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PHYSL 404 - Cardiovascular Physiology > Lecture 3 Electrophysiolgoy > Flashcards

Lecture 3 Electrophysiolgoy Flashcards

1
Q

What are ion channels?

A

Pore-forming proteins present in the plasma membrane of different cells

  • allow passage of ions down their electrochemical gradient
  • Help establish and control the voltage gradient across the plasma membrane
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2
Q

What are two important properties of ion channels?

A
  1. Gating
    • the process of channel activation (opening), inactivation (prevent flow through open channel) and deactivation (closing)
  2. Permation/Selectivity
    • Ion channel pores can allow over 10^6 ions to flow per second while at the same time selectively discriminate between different ions
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3
Q

In which 3 ways are ion channels classified?

A
  1. Ionic selectivity
    • eg K+, Na+, Ca++, Cl-, nonselective
  2. Functional properties (activation trigger)
    • voltage-gated
    • ligand-gated
    • mechanosensitive
  3. Molecular structure
    • 2 transmembrane channels
    • 2 transmembrane/2pore channels
    • 4 transmembrane channels
    • 6 transmembrane channels
    • 24 transmembrane channels
    • *** Anything greater than 24 requires 4 subunites
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4
Q

Define the cardiac action potential

A

A rapid and transient increase in the membrane potential that results in myocyte contraction

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5
Q

How is the cell membrane/AP similar to an electrical circuit?

Cm=

E=

gn =

gL =

Ip =

A

Cm= capacitance = lipid bilayer (structure that separates charges)

E= battery = electrochemical gradient (driving force)

gn = (t,v) = Non-linear electrical conductance, time (t) and voltage (v) dependent = gating property

gL = Linear electrical conductance, leak/ligand channels on in the background (independent of voltage)

Ip = Ionic pump

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6
Q

How is membrane potential calculated?

A

Membrane Potential = Vinterior - Vexterior

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7
Q

What is the Nernst potential of Ca++?

A

+150mV

Moved into cell during depolarization

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8
Q

What is the Nernst potential of Na+

A

+70mv

Into cell during depol

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9
Q

What is the nernst potential of Cl- ?

A

-30 to -65mv (moves into cell during repolarization and out of cell during depolarization)

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10
Q

What is the nernst potential of K+?

A

K+ is always higher intracellularly

Nernst potential (Erev) = -98mv

Moves out of cell during repol

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11
Q

In a single ion system, what controls the movement of ions across the cell membrane?

A

Nernst potetial:

AKA

  • Reversal potential
  • Equilibrium potential
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12
Q

What is Nernst potential?

A

A potential (for a single specific ion channel) at which there is no net flux of that particular ion across the cell membrane

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13
Q

What is the Nernst Equation?

A

E = (RT)/(zF) ln ([ion outside cell])/([ion inside cell])

or

E = 2.303 (RT)/(zF) log10 ([ion outside])/([ion inside])

E = nernst potential/membrane potential

R = the ideal gas constant (8.3145 J/molK)

T = temperature in Kelvin

F = faraday’s constant

z = ionic charge

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14
Q

How does Nernst equation change when dealing with a negatively charged ion? (eg Cl-)

A

Current is movement of POSITIVE charge so when dealing with a neg ion = outward current = invert the concentration relation

eg:

E = RT/zF ln ([Ion inside])/([ion outside])

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15
Q

What are the three types of Patch Clamp techniques?

A
  • Cell-attached
    • pull 1-2 ion channels into pipette
    • Influence environment inside the pipette
    • electrode acts as an ion channel
  • Inside out
    • Involves tearing piece of membrane to gain access to intracellular channels
    • Ion channel regulated by intracellular ligands
    • ability to manipulate intracellular enviornment
  • Whole-cell
    • Form giga-seal
    • Interupt membrane inside the pipette
    • Electrode is continuous with intracellular sol’n
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16
Q

Depolarization is the movement of _______ ions _______ the cell

A

Depolarization is the movement of positive ions into the cell

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17
Q

How does the membrane potential affect the movement of an ion across the cell membrane?

A

If Vm < Ek => K+ moves inward

If Vm > Ek => K+ moves outward

V = IR

V= I/G

I=VG

I = (Vm-E)G

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18
Q

What is the range of action for K+ channels in the heart?

A

-80mV to +40mV

Ek = -95mV

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19
Q

What happens to Na+ when the voltage is less than +70mV?

A

There will be an inward movement of Na+ (Na+ moves into cell)

ENa+= +70mV therefore when membrane current is less than +70, Na+ moves inward to increase membrane potential

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20
Q

What equation is used to measure Membrane Potential?

A

Goldman-hodgkin-katz (GHK) equation

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21
Q

Which 2 factors determing membrane potential (Em)?

A
  1. Concentraion gradients for K+, Na+ and Cl- across the membrane
  2. The relative permeability (electrical conductance / electrical gradient) of the membrane to each of these ions (regulated by ion channels)
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22
Q

What is channel gating?

A

Process of channel activation, inactivation, and deactivation

  1. Ligand-gated
    • require ligand to bind
  2. Voltage gated
    • respond to changes in membrane potential (Em)
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23
Q

The cardiac AP originates at the ______

A

Sinoatrial (SA) node

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24
Q

From the SA node, the AP travels:

A

From the SA node, the AP travels to the AV node, through the bundle of His, and into the right and left bundle branches before finally reaching the purkinje fibres

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25
Q

What is the role of each channel in the image?

  • INa
  • ICa,L
  • INCX
  • Ito,f
  • Ito,s
  • IKs
  • IKr
  • IKI
A
  • INa
    • ​Na Current (inward) - depolarizing
  • ICa,L
    • L-type Calcium channel (inward) depolarizing
  • INCX
    • Na+/Ca++ exchanger (3Na/1Ca) electrogenic current
  • Ito,f
    • Fast transient outward K+ current (outward) - repolarizing
  • Ito,s
    • Slow transient outward K+ current (outward) repolarizing
  • IKs
    • Slowly activating delayed rectifying K+ current (outward) - repolarizing
  • IKr
    • rapidly activating delayed rectifying K+ current (outward) - repolarizing
  • IKI
    • Inward rectifying K+ current - maintains resting membrane potential
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26
Q

Which ion channels are involved in depolarization?

A

INa - Sodium channel - Na in = depol

ICa,L - L-type Ca++ channel (inward) depol

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27
Q

Which channels are involved in repolarization? (4)

A

Ito,f - Fast transient outward K+ current

Ito,s - slow transient outward K+ current

IKs - slowly activating delayed rectifying K+ current

IKr - rapidly activating delayed rectifying K+ current

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28
Q

Which ion channel produced electrogenic current?

A

Na+/Ca++ exchanger (INCX) - 3Na+/1Ca++

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29
Q

Which ion channel maintains the resting potention?

A

IKl - Inward rectifying K+ current

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30
Q

What are the 5 stages of cardiac electrical propagation?

A
  1. Muscle impulse is generated at the sinoatrial node. It spreads throughout the atria and to the atrioventricular node
  2. Atrioventricular node fibres delay the muscle impulse as it passes to the atrioventricular bundle
  3. The atrioventricular bundle (Bundle of His) conducts the muscle impulse into the interventricular septum
  4. Within the interventricular septum, bundle branches split from the atrioventricular bundle
  5. The muscle impulse is delivered to purkinje fibers in each ventricle and distributed throughout the ventricular myocardium
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31
Q

What are four factors that determine the shape of the cardiac action potential?

A
  • Chamber
  • Development
  • Disease
  • Type
32
Q

Based on the shape of the action potential, determine which region of the heart it is from?

33
Q

Why do cardiac muscles have a longer refractory period?

A

to avoid tetanus of the heart

and ensures that each contraction is followed by enough time to allow the heart chamber to refill with blood before the next contraction.

34
Q

Voltage gated Na+ channels are important for which phase of the action potential?

A

The upstroke (phase 0)

35
Q

Na+ channels are most common in which myocytes? WHy?

A

Na+ channels are highly expressed in His-Purkinje cells compared to atrial or ventricular myocytes

  • important for rapid propagation of the AP
36
Q

Na+ channels are NOT found in _______

A

Na+ channels are NOT found in SA nodes

37
Q

Na+ channels are expressed more in ___________ (endocardium or epicardium)

A

endocardium

38
Q

Describe the make-up of voltage gated Na+ channels?

A
  • Only one alpha subunit
    • four domains (D1-D4)
    • 24 transmembrane spanning segments
      • 6 in each domain
    • Pore-forming loop between 5th/6th transmembrane spanning segment in each domain
    • Voltage sensor on 4th TMD
    • Accessory Beta subunit (modifies activity but is not essential for function)
39
Q

Which sodium channel is located in the heart and which gene transcribes it?

A

the Nav1.5 channel

gene: SCN5

40
Q

Is Nav1.5 sensitive to tetrodotoxin (TTX) (in puffer fish)?

A

No - heart keeps beating

Most other Na+ channels are sensitive to the toxin

41
Q

What two types of voltage-gated calcium channel contribute to cardiac action potential?

A
  1. T-type (ICa,T)
    • minimal to no contribution to upstroke during phase 0
    • Low threshold Ca++ channels
    • High expression in nodal and purkinje cells
    • Scarce in adult ventricular cells
  2. L-type (ICa,L)
    • important for maintaining the plateau phase (phase 2)
    • Allows Ca++ entry that is important for initiating cardiac contraction
42
Q
  1. T-type (ICa,T)
    • Active during which phase?
    • sensitivity?
    • Location?
  2. L-type (ICa,L)
    • important during which phase
A
  1. T-type (ICa,T)
    • minimal to no contribution to upstroke during phase 0
    • Low threshold Ca++ channels
    • High expression in nodal and purkinje cells
    • Scarce in adult ventricular cells
  2. L-type (ICa,L)
    • important for maintaining the plateau phase (phase 2)
    • Allows Ca++ entry that is important for initiating cardiac contraction
43
Q

What is the structure of voltage gated Calcium channels?

A
  • One alpha-1 subunit (pore forming unit, four domains)
  • 24 transmembrane segments
  • requires the accessory subunits (beta, alpha 2 omega in heart (skeletal muscle requires gamma as well))
  • Loop between 5th/6th tss
  • voltage sensor in 4th tss
44
Q

In addition to contributing to AP formation, L-type Ca++ channels are Key players in __________

A

In addition to contributing to AP formation, L-type Ca++ channels are Key players in excitation-contraction coupling

45
Q

Which three calcium channels (CavX.X) are located in the heart?

Are they L or T type

Blocked by?

A
  1. Cav1.2
    • L-type
    • blocked by nifedipine, verapamil, diltiazem
      • tx for heart conditions
  2. Cav3.1
    • T-type
    • blocked by mibefradil
  3. Cav3.2
    • T-type
    • blocked by mibefradil
46
Q

What are the three structural types of K+ channels in the heart

A
  1. Background K+ channels (2-pore channels)
  2. Voltage dependent K+ channels (Kv channels)
  3. Inward rectifiers (Kir channels)
47
Q

Based on the image, label each channel as a Kv channel, Kif channel or 2-pore channel

48
Q

What are the four key voltage-gated K+ channels in the heart?

A
  1. Transient outward (Ito)
  2. Rapidly activating delayed rectifying (Ikr)
  3. Slowly activating delayed rectifying (Iks)
  4. Inward rectifying (IKI)
49
Q

What is required to activate voltage gated K+ channels?

A

deopolarization

50
Q

Ito

Transient outward K+ current

  • active during which phase
  • molecular candidates in fetal/neonate vs adult
A

Ito

Transient outward K+ current

  • contributes to the initial repolarization during phase 1 of the action potential
  • Kv1.2 - fetal/neonate (slower)
  • Kv4.2 and Kv4.3 - adult (rapid current = steeper phase 1)
51
Q

What is C-type inactivation?

A

Conformation change after N-type inactivation

52
Q

How does the transient outward K+ current (Ito) vary between epicardial, midmyocardial and endocardial myocytes?

A

More Ito in epicardial and mid-myocardial myocytes = shorter phase 1 = shorter AP

53
Q
  • IKr , hERG
    • name
    • involved in which phase
    • hERG stands for:
    • Molecular clone
A
  • IKr , hERG
    • Rapidly activating delayed rectifying K+ current
    • involved in phase 2 and early and late repolarization of phase 3 of the action potential
    • hERG stands for: human homolog of ether-a-go-go gene
    • Molecular clone: hERG channels (Kv11.1, KCNH2)
54
Q

What happens if you take a IKr inhibitor (blocker) or if it is blocked by disease?

A

Prolonged action potential

Prolonged QT segment (LQT syndrome, LQT2)

  • QRS - ventricular depol (contraction)
  • T-wave = ventricular repol (relaxation)
55
Q

IKs

  • name
  • involved in which phase
  • Channel structure
A

IKs

  • Slowly activating delayed rectifying K+ current
  • Contributes to the late repolarization during phase 3 of the AP (required for termination of the action potential
  • Channel structure
    • heteromer formed by the assembly of an alpha-subunit, KvLQT1 (KCNQ1) and a beta-subunit, minK (KCNE1) subunits
56
Q

Which channel is required for termination of the action potential?

A

IKs

Slowly activating delayed rectifying K+ current

57
Q

Mutation in KCNQ1 leads to:

A

LQT1 - subtype of Romano–Ward syndrome

KCNQ1 is the alpha subunit of the IKs channel (slowly activating delayed rectifying K+ channel)

58
Q

mutation in minK leads to:

A

LQT5

minK encodes the beta-subunit of the IKs channel (slowly activating delayed rectifying K+ channel)

59
Q

Which K+ channels mediate inward current?

A

Inward rectifying K+ channel - IK1

60
Q

Which channel has only 2 transmembrane spanning segments and conducts a strong inward K+ current?

A

Inward rectifying K+ channel - IK1

61
Q

What happens at the inward rectifying K+ channels (IK1) when Em < Ek ?

A

When the membrane potential falls below the equilibrium potential for k+, the IK1 channel supports the flow of positively charged K+ ions into the cell, depolarizing the membrane potential back to the resting potential

(compensates for hyperpolarization)

62
Q

What Kir clones are important for IK1 channels in the heart?

A

Kir2.1

  • gene KCNJ2

Kir 2.2

  • gene: KCNJ12

Kir2.3

  • gene: KCNJ4

Kir2.4

  • gene: KCNJ14
63
Q

which gene encodes Kir2.1

A

KCNJ2

IK1 channel

64
Q

Which gene encodes Kir2.2

A

KCNJ12

IK1 ​channel

65
Q

Which gene encodes Kir2.3

A

KCNJ4

IK1 ​channel

66
Q

Which gene encodes Kir2.4

A

KCNJ14

IK1 ​channel

67
Q

Which ion channel maintains the resting membrane potential in cardiomyocytes?

A

Inward rectifier K+ channels - IK1

68
Q

Inward rectifier K+ channels (IK1) are absent in __________ cells where the resting membrane potential is modulated by ___ and ______

A

Inward rectifier K+ channels (IK1) are absent in SA and AV nodal cells where the resting membrane potential is modulated by If and IK-Ach

69
Q

Which channels modulate the resting membrane potential in SA and AV nodes?

A

If and IK-Ach

70
Q

Autorythmic cells (nodal cells) have an unstable resting membrane potential, what ion channel is responsible for this?

A

If

HCN channels (pacemaker channels)

71
Q

What are If or HCN channels?

  • structure
  • selectivity
  • molecular clones in heart
  • Activated by:
  • Regulated by:
A
  • Pacemaker channels
  • Non-selective
  • Structure similar to voltage-activated K channels
    • alpha subunit structure
    • 6TMD - pore forming between 5/6 / voltage sensor in 4
  • Molecular clones in the heart:
    • HCN1
    • HCN2
    • HCN4
    • HCN = hyperpolarization-activated Cyclic nucleotide-gated
  • Activated by hyperpolarization
  • Regulated by cAMP
72
Q

If is expressed in _________ cells

A

If​ is expressed in autorhythmic cells

  • highly expressed in nodal (SA and AV) and Purkinje cells
73
Q

What are If channels important for?

A

Important for initiation of the action potential (in nodal cells)

  • Pacemaker activity
  • Nonselective cation conductance (Na,Ca)
74
Q

What ions pass through the If channel?

A

Non-selective cation conductance (Na and Ca)

75
Q

What activates If channels?

A

If channels are activated when the membrane is hyperpolarized

  • mediate depolarization current
  • gradual depolarization
76
Q

What is KUR

  • Phase?
  • Location?
  • Molecular clone?
A

Ultrarapidly activating delayed rectifying K+ channels

  • involved in repolarization during phase 2
  • Present only in atrial (not ventricular) cells
  • Kv1.5

PHYSL 404 - Cardiovascular Physiology (17 decks)

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