DSA 2

Question 1

The jejunum is the major site for absorption of what ion? How is it absorbed?

Answer 1

Na
net absorbtion of NAHCO3
Apical membrane has Na/monosaccharide cotransporters
Na/a.a cotransporters
Na/H exchange (H comes from intracellular CO2 and H2O: carbonic anhydrase)

Question 2

What transporters are on the basolateral side of jejunum epithelial cells?

Answer 2

Na/K ATPase
Sugar or aa transporter
Bicarb transporter

Question 3

What transporters do the epithelial cells of the ileum contain?

Answer 3

Same as jejunum
plus Cl/HCO3 exchanger on apical membrane and
Cl transporter on basolateral membrane instead of bicarb transporter

Net absorption of NaCL

Question 4

The colon has cellular mechanisms similar to principal cells of the late distal tubule and collecting ducts of the kidney. Explain how.

Answer 4

Apical membrane contains Na and K channels, which are responsible for Na absorption and K secretion. Na channel synthesis induced by aldosterone

High flow rate of intestinal fluid causes increased colonic K secretion resulting in hypokalemia

Question 5

Compare the permeability of tight junctions between the epithelial cells of the small intestines and the epithelial cells of the colon

Answer 5

Small intestines: ‘leaky’

Colon: ‘tight’

Question 6

Do the epithelial cells lining the intestinal crypts secrete or absorb fluid and electrolytes?

Answer 6

secrete

Question 7

What are the transporters on the epithelial cells of crypts and how are they related to cholera ?

Answer 7

Blood side has Na/K ATPase, Na/K/2Cl cotransporter bringing in the ions into the cell

Apical side: Cl channels; usually closed by may open in response to ACh and VIP. Activate adenylyl cyclase and generates cAMP

Na and water follow Cl

Cholera: adenylyl cyclase is maximally stimulated and fluid secretion by the crypt cells overwhelms the absorptive capacity of the villar cells

Question 8

In diarrhea, the loss of extracellular-type fluid results in what?

Answer 8

decreased extracellular fluid volume, decreased intravascular volume, decreased arterial pressure
Loss of bicarb relative to Cl causes hyperchloremic metabolic acidosis with normal anion gap
hypokalemia

Question 9

What are the causes of diarrhea?

Answer 9

Decreased surface area for absorption: infection, inflammation
Osmotic diarrhea: nonabsorbable solutes in lumen of intestine (i.e. lactose not digested and holds onto water)
Secretory Diarrhea

Question 10

Describe secretory diarrhea

Answer 10

Not caused by inadequate absorption of fluid but from excessive secretion of fluid by crypt cells
Major cause: overgrowth of enteropatic bacteria (i.e. cholera or e coli)

Question 11

How does the cholera toxin cause secretory diarrhea ?

Answer 11

Enters intestinal crypt through apical membrane
A subinit detaches and moves across cell to basolateral membrane
catalyzes ADP ribosylation of alpha s subunit of Gs protein coupled to adenylyl cyclase
Inhibits GTPase activity : GTP cant be converted back to GDP
Adenylyl cyclase permanently active
cAMP levels high
Cl channels remain open
Na and H2O follow Cl into lumen