Cardiac action potential Dr Shreetal Rajan Nair

Introduction Ions Channels/Pores/Carriers & Pumps Channels- Aqueous channel/ Conformational change/ Action usually regulated/ Open to both environment/ Large number of molecules diffuse across Pores- Continuously open to both environment/ No conformational changes/ Always open. Carriers & Pumps- Not open simultaneously to both environments/ Binding sites/ Limited number of molecules diffuse across Carriers & Pumps maintain the concentration gradients Channels- contain an aqueous channel. Hydrophilic molecules of the right size and shape can diffuse through this channel. In many cases, whether a channel is open or closed can be regulated; they can be turned on and off. Channel is opened with a conformational switch, it is open to both environment simultaneously (extracellular and intracellular); channels are either in open state or closed state. Pores-Pores are continuously open to these both environment, because they do not undergo conformational changes. They are always open. Carriers & Pumps-Pathway in a carrier protein is not open smiltaneously to the both environment. Either its inner gate is open, or outer gate is open, or both gates are closed. Carrier has binding sites, but porins and channel proteins have not. When a channel is opened, thousands to millions of ions can pass through the membrane in one time, but only one or a small amount of molecules can pass through a carrier molecule. A carrier molecule can be a pump (active transport); cotransporter or antiporter Biological membranes leak {since semipermeable}– without the constant addition of energy, the energy stored in concentration gradients across a membrane would dissipate over time, Carriers & Pumps maintain the concentration gradients

What makes ions to move across? Steady state is reached when the magnitude of the chemical and electric gradients are equal

What makes ions to move across? Nernst equation EK =RT/ZF ln [K]2 / [K]1 Where, T is temperature [370 C] R is the gas constant F is the Faraday constant Z is the valence of ion [1] [K]2 and [K]1 are the final concentrations of potassium in compartments 2 and 1, respectively. [150mmol, 5 mmol] EK is the equilibrium potential for potassium [-90mV] At equilibrium potential net diffusion is 0 All ions try to reach equilibrium i.e., tries to drive the membrane potential towards its equilibrium potential At RMP, membrane is permeable mostly to potassium , hence RMP is close to the EK

Membrane Action Potential 2 factors Electromechanical gradient Open Channels MAP Sum of AP generated by different channels [amplitude & direction] Number of open channels

Action potential - definition Action potential is the net product of mutiple distinctive time and voltage dependent ionic currents. Complex interaction between depolarising inward currents and hyperpolarising outward currents

ACTION POTENTIAL Excitable tissues : Neurons Muscles Action potential duration (APD) Neurons and skeletal muscle cells (1–5 ms) Cardiac myocytes (200-400 ms)

Some terms Strength of stimulus required to elicit a response or to produce an action potential. Hyperpolarisation decreases excitability Small reductions in RMP increase excitability When RMP is less than the threshold potential the fibre becomes inexcitable Threshold potential- potential at which net inward membrane current becomes large enough to initiate autoregenerative depolarization

Sites of action potential genesis in heart Nodal tissue – AV node and SA node His - Purkinje fibres - Atrial muscle Ventricular muscle

Refractory period Refractory Period- - The interval of time during which the cell cannot be re-excited [Absolute RP] - Relative RP Effective refractory period : minimum interval between 2 propagating AP - Supranormal Excitability

Cardiac AP Pacemaker potential (slow channel) vs myocyte potential ( fast channel ) Pacemaker potential occurs in cells capable of producing spontaneous impulses – 1% Myocyte potential occurs in all other cells – contractile cells – 99%

How a cardiac myocyte is stimulated ? Spontaneous impulse generation - automatic ( pacemaker) cells Spread of depolarising current from neighbouring cells ( cardiac syncytium )

Ion channels in the heart Sodium channels – voltage gated and ligand gated Potassium channels - voltage gated, inward rectifier and background channels Calcium channels – transient and long lasting

Non pacemaker cell – fast channel AP

Phase 4 Represents resting membrane potential membrane potential when the cell is not stimulated. this phase will be a horizontal line caused by the difference in ionic concentrations and conductances across the cell membrane normal resting membrane potential in the ventricular myocardium is about -85 to -95 mV

Phase 4 IK1 Current- Membrane stabilizing current [inward rectification Others-TWIK-1/2 (KCNK1/6), TASK-1 (KCNK3), and TRAAK (KCNK4) Na/K Pump- 3/2 outward; At fast HR RMP more negative Low [K]o leads to less IK1 activity, more excitability

Action potential of a ventricular myocyte and how is it different from action potential of atrial myocyte The membrane of the contractile cells remain essentially at rest at about -90mv until excited by electrical activity propagated by the pacemaker cells.

Phase 0 Rapid depolarisation phase Opening of the fast Na+ channels causing a rapid increase in the membrane conductance to Na+ Rate of membrane depolarisation corresponds to conduction velocity

Na+ channels 3 States Open Closed Inactivated Depends on the RMP

Gating & Inactivation Closing and opening of channels Voltage, Metabolic, Stretch

Gating & Inactivation m gate h gate The N-terminal or “ball and chain” mechanism of Na channel inactivation

video

RMP and Na channels The slope of phase 0 represents the maximum rate of potential change and is known as dV/dtmax  If the membrane potential is at its baseline (about -85 mV) all fast Na+channels are closed and excitation will open all of them When membrane potential is less negative some of the fast Na+ channels will be in an inactivated state insensitive to opening lesser response to excitation of the cell membrane and a lower Vmax

When the resting membrane potential becomes too positive the cell may not be excitable and conduction through the heart may be delayed

SODIUM CHANNEL BLOCKERS IA – decrease conduction velocity and increase refractoriness I B – only increase refractoriness I C - decrease only conduction velocity

Phase 1 Phase 1 of the myocyte action potential occurs with the inactivation of the fast Na+ channels The transient net outward current causing the small downward deflection of the action potential is due to the movement of K+ and Cl- ions, carried by the Ito1 and Ito2 currents respectively

Ito currents determine the amplitude and the timing of Ca2+ release from the SR. In advanced HF Ito currents are downregulated and indirectly affect the Ca currents in phase 2 Ito currents express differentially at different parts of heart Maximum in RV, basal region and epicardium Ito current – ACE inhibitors

Phase 2 "plateau" phase of the cardiac action potential (absent in pacemaker cells) sustained by a balance between inward movement of Ca2+ (ICa) through L-type calcium channels and outward movement of K+ through the slow delayed rectifier potassium channels - IKs

Phase 3 L-type Ca2+ channels close slow delayed rectifier (IKs) K+ channels are still open. more types of K+ channels open.- Ikr,Ikur,Ik1 the rapid delayed rectifier K+ channels (IKr)  inwardly rectifying K+ current, IK1 Additional channel in atrial muscle ( I kur ) Vernakalant has action on this channel RESULT IN REPOLARISATION

Potassium channels

Source :Zipes textbook of electrophysiology

Refractory period - explanation Absolute refractory period - Sodium channels are in an inactivated state - Relative refractory period - sufficient number of sodium channels have transitioned back to their resting state – a stronger stimulus can generate AP

Refractory period Absolute refractory period - beginning of phase 0 until nearly the end of phase 2 Relative refractory period - phase 3 and phase 4 - caused by changes in the state of sodium and potassium channel molecules

Afterdepolarisations Positive potentials that occur during the repolarisation phase of the action potential Types : Early – phase 2 and phase 3 Delayed – phase 4 Important in the genesis of arrhythmias

Afterdepolarisations

Mechanism of arrhythmogenesis in DAD

AP in pacemaker cells

Action Potential of the pacemaker cells The pacemaker cells do not have a stable resting membrane potential like the nerve and the skeletal muscles. Instead they have an unstable membrane potential that starts at – 60mv and slowly drifts upwards towards threshold. Because the membrane potential never rests at a constant value, it is called a Pacemaker Potential rather than a resting membrane potential. Phase 1 and 2 typically absent.

IONIC BASIS OF ACTION POTENTIAL OF PACEMAKER CELLS Phase 4: Pacemaker Potential: Opening of voltage-gated Sodium channels called Funny channels (If or f channels ). Closure of voltage-gated Potassium channels. Opening of Voltage-gated Transient-type Calcium (T-type Ca2+ channels) channels . Phase 0: The Rising Phase or Depolarization: Opening of Long-lasting voltage-gated Calcium channels (L-type Ca2+ channels). Large influx of Calcium. Phase 3: The Falling Phase or Repolarization: Opening of voltage-gated Potassium channels Closing of L-type Ca channels. Potassium Efflux.

Phase 4 Diastolic depolarisation Chronotropism or the rate of heart beat depends on the slope of pacemaker potential modulation by the autonomic system of the cardiac SAN rate also takes place in this phase  Sympathetic stimuli induce the acceleration of rate by increasing the slope of the pacemaker phase Parasympathetic stimuli has opposite effect

Regulation by autonomic nervous system Parasympathetic activation - c AMP levels decrease and open additional K channels and produce more hyperpolarisation. - phase 4 takes longer to reach the threshold voltage Sympathetic activation - increase c AMP levels and this in turn open calcium channels

Autonomic nervous system modulates the frequency of depolarization of pacemaker Sympathetic stimulation (neurotransmitter = ); binds to b1 receptors on the SA nodal membranes Parasympathetic stimulation (neurotransmitter = ); binds to muscarinic receptors on nodal membranes; increases conductivity of K+ and decreases conductivity of Ca2+

Clinical aspects

Summary

Refractory period In fast channels ERP/APD < 1 : Na channels recover in a voltage dependent manner above the threshold potential In slow channels ERP/APD > 1 : Ca channels recover only in a time dependent manner progressively after the fiber has fully repolarised

CARDIAC AP – types Fast channel AP Slow channel AP Site Atria , Ventricle, Purkinje fibre SA node, AV node, around AV ring and coronary sinus opening Predominant ion in phase 0 Sodium Calcium Activation potential -60 to -70 -40 to -55 Conduction velocity 0.5 - 5.0 m/s 0.01 – 0.1 m/s ERP- APD relationship ERP<APD ERP>APD

Summary Potassium ion is the main determinant of RMP AP - fast channel ( Na mediated ) and slow channel ( Ca mediated ) Phase 0 – sodium Phase 1 – I to Phase 2 and 3 – potassium Pacemaker potential – funny channels

Summary

Phase 0 Phase 2&3 Phase 2&3 Phase 1 Phase 2&3 Phase 2&3 Phase 2&3 Phase 4