Cardiac Action Potential and Excitation-Contraction Coupling Flashcards
What law is biolectricity based on?
Ohm’s law - Current = Voltage/Restistance (I=V/R)
What is voltage?
Electrical driving force for current (ions) to flow
In bioelectricity, where does voltage come from?
Membrane potential
Membrane potential:
1. What is it?
2. What does it arise as a result of?
3. What does it determine?
- It is the electrical driving force for ionic fluxes
- Arises as a result of separation of charge across a biological barrier
- Determines the size and direction of current flow through ion channels alongside the chemical gradient
What is meant by voltage clamp and a current clamp?
Voltage clamp - if you fix the voltage you can get the current
Current clamp - if you fix the current you can get the voltage
Resting ionic distributions across sarcolemma:
Sodium is high/low on the outside and high/low on the inside
Sodium is high on the outside and low on the inside
Resting ionic distributions across sarcolemma:
Potassium is high/low on the outside and high/low on the inside
Potassium is low on the outside and high on the inside
Resting ionic distributions across the sarcolemma:
Calcium is high/low/very low on the outside and high/low/very low on the inside
Calcium is low on the outside and very low on the inside
Resting ionic distributions across sarcolemma:
Chloride is high/low on the outside and high/low on the inside
Chloride is high on the outside and low on the inside
Give the values for the resting ionic distributions across the sarcolemma on the outside and inside:
Na+
K+
Ca2+
Na+ - outside = 140, inside = 15
K+ - outside = 5, inside = 140
Ca2+ - outside = 2, inside = <0.001
What does the Nernst equation calculate?
The membrane potential in which forces driving ion movement are in equilibrium (no net movement of ions)
What determines the equilibrium potential of ions?
The Nernst equation
What is the equilibrium potential of sodium ions?
+59mV
What is the equilibrium potential of potassium ions?
-89mV
What is the equilibrium potential for calcium ions?
+140mV
What is the equilibrium potential for chloride ions?
-59mV
What is patch clamp electrophysiology used to measure?
Bioelectricity
Describe the technique of patch-clamp electrophysiology
- Polished pipette filled with a mock intracellular solution is lowered onto the cell
- Suction is applied to achieve a tight electrical and physical seal between the glass and cell membrane
- The voltage/current of this piece of membrane is now under your control
Describe the structure of a voltage-gated Na+ channel
4 homologous domains (D1-D4) with each domain containing 6 segments (S1-S6)
What segment is a “voltage-sensor”
S4 transmembrane segment
Describe the cycle of the voltage-gated Na+ channel
- At rest it is closed
- It is activated by applying a depolarising pulse, the channel opens and sodium goes into the cell
- It enters an inactivated state almost immediately after opening
- It enters intermediate inactivated state before going back to resting state
What shape will the current voltage relationship be if you plot it?
U-shaped
What is the functional unit of the Na+/K+ ATPase?
𝛂𝛃 subunit
How many domains are on an alpha subunit of the Na+/K+ ATpase?
10
What binds to the alpha subunit of the Na+/K+ ATPase?
Na+, K+ and ATP
How many domains are on the beta subunit of the Na+/K+ ATPase?
1 - single transmembrane domain
What is the function of the beta subunit of the Na+/K+ ATPase?
Trafficking of the complex to the sarcolemma - ensures that Na+/K+ ATPase ends up in the sarcolemma
What is the gamma subunit of the Na+/K+ ATPase also known as?
Phospholemman
What is the function of phospholemman?
Acts as on/off switch for Na+/K+ ATPase
What happens to the gamma subunit of Na+/K+ ATPase when phosphorylated/de-phosphorylated?
Phosphorylated - unbound
De-phosphorylated - binds and switches off function of Na+/K+ ATPase
The gamma subunit of Na+/K+ ATPase is unbound when phosphorylated/de-phosphorylated
Phosphorylated
What happens when the gamma subunit of the Na+/K+ ATPase is dephosphorylated?
It binds and switches off the function of Na+/K+ ATPase
In the Na+/K+ ATPase electrogenic pump, sodium ions move in to/out of the cell and potassium ions move in to/out of the cell?
Sodium ions move out of the cell and potassium ions move in to the cell
How many sodium ions move out of the cell and how many potassium ions move in to the cell via Na+/K+ ATPase
3 sodium ions out of the cell for every 2 potassium ions in to the cell
Where does the energy for each cycle of Na+/K+ ATPase pump come from?
Hydrolysis of ATP
What 2 things are responsible for setting the resting membrane potential?
Relative permeability and concentrations of Na+ and K+
What is the relative permeability of Na+:K+?
1:100
At rest, potassium ions are 100x more permeable to across the membrane than sodium ions
In the heart, what determines the resting membrane potential and why?
K+ movements because they are moving at rest
What are the 4 types of K+ channels?
Voltage-gated
Inwardly rectifying
Hyperpolarisation activated
Ca2+ activated
What is the main K+ channel responsible for maintaining resting membrane potential in non-modal cardiomyoctes?
Inwardly rectifying K+ channels
True or false:
In voltage-gated K+ channels, the current activity changes with voltage?
True
Describe the activity of voltage-gated K+ channels at resting membrane potential
They have no activity
How do inwardly rectifying K+ channels work?
Magnesium blocks pore and as the membrane depolarises the blockage is relieved
According to the Nernst equation, what is the equilibrium potential for Na+?
+70mV
According to the Nernst equation, what is the equilibrium potential for K+?
-88mV
Why is the membrane permeability for K+ higher than Na+?
Na+ channels are mostly closed at rest
What are the 5 phases of cardiac action potential?
Phase 0 - depolarisation
Phase 1 - early repolarisation
Phase 2 - plateau phase
Phase 3 - late depolarisation
Phase 4 - Stabilisation of the resting membrane potential
What happens in phase 0 of the cardiac action potential?
The opening of sodium channels causes depolarisation
Membrane potential goes from -80/-90mV to +40/+50mV
What protein underlies the depolarisation phase of the cardiac action potential?
NaV1.5
What is phase 1 of the cardiac action potential mediated by?
Transient outward K+ channels
What are the 2 components of the transient outward K+ currents of phase 1 of the cardiac action potential?
Fast (Kv4.2 and Kv4.3) and slow (Kv1.4)
True or false - there is a second transient outward current in phase 1 of the cardiac action potential?
True - carried by a Ca2+ activated Cl- current
What happens in phase 2 of the cardiac action potential?
There is a mixture of inward and outward currents.
The sum activity means membrane potential doesn’t really change resulting in plateau
What happens in phase 3 of the cardiac action potential?
There is Ca2+ dependent inactivation of L-type calcium channels which starts to bring the membrane potential back down
How does an action potential trigger a contraction?
It triggers a rise in cytoplasmic Ca2+ concentration
Stages of cardiac excitation and contraction coupling
- T-tubule depolarisation opens LTCC
- Ca2+ entry via LTCC raises conc. Ca2+ in junctional cleft
- Calcium activates clusters of RyR2 in junctional sarcoplasmic reticulum
- Ca2+ binds to myofilaments and initiates contractions
- Intracellular Ca2+ is removed
- Reduction of conc. Ca2+ causes unbinding of Ca2+ from myofilaments leading to relaxation
In what form are calcium ions released from the sarcoplasmic reticulum?
Released as sparks
True or false:
Calcium sparks only occur spontaneously
False - they can occur spontaneously or be evoked by depolarisation of cell patches
What are the 3 means by which Ca2+ can be removed from the cytoplasm to allow relaxation?
By SERCA (sarcoendoplasmic reticulum Ca2+ ATPase) back to SR
By NCX (sodium calcium exchanger) out of cardiomyocyte
By PMCA (plasmalemmal Ca2+ ATPase) out of cardiomyocyte
What isoform of SERCA is usually found in the cardiomyocyte?
SERCA2a
How does SERCA work to remove Ca2+ back into the sarcoplasmic reticulum?
Moves Ca2+ against the concentration gradient through hydrolysis of ATP
What does phospholamban (PLB) do?
Endogenous inhibitor which regulates the removal of Ca2+ via SERCA
How does NCX work to remove Ca2+ out of the cardiomyocyte?
Uses the sodium gradient (high on outside, low on inside) to move Ca2+ out of cell against concentration gradient
In the normal mode of the NCX, how many sodium ions are moved in to the cell for how many calcium ions moved out?
3 Na+ in for 1 Ca2+ out
3 positive ions in for 2 positive ions out resulting in depolarisation (the reverse is hyperpolarising)
How is Ca2+ balance in the SR maintained?
Ca2+ entry via LTCC and exit via NCX
What is the difference in cardiomyocyte movement when there is normal Ca2+ maintenance vs overload in the SR?
Normal Ca2+ maintenance = whole cell contraction
Overload = cardiomyocyte quivers rather than contracts
What is the issue caused by the cardiomyocyte quivering rather than contracting when the SR is overloaded with Ca2+?
Can lead to delayed after-depolarisations and lead to re-enrant arrhythmias
Where to neurogenic hearts receive their pacemaker commands from?
Neuronal innervation from an extracardiac source: the cardiac ganglion
Where do myogenic hearts receiver their pacemaker commands from?
They have their own intrinsic muscle-derived pacemaker
What is the difference between the contraction in neurogenic hearts vs myogenic hearts?
Neurogenic hearts - all parts of the heart tend to contract simultaneously
Myogenic hearts - tend to contract sequentially - wave of contraction spreads monotonically from one end to the other
Is the mammalian heart neurogenic or myogenic?
Myogenic - although is a functional mixture between a neurogenic and myogenic heart
True or false:
The action potential shape of the cardiomyocytes of atrial ventricular muscle is the same as the nodal action potential shape
False
True or false:
The pacemaker action potential is more positive than the resting membrane potential
True - -50mV compared to -90mV
Why is upstroke a lot slower in the action potential of nodal cells?
There are no voltage gated sodium channels
What are the phase of the action potential in nodal cells?
Phase 0
Phase 3
Phase 4
There are decreased/increased potassium channels in phase 3 of the action potential in nodal cells compared to the action potential of other cardiomyocytes
There are increased potassium channels
What is phase 4 of the action potential in nodal cells?
The pacemaker potential
What is the ‘funny current’?
A hyperpolarisation activated current which is carried by Na+ K+ and is involved in phase 4 of the action potential of nodal cells
