Electrolytes DSA

Question 1

Where does electrolyte ABSORPTION occur in the small intestine, specifically?

Answer 1

Absorption of fluid and electrolytes occurs across epithelial cells lining the intestinal villi

[contrast with secretion of fluid and electrolytes, which occurs across epithelial cells lining the crypts]

Question 2

Where does electrolyte SECRETION occur in the small intestine, specifically?

Answer 2

Secretion of fluid and electrolytes occurs across epithelial cells lining the intestinal crypts

[contrast with absorption of fluid and electrolytes, which occurs across epithelial cells lining the villi]

Question 3

Describe the resistance of tight junctions in the small intestine vs. the colon, and what this means for route of cellular transport

Answer 3

Tight junctions have low resistance (leaky) in SI, which permits significant paracellular movement

Tight junctions have high resistance in colon, which does NOT permit paracellular movement

Question 4

Which of the following is characteristic of absorption of fluid/electrolytes in the SI?

A. First is absorption of water, then absorption of solute; total absorbate is isosmotic
B. First is absorption of solute, then absorption of water; total absorbate is isosmotic
C. First is absorption of water, then absorption of solute; total absorbate is hyposmotic
D. First is absorption of solute, then absorption of water; total absorbate is hyperosmotic

Answer 4

B. First is absorption of solute, then absorption of water; total absorbate is isosmotic

Question 5

Which of the following is the major site for Na+ absorption?

A. Duodenum
B. Jejunum
C. Ileum
D. Ascending colon
E. Descending colon

Answer 5

B. Jejunum

Question 6

Describe transport mechanism of Na+ in jejunum, including apical and basolateral transporters present

Answer 6

Na+ enters the enterocyte via several Na-dependent coupled transporters in apical membrane:
Na-glucose cotransporter
Na-galactose cotransporter
Na-amino acid cotransporters
Na-H exchangers

After Na+ enters, it is extruded across basolateral membrane via Na/K-ATPase

There is also an HCO3 transporter in basolateral membrane

Question 7

The result of the transport mechanisms in the jejunum is net absorption of ______

Answer 7

NaHCO3

Question 8

There is an Na-H exchanger in the apical membrane of enterocytes in the jejunum, facilitating absorption of Na, and secretion of H.

What is the source of the H that is exchanged for Na?

Answer 8

H comes from intracellular CO2 and H2O, which are converted to H and HCO3 in the presence of carbonic anhydrase

When H is secreted into the lumen, HCO3 is absorbed into the blood

Question 9

The transporters in the ileum are similar to those in the jejunum, with what additions in the apical and basolateral membranes?

Answer 9

Apical:
Cl-HCO3 exchanger

Basolateral:
Cl transporter (replaces HCO3 transporter)

Question 10

In the ileum, the Cl-channel replaces the HCO3 transporter in the basolateral membrane. What does this mean for H and HCO3 generated in the ileal enterocytes?

Answer 10

H is secreted into lumen via Na-H exchanger

HCO3 is secreted into lumen via Cl-HCO3 exchanger (rather than going into the blood like in the jejunum)

Question 11

The result of the transport mechanisms present in the ileum are a net absorption of ______

Answer 11

NaCl

Question 12

What ion channels are present in the apical membrane of cells in the colon?

Answer 12

Na channels for Na+ absorption

K channels for K+ secretion

Question 13

Synthesis of Na channels in the apical membrane of cells in the colon is induced by _______, leading to increased Na+ absorption and secondarily increased K+ secretion

Answer 13

Aldosterone

Question 14

What effect does diarrhea have on K+ secretion in the colon?

Answer 14

High flow rate of intestinal fluid causes increased colonic K+ secretion, resulting in increased K+ loss in feces and possible hypokalemia

Question 15

Transporter mechanisms for secretion in small intestine, including apical and basolateral membranes

Answer 15

Apical:
Cl-channels

Basolateral:
Na/K ATPase
Na-K-2Cl cotransporter

Question 16

Describe the Cl-channels present in the apical membrane of epithelial cells lining intestinal crypts

Answer 16

Usually closed, but may open in response to binding of various hormones and NTs, such as ACh and VIP, to their receptors on basolateral membrane

When they bind their receptors on basolateral membrane, adenylyl cyclase is activated, generating cAMP in crypt cells, which opens the Cl channels on the apical side, initiating Cl secretion. Na and water will follow

Question 17

Describe the Na-K-2Cl cotransporter present in the basolateral membrane of epithelial cells lining intestinal crypts

Answer 17

Brings Na, K, and Cl into cell from blood

Cl moves into the cells, then diffuses into lumen thru Cl channels in apical side. Na follows Cl through passively, moving paracellularly

Question 18

How does lactase deficiency cause osmotic diarrhea?

Answer 18

Lactose cannot be digested into its absorbable components: glucose and galactose. It remains in lumen undigested, where it retains water and causes osmotic diarrhea

Question 19

Normally, electrolytes and water secreted by intestinal crypt cells are absorbed by intestinal villi cells to avoid excess fluid loss.

How does the cholera toxin affect this process, causing secretory diarrhea?

Answer 19

Cholera toxin enters by crossing apical membrane. Once inside cell, A subunit detaches and moves to basolateral membrane where it catalyzes ADP ribosylation of alpha-s subunit of the Gs protein that is coupled to adenylyl cyclase, inhibiting GTPase activity

With GTP permanently bound to alpha-s subunit, adenylyl cyclase is permanently activated and cAMP levels remain high —> Cl channels in apical membrane remain open

Resulting Cl secretion is accompanied by secretion of Na and H2O. Volume of fluid secreted into intestinal lumen overwhelms absorptive capacity of SI and colon, leading to massive diarrhea