1. Electrical Activity에 관한 공부
(Cardiac Electrophysiology)
1) Generation and Conduction of Action Potential
Generation of ECG:
- conduction of depolarization wave
- electrical heterogeneity of AP
3) Ionic Basis of Action Potentials: How to understand the generation of electrical signal (V) from the characteristics of ion channels and currents (I): I vs V
4) Abnormal electrical activity: Abnormal AP//Abnormal conduction --- Arrhythmias

2. Ion current
through ion channels
electrical signal

3. Since, Iion = Iinward + Ioutward ,
Equivalent Circuit Model
Im = Iion + Cm*dV/dt = 0
dVm = -∫ Iion dt / Cm
Since, Iion = Iinward + Ioutward ,
Iinward > Ioutward depolarization
Ioutward > Iinward hyperpolarization

4. Ionic Basis of Action Potentials of a single cell:
Specific form of AP can be understood by
Biophysical characteristics of individual ion channels
Distribution of various channels specific to the cell type
General principle: dV/dt = −Iion/Cm
Outward Current ;
Cause repolarization
or hyperpolarization
Cause depolarization
Inward Current:
I(Ca)
I(Na/Ca)
I(back)
I(Na)
I(K)
I(to)
I(pump)

5. Ion Channels
Selectivity for a specific ion: Na channel, Ca channel, K channel, Non-selective cationic channel, Cl channel.
Gating by a specific signal: Voltage-gated, Ligand-gated, Ca2+-activated, ATP-dependent...

6. Selectivity of ion channels
determines the direction of the current flow.
Channels selective for Na+ or Ca2+, or non-selective to cations produce
inward currents, thus cause depolarization.
Channels selective for K+ produce outward currents,
thus cause repolarization
or hyperpolarization.
Current direction depends on electrochemical gradient

7. If membrane is selectively permeable to a specific ion, X;
Na+
Ca2+ K+
Membrane potential approaches to EX

8. X+ moves outward when the membrane potential, Em > Ex
X+ moves inward when the membrane potential , Em < Ex K+
Na+
If gNa=gK,
net current flow = 0
at (ENa+EK)/2
Outward
Inward

9. Resting membrane potential is the potential
where inward current equals to outward current. K+
Na+
RMP
Since gK>>gNa at resting state, RMP is close to EK.

10. If conductance changes to make gNa>>gK, Em moves toward ENa.

11. Without current injections, membrane potential changes
in the range of reversal potentials for major ions:
EK < Em (RMP< AP) < ENa

12. Ion channel conductance is changed by Gating mechanisms
Gating signals
Voltage-gated: depolarization/hyperpolarization
Receptor-gated
Ca2+-activated
ATP-sensitive
Stretch-activated
H+, O2, other ligands
Temperature: heat or cold
Gating kinetics
Time dependence: transient, slowly activating, rapidly activating
Time-independent

13. Analysis of current recordings to understand ion channel properties
Dissection of an individual channel:
Pharmacological methods
Ion substitution: external solution/pipette solution
Electrophysiological methods: various pulse protocol
Analysis of channel properties:
Current-voltage relationship (I-V curve): Erev, slope
Activation curve/Inactivation curve: V1/2, slope
Time constant

14. Dissection of membrane currents using electrophysiological methods
Pipette solution
Pulse protocol
K+ pipette
Cs+ pipette
+50 mV
-40 mV
40 mV
-80 mV
-50 mV
-80
-70
6000 IK
5000
4000
3000
2000
1000
200
400
600
800
1000
1200
1400
-1000
ICa,L ms
-2000
-3000
INa
-4000 pA
-5000
-6000

15. Current-voltage relationship
Can be drawn by measuring:
Instantaneous current
Peak current
Steady state current
Analysis parameter:
Reversal potential --- represents ionic selectivity
Conductance: slope/chord
Shape: linear/rectification/bell

16. Current-voltage relationship for K+ current
Inward Rectification
Outward Rectification
Inward rectifing K+ EK
Voltage-gated K+ EK
Background K EK
Linear

17. Current-voltage relationship for Na+ or Ca2+currents
Bell shape
Voltage-gated Ca2+, low threshold
Voltage-gated Ca2+, high threshold
Voltage-gated
Na+

18. Current-voltage relationship for nonselective cation currents
Reversial Potential (Erev) = gNa*ENa/(gNa+gK) + gK*EK/(gNa+gK)
Linear: Background
Hyperpolarization-activated

19. a(V) Open Close n b(V) 1-n
Gating analysis for voltage-gated channel
a(V) Open Close n b(V) n
Open 1 I V a b
Close 0 mV
V1/2
1. Kinetic analysis
time constant =1/(a+b)
2. Steady state analysis: Activation curve
n = a/(a+b) = 1/(1+exp((V-V1/2)/dx))
(Boltzmann function)
fast activation / slow activation
voltage-dependence of activation

20. Function of Ion Channels can be understood…
Biophysical properties --- simulation
Diseases
Drugs
Knock-out of specific genes

21. Ventricular
myocyte
SA node cell

23. Cardiac Ion Channels
 Electrical properties (Resting Membrane Potential, Action Potential)를 결정지을 뿐 아니라, 수축의 발생 및 조절과도 밀접한 관계.
 Pathophysiology of Diseases, 또는 side effect of drug 와 관련됨.
 Target of therapeutics: Ion channel blocker, Ion channel opener들이 부정맥, 고혈압의 치료제로 쓰임.

24. depolarization-activated hyperpolarization-activated
-40 mV
40 mV
-80 mV
-50 mV
B-1
-40
-50
-90 -2
200 ms nA
ICa,L Ih
INa
B-2
A-2 5
-50
50 mV
-100
500 pA
-1000
-2000
-50 50
100 mV -5
ICa,L
INa
-10
-15 nA

25. Analysis of Na+ current
-80 to +40 mV
-120 mV
activation
inactivation
g=I/(Vm-ENa)
Activation curve
I-V curve

27. TTX
h-gate

28. Analysis for Na+ channel inactivation
Inactivation curve
Inactivation time constant

29. Contribution of INa : Simulation study
Ventricle
SA node

30. Na channel inactivation을 느리게 하는 약물

31. LQT3 Syndrome
Brugada Syndrome
Conduction disturbance

32. Na channel toxins Tetrodotoxin, saxitoxin, -conotoxin:
outer pore occlusion
Veratridine, aconitine:
persistent activation due to negative shift of m()
Local anesthetics
binding to inner surface of IV-S6 (IV)
 resting and use-dependent block

33. Specific tissue localization of Na+-channel  subunits
NaV1.1 – 1.3: CNS, PNS, embryonic(1.3)
NaV1.4: skeletal m.
NaV1.5 (SCN5A): heart m.
NaV1.6: CNS, PNS, node of Ranvier, glia
NaV1.7: PNS Schwann cell
NaV : PNS
Nax: heart, uterus, smooth m., glia
(TTX-resistant: underlined)

34. Cardiac Na Channel
 활성화(activation)되면 전기화학적 경사에 의해 Na 이온이 세포내로 유입되어 내향전류가 발생.
 심실근 활동전압의 빠른 upstroke (수십 V/s)는 Na 통로의 활성화에 기인:
fast action potential fast conduction velocity
 Na 통로는 수 ms내에 곧 비활성화(inactivation)되므로 지속적으로 내향전류를 발생하지는 않음.
 비활성화(inactivation)의 장애 --재분극 지연 -- APD 증가로 인한 long QT
syndrome의 원인.
 Na channel blocker: - TTX (high concentration), local anesthetics (lidocain, quinidine등은 Na 통로의 비활성화를 negative로 shift -- 부정맥 치료에 쓰임)

35. Ca channels L-type Ca current recording Present in heart T-type L-type
-35 mV
40 mV
-80 mV
-50 mV
-60
-40
-20 20 40
-2500
-2000
-1500
-1000
-500 pA mV
T-type
L-type
Present in heart

36. A. SA-Node
B. Ventricle

37. Effect of Ca channel blocker

38. Ca2+ currents  활성화되었을 때 Ca 이온이 세포내로 유입되며 내향전류가 발생한다.
 Na 통로에 비해 activation, inactivation이 느림
 심실근, 심방근에서의 Ca 전류는 활동전압의 plateau유지에 기여
 동방결절이나 방실결절 같이 안정막전압이 낮아서 Na channel은 비활성인
세포에서는 활동전압의 upstroke에 기여: upstroke dV/dt 느림 --- slow AP
--- slow conduction
 유입된 Ca은 흥분-수축 연결에서 작용 : 수축의 유발, 수축 크기 결정에 기여.
L-type Ca channel blocker
- inorganic blocker: Mn, Co, Ni
- organic blocker: verapamil, D-600, diltiazem, nifedipine 등.
- 부정맥, 고혈압 치료에 쓰임.

39. Excitation-secretion
gene name
Pre-synaptic terminal
Excitation-secretion
coupling
present in heart

40. L-type (Long lasting) Ca2+ channel
Subunits: α1C, α1D, α1F, or α1S, α2δ, β3a
Blockade
Sensitive to dihydropyridine (DHP) agonists and antagonists
Also blocked by phenylalkylamines (verapamil), benzothiazepines (diltiazem) & calciseptine
Activation: Strong depolarization
Inactivation by depolarization: Little
Localization
Skeletal muscle: α1S
Brain (Neuronal soma & proximal dendrites): α1D
Cardiac muscle: α1C
Neuroendocrine: α1D
Retina: α1F
General function in muscle: Excitation-contraction coupling
- Cav 1.1 (A1S): Skeletal muscle; Also acts as voltage sensor
Cav 1.2 (A1C): Heart & Smooth muscle

41. T-type (Transient) Ca2+ channel
- Activation
By depolarization near resting potential
Low voltage activation (LVA) threshhold
- Inactivation
Rapid
Steady-state inactivation occurs over a similar voltage range as activation
Window current: Small range of voltages where T-type channels can open, but do not inactivate completely
Rapid deinactivation
- Reactivation: Requires strong hyperpolarization
- Blockade
Nickel ions: Especially Cav 3.2
Mibefradil
Do not bind dihydropyridines
- Tissue localization: Cardiac & vascular smooth muscle; Nervous system
- Function
Rhythmic action (pacemaker) potentials in cardiac muscle & neurons
Burst firing mode of action potentials
Regulate intracellular Ca++ concentrations

42. SA node cell Pacemaker current (If, Ih) :IK
ICa
Pacemaker current (If, Ih) :
hyperpolarization-activated inward currents

43. Functional role of hyperpolarization-activated inward current in cardiac pacemaker activity.
SA node
Purkinje fiber

44. HCN channels (Pacemaker channels)
Hyperpolarization-activated, Cyclic nucleotide-gated
(PK+/PNa+ = 5), Erev = –30 mV
Generation of repetitive spontaneous electrical activity
Found in pacemaking cells in the heart and the brain
Cyclic nucleotide binding domain
Regulated by neurotransmitters:
faster activation by sympathetic (cAMP)
slower activation by parasympathetic (cGMP)

45. Hyperpolarization-activated inward current is regulated by neurotransmitters

46. Species difference in the density of Hyperpolarization-activated inward current

47. Hyperpolarization-activated inward current
Cyclic nucleotide binding domain
HCN isoforms

48. Ventricular AP Sinoatrial AP INa ICa,L Ih , ICa,T MDP -65 mV RMP
Repolarization
INa
ICa,L
MDP
-65 mV
RMP
-90 mV Ih
, ICa,T