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BMS 242 - Phys and pharm of cells > Introduction > Flashcards

Introduction Flashcards

1
Q

What are channels?

A

GATED pores in the membrane

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2
Q

What happens when channels open?

What does this generate?

A

Ions flow through them
Get CONDUCTANCE

Generates a CURRENT

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3
Q

What happens when the channels are closed?

A

No ions through
No conductance
No current

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4
Q

What is important about the opening of the channels?

A

Timing of the opening

Disruption can have an adverse effect on the normal physiology of nerves and muscles

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5
Q

How did ion channels use to be classified?

A

Through 3 main features:

1) Selectivity
- What is the main ion that moves through the pore?

2) Gating
- What is needed to open the channel

3) Regulation

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6
Q

What could be in control of gating the ion channel?

A

Voltage
Ligand
Mechanical

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7
Q

What could regulate the channel?

A

ATP
G proteins
Ca2+

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8
Q

Now, how are ion channels classified?

A

By the there MOLECULAR STRUCTURE (amino acid sequence and structure):
- Put into groups where there are high similarities in the amino acid sequence

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9
Q

Describe the distribution of K+ channels

What does this mean for mutations?

What other channels is this true for?

A

Many K+ channels with high similarities in amino acid sequence

But they are DIFFERENTIALLLY EXPRESSED in different tissues

If have a mutation effecting one K+ channel - will impact ONLY the cells where that channel is expressed

True for any other type of channel

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10
Q

When a channel is open, what does it drive the membrane potential to?

A

The Nerst (reversal) potential for that channel

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11
Q

What is the nerst potential?

A
  • Potential where there is no net current flow across the membrane for that ion
  • Ions moving in balance ions moving out
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12
Q

What is Rt/Zf at BODY TEMPERATURE?

A

61.5

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13
Q

What is Rt/Zf at ROOM TEMPERATURE?

A

58.2

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14
Q

What is the intraceullar [K+]?

A

150mM

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15
Q

What is the extracellular [K+]?

A

5mM

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16
Q

What is the Ek?

What does this show about the membrane permeability at rest?

A

-89mV

At rest - membrane is -70mV, showing high permeability to K+

BUT, must be other channels open in the membrane (drive the membrane potential to their nerst potential) as the potential is not exactly -89mV

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17
Q

Why does the Nerst potential of the K+ channel sit slightly more positive than the Nerst potential for the K+ ion?

A

K+ channels have an Na+ leak

Reversal potential of a channel is dependant on ALL the ions that move through the ion channel

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18
Q

What is the extracellular [Na]?

A

150 mM

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19
Q

What is the intracellular [Na}?

A

15mM

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20
Q

What is the Ena?

A

+66mV

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21
Q

Why is the nerst potential of the Na+ channel slightly hyper polarised compared to the nerst potential for the Na+ ion?

A

Na+ channel has a K+ leak

Nerst/reversal potential of a channel is dependant on all the ions that move through the channel

22
Q

What is the resting potential of the membrane?

What does this show?

A

-70mV

Shows that the membrane is more permeable to K+ than is to Na+, as the membrane potential sits closer to Ek

(50x more permeable to K+)

23
Q

How much more permeable to Na is the membrane at the peak of the AP?

A

5 x more permeable than K+

24
Q

What is the intracellular [Cl-] concentration?

A

6mM

25
Q

What is the extracellular [Cl-] concentration?

A

100mM

26
Q

What is the Ecl?

A

-87mV (very close to Ek = - 89mV)

27
Q

Why is looking at the nerst potential for a cell insufficient to be SURE of the ion channel that is open in the membrane?

What can be used to back up the ideas made using the Nerst eqn?

A

Nerst potentials of different channels can be very close together - unsure which one is open

(Ek = -89mV, Ecl = -87mv)

Patch clamp technique used to back up

28
Q

What is the current equation?

A

I = N.Po.g. (Vm-Ei)

N = number of channels
Po = open probability of the channels
g = a constant (open channel conductance) 
Vm-Ei = driving force of an ion
29
Q

What is the basis of the patch clamp technique?

A

Change and clamp the Vm (potential)

Measure the current (see how change in Vm effects current)

30
Q

How is the current through a channel regulated?

How can these be changed?

A
  • Number of channels (change by membrane shuttling)
  • Open probability of channels (change by ATP,GTP,phos, Ca2+ etc.
  • Potential (change through opening/closing different channels)
31
Q

What does changing the membrane potential change?

A

Changes the DRIVING FORCE

32
Q

What is the current recorded with the patch clamp technique called?

What is this?

A

Itotal (total current across the WHOLE membrane)

33
Q

What is Itotal equal to?

What is this in a cell with Na channels and K channels only?

A

The SUM of the currents through the open channels

Itotal = Ina + Ik

34
Q

What is each of the current in the eqn Itotal = Ina + Ik described by?

A

Ina or Ik = N.Po.g. (Vm-Ei)

35
Q

How can the different channel types be found in the membrane using the patch clamp technique?

A

1) Itotal (current through all the channels in the cell) measured when clamp the potential of the cell
2) Itotal = I1 + I2 +I3 etc….
3) Each I is described by I = N.Po.g. (Vm-Ei)

36
Q

What does each horizontal line on the whole cell current recording represent?

A

The current recorded at each different potential (change in Vm)

37
Q

How can the ion channels that are causing the Itotal be pharmologically identified from the whole cell current recordings?

A

Can add a BLOCKER for a specific channel and see if the current decreases

38
Q

What is an example blocker of K+?

What happens when this is used?

A

Barium

When used - sends the current flow towards 0 (blocking the K+ channels)

39
Q

Why are Nav channels difficult to look at?

A

They are CLOSED at -ve potentials (current= 0)

Activate QUICKLY with depolarisation - giving an increase in current

CLOSE with depolarisation slowly

40
Q

What are the 3 separate states of configuration of the Na+ channels

A

1) Closed
- Pore not open

2) Open
- Activated by +ve shift in the membrane potential

3) Inactivated

41
Q

What triggers the inactivated state of the Na+ channel?

A

Depolarisation of the membrane

However, this takes longer to occur than activation - happens afterwards

42
Q

How is the structure of an inactivated Na+ channel different to the closed channel?

A

In an inactivated channel - pore is open but there is a BLOCKAGE in the channel

Blockage - ball of amino acids

43
Q

What is shown on the whole cell patch clamp recording for Na+?

Why?

Which channels show the same profile as this?

A

Increase and then decrease in the recording

Na+ channels first open and then close - due to the ball of amino acids blocking the pore of the channel in the inactivation state

Ca2+ voltage gated channels show the same profile

44
Q

How can Nav be distinguished from Cav in the whole cell patch clamp technique?

A

Nav is blocked by TETRODOTOXIN

Cav are not

45
Q

What is FHEIG?

A
  • Bi-temporal narrowing
  • Excess body hair
  • Thin upper lip
  • Overgrowth of tissues inside the mouth

Caused by:

  • GENETICALLY inherited mutation in KCNK-4 (K+ chanenl)
  • Single amino acid change in this channel
  • GOF mutation
46
Q

Where is the KCNK-4 K+ channel normally expressed?

A

In the CNS and PNS

47
Q

What does over expression of the KCNK-4 K+ mutants cause?

What does this show?

A

LARGER CURRENTS

Shows that this is a GOF mutation

48
Q

What technique proved that FHEIG was due to a GOF mutation?

A

Patch clamp technique

49
Q

What was it hypothesised about the cause of FHEIG?

A
  • KCNK-4 K+ channel found in the CNS and the PNS (but not ALL the cells)
  • GOF –> Excess loss of K+ from the cells within the mutant channel into the interstitial space (where there is normally LOW K+)
  • As K+ accumulates here - Ek becomes less negative (depolarised)
  • Resting potential more positive
  • Neighbouring cells more depolarised - closer to threshold potential
  • More likely to fire action potentials (normally wouldn’t)
50
Q

Why is K+ accumulates in the interstitial fluid does the Ek become less negative?

A

Ei = 61.2 x log ( [K]out/ [K]in)

Higher [K]out = higher ( [K]out/ [K]in) ratio –> when ‘logged’ = more positive

BMS 242 - Phys and pharm of cells (13 decks)

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