Lecture 12 (DSA): Fluid and Electrolyte Absorption in the GI Tract Flashcards
What are the tight junctions in the small intestine like compared to the colon and how does this affect the route of fluid and electrolyte absorption and secretion?
Small intestine: are “leaky” w/ low resistance and permit significant paracellular movement
Colon: are “tight” w/ high resistance and do not permit paraceullar movement
What is first absorbed by the epithelial cells lining the vili and what comes second; the absorbate is always what tonicity?
- Solute absorbed first
- Fluid absorbed second
- Absorbate is ALWAYS isosmotic
Where is the major site of Na+ absorption in the small intestine?
Jejunum
What are the transporters located on the apical side of the epithelial cells of the jejunum?
- Na+-monosaccharide co-transporters (Na+-glucose or Na+-galactose)
- Na+- amino acid co-transporters
- Na+-H+ exchanger
How does carbonic anhydrase play a role in the epithelial cells of the jejunum; what occurs to the Na+ in the cell?
- Converts H2O + CO2 –> H2CO3 –> HCO3- and H+
- H+ is used for the Na+-H+ exchanger
- HCO3- is absorbed into the blood
- Na+ that entered the apical side is extruded across the basolateral membrane using the Na+-K+ ATPase
What is the final net absorption of the jejunum?
NaHCO3-
What are the transport mechanisms on the apical side of the ileum?
- Na+-monosaccharide co-transport
- Na+-amino acid co-transport
- Na+-H+ exchanger
- Cl-HCO3- exchanger
How does carbonic anhydrase play a role in the enterocytes of the ileum, what will be secreted and absorbed (hint: is different from the jejunum)?
- H2O + CO2 –> H2CO3 –> H+ + HCO3-
- HCO3- is secreted into lumen via Cl-HCO3- exchanger
- H+ is secreted into lumen via Na+-H+ exchanger
- Cl- will be absorbed into blood
What is the final net absorption of the ileum?
NaCl
What are the cellular transport mechanisms of the apical side of the enterocytes of the colon?
- Na+ channel for absorption
- K+ channel for secretion
What affect does aldosterone have on the enterocytes of the colon?
- Increases synthesis of Na+ channels, which leads to increased Na + absorption and increased K+ secretion
- More Na+ across apical membrane of the enterocyte will increase the Na+ pumped out across the basolateral membrane via the Na+-K+ ATPase, in turn, increasing the K+ inside the cell, which is secreted across the apical membrane
Explain the mechanism of fluid and electrolyte secretion by epithelial cells lining the intestinal crypts, describing both the transporters on the apical and basolateral side.
Apical membrane: Cl- channels (typically closed)
Basolateral membrane: Na+-K+ ATPase and Na+-K+-2CL- cotransporter
- Na+, K+, Cl- move into the cell from the blood and Cl- then diffuses into the lumen thru the Cl- channels of apical membrane
- Na+ passively follows the Cl- and H2O follows the NaCl into the lumen
What is the typical configuration of the Cl- channels of the apical membrane of the intestinal crypt cells; what activates them?
- They are usually closed
- Activated by VIP and ACh
- NT’s bind basolateral membrane receptor, activate adenylyl cyclase and generate cAMP which opens the Cl- channels and initiate Cl- secretion; Na+ and H2O follow Cl- into the lumen
What typically occurs to the electrolytes and water secreted by intestinal crypt cells?
Absorbed by intestinal villar cells
How does lactase deficiency cause osmotic diarrhea; how do bacteria contribute?
- Lactose is not digested and remains in the intestinal lumen, where it retains water
- Bacteria in the intestine may degrade lactose to more osmotically active solute particles, exacerbating the problem
Explain the 4 steps involved in Cholera toxin causing secretory diarrhea?
1) Inside cell A subunit of the toxin detaches and moves to the basolateral membrane where it catalyzes ADP ribosylation of the αs subunit of the Gs protein that is coupled to adenylyl cyclase
2) ADP-ribosylation of the αs subunit inhibits its GTPase activity, which results in GTP not being converted back to GDP and adenylyl cyclase being permanently activated
3) cAMP levels remain high, and Cl- channels in apical membrane are kept open
4) Resulting Cl- secretion accompanied by Na+ and H2O into lumen overwhelms the absorptive mechanism of the small intestine and colon, leading to massive diarrhea.
