Ionic Basis Of The Cardiac Action Potential Flashcards
In regards to a Na+ ion channel: how many transmembrane segments are there per alpha subunit
24 transmembrane segments per alpha subunit
In regards to a Na+ ion channel: how many alpha subunits are there per Na+ channel
1 alpha subunit per Na+ channel
How is the Na+ ion channel coded by genes
Coded as one long strand
In regards to a K+ ion channel: how many transmembrane segments per alpha subunit
6 transmembrane segments per alpha subunit
In regards to a K+ ion channel: how many alpha subunits per K+ channel
4 alpha subunits per K+ channel
How do genes code for a K+ ion channel
Code in 4 separate sections and cell puts together
Role of beta subunits in ion channels?
Modify membrane expression and modify behaviour of alpha subunits
What are the auxiliary (accessory-other than alpha) subunits of the Ca2+ ion channel
Beta subunit
Gamma subunit
Alpha2-delta subunit (joined by s-s bond)
Ohms law equation and the current equation we derive from it
Ohms law: V= I x R
Derive: I= V x G
* R= 1/G *
What does G stand for
Conductance of ions
What makes up delta V (voltage)
Delta V= Vm-Eion
Vm- membrane potential
Eion- equilibrium potential for that ion
What forces act on an ion when it moves?
Electrical gradient
Concentration gradient
together its called the electrochemical gradient
How does the nernst equation change units?
Turns chemical units into electrical units
Provide the nernst equation
Eion = (RT)/(zF)*ln(ion-out/ion-in)
What are the equilibrium potentials of: K, Na, Ca
Ek= -90 mV
Ena= +65 mv
Eca= +120 mV at rest AND +90 mV during contraction
At rest what ions have the most conductance (G) through the membrane
Potassium
What phenomenon does the goldman-hodgkin-katz equation explain
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)
What does the conductance of an ion depend on
If channel is open
Number of ion channels
When you are depolarizing the membrane, what charge is moving and what direction
Positive charge is moving into the cell
What events occur in skeletal muscle that do not occur in cardiac muscle and why do they not occur in cardiac muscle
- Variable recruitment of muscle fibres
- Frequency dependence- summation and tetanus
Cant occur because: - Functional syncytium so 100% recruitment with every beat
- No summation or tetany due to long AP duration
Name the 5 phases of cardiac action potential
4- resting membrane potential
0- depolarization
1- rapid repolarization
2- plateau
3- repolarization
What channels control phase 4 and what direction do the ions flow
4- Resting membrane potential
Kir2.1 channels
Ik1 current flows out
What channels control phase 0 and what direction do ions flow
0- depolarization
Nav1.5 channels
Ions flow into cell
What channels control phase 1 and what direction do ions flow
1- rapid repolarization (epicardium, surface of myocyte)
Kv4.3 channels
Transient K+ current out of the cell (Ik,to current)
What channels open during phase 2 and what direction do ions flow
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
What channels open during phase 3 and what direction do ions flow
3- repolarization phase
Ik,DR- delayed rectifier returns cell to resting membrane potential
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?
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.
Name the different potassium currents and the phase of cardiac action potential they are contributing to.
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
Define rectification
Rectification- conductance of ions through a channel is greater in one direction than the other
What direction do ions flow from the inward rectifier Ik1 under physiological conditions and under preferred?
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)
What are the three delayed rectifier currents Ik,DR
- Slow delayed rectifier- Iks, Kv7.1
- Rapid delayed rectifier Ikr, Kv11.1
- Ultra-rapid delayed rectifier Ikur, Kv1.5
Name the depolarizing currents and the repolarizing currents (neglect phases)
Depolarizing- INa, ICa, Incx (inward current)
Repolarizing- IK1, IKr, IKs, IKur, IK,to, (outward current)
What determines if the delayed rectifier is “delayed” and what kind of current does it pass?
Delayed condition means that the current is slower than INa
Passes outward current
Define absolute refractory. Condition for absolute refractory?
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
Define relative refractory. Condition for relative refractory
Define- second AP can be elicited but at a greater cost (strength/duration)
Condition- some Na channels still inactivated
