Download The action potential and the underlying ionic currents

The action potential and the underlying
ionic currents
Norbert Jost, PhD
The propagation of the stimulation in the heart
Left atria
Sinus node
His Bundle
Left ventricle
Conduction velocity
in m/s
AV node
Left Bundle
branch
Time to arrive from AV
to the respective place (ms)
Right atria
Right ventricle
Right Bundle
branch
Purkinje
fibres
Bovine Purkinje fibres system marked with ink
The action potential in a ventricular myocyte
1. EXCITABILITY
Small triggering stimulus

large response
„Action Potential”
Action potential
Threshold and autogenerative
excitation
Ca- channel / Ca- current
to intracellular space
K- channel / K- current
from intracellular space
to extracellullar space
4. Re-establish of the diastolic (resting) membrane potential
ION
AT
3. REPOLARIZATION
IZ
AR
from extracellullar space
L
PO
RE
Na-channel / Na-current
DEPOLARIZATION
2. DEPOLARIZATION
The ECG and the action potential I.
The ECG and the action potential II.
REFRACTORINESS
ERP = Effective Refractory Period
The shortest time needed for reactivation of the heart muscle
Depends on ?
1. Repolarization of the myocytes (K-channels)
2. The actual size of the depolarizing currents (Na- and Ca - channels)
Ventricle/atria
Na-channel
SA/AV node
Ca-channel
Multi-cellular Organization
Functional syntitium
=
The ability for cardiomuscles to contract all at once
= Gap Junction Channel
(small resistance)
IMPULSE CONDUCTION
Direction of the impulse propagation
1. The speed of depolarization (Vmax)
- depends on fast sodium current
Velocity at which each domino falls
2. Action potential amplitude
- depends on fast sodium current
Height of the domino
3. Treshold of activation
- depends on fast sodium current
The energy needed to push the domino
4. The cells internal resistance / the resistance
between the cells (ri)
- depends on the gap junctions
What is the medium resistance
(water, air, vacuum)
Sodium channels (atria, ventricle) or calcium channels (sinus and AV- node)
Outline of membrane currents of sinus node cells: current profiles
(drawn by hand) are time aligned with the action potential.
Purkinje fibre
Diastolic
depolarization
The main potassium currents in the ventricular and atrial muscle
And many other currents and mechanisms !!!
Species differences in APD and ERP
Action potential and currents in sinus node and Purkinje fibre
Diastolés
depolarizáció
Action potential and currents in sinus node
A more positive resting potential !
The pacemaker current (If)
To induce the spontanous diastolic depolarization
If
Action potential and fast sodium current (INa) in atria and ventricle
The fast sodium channel (INa)
50 mV
Wu et al, Heart Rhythm, 2008, 5(12):1726-34
INa
100 ms
Resting state
Na
+
Active state
Na
+ Na
+ Na
outside
m
h
inside
-
h
inside
Na
+
h
-
m
m
inside
+ Na
+
outside
outside
Inactive state
+
Blockers of the fast sodium channel (Ina)
Na-channel
Tetrodotoxin (TTX) binds specifically to sodium channels by mimicking
the hydrated Na+ ion, denying entry to Na+ ions.
TTX binding
site
Na channel
The slow (late) sodium channel
INaL
50 mV
ATX
100 ms
Wu et al, Heart Rhythm, 2008, 5(12):1726-34
Action potential and the L type calcium current (ICaL) in atria and ventricle
Atria
L type calcium current (ICa)
50 mV
ICaL
100 ms
Varro et al, Br. J. Pharmacol, (2001) 133, 625 – 634.
Activation kinetics 
Resting potential
+ Ca
Ca
+
outside
Inactivation kinetics 
Actíve
Ca
outside
h
Re-activation kinetics
m
-
inside
Ca
+
h
h
inside
+ Ca
outside
m
-
+ Ca
+
m
inside
Inactíve
+
L type calcium current (ICa)
S4 – voltage sensor
Loop S5-S6 – ion conductance and selectivity
2 subunit complex
Intracellular -subunit
Action potential and the transient outward potassium current (Ito) in atria and ventricle
Csatorna fehérje
Transient outward potassium current (Ito)
50 mV
Virag et al, unpublished
100 ms
Resting potential
Activation
Inactivation
„Notch”
Effect of selective Ito blockade on the action potential
Repolarization lengthens
The notch disappears
Virag et al, unpublished
Action potential and the rapid and slow componets of the delayed rectifier potassium
currents (IKr and IKs) in atria and ventricle
The fast and slow delayed rectifier potassium currents (IKr and IKs)
1000 ms
5000 ms
30 mV
30 mV
-40 mV
-40 mV
100 pA
50 pA
There is a fast inactivation also !
2500 ms
Activation
25 pA
Resting potential
2500ms
Deactivation
500 ms
The fast and slow delayed rectifier potassium currents (IKr and IKs)
Controll
1 µM E-4031
+30 mV
-40 mV
-80 mV
250 ms
Difference current
50 pA
E-4031 sensitive (IKr)
Controll
100 nM L-735,821
0 pA
L-735,821 sensitive (IKs)
50 mV
200 ms
Varro et al, J.Physiol. 2000; 523.1: 67-81
200 ms
Action potential and the inward rectifier potassium current (IK1) in atria and ventricle
The inward rectifier potassium currents (IKr and IKs)
The ” inward” rectification is regulated (inhibited) by intracellular cations (Mg2+, Ca*, polyaminok) under
depolarization
10 M BaCl2
Control
0 mV
Control
0 pA
60 mV
-90 mV
36 s
-
50 mV
1000 pA
10 M BaCl2
-120 mV
cycle length = 1000 ms
-120
-80
-40
0
40 (mV)
200 ms
Biliczki et al, Br. J. Pharmacol, 2002,137(3):361-368.
Resting potential
Activation
Deactivation
Summary – the four main repolarizing current under the action potential
Atria specific currents: The ultrarapid delayed rectifier potassium current (IKur)
Kamra
Pitvar
?
Ionáram
Csatorna fehérje
Ionáram
Atria specific currents: The ultrarapid delayed rectifier potassium current (IKur)
IKur
Gao et al, Br. J. Pharmacol, 2005; 144, 595-604
Resting potential
Activation
Inactivation
IKur szelektív gátlásának hatása az akciós potenciálra
Wang et al. Circ. Res. 1993, 73: 1061
Wettwer et al. Circulation 2004;110:2299-2306
Pitvarszelektív áramok – az acetilkolin függő káliumáram (IK,Ach)
Dobrev et al. Circulation 2005;112:3697-3706
Az IK,ACh gátlása lehetséges kezelési mód
a krónikus PF esetében ?!?
Other ligand dependent current: the ATP sensitive potassium current (IKATP)
SARCOLEMMAL CHLORIDE CHANNELS
Cl- channels Activated by PKA (ICl.PKA) and PKC (ICl.PKA)
Cl - Channels Regulated by Cell Volume (ICl.vol)
Other Cl - Channels (activated by Cytoplasmic Ca2+ (Ito2), purinergic
Receptors (ICl,ATP), etc)
ECl under normal physiological conditions in the range of
-65 to -45 mV
thus membrane Cl- channels have the unique ability,
compared with cation channels, to contribute both
inward as well as outward current during the cardiac AP
Cl- channels activated by PK (ICl.PK)
! 21 % of the studied atrial myocytes
James et al, Circ Res, 79, 201-207, 1996
Cl- channels activated by PK (ICl.PK)
Levesque et al, Pflug Archiv, 424, 54-62, 1993
Molecular background, tissue and species distribution of ICl.PK
Gene encoding: (CFTR)
(cystic fibrosis transmembrane conductance regulator)
Hume et al, Physiol Rev, 80, 31-81, 2000
Present in:
- adult ventricular, but not in atrial (?) or sinoatrial nodal cells in guinea pig, rabbit, and cat and human
(?!)
Cl - Channels Regulated by Cell Volume (ICl.vol)
Volume-regulated anion channels are now known to be ubiquitously expressed
in mammalian cells and play an important role in cell volume homeostasis.
An increase in cell volume activates outwardly rectifying chloride channel,
which inactivate at positive membrane potentials.
Exposure to hypotonic solutions is the most common technique used to swell cells and
activate ICl.vol
Current sensitive to SIDS and 9-AC.
Exists also a basally active current
Cl - Channels Regulated by Cell Volume (ICl.vol)
Sorota, Circ res, 70, 679-687, 1992
Main anion channels and transporters
Hume et al, Physiol Rev, 80, 31-81, 2000
-Receptors and KV Channels
 permeability for K+
 agonist
K+
Adenylate cyclase
Out
Gs

PKA
+
ATP
In
+
cAMP
IKS Channel
(KCNQ1+KCNE1)
Muscarinic Receptors and Pacemaker
Channels
ACh
Adenylate cyclase
Out
M2 
Gi
cAMP
ATP
cAMP
In
Pacemaker Channel
(HCN4)
-Receptors and Ca2+ Channels
 agonist
Adenylate cyclase
Out
Gs

+
+
+
ATP
cAMP
PKA
In
Ca2+
L-type Ca2+
Channel
Correspondence between cardiac ionic currents and channel proteins
Computer modelling
V (mV)
50
epi
epi 8 0 % I to1 block
endo
endo 8 0 % I to1 block
•
•
•
0
•
-50
•
-100
0
100
200
T im e (m s)
300
400
Decker 2009 canine model
1000 beats, 1 Hz
"epi" is the standard model
(which was built to
represent epi data)
"endo" has Gto reductions
according to Liu, Gintant
and Antzelevitch Circ Res
1992 (1/5th as much Ito
and epi)
"endo" has Gks reductions
according to Liu &
Antzelevitch Circ Res 1995
(11/35 as much Iks and epi)
Control human and dog APs
BCL = 1000 ms
Human model is based on the dog model by scaling:
factor_IK1 = 0.3779; %0.65 pA/pF vs 1.72pA/pF
factor_Ito = 0.9*0.77; % dog: 0.9 x normal HR model
factor_ICaL = 1.3;
factor_IKr = 1;
factor_IKs = 0.22; % dog = 4.5 x human
human
IK1 block (0.3xgK1)
dog
IKr block (0.25xgKr)
human
dog
human
IKs block (0.5 x gKs)
dog
human
dog
IKr + IKs
human
IKr + IK1
dog
THANK YOU FOR YOUR
ATTENTION !