20110113 Lecture 10 notes

From Iusmphysiology

• started here on 01/13/2011 at 8AM (not 11AM like the schedule says)

Contents 1 The Cardiac AP 1.1 References 1.2 Overall Learning Objective 1.3 Specific Learning Objectives 1.4 Problem-Based Examples 1.5 Outline 1.6 Introduction and overview, resting, and action potentials 1.7 Ionic basis, ion channels 1.8 Pacemaker potentials 1.9 Cellular heterogeneity (specialization) 1.10 Cellular basis for electrocardiogram (ECG, EKG)

The Cardiac AP

• Michael Sturek, Ph.D., Professor and Chair
• Department of Cellular & Integrative Physiology, MS 385, 274-7772, Email msturek@iupui.edu

References

• Boron, W.F. and E.L. Boulpaep. (“B&B”) Medical Physiology. Philadelphia: Saunders, 2005.
• Relevant reading –
  • B&B, Lederer, Chapter 20, Cardiac Electrophysiology and the Electrocardiogram
  • Berne RM and Levy MN. (“B&L”) Physiology. St. Louis, MO: CV Mosby, 1997 (or most recent edition)
  • Rhodes, R.A. and Bell, D.R. Medical Physiology: Principles for Clinical Medicine. Baltimore: Lippincott, 2009; Chapter 12.

Overall Learning Objective

• Understand the electrical signaling system and underlying cellular ionic mechanisms in the heart, which are essential for the coordinated function of pumping blood.

Specific Learning Objectives

• Know the electrical coupling between cardiac cells and its function.
• Know how excitation spreads from the SA node to other regions of the heart.
• Differentiate the shapes and the durations of action potentials in different regions of the heart.
• Know the 3 physiological functions of cardiac action potentials.
• Understand the ionic mechanisms underlying the different phases of an action potential in a myocardial and pacemaker cells.
• Know the mechanisms of pacemaking by which the rhythm of the heart can be altered.
• Understand the origin and significance of plateau phase of cardiac action potential relevant to the electrical refractoriness of cell.
• Understand how ionic equilibrium potentials contribute to excitation of the heart.
• Know the major ion channel types in myocardial cells.
• Know how intracellular action potentials contribute to integrated extracellular electrical activity, which is the EKG.

Problem-Based Examples

• Would Dr. Kevorkian use Na or K in his euthanasia cocktail and why?
  • K because high extracellular K reduces the potential across the membrane and decreases production of APs, thus stopping heart contraction.
• Why does coronary artery disease, which results in increased extracellular K, slow electrical conduction?
  • Higher ECF K concentrations decrease the potential across the membrane, thus ....
• What would be the composition of a cardioplegia solution that you would use to arrest the heart for open-heart surgery?
• Long QT syndrome leading to sudden death – Na and K channel mutations
• "Use-dependent" block of Na channels by anti-arrhythmic drugs (e.g. lidocaine)
• Wolf-Parkinson-White syndrome – AMP kinase mutation leading to glycogen accumulation and accessory conduction pathway
• Mishap in execution by lethal injection in Florida

Outline

• Introduction and overview, resting, and action potentials
• Ionic basis, ion channels
• Pacemaker potentials
• Cellular heterogeneity (specialization)
• Cellular basis for electrocardiogram (ECG, EKG)

• These notes contain all the information on cardiac electrophysiology and the ECG that is needed for this course.
  • Little red flags mean know it!

Introduction and overview, resting, and action potentials

Ionic basis, ion channels

Pacemaker potentials

Cellular heterogeneity (specialization)

Cellular basis for electrocardiogram (ECG, EKG)

Intracellular

idealized anatomy

automaticity spontaneous depolariz. fibrous

& endocardial ventricular

epicardial

I. Introduction and overview, resting, and action potentials

Human heart – sequential electrical activation of atria and ventricles, increased intracellular Ca, contraction. 

A.  Conduction of action potential via gap junctions  
    

Electrical coupling of cardiac myocytes (B&B, Fig. 9-1)

Functional syncytium of myocardium requires coupling of cells.

Intercalated disks are main site of coupling.

Desmosomes provide stability.

Gap junctions consisting of connexin proteins are permeable to ions.

Note actin and myosin filaments in very ordered arrays.

Mitochondria and other organelles

B&B Fig9-1v1

S23283-020-f002a

A.  Conduction of action potential via gap junctions  
   Electrical synapse (B&B, Fig. 20-2A) 

 This is one of few confusing concepts in B&B.  

 Delete the extracellular current flow; just consider depolarizing current through ion channels resulting in depolarization and the current flow through gap junctions. 

 Net increase in (+) charges leaves outside relatively (-).  

 Conduction (ion permeability) through gap junctions to adjacent cardiomyocytes. 

+

+

-

-

+

+

-

-

 Electrotonic spread of potentials (B&B, Fig. 20-2B) 

S23283-020-f002b functional syncytium

AP

time

X

threshold = Iin > Iout = depolarizing > repolarizing

Myelinated nerves have more passive conduction; gap junctions and more regenerative action potentials needed in cardiac muscle.  

Remember passive membrane properties from Dr. Obukhov. 
A single ventricular cell action potential (AP; left, blue trace) occurs and depolarizes cell A coupled via a gap junction. If the Vm of cell A is above threshold a regenerative AP will fire.  
If the Vm of cell A is subthreshold, the Vm will spread to coupled cardiomyocytes B, etc. passively (X), which is slower. 

B. Resting and action potentials  
 Easiest cell – ventricular cell  
 Intracellular electrode  
 Transmembrane potential – intracellular relative to extracellular; negative 
 Not performed clinically 

B. Resting and action potentials (continued)  
 Ventricular action potential (AP)  
 quiescent at rest  
 Phases 0, 1, 2, 3, 4 
 Note the action potential duration of ~200-300 ms vs. ~2 ms for nerve. Properties of ion channels result in longer AP duration in cardiac muscle.  

Slide11 ICM Byers 2008IntroECG.ppt – slide 7 (modified)

Vm

~200-300 ms

~100-150-fold > nerve

B.  Resting and action potentials (continued) 

 ventricular AP and contraction 
 electromechanical delay  
 Action potential is not absolutely indicative of contraction.  

Berne&Levy p Vm

B.  Resting and action potentials (continued)  

 Nernst equation for ELECTROCHEMICAL equilibrium 

 Two factors comprise the "driving force" that influences diffusion.   

 1. Since ions free in solution are charged, there is an ELECTRICAL force resulting from attraction and repulsion of charges.   

 2. Also, a CHEMICAL force exists because of concentration gradients.   
                  
  Eion in mV  = 60 log [Ion]o    
           z [Ion]i   
 
  where: 2.303 RT/F = 60 mV at 37° C; z = valence of ion, R = gas constant, T = temperature, F = Faraday constant 
 
 Definition of Nernst potential (electrochemical equilibrium potential) for a specific ion (in words):  
  
 Membrane potential at which the electrical force and chemical concentration gradient are equal, resulting in no net movement of a specific ion across the membrane.  

Other sources may say 61. Who’s correct?

 Normal physiological values for free ionic concentrations and equilibrium potentials for mammalian cardiac muscle  
 (based on values taken from B&B and B&L; [Ion] in mM, Eion in mV)  

EK = -91 mV

Ion

[Iion]o 
[Ion]i 
[Iion]o / [Ion]i 
log 
Eion 

Na+

142 
10 
14.2 
1.15 
+69 mV 

K+

4 
140 
0.03 
-1.52 
-91 

Ca2+

2 
~ 10-7 M 
20,000 
4.3 
+129 

Cl-

116 
35 
3.3 
0.52 
-32 

Note: Ca2+ has +2 charge

Pathological values for free ionic concentrations and equilibrium potentials for mammalian cardiac muscle

Pathological State

Ion 
[Iion]o 
[Ion]i 
[Iion]o / [Ion]i 
log 
Eion 

Cardiac glycosides

Na+ 
142 
15 (.) 
9.5 
0.98 
+59 mV 

Myocardial ischemia

K+ 
20 (.) 
140 
0.14 
-0.85 
-51 

Cardiac glycosides (oubain, digitalis, strophanthidin are positive inotropic agents (increase myocardial contractile force) by blocking the Na pump and increasing intracellular [Na].

 . ENa +59 mV 

Myocardial ischemia (lack of blood flow) can damage cardiac myocytes and dump intracellular K, thereby increasing in localized regions.

 . EK -51 mV . enormous influence on cardiac muscle excitability 
 

Driving force . force driving Vm toward Eion 
 Basically, all ion concentrations tend toward equilibrium and the further from equilibrium the greater the force to achieve equilibrium.   
Vm will go toward Eion if membrane is permeable to the ion (conducts the ion). 
  
Two basic properties account for resting Vm:   
1. selective permeability (~conductance) of the membrane to various ions, mainly K+ 
2. ionic concentrations differences across the membrane that are maintained by the Na-K pump, which actively transports Na+ out of the cell and K+ into the cell.  
 

GK >>> GCa, GNa, GCl

Vm

Don’t calculate. Conceptualize!

ACTIVATED Vm in ACTIVATED CELL will go toward Eion if membrane permeable (conducts) the ion.

• • • IMPORTANT POINT*** Ion with highest G is one that makes largest contribution to Vm.

 At rest GK >>> GNa + GCa + GCl 
 When activated GCa, GNa > GK, GCl 
ACTIVATED ventricular delineated by the blue box with intracellular ion concentrations shown by the bars.   
Overshoot (plateau) of the action potential ~+20 mV.   
 ECa, ENa are very postitive and GCa, GNa > GK, GCl, thus the balance . +Vm.     

GCa, GNa > GK , GCl

Vm

Don’t calculate. Conceptualize!

II. Ionic basis, ion channels

Conductile, contractile – Purkinje, atrial, ventricular  
Vm similar to previous figures.  Corresponding movement of ions through channels vs. time of the action potention are shown by voltage-clamp.   
Conventional upward deflection for outward ion currents, downward deflection for inward ion currents.   
Fast Na channels are activated first; thus, responsible for upstroke of action potential (phase 0).  Rapid inactivation (closing), similar to neurons, is partly responsible for phase 1; also, small component of IK(transient outward; ITO)   
Delayed K channel activation responsible for repolarization (phase 3) and plateau (phase 2).   

S23283-020-f003b (See Netter, p. 4)

absolute RP

slower, “long-lasting” (L-type)

fast!

delayed

Voltage-clamp convention

CICR

ITO

II. Ionic basis, ion channels (continued)

Slow Ca channels (L-type) are slower to activate and inactivate and largely responsible for plateau of action potential (phase 2).   
CICR (Ca-induced Ca release) not shown, but occurs from Ca influx through voltage-gated Ca channels; elicits large release of Ca from sarcoplasmic reticulum and contraction. 
Clinical example: What happen if there was not rapid inactivation of fast Na channels? Answer: More sustained depolarization and/or after depolarization . arrhythmias, long QT syndrome, possibly death. 

Slide11 ICM Byers 2008IntroECG.ppt – slide 7 (modified)

II. Ionic basis, ion channels (continued)

Berne&Levy p

ERP (effective or absolute refractory period) results from inactivation of Na channels 

Vm~-51 mV

Vm~-62 mV

ERP

More time, more negative Vm . more recovery from inactivation

1. ms?

RRP

Prevents: 1) ectopic beats / pacemakers, 2) tetany Stimulate cell (“lightening rod”), record action potential, then stimulate again after varying times of recovery (repolarization). See blue dots showing longer recovery time before stimulus . greater AP amplitude.

II. Ionic basis, ion channels (continued)

"Slow" AP responses with increasing [K]o    
Note diastolic Vm increases (more depolarized). 
Upstroke velocity and overshoot decrease.   
What will happen to conduction velocity in the heart? 
Answer: Decrease velocity because not reach threshold in neighboring (coupled) myocytes.  
Ischemia – Na channels inactivated, refractory 

Berne&Levy p

ischemia, injury

Modified from B&B, Fig. 7-12A

ß1

ß2

a

A

I

Intracellular

Extracellular

Resting Vm ~-80 mV

STATES OF THE CARDIAC VOLTAGE-GATED Na CHANNEL

Activated Vm ~>-50 mV few ms

Inactivated Vm ~>-50 mV many ms, s, min

ß1

ß2

a

A

ß1

ß2

a

I

I

A

Na+

Na+

Na+

Na+

Na+

Ca channel blockade; effect of drug on contraction is greater than effect on Vm.

Berne&Levy p CICR

~full block ICa

Long plateau normally in ventricular cells; partly due to delayed IK, not just ICa

Nifedipine, verapamil are clinically used Ca channel blockers. Increasing doses progressively from 0 (C=control) to 3, 10, and 30 cause greater block (dose-response relationship).

electromechanical delay

Action potential is not absolutely indicative of contraction. Multiple steps in excitation-contraction (E-C) coupling.

Blocking sarcolemmal Ca channels inhibits CICR through ryanodine receptors in sarcoplasmic reticulum (SR), which is the major source of Ca for activation of contractile filaments.

Puglisi and Bers. Am. J. Physiol.: Cell Physiol. 281: C2049, 2001

SR

S23283-020-f003a Pacemaker – SA node, AV node (B&B, Fig. 20-3)

IK – decreases; decreasing normally hyperpolarizing outward current . depolarization If – increases; permeable to Na, K; “funny” because non-selective, hyperpolarization activated; increasing depolarizing current . depolarization ICa – Ca (Ca-dependent AP); voltage-dependent; increased by depolarization . further depolarization

spontaneous phase 4

CURRENT

NAME 
CHANNEL 
IONS THAT CARRY CURRENT 
REVERSAL POTENTIAL OF CURRENT (mV) 
INHIBITORS 

INa

Na+current 
Voltage-gated Na+channel 
Na+ 
~ +60 
TTX Local anesthetics 

ICa

Ca2+current 
L-type Ca2+channel 
Ca2+ 
~ +120 
nifedipine verapamil 

IK

IKR 
HERG +miRP1* 
K+ 
~ -100 
Ba2+, Cs+, TEA 

 
IKS 
KvLQT1 +minK* 
  
  
  

 
Ito 
Kv4.3 
  
  
  

 
G protein activated 
GIRK* 
  
  
  

 
ATP-sensitive 
KATP* 
  
  
  

If

Pacemaker current 
HCN 
Na+, K+ 
-35 
Cs+ 

B&B Table 20-1 Summary major cardiac ion currents; time-dependent and voltage-gated

B&B Table 20-1 Summary major cardiac ion currents; time-dependent and voltage-gated

INa – TTX = tetrodotoxin; lidocaine = major therapy for life-threatening arrhythmias

ICa – ~6 subtypes / isoforms

IKR – relatively rapid; all K currents blocked by TEA (tetraethylammonium)

IKS – relatively slow

ITO – transient outward . phase 1 of ventricular AP

G protein activated – parasympathetic stimulation by acetylcholine

ATP-sensitive – ischemia (. blood flow) results in decreased ATP; results in need to slow down heart, so increase IK(ATP); blocked by glibenclamide

HCN – Hyperpolarization-activated, Cyclic Nucleotide-gated

For your information (FYI)

INa-Ca – Na-Ca exchange; 3Na, 1Ca transported 
INa-K – Na-K ATPase (Na pump); 3Na, 2K transported 
ICl – minimal role in heart 

III. Pacemaker potentials

Pacemaker – SA node, AV node, and Purkinje (tertiary pacemaker) 

Integrated effects of sympathetic and parasympathetic stimulation  
 inotropic – contractile force  
 chronotropic – rate; modulation of pacemaker currents alter rate 
 acetylcholine . increased If, decreased IK 

S23283-020-f005a

.If, .IK

III. Pacemaker potentials (continued)

Pacemaker – SA node, AV node, and Purkinje (tertiary pacemaker) 

Integrated effects of sympathetic and parasympathetic stimulation  


 acetylcholine . increased GIRK 

S23283-020-f005b

.GIRK

III. Pacemaker potentials (continued)

Pacemaker – SA node, AV node, and Purkinje (tertiary pacemaker) 

Integrated effects of sympathetic and parasympathetic stimulation  


 acetylcholine . . ICa  

 epinephrine, norepinephrine . . If, . IK (opposite of acetylcholine) 

S23283-020-f005c

.ICa

IV. Cellular heterogeneity (specialization)

Ventricular AP and contraction 
Electromechanical delay  
FIGURE (B&B, Fig. 20-4) – Sequential depolarization in cardiac tissue 
Netter FIGURE "Take-home" message – gap junctions, functional syncytium, delays in AP in regions of the heart  

S23283-020-f004

IV. Cellular heterogeneity (specialization)

 B&B Table 20-3 Electrical properties of different cardiac tissues   

TISSUE

FUNCTION 
CURRENTS 
ß-ADRENERGIC 

(epinephrine)

CHOLINERGIC 

(acetylcholine)

SA node

Pacemaker 
ICa, IK, If 
. conduction velocity 

. pacemaker rate

. conduction velocity 

. pacemaker rate

Atrial muscle

expel blood from atria 
INa, ICa, IK 
. strength contraction (contractility) 
~little effect 

AV node

secondary pacemaker 
ICa, IK, If 
. conduction velocity 

. pacemaker rate

. conduction velocity 

. pacemaker rate

Purkinje fibers

rapid conduction AP 

tertiary pacemaker

INa, ICa, IK, If 
. pacemaker rate 
. pacemaker rate 

Ventricular muscle

expel blood from ventricle 
INa, ICa, IK 
. contractility 
~little effect 

V. Cellular basis for electrocardiogram (ECG, EKG)

Extracellular recording 
Ideal components – P, QRS, T, U (rare; papillary muscle repolarization) 
Not enough mass in SA, AV, Bundle of His, bundle branches, Purkinje to contribute. 
Q wave – septal depolarization 
S wave – basal depolarization  
Rate – 1 second RR interval = 60 / min; paper speed 25 mm/s  

S23283-020-f006

U wave

Translating single cell action potentials to the ECG  
2 ideal cells: intracellular action potentials (B&B, Fig. 20-9A,B) 

Positive (+) electrode minus negative (-) electrode = net polarity. Vm are recorded in cell A (endocardial) and cell B (epicardial) simultaneously. First examine depolarization of cells A and B at times 1-5 labeled on the Vm in Panel A. The delay in depolarization of cell B is due to the gap junction, the resulting net polarity between cells A and B being shown in Panel B. Repolarization of cells A and B at times 1-4 are labeled on the Vm in Panel A for cells having the same AP duration and the resulting (-) deflection is shown in Panel B for the subtracted AP.

Repolarization of cells A and B at times 1-5 are labeled on the Vm in Panel A for cell A (endocardial; green curve) having a greater AP duration than cell B (epicardial) and the resulting (+) deflection is shown in Panel B for the subtracted AP (green curve). This is a “normal” control. In the subtracted AP in panel B depolarization is rapid similar to the QRS complex. The same overshoot of the action potentials results in zero difference between the potentials, similar to the isoelectric ST segment. Repolarization is slower similar to the T wave in the ECG.

SUMMARY: Duration of action potential in ventricular endocardial cells is greater than epicardial cells due to ion channel differences.

Thus, time sequence:  
 Ventricular depolarization is from endocardium to epicardium.  
 Ventricular repolarization is from epicardium to endocardium.  

S23283-020-f009a S23283-020-f009b Vm

~-80

~+20

endocard.

epicard.

1

2

3

4

X

XX

1

2

3

4

1

2

3

4

1

2

3

4

1

2

3

4

5

1

2

3

4

5

~QRS

~T

5

5

0 0.2 0.4 0.6 s

Extracellular action potentials (electrical activity) (B&B, Fig. 20-9C,D,E) 

Extracellular is opposite of intracellular. Use polarity at positive electrode minus polarity at negative electrode . net polarity

[(+)electrode  -  (-)electrode = net polarity] 
Panel C:  VB – VA = (+) – (-) = (+)  
Panel E:  VA – VB = (-) – (+) = (-) 

Analogous to multicellular nerve action potential recordings.

Intracellular +20 mV .

   -80 mV . 

Extracellular

          ~1 or 2 mV . 

 
 `          0 . 

2 ms

Extracellular action potentials (electrical activity) (B&B, Fig. 20-9C,D,E) 

S23283-020-f009c Depolarization from cell A to cell B. Lead axis 0° (yellow arrow). Depolarization yields (-) extracellular relative to intracellular, thus (+) deflection when cell A is depolarized (blue dot) and cell B is resting potential: VB – VA = (+) – (-) = (+) If cell A and B action potentials are same duration, then (-) deflection when cell B is depolarized and cell A is repolarized; (+) deflection when duration AP cell A > cell B.

lead axis 0°

+

+

-

-

+

+

-

-

S23283-020-f009e

Extracellular action potentials (electrical activity) (B&B, Fig. 20-9C,D,E) [continued] 

Depolarization from cell A to cell B. Lead axis 180° (yellow arrow); switch polarity of recording. Depolarization yields (-) extracellular relative to intracellular, thus (-) deflection when cell A is depolarized (blue dot) and cell B is resting potential: VA – VB = (-) – (+) = (-) Repolarization of cell A relative to cell B also shows opposite polarity deflection vs. lead axis 0°.

lead axis 180°

+

+

-

-

+

+

-

-

S23283-020-f009d

Extracellular action potentials (electrical activity) (B&B, Fig. 20-9C,D,E) [continued] 

Depolarization from cell A to cell B. Lead axis 90° (yellow arrow); recording perpendicular to depolarization. Depolarization yields (-) extracellular relative to intracellular, thus (0) deflection when cell A is depolarized (blue dot) and cell B is resting potential: VB – VA = (-) – (-) = (0) or

 VB – VA = (+) – (+) = (0).  

Repolarization of cell A relative to cell B also shows (0) polarity, i.e. isoelectric lead.

lead axis 90°

+

+

-

-

+

+

-

-

isoelectric

CARDIAC ACTION POTENTIAL Specific Learning Objectives 1. Know the electrical coupling between cardiac cells and its function. 2. Know how excitation spreads from the SA node to other regions of the heart. 3. Differentiate the shapes and the durations of action potentials in different regions of the heart. 4. Know the 3 physiological functions of cardiac action potentials. 5. Understand the ionic mechanisms underlying the different phases of an action potential in a myocardial and pacemaker cells. 6. Know the mechanisms with which the rhythm of the heart can be altered. 7. Understand the origin and significance of plateau phase of cardiac action potential relevant to the electrical refractoriness of cell. 8. Understand how ionic equilibrium potentials contribute to excitation of the heart. 9. Know the major ion channel types in myocardial cells. 10. Know how intracellular action potentials contribute to integrated extracellular electrical activity, which is the EKG. Problem-Based Examples Would Dr. Kevorkian use Na or K in his euthanasia cocktail and why?

Why does coronary artery disease, which results in increased extracellular K, slow electrical conduction?

What would be the composition of a cardioplegia solution that you would use to arrest the heart for open-heart surgery?

. depol. . inactivate ventr. Na channels

K, etc.

. . depol. current . . propagation