Cardiac Ion Channels & Action Potentials

Question 1

T or F: Electrical activity propagates within myocardium AND via specialized conductive pathways

Answer 1

True

Question 2

What cardiac structure serves as the main control for HR?

Answer 2

pacemaker cells in the sinoatrial node (SA node).

• fire intrinsically at ~100/min.
• Rate modulated by ANS.

Question 3

What slows down the intrinsic HR of the SA node?

Answer 3

Parasympathetic tone slows rate to 60-80/min.

Question 4

T of F: AV node is also capable of spontaneous activity

Answer 4

True

Question 5

What is overdrive suppression?

Answer 5

Describes how the AV node is driven by action potentials originating in the SA node since their firing frequency is less than the SA node.

Question 6

What are ectopic pacemakers?

Answer 6

Self firing cells that (or others, especially cells in damaged regions of the myocardium) can take over initiation of the heartbeat Under abnormal circumstances

Question 7

How does cell-to-cell conduction take place?

Answer 7

gap junctions

Question 8

How is the direction of electricl propagation controlled?

Answer 8

by gap junction position and by connective tissue “insulation” (e.g., between the atria and ventricles).

Question 9

T of F: The AV node is the only place where action potentials can propagate from the SA node to the ventricles.

Answer 9

True

Question 10

Which structures are characterized as having a fast action potential?

Answer 10

• Atrial and ventricular muscle

- Purkinje fibers

Question 11

Which structures are characterized as having a slow action potential?

Answer 11

SA & AV nodes

Question 12

Direction of current flow depends on what?

Answer 12

• membrane potential (Vm) and

- ion gradient (Nernst Potential, Eion)

Question 13

T or F: If Vm<Eion, current flows into cell

Answer 13

True

Question 14

T or F: If Vm>Eion, current flows out of cell

Answer 14

True

Question 15

What causes depolarization?

Answer 15

Current flowing into cell causes depolarization

Question 16

What causes hyperpolarization?

Answer 16

Current flowing out of cell causes hyperpolarization

Question 17

T or F: ion channels serve as gates; open upon depolarization and close upon repolarization

Answer 17

True

Question 18

T of F: Current is constant with voltage

Answer 18

True

Question 19

T of F: If we keep the voltage positive inactivation gate closes with time

Answer 19

True

Question 20

What is the characteristic subunit in cardiac sodium channels?

Answer 20

NaV1.5 , cardiac Na channels show Voltage-dependent inactivation

Question 21

What are the types of Ca++ channels in the heart?

Answer 21

L & T types. Also, Ryanodine Receptors (intracellular- SR)

Question 22

What are the characteristics of the L type Ca++ channels

Answer 22

-L-type current; dihydropyridine receptor (DHPR)
-(CaV1.2, CaV1.3)

Question 23

What are the characteristics of the T type Ca++ channels

Answer 23

• T-type current
• (CaV3.1, CaV3.2)
• Low-Voltage Activated (LVA)
• Voltage-dependent Inactivation

Question 24

List the Time-dependent potassium currents

Answer 24

-IKto (Kv4.3 tetramer + KChiP2): voltage-dependent inactiv.
-IKr (HERG tetramer + miRP1): “rapid” delayed rectifier
-IKs (KvLQT1 tetramer + minK):“slow” delayed rectifier
Depolarization causes activation of IKr and IKs

Question 25

What are the characteristics of the time dependent cation current?

Answer 25

If (or Ih) (HCN tetramer)
Evoked by hyperpolarization
Na+ & K+ permeable, Erev  -30 mV

Question 26

What are the characteristics of the Inward Rectifier potassium current

Answer 26

IK1 (Kir tetramer)
IKACh (GIRK tetramer)
  Increased by activation of
   muscarinic receptors; 
   slows pacemaking

Question 27

The categorization of cardiac action potentials as fast or slow is based on _________

Answer 27

on whether the initial upstroke is rapid or slow

Question 28

Phase 0, fast

Answer 28

rapid depolarization caused by the entry of sodium ions (INa) through voltage- activated sodium channels

Question 29

Phase 1, fast

Answer 29

partial repolarization produced by inactivation of sodium current and activation of a transient potassium current IKto.

Question 30

Phase 2, fast

Answer 30

prolonged plateau, voltage-activated, L-type calcium channels are open. influx of calcium = efflux of potassium ions (IKr and IKs) so that membrane potential remains at a roughly constant level (near 0 mV)

Question 31

phase 3, fast

Answer 31

rapid repolarization due to inactivation of (ICa) and increasing activation of IKr and IKs causes termination of the plateau

Question 32

Phase 4, fast

Answer 32

IKr and IKs are de-activated, and inactivation of INa and ICa is removed; the cell is held near EK (phase 4) by the inward rectifier (IK1).

Question 33

What are the notable difference b/t currents in myocardial cells and the nodes?

Answer 33

• pacemaker cells have reduced INa and little IK1;

- pacemaker cells express If and ICa-T which are essentially absent in myocardial cells

Question 34

T of F: there is no stable resting potential in pacemaker cells

Answer 34

True

Question 35

Phase 0, slow

Answer 35

upstroke due to activation of ICa-T and ICa-L and is relatively slow owing to the absence of INa

Question 36

Phase 3, slow

Answer 36

repolarization occurs shortly after the peak of the action potential due to balance between ICa and delayed rectifier current (IKr and IKs)

Question 37

Phase 4, slow

Answer 37

repolarization is followed by a slow depolarization (the “pacemaker potential”)

Question 38

What induced pacemaker potential?

Answer 38

funny current (If) which is induced by hyperpolarization

Question 39

HERG produces what?

Answer 39

IKr, Impt. for duration of slow and fast action potentials

Question 40

Reducing IKr can result in?

Answer 40

arrhythmias