Lecture 8: Epithelia 2

Question 1

What happens if Na+/K+ ATPase is damaged?

Answer 1

from eg. no O2

We can’t make ATP and so some basic functional processes are broken down

Question 2

What are the two mechanisms for apical membrane transport for Na+ in leaky absorptive epithelia?

Answer 2

• Na+ coupled cotransport

- Na+ coupled counter-transport

Question 3

Na+ coupled counter transport is an example of what type of transport?

Answer 3

secondary active transport

Question 4

Give an example of an Na+ coupled counter transport and describe how it works

Answer 4

Na+/H+ exchanger

this is in the apical membrane and it uses the energy from the gradient pushing Na+ into the cell to pump H+

Question 5

What is the main mechanism for apical membrane transport of Na+ in tight absorptive epithelia?
Is this a passive or active process? Why?

Answer 5

channels

this is a passive process because they are using their gradient to be transported in a certain direction

Question 6

Give an example of a channel that facilitates Na+ transport in tight absorptive epithelia and describe how it works

Answer 6

ENaC

This uses Na+ electrochemical gradient to passively get it into the cell.

Question 7

Is the transport of Na+ into the tight absorptive epithelia via ENaC an electrogenic or an electroneutral process?

Answer 7

electrogenic

Question 8

What can block ENaC and what cannot?

Answer 8

amiloride blocks it and TTX does not

Question 9

How many subunits does ENaC have and what are these called?

Answer 9

3 subunits:
α ENaC subunit
β ENaC subunit
γ ENaC subunit

Question 10

Where can you find a good summary of Na+ transport across the apical membrane?

Answer 10

slide 38

Question 11

Tight absorptive epithelia express what?

Answer 11

ENaC

Question 12

The transport of ions is linked to the transport of what?

Answer 12

water

Question 13

What is the driving force for the movement of water in the body?

Answer 13

osmosis

Question 14

Absorptive epithelia absorb Na+ and _________

Answer 14

water

Question 15

Na+ can enter the cell via which two things?

Answer 15

channels or transporters

Question 16

Describe the Na+/glucose cotransport in leaky absorptive epithelia and how it leads to the movement of water across the membrane

Answer 16

On the apical membrane, there is the Na+/glucose cotransporter (SGLT1) which brings glucose into the cell. Glucose is pumped out of the cell on the basolateral membrane via a transporter (GLUT2). Na+ is pumped out of the cell actively via the Na+/K+ ATPase in basolateral membrane. K+ in being brought into the cell in this exchange and so it is removed passively via a channel in the basolateral membrane.
Na+ is now in the interstitial fluid and Cl- follows paracellularly to maintain charge neutrality and water follows

Question 17

Is glucose being brought in via the Na+/glucose co-transporter an electrogenic or electroneutral process?

Answer 17

electrogenic

Question 18

In the early proximal tubule ________ reabsorbs 90% of the filtered glucose, whereas in the late proximal tubule ________ is the main glucose transporter

Answer 18

SGLT2

SGLT1

Question 19

Describe the Na+ transport in tight absorptive epithelia and how it leads to the movement of water across the membrane

Answer 19

Na+ passively enters the cell on the apical surface and then it is pumped out of the basolateral membrane into the interstitial fluid by Na+/K+ ATPase in basolateral membrane. K+ in being brought into the cell in this exchange and so it is removed passively via a channel in the basolateral membrane.
Na+ is now in the interstitial fluid and Cl- follows paracellularly to maintain charge neutrality and water follows

Question 20

Glucose is transported against its gradient at the ________ membrane

Answer 20

apical

Question 21

What is the difference between absorb and secrete?

Answer 21

absorbing is taking it up whereas secreting is producing it and discharging it

Question 22

What are the electrical properties of secretory epithelia?

Answer 22

• leaky for water secretion

- tight

Question 23

Secretory epithelia actively secrete ________ ions and then Na+ and _________ follow passively which leads to a secretion of what?

Answer 23

Cl-
water
NaCl solution

Question 24

The apical membrane has a high permeability to Cl- due to the presence of Cl- ___________

Answer 24

channels

Question 25

Why does Cl- exit through the channels across the apical membrane, even though this is against its concentration gradient?

Answer 25

because the inside of the cell is negatively charged and the electrical gradient pushes it out of the cell

Question 26

What are the two important types of Cl- channels? Give an example of each

Answer 26

- cAMP-activated Cl- channel eg. CFTR | - Ca2+ activated channel (TMEM16A)

Question 27

Ca2+ activated Cl- channels are activated by what?

Answer 27

an increase in intracellular Ca2+

Question 28

Is the hydraulic conductivity high or low in secretory epithelia?

Answer 28

high

Question 29

Describe the hormonal/nervous control of secretory epithelia this is under ________ control there is a _______ basal rate of secretion that are _______ by __________ or ___________ control as recovered

Answer 29

this is under precise control | there is a low basal rate of secretion that are increased by nervous or hormonal control as recovered

Question 30

What is the physiological function of the secretory respiratory epithelia?

Answer 30

to keep the surface moistened to maintain mucociliary clearance

Question 31

________ secretion drives water secretion

Answer 31

Cl-

Question 32

Describe the Cl- secretion

Answer 32

NKCC in the basolateral membrane uses the energy from the Na+ gradient to take Na+, a K+ and 2 Cl- into the cell. Na+ is pumped out of the cell via Na+/K+ ATPase in the basolateral membrane. K+ passively leaves the cell via the K+ leak channel in the basolateral membrane. Cl- is driven out of the cell because of its electrical gradient through a channel on the apical membrane. Na+ follows paracellularly and water follows

Question 33

What determines the transport of ions?

Answer 33

electrochemical gradients

Question 34

Water secretion is mainly facilitated by which ion?

Answer 34

Cl-

Question 35

What is impaired during cystic fibrosis?

Answer 35

Cl- secretion

Question 36

Describe how cystic fibrosis impairs Cl- secretion and what effect that has

Answer 36

Due to a mutation, CFTR is not working properly and so there is decreased Cl- secretion via the apical membrane and therefore there is reduced water secretion so the mucus is too thick in various organs