CS 2015

The Cardiac Action PotentialChristian Stricker

Associate Professor for Systems PhysiologyANUMS/JCSMR - ANU

Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Cardiac_Action_Potential.pptx

THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2015

CS 2015

Cardiovascular Part in Block 2• Week 1

1. Cardiac Action Potential

2. Modulation of the Action Potential

3. Basis of the ECG• Week 2

4. Pressures, Flows and Volumes during the Cardiac Cycle- Practical: ECG

5. CO as HR·SV and Introduction to Starling's Law

6. Cardiac Work and Coupling of Venous Return and CO• Week 3

7. Regulation of Blood Pressure.• Week 4

8. Neural and Humoral Regulation of Blood Pressure

9. PV Loops and Principles of Left and Right Heart Failure

10. Vessels of the Systemic Circulation• Week 5

11. Special Circulations (coronary, etc.)– Practical: Cardiovascular Simulations

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AimsAt the end of this lecture students should be able to

• identify the cellular specialisations supporting AP spread;

• distinguish the AP phases in cardiac myocytes and ICS;

• describe important in- and outward currents, how they

contribute to cardiac AP and what their properties are;

• outline determinants of the refractory period;

• recognise determinants of AP propagation in the tissue;

• argue why there is a hierarchy of pacemakers;

• explain different AP shapes and repolarisations; and

• point out drugs that target ion channels involved in AP.

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Contents• Two types of cells in heart

– Relevant anatomy incl. histological specialisations• Myocytes – coupled via gap junctions forming a functional syncytium

• Cardiac impulse conduction system (ICS)

• Cardiac myocyte– RMP, AP and currents involved

5 phases and determinants, relationship to contraction,

refractory periods, determinants of action potential propagation

• Cardiac ICS– RMP, AP and currents involved in ICS

3 phases and determinants, pacemaker currents incl. If and ICaT

determinants of action potential propagation

• AP and drug targets

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Cardiac Specialisations• Cardiac myocytes

– Intercalated discs• Desmosomes

– complexes of cell adhesion proteins

and linking proteins

• Gap junctions: connexins– “electrical syncytium”

• Impulse conduction system (ICS)– Specialised cells expressing

different ion channels and amounts

of contractile protein.• Sinoatrial node (SA)

– Round cells: pacemakers

– Elongated cells: conductors

• Atrioventricular node (AV)

• Purkinje fibres

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I. AP in the Cardiac Myocyte

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RMP and AP of Cardiac Myocyte• Ventricular myocyte (~80 BPM)

– RMP: ~ -90 mV, ± constant!– AP peak: 20 – 30 mV; amplit.: ~120 mV.– Duration: 300 – 450 ms → 200 x longer than

neuronal AP!– Shape depends on location in heart.

• Net inward currents depolarise.– On: Na, Ca channels (voltage-gated).– Off: Some K channels

• Net outward currents hyperpolarise.– On: various different K channels (voltage-dep., G

protein coupled and ATP gated).– Off: Na, Ca channels

• 5 Phases of AP (0 – 3 ± fixed dur.)0. Fast depolarisation: Na+ “overshoot” – NaV 1.5- Not a “neuronal” Na channel – different properties.

1. Early (partial) repolarisation (fast)

2. Plateau (Ca2+ shoulder)

3. Final repolarisation (slow)

4. RMP restoration: “diastole”, duration variable

Draper & Weidmann (1951), J Physiol 115:74-94

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Currents in Different Phases• Phase 0

– Threshold @ -60 – 70 mV (negative!)

– Activation of fast INa; deactivation of IK1

– Overshoot determined by [Na+]

• Phase 1– Inactivation of fast INa (NaV 1.5)

– Activation of K channels: Ito,f&s

– INCX (Na-Ca exchange – early hyperpolarising)

• Phase 2– ICaL (L-type, blocked by nifedipine)

– INCX (Na-Ca exchange – late depolarising)

• Phase 3– Deactivation of ICaL

– Activation of IK (del. rectifier (DR), IKs, IKr)

– Re-activation of INa and ICaL

• Phase 4– Activation of IK1 (inward rectifier, IR)

– Re-establishes “constant” RMP

Mod

ified

from

Rho

ades

& B

ell (

2009

), 3

rd E

d.

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Relationship to Contraction

• AP peaks before Ca2+ rise.

• Peak of Ca2+ signal during phase 2.– ICaL.

– INCX (adds to early Ca2+ influx – “reversed”

as Na+↑; later causes Ca2+ efflux).

• Twitch peaks when Ca2+ is already

decaying.– Highly efficient Ca2+ extrusion

mechanisms.

– Excitation-contraction coupling (see next

lecture).

Spurgeon et al. (1990), Am J Physiol 258:H574

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Refractory Periods in Myocyte• Absolute refractory period (ARP): Very

few Na+ channels can be reactivated

above Vm > -50 mV (technical term).

• Effective refractory period (ERP): before,

myocytes nearby cannot be physiologi-

cally activated as early current spread is

too small for activation.

• Relative refractory period (RRP): follows

ARP; during this time, no full AP can be

generated (smaller amplitude and slower

rise).

• Full AP generated again after Vm

hyperpolarised to ~ <-70 mV.

Ber

ne &

Lev

y, 2

008

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Action Potential Propagation

• Passive spread of AP: “slow” – about 1 m/s

• Depolarisation spreads passively, accelerated by gap

junctions between cells (intercellular current ahead).

• Extracellular current spread (extracell. current back).

• Propagation speed dependent on– Gap junction conductance: the larger – the faster.

– Fibre diameter: the larger – the faster (Ri↓).

Mod

ified

from

Raf

f & L

evitz

ky (

2011

)

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II. AP in the Cardiac ICS

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RMP and AP in SAN and AVN• Atrial ICS

– (Rabbit heart; ~ 96 BPM).

– RMP: variable, and much more

positive than in myocyte; when all

conductances blocked, ~ -35 mV.

– Slow rising phase: Not INa.

– Peak amplitude: ~20 mV (depends

on location within node; less positive

in centre).

– Duration: 200 – 250 ms.

– Threshold for AP: -50 – -40 mV.

• 3 Phases of AP (0 / 3 ± fixed; 4 var.)

0. Depolarisation (ICaL much

slower)

3. Repolarisation (slow)

4. Variable RMP (linear increase in

time to threshold).

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Currents in Pacemaker Cells• Phase 0 (rise)

– Mostly Ca2+ spike.• Amplitude is [Ca2+]-dependent.

• ICaL (L-type) blocked by nifedipine; RMP).

– Not much INa involved (blocked by TTX).

• INa remains inactivated as hyperpolarisation not

sufficiently big and long – it is there…

• Phase 3

– Inactivation of ICa.

– Activation of IK (delayed rectifier, DR).

• Phase 4 (“Pacemaker potential” - PMP)

– Early de-activation of fast IK (del. rectifier).• Turning off an outward is seen as net inward

current.

– Followed by opening of a set of channels

that generate inward currents –

predominantly carried by Na+ (see next).

Kohnhardt et al. (1976), Basic Res Cardiol 71:17

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Pacemaker Currents/Potential• = decay of “RMP” until Ca2+ spike.• Mixture of at least three currents:

– If - Current has “funny” properties – largest.• Hyperpolarisation-activated cation current (activated

during late phase 3).– Mixed cation current: largely Na+ inward current →

depolarisation; slow activation / deactivation.

– Erev = -20 – -10 mV.

• Tetrameric channel, gated via cyclic nucleotides (cAMP/cGMP bind to intracellular tail to close channel).

– cA/GMP↑: channel opened and vice versa.– Current modulated fast with cA/GMP↓↑ (see next lecture).

• 4 genes (HCN1 - 4).– In humans, mostly HCN4 – “sick sinus” if “blocked”.– HCN4 viral transfection in dogs can restore “sick sinus” to

normal pacing.

• Blocked by ivabradine – but PMP not fully blocked.

– ICaT – T-type Ca2+ current > -55 mV (late 4).• There is small amount of L-type current present, too.

– INCX – in reverse mode (3 Na+ vs 1 Ca2+) as

[Na+] high due to entry via HCN channels: hyperpolarisation → slows decay.

– Likely other Na+ conductance(s) involved.• IbNa (leak; TRPM4, NaV1.5; see next lecture).

Ber

ne &

Lev

y, 2

008

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Hierarchy of Pacemakers• Normally, 3-tiered system for pacing

– SA node: 60 – 100 BPM• Natural rate around 100 BPM

• Slower due to ANS (parasympathetic innervation).

– AV node: 40 – 60 BPM

• Smaller If → slower depolarisation → HR↓.

• Is typically subservient to SA node; takes over if

not paced in time (supraventricular pacing).

• Delay of ~160 ms: gap junction coupling↓ and

small cells (delay line): impulse propagation slow.

– Purkinje cells: 20 – 40 BPM

• Even slower If than AV node; plus

• RMP much more hyperpolarised (-90 mV).– Cells have Na+ current in them (reactivated…).

• Cells with highest AP rate set heart rate as

all other ICS cells are depolarised by them

and functionally rendered “inactive”.

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Spread of Excitation

• Atria excited within 80 – 90 ms (right earlier than left).

• First excitation seen with 140 ms delay in top septal areas: AV delay.

• Ventricles fully excited within 50 – 60 ms; right slightly earlier than left.– Compared to atria, speedup due to larger and better coupled Purkinje

fibres generating bigger currents → faster depolarisation wave.

Rho

ades

& B

ell (

2009

), 3

rd E

d.

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AP Time-Courses in Heart

• Purkinje cells have longest AP: prevents ventric. arrhythmia.– Longest APs are subendocardial, shortest subepicardial.

• Last ventricular cells to depolarise are the first to repolarise.– Mechanism: ? – gradients of channel expression

Mod

ified

from

Bar

rett

et a

l. (2

010)

, 23rd

Ed.

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III. Drug targets in AP

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Currents as Therapeutic Targets

• Several currents involved in AP generation are targeted

via drugs used in clinical settings.

• Some drugs used for cardioversion may target several

currents unspecifically.

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Take-Home Messages• Gap junctions are instrumental in spreading APs.

• Cardiac myocyte and ICS have different ionic currents.

• AP in myocytes has 5 phases during which specific currents are

activated/inactivated.

• AP propagation speeds up in larger fibres and when gap junction

conductance is large.

• Refractory period is determined by extent of repolarisation (re-

activation of Na+ channels).

• Pacemaker current is largely carried by HCN channels with no

NaV current.

• There is a 3-tiered hierarchy of pacemakers running at different

frequencies: SAN > AVN > Purkinje fibres.

• A multitude of drugs is used that affect AP.

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MCQ

Joe Parsons, a 23 year-old very fit medical student participates in

a study by a drug company, which aims to block HCN channels.

Which of the following statements best describes the expected

effect of an HCN block on the heart?

A. Drop in heart rate (bradycardia).

B. Lengthening of the action potential in Purkinje cells.

C. Decreased depolarisation rate in atrio-ventricular cells.

D. Early delayed rectifier activation in sino-atrial node cells.

E. Increased T-type calcium current in sino-atrial node cells.

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That’s it folks…

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MCQ

Joe Parsons, a 23 year-old very fit medical student participates in

a study by a drug company, which aims to block HCN channels.

Which of the following statements best describes the expected

effect of an HCN block on the heart?

A. Drop in heart rate (bradycardia).

B. Lengthening of the action potential in Purkinje cells.

C. Decreased depolarisation rate in atrio-ventricular cells.

D. Early delayed rectifier activation in sino-atrial node cells.

E. Increased T-type calcium current in sino-atrial node cells.