The action potential and the underlying ionic currents. Norbert Jost, PhD

Transcription

1 The action potential and the underlying ionic currents Norbert Jost, PhD

2 The propagation of the stimulation in the heart Sinus node Left atria His Bundle Conduction velocity in m/s Time to arrive from AV to the respective place (ms) AV node Right atria Left ventricle Left Bundle branch Right ventricle Right Bundle branch Purkinje fibres

3 Bovine Purkinje fibres system marked with ink

4 The action potential in a ventricular myocyte 1. EXCITABILITY Small triggering stimulus Threshold and autogenerative excitation large response Action Potential Action potential 2. DEPOLARIZATION REPOLARIZATION Na-channel / Na-current from extracellullar space Ca- channel / Ca- current to intracellular space 3. REPOLARIZATION K- channel / K- current from intracellular space DEPOLARIZATION to extracellullar space 4. Re-establish of the diastolic (resting) membrane potential

5 The ECG and the action potential I.

6 The ECG and the action potential II.

7 ERP = Effective Refractory Period The shortest time needed for reactivation of the heart muscle Depends on? REFRACTORINESS 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

8 Multi-cellular Organization Functional syntitium = The ability for cardiomuscles to contract all at once = Gap Junction Channel (small resistance)

9 IMPULSE CONDUCTION Direction of the impulse propagation 1. The speed of depolarization (V max ) - depends on fast sodium current 2. Action potential amplitude - depends on fast sodium current 3. Treshold of activation - depends on fast sodium current Velocity at which each domino falls Height of the domino The energy needed to push the domino 4. The cells internal resistance / the resistance between the cells (r i ) - depends on the gap junctions What is the medium resistance (water, air, vacuum) Sodium channels (atria, ventricle) or calcium channels (sinus and AV- node)

10 Outline of membrane currents of sinus node cells: current profiles (drawn by hand) are time aligned with the action potential. Purkinje fibre Diastolic depolarization

11 The main potassium currents in the ventricular and atrial muscle And many other currents and mechanisms!!!

12 Species differences in APD and ERP

13 Action potential and currents in sinus node and Purkinje fibre Diastolés depolarizáció

14 Action potential and currents in sinus node A more positive resting potential!

15 The pacemaker current (If) To induce the spontanous diastolic depolarization If

16 Action potential and fast sodium current (I Na ) in atria and ventricle

17 Wu et al, Heart Rhythm, 2008, 5(12): The fast sodium channel (I Na ) I Na 50 mv 100 ms outside Resting state Na + + Na outside Active state Na + + Na outside Inactive state + Na m m m inside - h inside - h inside Na + h +

18 Blockers of the fast sodium channel (I na )

19 Na-channel Tetrodotoxin (TTX) binds specifically to sodium channels by mimicking the hydrated Na + ion, denying entry to Na + ions. TTX binding site

20 Na channel

21 The slow (late) sodium channel I NaL ATX 50 mv 100 ms Wu et al, Heart Rhythm, 2008, 5(12):

22 Action potential and the L type calcium current (I CaL ) in atria and ventricle Atria

23 L type calcium current (I Ca ) I CaL 50 mv Varro et al, Br. J. Pharmacol, (2001) 133, ms Activation kinetics Inactivation kinetics outside Resting potential Ca + Ca + outside Actíve Ca + + Ca outside Inactíve + Ca m m m inside - h inside - h Re-activation kinetics inside Ca + + h

24 L type calcium current (I Ca ) S4 voltage sensor Loop S5-S6 ion conductance and selectivity 2 subunit complex Intracellular -subunit

25 Action potential and the transient outward potassium current (I to ) in atria and ventricle Csatorna fehérje

26 Transient outward potassium current (I to ) Virag et al, unpublished Notch 50 mv 100 ms Resting potential Activation Inactivation

27 Effect of selective I to blockade on the action potential The notch disappears Repolarization lengthens Virag et al, unpublished

28 Action potential and the rapid and slow componets of the delayed rectifier potassium currents (I Kr and I Ks ) in atria and ventricle

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30 The fast and slow delayed rectifier potassium currents (I Kr and I Ks ) 30 mv 1000 ms 30 mv 5000 ms -40 mv -40 mv There is a fast inactivation also! 50 pa 100 pa 2500 ms 2500ms Resting potential Activation Deactivation 25 pa 500 ms

31 The fast and slow delayed rectifier potassium currents (I Kr and I Ks ) +30 mv Controll 1 µm E mv -80 mv 250 ms Difference current E-4031 sensitive (I Kr ) 50 pa Controll 100 nm L-735,821 0 pa L-735,821 sensitive (I Ks ) 200 ms Varro et al, J.Physiol. 2000; 523.1: mv 200 ms

32 Action potential and the inward rectifier potassium current (I K1 ) in atria and ventricle

33 The inward rectifier potassium currents (I Kr and I Ks ) The inward rectification is regulated (inhibited) by intracellular cations (Mg 2+, Ca*, polyaminok) under depolarization Control 10 M BaCl 2 0 mv 0 pa 1000 pa 60 mv 36 s -90 mv -120 mv (mv) 50 mv 200 ms Control 10 M BaCl 2 cycle length = 1000 ms Biliczki et al, Br. J. Pharmacol, 2002,137(3): Resting potential Activation Deactivation

34 Summary the four main repolarizing current under the action potential

35 Atria specific currents: The ultrarapid delayed rectifier potassium current (I Kur ) Pitvar? Kamra Ionáram Csatorna fehérje Ionáram

36 Atria specific currents: The ultrarapid delayed rectifier potassium current (I Kur ) I Kur Gao et al, Br. J. Pharmacol, 2005; 144, Resting potential Activation Inactivation

37 I Kur szelektív gátlásának hatása az akciós potenciálra Wang et al. Circ. Res. 1993, 73: 1061 Wettwer et al. Circulation 2004;110:

38 Pitvarszelektív áramok az acetilkolin függő káliumáram (I K,Ach ) Dobrev et al. Circulation 2005;112: Az I K,ACh gátlása lehetséges kezelési mód a krónikus PF esetében?!?

39 Other ligand dependent current: the ATP sensitive potassium current (I KATP )

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42 SARCOLEMMAL CHLORIDE CHANNELS Cl - channels Activated by PKA (I Cl.PKA ) and PKC (I Cl.PKA ) Cl - Channels Regulated by Cell Volume (I Cl.vol ) Other Cl - Channels (activated by Cytoplasmic Ca 2+ (I to2 ), purinergic Receptors (I Cl,ATP ), etc) E Cl 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

43 Cl - channels activated by PK (I Cl.PK )! 21 % of the studied atrial myocytes James et al, Circ Res, 79, , 1996

44 Cl - channels activated by PK (I Cl.PK ) Levesque et al, Pflug Archiv, 424, 54-62, 1993

45 Molecular background, tissue and species distribution of I Cl.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 (?!)

46 Cl - Channels Regulated by Cell Volume (I Cl.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 I Cl.vol Current sensitive to SIDS and 9-AC. Exists also a basally active current

47 Cl - Channels Regulated by Cell Volume (I Cl.vol ) Sorota, Circ res, 70, , 1992

48 Main anion channels and transporters Hume et al, Physiol Rev, 80, 31-81, 2000

49 -Receptors and K V Channels permeability for K + agonist Adenylate cyclase K + Out G s + + PKA In ATP camp I KS Channel (KCNQ1+KCNE1)

50 Muscarinic Receptors and Pacemaker Channels ACh Adenylate cyclase Out M2 G i - camp In ATP camp Pacemaker Channel (HCN4)

51 -Receptors and Ca 2+ Channels agonist Adenylate cyclase Out G s + + In ATP camp + PKA Ca 2+ L-type Ca 2+ Channel

52 Correspondence between cardiac ionic currents and channel proteins

53 Computer modelling V (mv) 50 epi epi 80% I to1 block endo endo 80% I to1 block 0-50 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) Time (ms)

54 Control human and dog APs BCL = 1000 ms Human model is based on the dog model by scaling: factor_ik1 = ; %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

55 human IK1 block (0.3xgK1) dog

56 human IKr block (0.25xgKr) dog

57 human IKs block (0.5 x gks) dog

58 human IKr + IKs dog

59 human IKr + IK1 dog

60 THANK YOU FOR YOUR ATTENTION!