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60 Cards in this Set

  • Front
  • Back
Undifferentiated cells in the small intestine migrate along
villous-crypt axis and become villous absorptive cells, globet, enteroendocrine cells, paneth cells.
Colon crypts feats
No paneth cells here. Possibly the reason why colon infections are more prevalent.
Cell junction in intestinal epithelia formed by
tight junction
Adhering junction
Gap junction
Desmosome
Tight junctions in intestinal epithelia has 3 function
1-provide selectivity for transport
2-make a fence: separate the membrane of the epithelial cells into apical vs basolateral
3-define polaririty
Adhering junction jobs
Mediates cell-cell contact via Cell Adhesion Molecules (CAM)

Essential for proper organization of polarized epithelium

Initiates assembly of subcortical cytoskeleton
Tight junctions of leaky epithelia are relatively
more permeant to diffusion of ions than tight junctions of tight epithelia
Which one is more leaky, jejunum or distal colon?
jejunum
Epithelia can become more leaky by factors such as
proinflammatory cytokines
Intercellular (Paracellular) transport process
Solutes move passively between adjacent epithelial
cells via the tight junctions
Driven by diffusion and convection
Transcellular transport process
Solutes have to pass through two membranes (apical and basolateral) of the epithelial cells
Fluid and electrolyte absorption and secretion occurs in both
small and large intestine
Net nutrient absorption occurs in which part of the GI?
small intestine..proximal part
GI feats regarding absorption
1-tremendous capacity to absorb fluid
2-Net balance is towards absorptioon and very efficient
Absorption of electrolytes is proportional to
water absorption
Molecules that show parallel absorption along with water
Na, Cl, HCO3
Most of the Na+ is absorbed by
villous cells of the small intestine
Movement of Na+ from cell to blood due to
action of Na+/K+ ATPase pump on the basolateral membrane
Inwardly directed electrochemical gradient for Na+ is created by
Na+/K+ ATPase pump across the apical membrane of epithelial cells
Na/Amino acid transporter is electrogenic because
it creates a current when Na goes across membrane
Primary mechanism for Na+ absorption after a mediated by
apical membrane transporters
-Na/Glucose cotransporter (SGLT1)
-Na/Amino Acid cotransporters
Na-glucose, Na-AA transporters found in
Jejunum and Ileum
Na-H exchangers found in
Duodenum
Jejunum
Types of Na-H exchanger
-NHE3 and NHE2 isoforms on apical membrane
-NHE1 isoform on basolateral membrane
Na-H exchanger stimulated by
luminal HCO3
Na-H Exchange is uncoupled to
Cl/HCO3 exchange process
Na-H exchange is classified as...what type of transport
secondary
Na-H exchanger in terms of electric changes is
electroneutral
Na-H exchanger coupled to Cl/HCO3 process
Na goes in through transporters
H+ goes out of cell
More Alkaline in the cell since H+ is going out,leads to HCO3 going out and Cl coming in
Net effect, Na and Cl come into the cell
Major mechanism of NaCl absorption in intervals between meals
Epithelium Na channel (ENaC) restricted to
very distal part of the colon close to rectum
Cellular mechanisms of Cl absorption
Voltage-dependent Cl- absorption coupled to electrogenic Na+ absorption (Na-nutrient cotransporters and ENaC)

Cl-/HCO3- exchange process mediated by members of SLC26 gene family of anion exchanger
SLC26A3 is down regulated during
SLC26A6 is a
adenoma (DRA)
putative Anion Transporter-1
Cl absorption in passive in
Jejunum, distal colon, ileum. Passive due to pores,opened channels
Cl/HCO3 for Cl transport used in
Ileum, proximal colon, distal colon
Cl/HCO3 function
Cl goes in, bicarb out. Electroneutral, can work by itself or couple to Na/H exchanger
Na/H and Cl/HCO3 work in parallel in
Ileum, proximal colon
Electrogenic Cl secretion occurs nmostly in
crypt
Cloride secretion is stimulated by
secretagogues:
Bacterial exotoxins (Cholera toxin)
Hormones and neurotransmitter (acetylcholine, VIP)
Product of the immune system (histamine)
Laxatives (bile acids)
Second messengers role in Cl secretion
is stimulated by second messengers: ^[Ca++]i, ^cAMP and ^cGMP
Chloride secretion across the apical membrane is via
apical chloride channels
CFTR and/or calcium dependent-channel CIC
CTFR requires for proper Cl secretion
ATP and phosphorylation
Cl transport in basolateral membrane vua
Na+/K+ATPase (Ouabain-sensitive)
NKCC (Na+-K+-2Cl- cotransporter,Furosemide-sensitive)
K+ channels
HCO3- transport may occur via
-anion exchangers such as Cl-/HCO3- exchangers: net Cl- absorption and HCO3- secretion.
-may also occur via channel (possibly CFTR)
HCO3 loading occurs via
Na+-HCO3- cotransporters localized to the basolateral membrane of the epithelial cells
HCO3 converted from CO2 via
Carbonic Anhydrase
Net K+ movement is
absorptive in the small intestine and secretory in the colon
Solvent drag
K+ absorption in the small intestine
Primary mechanism of K+ secretion in the colon
Passive
Active secretion of K+ in colon induced by
aldosterone and cAMP
Distal part of the colon, K+ net transport is
active absorption
cGMP net effect in Ion transport
Decreases NaCl absorption
Excess fluid in the stool is caused by
Increase in fluid secretion or decrease in fluid absorption or both
Osmotic Diarrhea
Excess of non absorbed solutes in the lumen
example: lactase deficiency
Secretory Diarrhea
Excessive secretion of fluid into the lumen
Infection: E.coli and cholera
Secretagogue-producing tumors
VIP, Serotonin, Glucagon
Congenital Chloride Diarrhea
Rare genetic disease
Voluminous diarrhea
Distended abdomen
Fecal chloride concentration > 90 mmol/L vs 10-15 mmol/L in normal individuals
Treatment: Salt replacement therapy
Defect in Congenital chloride diarrhea
DRA is mutated.
Cl accumulates in lumen (luminal acidification)
Diarrhea ensues when
the absorptive capacity of the GI tract has been surpassed
Functions of Microflora or commensal microorganisms:
Essential for the development of mucosal immune system

Help in performing reactions that do not occur in mammalian cells

Inhibit the growth of competitor pathogenic bacteria
Microflora produced short chain fatty acids such as
butyrate, propionate, acetate
Why is butyrare important?
It's the mos abundant anion in colonic lumen and it's the preferential energy source for the colonocytes
Short fatty acids are transported across the membrane via
HCO3 exchanger.