Ionic Basis Of The Cardiac Action Potential

Question 1

In regards to a Na+ ion channel: how many transmembrane segments are there per alpha subunit

Answer 1

24 transmembrane segments per alpha subunit

Question 2

In regards to a Na+ ion channel: how many alpha subunits are there per Na+ channel

Answer 2

1 alpha subunit per Na+ channel

Question 3

How is the Na+ ion channel coded by genes

Answer 3

Coded as one long strand

Question 4

In regards to a K+ ion channel: how many transmembrane segments per alpha subunit

Answer 4

6 transmembrane segments per alpha subunit

Question 5

In regards to a K+ ion channel: how many alpha subunits per K+ channel

Answer 5

4 alpha subunits per K+ channel

Question 6

How do genes code for a K+ ion channel

Answer 6

Code in 4 separate sections and cell puts together

Question 7

Role of beta subunits in ion channels?

Answer 7

Modify membrane expression and modify behaviour of alpha subunits

Question 8

What are the auxiliary (accessory-other than alpha) subunits of the Ca2+ ion channel

Answer 8

Beta subunit
Gamma subunit
Alpha2-delta subunit (joined by s-s bond)

Question 9

Ohms law equation and the current equation we derive from it

Answer 9

Ohms law: V= I x R
Derive: I= V x G
* R= 1/G *

Question 10

What does G stand for

Answer 10

Conductance of ions

Question 11

What makes up delta V (voltage)

Answer 11

Delta V= Vm-Eion
Vm- membrane potential
Eion- equilibrium potential for that ion

Question 12

What forces act on an ion when it moves?

Answer 12

Electrical gradient
Concentration gradient
together its called the electrochemical gradient

Question 13

How does the nernst equation change units?

Answer 13

Turns chemical units into electrical units

Question 14

Provide the nernst equation

Answer 14

Eion = (RT)/(zF)*ln(ion-out/ion-in)

Question 15

What are the equilibrium potentials of: K, Na, Ca

Answer 15

Ek= -90 mV
Ena= +65 mv
Eca= +120 mV at rest AND +90 mV during contraction

Question 16

At rest what ions have the most conductance (G) through the membrane

Answer 16

Potassium

Question 17

What phenomenon does the goldman-hodgkin-katz equation explain

Answer 17

The membrane potential (Vm) is closest to the equilibrium potential of the ion that permeates the membrane most freely (ex. Only K permeates, Vm= Ek)

Question 18

What does the conductance of an ion depend on

Answer 18

If channel is open
Number of ion channels

Question 19

When you are depolarizing the membrane, what charge is moving and what direction

Answer 19

Positive charge is moving into the cell

Question 20

What events occur in skeletal muscle that do not occur in cardiac muscle and why do they not occur in cardiac muscle

Answer 20

1. Variable recruitment of muscle fibres
2. Frequency dependence- summation and tetanus
   Cant occur because:
3. Functional syncytium so 100% recruitment with every beat
4. No summation or tetany due to long AP duration

Question 21

Name the 5 phases of cardiac action potential

Answer 21

4- resting membrane potential
0- depolarization
1- rapid repolarization
2- plateau
3- repolarization

Question 22

What channels control phase 4 and what direction do the ions flow

Answer 22

4- Resting membrane potential
Kir2.1 channels
Ik1 current flows out

Question 23

What channels control phase 0 and what direction do ions flow

Answer 23

0- depolarization
Nav1.5 channels
Ions flow into cell

Question 24

What channels control phase 1 and what direction do ions flow

Answer 24

1- rapid repolarization (epicardium, surface of myocyte)
Kv4.3 channels
Transient K+ current out of the cell (Ik,to current)

Question 25

What channels open during phase 2 and what direction do ions flow

Answer 25

2- plateau phase
Ica- ca2+ enters cell to depolarize Cav1.2 channel
Incx- 3Na+ in/ 1Ca2+ out
Ik1- blocked (allows plateau)
Ik,DR- K+ exits cell, repolarizing drive

Question 26

What channels open during phase 3 and what direction do ions flow

Answer 26

3- repolarization phase
Ik,DR- delayed rectifier returns cell to resting membrane potential

Question 27

Name a drug that can inhibit the Ca2+ current. What happens to the cardiac action potential if Ica is inhibited? How does this effect the force of contraction?

Answer 27

Drug = diltiazem
The Ica depolarizes the cell, with ca entering the cell. Calcium current’s role is to increase the duration of the plateau phase. The plateau phase is in charge of prolonging the AP. Therefore, less current will reduce the plateau phase and will decrease the cardiac action potential. Reducing the AP means that less ca2+ influx has occurred which would in turn decrease the force of contraction.

Question 28

Name the different potassium currents and the phase of cardiac action potential they are contributing to.

Answer 28

IK,to- phase 1 (rapid repolarization). transient outward current
IK1- phase 4 (resting membrane potential), phase 2 (if long plateau?). inward rectifier
IK,dr- phase 2 (plateau), phase 3 (repolarization). Delayed rectifier

Question 29

Define rectification

Answer 29

Rectification- conductance of ions through a channel is greater in one direction than the other

Question 30

What direction do ions flow from the inward rectifier Ik1 under physiological conditions and under preferred?

Answer 30

Physiological range- k+ exits the cell during resting membrane potential
Preferred- k+ enters cell, this condition occurs outside of myocyte range (voltage needed is below -80mV)

Question 31

What are the three delayed rectifier currents Ik,DR

Answer 31

1. Slow delayed rectifier- Iks, Kv7.1
2. Rapid delayed rectifier Ikr, Kv11.1
3. Ultra-rapid delayed rectifier Ikur, Kv1.5

Question 32

Name the depolarizing currents and the repolarizing currents (neglect phases)

Answer 32

Depolarizing- INa, ICa, Incx (inward current)
Repolarizing- IK1, IKr, IKs, IKur, IK,to, (outward current)

Question 33

What determines if the delayed rectifier is “delayed” and what kind of current does it pass?

Answer 33

Delayed condition means that the current is slower than INa
Passes outward current

Question 34

Define absolute refractory. Condition for absolute refractory?

Answer 34

Define- a second response is not possible regardless of strength or duration of stimulus
Condition- Na channels are inactivated so another AP can not occur

Question 35

Define relative refractory. Condition for relative refractory

Answer 35

Define- second AP can be elicited but at a greater cost (strength/duration)
Condition- some Na channels still inactivated