Lecture 8: Epithelia 2 Flashcards
What happens if Na+/K+ ATPase is damaged?
from eg. no O2
We can’t make ATP and so some basic functional processes are broken down
What are the two mechanisms for apical membrane transport for Na+ in leaky absorptive epithelia?
- Na+ coupled cotransport
- Na+ coupled counter-transport
Na+ coupled counter transport is an example of what type of transport?
secondary active transport
Give an example of an Na+ coupled counter transport and describe how it works
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+
What is the main mechanism for apical membrane transport of Na+ in tight absorptive epithelia?
Is this a passive or active process? Why?
channels
this is a passive process because they are using their gradient to be transported in a certain direction
Give an example of a channel that facilitates Na+ transport in tight absorptive epithelia and describe how it works
ENaC
This uses Na+ electrochemical gradient to passively get it into the cell.
Is the transport of Na+ into the tight absorptive epithelia via ENaC an electrogenic or an electroneutral process?
electrogenic
What can block ENaC and what cannot?
amiloride blocks it and TTX does not
How many subunits does ENaC have and what are these called?
3 subunits:
α ENaC subunit
β ENaC subunit
γ ENaC subunit
Where can you find a good summary of Na+ transport across the apical membrane?
slide 38
Tight absorptive epithelia express what?
ENaC
The transport of ions is linked to the transport of what?
water
What is the driving force for the movement of water in the body?
osmosis
Absorptive epithelia absorb Na+ and _________
water
Na+ can enter the cell via which two things?
channels or transporters
Describe the Na+/glucose cotransport in leaky absorptive epithelia and how it leads to the movement of water across the membrane
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
Is glucose being brought in via the Na+/glucose co-transporter an electrogenic or electroneutral process?
electrogenic
In the early proximal tubule ________ reabsorbs 90% of the filtered glucose, whereas in the late proximal tubule ________ is the main glucose transporter
SGLT2
SGLT1
Describe the Na+ transport in tight absorptive epithelia and how it leads to the movement of water across the membrane
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
Glucose is transported against its gradient at the ________ membrane
apical
What is the difference between absorb and secrete?
absorbing is taking it up whereas secreting is producing it and discharging it
What are the electrical properties of secretory epithelia?
- leaky for water secretion
- tight
Secretory epithelia actively secrete ________ ions and then Na+ and _________ follow passively which leads to a secretion of what?
Cl-
water
NaCl solution
The apical membrane has a high permeability to Cl- due to the presence of Cl- ___________
channels
Why does Cl- exit through the channels across the apical membrane, even though this is against its concentration gradient?
because the inside of the cell is negatively charged and the electrical gradient pushes it out of the cell
What are the two important types of Cl- channels? Give an example of each
- cAMP-activated Cl- channel eg. CFTR
- Ca2+ activated channel (TMEM16A)
Ca2+ activated Cl- channels are activated by what?
an increase in intracellular Ca2+
Is the hydraulic conductivity high or low in secretory epithelia?
high
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
this is under precise control
there is a low basal rate of secretion that are increased by nervous or hormonal control as recovered
What is the physiological function of the secretory respiratory epithelia?
to keep the surface moistened to maintain mucociliary clearance
________ secretion drives water secretion
Cl-
Describe the Cl- secretion
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
What determines the transport of ions?
electrochemical gradients
Water secretion is mainly facilitated by which ion?
Cl-
What is impaired during cystic fibrosis?
Cl- secretion
Describe how cystic fibrosis impairs Cl- secretion and what effect that has
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
