epithelial function

Question 1

Sodium channels . . . Explain

Answer 1

Na/K pump on and K channel on BL surface.

High sodium concentration in the lumen. It is pumped out across BL membrane into the blood or interstitial compartment. Now sodium is low in the luminal fluid.

Luminal compartment becomes now negative compared to interstitial compartment.

This created driving force for the chloride ions to flow paracellularly.

Question 2

Potassium

Answer 2

**Path is from blood or interstitial compartment to luminal compartment.

Sodium flows in , more potassium flows in and potassium wants to be secreted.

** Key point : Inward movement of sodium then outward movement of potassium.

Question 3

Glucose

Answer 3

Glucose moves in via a co transporter with sodium. Sodium is moving across apical membrane with its gradient. Glucose hitches a ride with sodium. Glucose exits through BL membrane.

Question 4

Chloride

Answer 4

Na K 2Cl is expressed on the BL membrane. Chloride is moving in from the BL side. Sodium also comes in due to following its gradient. The chloride moves in with the sodium. Chloride builds up and flows out.

Question 5

Explain the movement of water across a leaky potassium channel.

Answer 5

Apical membrane contains Na/H exchangers - Sodium in proton out.

The BL side contains Na/K pump. Sodium goes in and then exits lateral and basolaterally.

NaCl moves paracellularly.

Movement of these ions causes lateral spaces to be hyper osmotic and the lumen to be hypoosmotic.

Both apical and BL are very permeable to water.

Water flows in through aquaporins and then it flows out.

Water is going from lumen (hypo) to BL and lateral (hyper).

Water also flows paracellularly.

Net result : the fluid that has been reabsorbed is going to be isosmotic with the interstitial or the blood.

Occurs in PCT.

Question 6

Explain how trans epithelial transport is dynamically regulated in response to physiologic stimuli.

Answer 6

1. Recruitment of proteins that can be inserted into apical of BL surface. Stimulus can be ADH.
2. Post translational modification such phos or dephos.
3. Increased degradation (ubiquitin)
4. Gene transcription or translation. (Aldosterone)