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A BMS346 Epithelial Physiology > Overview and review of basic concepts > Flashcards

Overview and review of basic concepts Flashcards

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1
Q

What is a key property of epithelial cells?

What does this allow?

A

They are POLARISED - have different transport proteins on their apical and basolateral surfaces

Allows NET transport of ions and water

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2
Q

How can epithelial cells be arranged?

A

In sheets or in a tubular shape

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3
Q

How are epithelial cells arranged in the upper airways?

A

In tubular structures

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4
Q

What are the 2 ways that net transport can occur?

A

TRANSCELLULAR - across the cell (through ion channels in the apical and basolateral membrane)

PARACELLULAR - between the cells

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5
Q

What is a key difference between transcellular and paracellular transport?

A

Transcellular transport –> CONTROLLED/REGULATED transport of ions, solutes and water

Paracellular - no channels –> no regulation

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6
Q

What determines if an epithelium is tight or leaky?

What is this?

A

The transepithelial resistance (Rte)

The resistance across the epithelium to movement

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7
Q

What does the type of epithelium determine?

A

Which DIRECTION different ions and solutes move

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8
Q

What does a high Rte mean?

A

Harder to move ions across the epithelium - not a lot of transport occurs across the epithelium (in terms of paracellular transport)

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9
Q

What is the difference between the Rte of a leaky and a tight epithelium?

A

Leaky Rte < 200 Ohmscm2 (low resistance - lots of transport across the epithelium)

Tight Rte >2000 Ohmscm2 (high resistance - not a lot of transport across the epithelium)

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10
Q

What are 4 examples of leaky epithelia?

A
  • Proximal tubule (kidney)
  • Gallbladder
  • Small intestine
  • Choroid plexus
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11
Q

What are 3 examples of tight epithelia?

A
  • Distal tubule (nephron)
  • Stomach
  • Frog skin
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12
Q

What is Rte determined by?

Why?

A

PARACELLULAR permeability

As the net transport in terms of TRANSCELLULAR permeability is similar for all different types of epithelium

SO, what makes cells tight or leaky –> how much transport is occurring BETWEEN the cells

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13
Q

What are the contacts between the cells in the epithelium called?

How are they different in tight and leaky epithelia?

A

Tight junctions

Tight epithelia - TIGHT tight junctions (not a lot of paracellular transport)

Leaky epithelia - LEAKY tight junctions (a lot of paracellular transport occurring)

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14
Q

What is Vte?

A

The transepithelial potential - potential that exists ACROSS the epithelium

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15
Q

What determines the transepithelial potential?

A

The SUM of the membrane potentials across the 2 epithelial membranes (apical and basolateral)

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16
Q

How is the Vte generated?

A

Net ion/charge flow across the membranes

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17
Q

What is the difference between the Vte in tight and leaky epithelium?

A

Leaky:
- Have a very small (1/2mV) or absent transepithelial potential

Tight:
- Vte ~50mV (LARGE)

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18
Q

Why is the transepithelial potential for leaky epithelial absent/low?

A

Vte is generated by TRANSCELLUAR transport (across the cell)

But in a LEAKY epithelium - leaks back again (paracellularly) as soon as transported across the cell –> cannot sustain the potentials

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19
Q

What is the difference between flux in a tight and leaky epithelium?

A

Leaky:

  • Large IOSMOTIC (followed by water PARACELLULARY)
  • Transcellular and paracellular transport

Tight:

  • Small flux
  • Only TRANSCELLULAR
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20
Q

How can the Vte be measured?

When is Vte measured?

A
  • Reference electrode at 0mV
  • Electrode in the APICAL surface of the epithelium

Vte measured - TIGHT epithelium

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21
Q

Is Vte +ve or -ve?

A

Can be +ve OR -ve depending on the net movement of charge across the membrane:

  • More anions or less cations (at the APICAL membrane)–> -ve Vte
  • More cations or less anions (at the APICAL membrane) –> +Vte
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22
Q

Describe the epithelium of the principle cell of the kidney

A
  • ve and large Vte:

- Tight epithelium

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23
Q

Describe the principle cell model in the kidney in terms of Vte

A

Basolateral membrane:

  • NaKATPase
  • K+ channel

Apical membrane:
- ENaC (Na channel)

Na through apical and basolateral membrane (caries a +ve charge)

  • -> loss of +ve charge from the apical side to the basolateral side
  • -> leaves behind -ve charge at the apical membrane

–> -Vte

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24
Q

What does the NaKATPase pump and the K channel in the basolateral membrane do?

How?

A

Sets up and maintains the electrochemical driving force for Na uptake across the APICAL membrane:

1) -ve intracellular potential
Net loss of 1 +ve charge

2) Low intracellular Na
Pumps 2 x Na out and 1 x K into the cell

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25
In which direction does Na travel through ENaC? Where does this then go?
INTO the cell from the APICAL environment Na then PUMPED by the NaKATPase channel across the basolateral membrane
26
Describe the epithelium of the thick ascending limb of the kidney
Basolateral: - NaKATPase - K channel - Cl channel (Cl out) Apical: - NKCC2 - ROMK (K out)
27
What is the function of the NKCC2 channel?
``` Transport: 1 x Na 1 x K 2 x Cl INTO the cell ```
28
What is the drives the function of the apical NKCC2 channel?
- Low intracellular Na - -ve intracellular potential Both of which are set up and maintained by the NaKATPase pump in basolateral membrane
29
What is the net charge through the NKCC2 channel?
ElectroNEUTRAL (no net movement of charge)
30
What happens to the ions once they have been absorbed into the cell through the apical NKCC2 channel? What does this mean for the Vte of the cell?
- Na and 2 x Cl are reabsorbed at the basolateral membrane (through the NaKATPase and the Cl) - K ions are RECYCLED through the apical ROMK channels Therefore only 1 net +ve charge across the basolateral membrane but 2 net -ve Vte = +ve (due to loss of more -ve charge from the apical membrane)
31
How can we determine changes in epithelial function?
Monitor changes in Vte - change in response to manipulation
32
What is electrophysiology used for?
To look at the FUNCTION and PHYSIOLOGY of epithelia through measurements of: 1) Current 2) Potential
33
How is a current generated?
Ion movement through a channel
34
What are 4 electrophysiology techniques and what do they measure?
1) Intracellular micro electrode - measure IC membrane potentials (Vm) 2) Patch clamp - single channel or cell current, Po 3) Two electrode voltage clamp - total cell current 4) Ussing chamber - WHOLE epithelial function (Vte, Rte and single cell current (SCC))
35
What are similarities/differences between the patch clamp and the two electrode voltage clamp method?
Similarities: - Clamp the potential and measure the CURRENT - Measure total cell current Differences: - Patch clamp - can measure single cell currents? - Patch - 1 electrode, smaller cells (mammalian) - 2 electrode - 2 electrodes, larger cells (xenopus)
36
What can the Ussing chamber technique used for?
- Looking at WHOLE epithelial function/behaviour (not single cell) - POPULATION of cells
37
As well as identifying in the epithelium is tight or leaky, what does the Rte identify?
If the tissue is VIABLE - if the Rte is not in the range you would expect --> tissue is dead
38
What is the SCC?
Short circuit current: - Indirect measurement of the net flux of ions across the membrane - Amount of current needed drive Vte --> 0mV - EQUAL and OPPOSITE to the net current flowing across the epithelium in the ussing tecnique
39
How is SCC calculated? Why calculated in this way?
Using the Vte and Rte values recorded from the Ussing chamber technique Cannot directly measure the net current flow across an epithelium (have to work out the equivalent)
40
What is the Nernst of K?
-90mV
41
What is the Nernst of Na?
+61mV
42
If measure Vm close to the Nernst of ion X what is this an indication of? Why in reality is the Nernst not equal to the Vm of ion X?
Indication that the membrane has a high selectivity for the ion X (dominant channels open in the membrane are X channels) In reality: - More than one ion channel open in the membrane --> drives the Vm towards the Nernst of that ion
43
How can the protein channels in the membrane be determined?
1) Measuring the Vm and seeing what Nernst it is close to | 2) Adding ion channel blockers and identifying the shift in the Vm
44
If Na channels are open in the membrane, what happens to the Vm when add amiloride?
Amiloride blocks Na channels: - No longer contribute to Vm - Vm moves AWAY from the Nernst of the ion moving through the blocked channel (Na) - Vm moves TOWARDS the Nernst of the ion moving through other channels in the membrane (membrane becomes more selective for these ions) - Eg. Ek
45
What is the selectivity of an ion channel?
?
46
What is Vrev?
?
47
What does it mean if the Vrev is close to the Nernst of an ion?
?
48
If add barium to a cell and it decreases the size of the currents, what does this show?
Shows K channels mediate the current as barium block the K channels
49
Describe the Ussing chamber technique
- 2 chambers with sheet of epithelium in the middle - Experimental solutions either side of the sheet of epithelium ``` 4 electrodes (2 each side): - First pair (reference and recording) - measure the Vte ``` - Second pair - injects the current across the epithelium (magnitude of this current is determined (SET/KNOWN) by a current injection box)
50
What happens when inject the current in the Ussing chamber experiment?
The Vte shifts
51
What is the shift of Vte when a known current is injected (in the Ussing chamber) dependant on? Why?
How much the Vte shifts is dependant the RESISTANCE Ohms law: Vte = IR (I is known)
52
How work out the Vte of the epithelium from ussing chamber technique?
Directly measured from the electrodes
53
How can the Rte of a the epithelium from ussing chamber technique?
Rte = change in V/ I injected | Rearrangement of Ohms law Vte = IR
54
How can Isc be calculated from the ussing chamber technique?
Isc = Vte (in the absence of current injection) / Rte
55
Describe the graph that is recorded from the ussing chamber technique
Line at the top - Absolute Vte (when no current injected) Deflections downwards - Change in Vte when there is an injection of a known current (change/known current = Rte) (Isc = Absoulte Vte (line @ the top) / Rte)
56
What happens to Vte in the presence of Lub? Why?
Vte increase: - Lub is a prostaglandin mimic --> activates receptors - Stimulates cAMP - Activates PKA - Activates the CFTR Cl receptors
57
What happens to Vte in the presence of CFTR inhibitor after Lub addition? Why?
Increase in Vte (with Lub) is REVERSED Due to a blockage of Cl secretion
58
What are the ion channels present in the upper airway cell?
Basolateral: - NaKATPase - K channel - NKCC1 Apical: - CFTR - ENaC
59
What happens to the ions that come through the NKCC1 channel in the basolateral membrane of the upper epithelial cell?
- Na recycle across the basolateral membrane - K recycles across the basolateral membrane - BUT the Cl ACCUMULATES inside the cell
60
What happens in the upper airway epithelium cell when CFTR channel opens? Why?
Cl LEAVES the cells (net Cl secretion) as Cl has accumulated inside the cell from the NKCC1
61
What happens in the upper airway epithelium cell when the ENaC channel opens? Why?
Na influx through ENaC Due to the electrochemical potential (low Na and -ve IC0 set up by NaKATPase and K channels in the basolateral membrane
62
What is the function of CFTR and ENaC in the upper airway epithelium cell?
BALANCE between Cl secretion (through CFTR) and Na absorption (through ENaC) Sets the HEIGHT of the periciliary layer (PCL) that is SPECIFIC and OPTIMUM
63
What is the function of the PCL?
- Layer which the cilia (from the bronchial epithelial cells) project into First line of defence against infection: - Cilia beat and move the PCL and the mucus on the top of the PCL up the respiratory tract - Swallow this mucus (contains trapped viruses and bacteria and anything else breathed in)
64
Where is the PCL present?
Sits on top of the epithelial cells of the respiratory tract (upper airway and alveolar cells)
65
What feature of the PCL is important in mucous clearance? What happens if this is disrupted?
The HEIGHT Disruption of the height - impacts on the ability to clear pathogens from the respiratory tract

A BMS346 Epithelial Physiology (13 decks)

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