the 3 types of transport, describe each

Answer 8

Passive
active - Primary-uses ATP, against electrochem gradient: ie Na-ATPase channel
Secondary- Cotransport- Na-glucose transporter

solvents drag-water leaks from lumen, through paracellular space to reach osmotic equilibrium on basolateral side. -upper small intestine
- moves/pulls additional solutes

Question 9

For all concepts look at the diagrams from slides 36 to 41

Describe concept 1

Answer 9

NA/K ATPase- Basolateral active transport. Creates Na electrochemical gradient, between enterocyte and lumen

Question 10

Concept 2

Answer 10

Na coupled transport-AA or glucose-Apical
the NA gradient from C1 allows coupled Na transport from lumen.
secondary transport- couples uphill movement of AA or glucose to downhill movement of NA, helping to set up Na/K-ATPase
Process is electrogenic as lumen becomes more negative

Question 11

Concept 3

Answer 11

Apical side: Separate NA and cl antiporters.

Na-H exchanger + HCO3-Cl exchanger, creating NACL

Question 12

Concept 4

Answer 12

Apical and Basolateral: CL- secretion occurs in conjunction with basolateral NA K CL absorption
Na-k-ATPase drives NA gradient allowing Apical Cl- secretion through CFTR channel and coupled import of k cl and na from basolateral
Due to high chloride conc CFTR widely expressed in colon

Question 13

concept 5

Answer 13

Water will travel through intercellular tight junctions in the setting of NACL absorption
from lumen to blood

Question 14

H20 Absorption in the small intestine

Answer 14

Basolateral Na/k-ATPase pumps
Leads to build up of NACL in paracellular spaces between enterocytes
Provides osmotic gradient that draws water into paracelluar spaces

Question 15

Oral rehydration therapy

Answer 15

Utilises mechanism of glucose-coupled NA absorption
oral rehydration solutions to promote fluid absorption by coupling Na with glucose in solution
Channel SGT-1 binds 2 Na molecules to 1 glucose, transporting them into cell.

Question 16

How do GI cells secrete water

Answer 16

In small intestine cl- secretion draws Na and H20 across tight junctions

Question 17

How does the jejunum absorb water

Answer 17

Solvent drag

Highest NA absorption coupled with nutrient absorption

Question 18

How does the ileum absorb water

Answer 18

Highest NACL absorption

Question 19

How does the colon absorb water

Answer 19

Apical NA and k

Aldosterone increases synthesis of Na channels and increased k secretion

Question 20

What are the effects of the cholera toxin

Answer 20

Stimulates excess cAMP in crypt cells 
cAMP activates CFTR channel 
therefore massive influx of cl-
Na follows forming NaCL
Followed by H20 secretion, more than can be reabsorbed 
Result: H20 and electrolyte loss-fatal

Question 21

Large Intestine: regulated by what hormone

Answer 21

Aldosterone

Question 22

What is the function of secreting cl- and HCO3- into lumen

Answer 22

Buffers for acid produced by bacteria

Question 23

what are the Na channels called

Answer 23

Basolateral-NHE1
Apical: NHE 2&3
For Na/H exchanger

Question 24

Na+ and H+ exchangers coupled with cl-/HCO3- result in

Answer 24

NaCl absorption

Question 25

CL- through

Answer 25

Lc16 channel

CL/HCO3

Question 26

What systems interact with the regulation of ion transport and secretion

Answer 26

Hormonal-enteroendocrine cells- Endocrine system
Immune cell- Derived factors-Immune system
Bioactive substances secreted into lumen- neurotransmitters -Enteric Nervous system

Question 27

Pathophysiological changes of transepithelial transport

Answer 27

Congenital-genetic
Acquired- via infection, inflammation, hormonal abnormalties

1.Stim of net fluid & electrolyte secretion
increased propulsive muscle contraction
Mucosal destruction & increased permeability
Nutrient malabsorption

Question 28

Gastrointestinal infections

Answer 28

Diarrhoea- bacterial enterotoxin- cholera, ecoli

Question 29

How do enterotoxins work?

Answer 29

Cause secretory diarrhoea by interacting with receptors & signal transduction pathways in enterocytes

Question 30

Cystic fibrosis

Answer 30

Autosomal Recessive disease
Deletion in gene for Transmembrane regulator CFTR channel; and therefore sticky mucus and high viscosity of luminal contents.
As no paracellular Na and H20 as no gradient set up
Presents as intestinal obstruction & meconium in newborns

Question 31

What is lactose intolerance

Answer 31

Lactose not digested into glucose and galactose and therefore not absorbed & remains in lumen - causing osmotic diarrhoea