Electrical Activity of the Heart I, II, and III Flashcards Preview

Cardiology > Electrical Activity of the Heart I, II, and III > Flashcards

Flashcards in Electrical Activity of the Heart I, II, and III Deck (42):
1

Rhythmic Activity of the Heart

  • Primary function of the heart: pump blood through arteries & veins to deliver nutrients & wash out breakdown products to the body
  • Action potentials: control heart rate & initiate contractions
  • SA node: pacemaker located above the right atrium that varies its rhythm & adjusts to different environmental conditions
  • Bachman bundle: conduction pathway for rapid transmission/propagation of electrical signals within the atira
  • AV node: pacemaker located b/n the atria & ventricles that propagates action potential from the right atrium, after a delay, to a specialized conduction system
  • Specialized conduction system: rapidly transmits the signal from teh base to the apex of the ventricles
    • His bundle: between the AV node & the ventricular septum
    • Purkinje fibers: course along both sides of the ventricular septum, trigger action potentials in ventricular myocytes via electrical coupling
  • Action potentials trigger contraction in ventricular myocytes --> atrial & ventricular contractions
    • Contractions: powerful enough to generate BP to the head, but gentle enough to avoid RBC hemolysis
  • When the SA node fails, the AV node becomes the primary pacemaker

2

Structure & function of cells comprising the heart

  • Ventricular cells
  • Purkinje fibers
  • SA node cells
  • AV node cells

  • Ventricular cells
    • Precise actin, myosin, & z lines
    • 3D structure w/ branches
  • Purkinje fibers
    • Largest cells
    • Fastest conduction velocity
  • SA & AV node cells
    • Torturous network of small cells w/ sparse striations
    • Embedded in connective tissue w/ transitional, electrically non-excitable cells
    • Slower conduction velocity (velocity slows as diameter gets smaller & more torturous

3

Gap Junctions, Gap Junction Protein, & Gap Junction Conductance

  • Cell are coupled via gap junctions
    • ​Large channels that connect cardiac cells to provide electrical & ionic coupling between cells
    • Permit the diffusion of small molecules from one cell to a neighboring cell
  • Gap junction protein: comprised of 2 hemichannels
    • Hemi-channel: comprised of 6 connexin protein monomers, linked by covalent disulfide bonds
    • Connexin 43: predominant connexin isoforms in ventricular cells
    • Connexin 40: predominant connexin isoforms in Purkinje fibers
  • Gap junciton conductance: measure of how readily ions & small molecules diffuse from cell to cell across cardiac tissue
    • Decreases in the presence of high Ca2+ & low pH in ventricular cells
    • Accounts for electrical isolation of ischemic heart muscle from "heatlhy" muscle in pathologic conditions
    • Micro-injection of dyes like Fluorescein readily diffuse to adjacent cells

4

Specialized Conduction Pathways

  • Atria
  • Ventricles

  • Atria
    • Electrical impulses from the SA node (primary pacemaker) stimulate atrial myocytes to fire electrical impulses to the AV node
    • Bachman's bundle: larger diameter myocytes w/ faster electrical prpoagation
  • Ventricles
    • Endocardium (inner ventricular wall) is lined w/ Purkinje fibers (specialized conduction muscle fibers)
      • Emerge from the AV node to form the His bundle
        • Form right & left bundle branches along the right & left sides of the septum
      • Course along both sides of the septum to reach the apex of the ventricles
      • Continue along the right & left endocardium of the ventricular free walls
      • Coupled to the papillary muscles & ventricular endocardial myocytes to increase the propagation velocity of electrical impulses
        • Ventricular cells: also transmit the electrical signal from cell to cell

5

Amplifying and Controlling Station

  • AV node is located at the AV junction of the right atrium
  • All electrical impulses from the atria to the ventricles pass through the AV node
  • The atria must fully contract to fill the ventricles before the ventricles contract
  • AV node delays the signal 60-120ms to ensure that this occurs
  • AV node may protect the ventricles from rapid arrhythmic beats

6

The mass of ventricular muscle is the contracting tissue that pumps blood

  • Heart cells
  • Cardiac calcium-dependent adhesion molecules (N-Cadherin)
  • Gap junctions
  • Intercalated discs
  • Functional electrical and mechanical syncytium

  • Heart cells: attached to each other end-to-end at intercalated discs
  • Cardiac calcium-dependent adhesion molecules (N-Cadherin): part of the intercalated disc junction
    • Essential for the adherens junctions in myocytes
  • Gap junctions: channels formed b/n adjacent cells
    • Low resistance pathways for the flow of current & movement of solutes
  • Intercalated discs: tight mechanical coupling between cells
  • Functional electrical and mechanical syncytium: heart muscle cells function as a unit
    • Cells that are part of the specialized conduction system also contain contractile protein & contract upon depolarization

7

Pacemaker of the Heart

  • SA node
  • Neurotransmitters

  • SA node: pace-setter of the heart
    • Conglomeration of flat mycoardial cells that act as the pace-setter of the heart
    • Located at the salcus terminalus near the junction of superior vena cava and the right atrium
    • Innervated by sympathetic & parasympathetic nerves
  • Ach & adrenaline: high concentration in the micro environment of pacemaker cells
    • Modify the inherent rhythm of the pacemaker cells

8

Cardiac Action Potential

  • Recording
  • Cardiac vs. Neuronal Action Potential
  • Image

  • Intracellular microelectrode measures the time course & magnitude of a ventricular action potential
    • Height: similar to nerve or skeletal muscle
    • Duration: 200-1500x longer
  • Cardiac action potential is faster & lasts longer than neuronal action potentials
    • Issue w/ how many ions go through b/c cardiac action potentials last so long
  • Image
    • A: both electrodes are extracellular, no difference in potential
    • B: one electrode is intracellular, resting potential = -90mV
    • C: apply small depolarization pulse, upstroke of action potential, peak = 30mV
    • D: repolarization
    • E: resting potential

9

SA & AV node vs. atrial & ventricular action potentials

  • SA & AV nodes
    • More positive resting potentials (-60 mV)
    • Slower rise-times
    • Shorter durations
    • "Unstable" resting ("pacemaker") potential provides the signal for rhythmic pacemaking activity (continuously fire action potentials)
  • Atrial & ventricular action potentials
    • More negative resting potentials (-80 to -100 mV)
    • Rapid upstrokes
    • Stable baselines

10

Phases of Ventricular Action Potentials

  • Rest: ventricular myocyte is quiescent
    • Resting potential = -90mV
  • Phase 0: upstroke, Na+ flows in
    • Depolarization: change in membrane potential away from the resting potential toward 0
  • Phase 1: reversal, overshoot, K+ transiently flows out
    • Peak potential = 30mV
  • Phase 2: plateau, Ca2+ flows in
  • Phase 3: rapid repolarization, K+ flows out
    • Hyperpolarizatoin: change in membrane potential that makes the inside of the cell more negative
  • Refractory period
    • Action potential triggers contraction & controls its duration & magnitude
    • Duration is almost as long as duration of contraction
    • Long duration prevents initiation of another signal until contraction is terminated

11

Nernst Equilibrium and Resting Potential w/ K+

  • Nernst equilibrium: Ei = (RT / zF) ln(Xo / Xi)
    • R = Rygdberg constant
    • T = temperature in kelvin
    • z = ion valence
    • X0 & Xi = concentrations of ion X outside & inside cardiac cells
  • Cardiac muscle has high permeability to K+ at rest
    • EK = -90mV = value of membrane potential if membrane is only permeable to K+ and n oother ions
  • At resting membrane potential, the dominant conductance for K+ is maintained by a K+ channel protein: Kir2.1
    • Kir2.1 is responsible for the K+ current (IK1) & the K+ conductance (GK1) at resting membrane potential

12

Why the resting potential deviates from a perfect Nernst relation

  • Low Na+ permeability
    • PK / PNa = 100 / 1
  • Na/K-pumps transport 3 Na+ out for 2 K+ in the cell using ATP for energy
    • Hyperpolarizes membrane potential by 5-6mV
  • Small conductances exist for anions through Cl- channels & non-specific cationic channels or low background leaks across the membrane

13

Permeability & Conductance

  • Permeability
    • Probability of diffusion of a particle across a diffusion barrier (ex. cellular membrane)
    • Applies to charged & neutral organic molecules
  • Conductance (G)
    • Inverse of resistance (R) in units of Siemens (1 Ω = 1/S)
    • V = I * R
      • V = voltage in volts
      • I = current in amperes
      • R = resistance in ohms
    • G = 1 / R = I / V
    • Applies to charged particles or ions

14

Upstroke, Tetrodotoxin, & INa Threshold

  • Upstroke
    • Fast (1-5ms), abrupt increase in Na+ conductance / inward Na+ current
    • Max rate of rise depends on [extracellular N+] = 140mM
  • Tetrodotoxin (TTX)
    • Toxin found in puffer fish
    • Blocks fast inward current & the upstroke
    • Has lower afifnity to cardiac than neuronal cells, so higher [TTX] is needed to fully block cardiac Na+ channels
  • Threshold potential: voltage that must be reached to open the activation (m) gate of voltage-gated Na+ channels
    • Threshold for INa (current) = threshold for action potential generation = -65mV
    • Membrane potential max value = ENa = 40mV
    • Once voltage-gated Na+ channels are activated, the channels automatically inactivate after a brief time-delay
    • Na+ channels close, & Na+ conductance drops back to its resting value

15

"All or None" Action Potential

  • Once some Na+ channales are activated, Na+ ions flow into the cell, causing further depolarization
    • Results in a positive feed-back, all-or-none effect
    • Only a small percentage of all the Na+ channels hav eto open to cause more to open
  • Healthy hearts
    • Rapid upstroke of ventricular APs is entirely due to the Na+ current (INa)
  • Ischemic hearts
    • Extracellular K+ becomes elevated
    • Resting membrane potential becomes depolarized
    • Na+ channels don't fully recover from inactivation
    • Ca2+ influx contributes ot the AP upstroke via the activation of voltage-gated Ca2+ channels

16

Plateau of the Cardiac Action Potential: GK1, TTX, & Ca2+ Channels

  • Total conductance of the membrane decreases by 300% during the plateau
  • GK1: controls dominant conductance
    • Decreases strongly at more + potentials b/c of Kir2.1 cardiac K+ channels
    • Voltage-dependent channel that's never fully closed
    • Conductance varies w/ voltage
  • Tetrodotoxin (TTX): Na+ channel blocker
    • Doesn't affect duration or amplitude of the plateau b/c fast inward Na+ current contributes little
  • In TTX blocked preparatoins, transient slow inward current ("secondary current") is sensitive to varying [Ca]o
    • Current is inactivated to maintain the plateau potential
    • Addition of TTX abolishes the rapid upstroke so that the action potential is dependent on voltage-gated L-type Ca2+ channels
  • Ca2+ channels
    • L (Large) Type: predominant isoforms in the heart
      • Can be blocked by other divalent cations (ex. Mn2+) and pharmacological Ca2+ channel blockers (ex. Diltiazem, Nifedipine, Verapamil)
    • T (Tiny) Type
    • N (Normal) Type
  • During the plateau, ICa,L (inward) = IK1 (outward)
    • Controls the duraiton of the action potential & msucle contraction

17

Inward and Outward Ionic Current during Ventricular Action Potentials

  • Inward current: movement of + charges from outside to inside the cell (depolarizing)
    • INa: dominant, greater amount
    • ICa(L): dominant, greater duration
    • ICa(T): weak
    • INa/Ca: weak, electrogenic exchanger (3 Na+ go in, 2 Ca go out)
  • Outward current: movement of + charges from inside to outside the cell (repolarizing)
    • Iss (Ito): brief, accounts for notch following upstroke
    • IK1: dominant, decreases during plateau phase
    • IKr & IKs: delayed K+ repolarizing current, important for downstroke
    • INa/K: small, continuous repolarizing current
  • Density associated with current: flow (pA) / capacitance (pF)
    • Because the capacitance is a measurement of the surface area of the cell's membrane or the size of hte cell

18

Repolarization Phase

  • Brought about by a time-delayed, rectifying K+ current
    • Similar to skeletal & nerve cells except for time delay
    • Time delay ensures the plateau phase is long & stable
    • Time-delayed rectifying K+ channels (IKr & IKs) contribute to repolarization
  • Brought about b/c membrane potential slowly decreases to the voltage range where the IK1 K+ current becomes larger
    • Drvies the voltage to the Nernst equilibrium potential for K+

19

Refractory Period

  • Duratio of plateau phase protects the myocardium from ectopic & aberrant stimulation
    • Protects against extra beats & arrhythmias
  • Effective refractory period: electrical stimuli int eh range of physiological impulses aren't able to elicit the firing of an additional AP
    • Occurs at a membrane potential around -50mV
  • Strong defibrillation shocks overcome refctoriness
    • During relative refractory period, stronger stimuli are necessary to produce an AP
    • Refractory period protects against stimuli that can be generated by other cells in the heart, not paddle electrodes used in the ER

20

Excitability during the Cardiac AP

  • Effective refractory period (ERP)
  • Relative refractory period (RRP)
  • Full recovery time (FRT)

  • Effective refractory period (ERP)
    • Most stimuli aren't able to initiate a propagated AP
  • Relative refractory period (RRP)
    • Only stimuli greater than those which normally reach threshold can cause a propagated AP
    • Na+ & Ca2+ channels havne't fully recovered form inactivation so aren't available to be activated or re-opened
    • APs generated propagate slower
  • Full recovery time (FRT)
    • interval following depolarization
    • Threshold returns to normal
    • Stimulation produces a normal propagated AP

21

Activation and Inactivation Properties of Voltage-Gated Na+ Channels

  • Voltage-gated channels
  • Threshold potential
  • Voltage-sensor
  • m gate
  • Inactivaiton (h) gate
  • Inactivation
  • Recovery form inactivation

  • Voltage-gated channels: triggered by an abrut change in membrane depolarizaiton or injection of + charge in the cell
    • Shifts the channel from closed to open
  • Threshold potential: minimum membrane voltage needed to open/activate the channel
  • Voltage-sensor: amino acid sequence linked to m gate
  • m gate: a region of the channel protein that acts like a gate to open/activate the channel
  • Inactivation (h) gate: internal mechanism to automatically close through a different gate
  • Inactivation: shifts the conductance of hte channel back to a 0 conductance
    • m gate closes, so the pore of the channel remains blocked
    • h gate reamins in the closed position as long as the voltage across the membrane remains depolarized
    • Inactivaiton determines the excitability of the cardiac muscle, since firing an extra stimulus when the channel is inactivated can't open the Na channels b/c the h gate hasn't yet recovered
  • Recovery from inactivation: resetting of the h gate when the voltage returns to -80mV to -90mV

22

Activation (m gate) & Inactivation (h gate) of Voltage-Gated Na+ Channels

  • Resting membrane potential
  • Electrical stimulation
  • Inactivation
  • Reset

  • Resting membrane potential
    • m gate is closed
    • h gate is open
    • Na+ conductance (GNa) = 0
  • Electrical stimulation
    • Vm passes through threshold voltage
    • m gate opens
    • GNa increases
  • Inactivation
    • m gate is still open
    • h gate closes within ms
    • GNa = 0
  • Reset
    • m gate resets
    • h gate remains closed until Vm returns to -90mV

23

Activation and Inactivation for Voltage-Gated Na+, Ca2+, & K+

  • Na+
    • Activation: fast
    • Inactivation: fast
  • Ca2+
    • Activation: slower
    • Inactivation: slower
  • K+
    • Activation: delayed
    • Inactivaiton: fast or slow

24

Voltage-Gated Ca2+ Channels

  • 3 types of Ca2+ channels
    • L (large): dominant isoform expressed in ventricular myocytes
    • N (normal)
    • T (tiny)
  • L-type Ca2+ channels
    • Threshold potential is more positive (-45mV) than Na+ channels (-65mV)
    • Voltage-gated activation is slower than for Na+ channels
  • Inactivation of ICa(L)
    • Ca2+ dependent negative feed-back system: when [Ca2+]i is high...
      • Ca2+ dependent inactivation turns off ICa(L) faster
      • AP duration becomes shorter
      • Total influx of Ca2+ via ICa(L) is suppressed
    • Voltage-dependent
  • Inactivation of Na+ channels
    • Voltage-dependent only

25

Protein & Current for the following Genes

  • KCNJ2
  • KCNH2
  • KCNQ1
  • SCN5A
  • Kcnd2
  • CACNA1C

  • KCNJ2
    • Protein: Kir2.1
    • Current: IK1
  • KCNH2
    • Protein: HERG1
    • Current: IKr
  • KCNQ1
    • Protein: KvLQT1
    • Current: IKs
  • SCN5A
    • Protein: Nav1.5
    • Current: INa
  • Kcnd2
    • Protein: Kv4.2
    • Current: It.o
  • CACNA1C
    • Protein: Cav1.2
    • Current: ICa(L)

26

Properties of Pacemaker Cells

  • Pacemaker cells: exhibit spontaneous oscillatory electircal activity around the resting potential
    • APs are generated spontaneously w/o an external electrical stimulus
    • Pacemaker current depolarizes until the threshold potential is reached
    • When the AP recovers towards baseline, the membrane potential doesn't remain stable & slowly depolarizes until it reaches the threshold
  • Pacemaker potential (slow diastolic depolarization): slow depolarizatoin of the resting potential
  • Maximum diastolic potential (depolarization): most negative value of SA node cells
    • More negative = longer time for pacemaker potential to reach threshold = slower HR
    • Less negative = shorter time for pacemaker potential to reach threshold = faster HR
  • SA & AV node cells v.s ventricular cells
    • More positive threshold (-45mV)
    • Slower upstroke b/c of an inward Ca2+ current generated by L-type (& T-type) voltage-gated Ca2+ channels

27

Effect of interventions on SA nodal pacemaker potential

  • Increase temperature
  • Decrease temperature
  • Increase Ach
  • Increase epinephrine
  • Increase [Ca]o
  • Decrease [Ca]o
  • Increase [Na]o
  • Decrease [Na]o
  • TTX

  • Increase temperature
    • Increase rate diastolic depolarization & HR
  • Decrease temperature
    • Decrease rate diastolic depolarization & HR
  • Increase Ach
    • Decrease rate diastolic depolarization & HR
  • Increase epinephrine
    • Increase rate diastolic depolarizatoin & HR
  • Increase [Ca]o
    • Increase rate diastolic depolarization & HR
  • Decrease [Ca]o
    • Decrease or no effect on rate diastolic depolarization & HR
  • Increase [Na]o
    • No effect on rate diastolic depolarization or HR
  • Decrease [Na]o
    • No effect on rate diastolic depolarization or HR
  • TTX
    • No effect on SA node

28

How electrical activity triggers and controls force generation

  • Extracelluar Ca2+
  • Intracellular Ca2+

  • Extracellular Ca2+ is required for normal force generation
    • Ca2+ entry into heart cells via voltage-gated Ca2+ channels provides Ca2+ to activate force generation by sarcomeres (actin-myosin system)
    • Extracellular Ca2+ is necessary for heart contractions but not skeletal contractions
  • Advantage of SR as a store for intracellular Ca2+
    • Ca2+ release occurs near myofibrils, so diffusion distances & times are short
      • Cardiac muscle contractions depend on external & internal Ca2+
    • SRs are also near transverse tubules
      • Less well-developed in cardiac than skeletal muscle

29

How Ca2+ is transported out of cardiac cells

  • SR Ca2+ pumps
  • ATP-dependent Ca2+ transporters
  • Na+/Ca2+ exchanger

  • SR Ca2+ pumps
    • Pump 2 Ca2+ out for 1 ATP hydrolyzed to ADP
    • Replenishes internal Ca2+ in the SR
    • Resets the SR for Ca2+ release at the next AP
    • Serca2: Ca2+, Mg2+-ATPase responsible for translocating Ca2+ across the SR membrane
  • ATP-dependent Ca2+ transporters
    • Transports Ca2+ to external spaces
    • Homologous to transporters on the SR membrane
  • Na+/Ca2+ exchanger
    • Pumps 1 Ca2+ out for 3 Na+ in
    • Electrochemical gradient of Na+ out-to-in drives Ca2+ against its electrochemical gradient in-to-out
    • Influx of Na+ is balanced by the ATP-driven Na+/K+ pump

30

Ryanodine Receptors (RyRs) & Phospholamban (PLB)

  • Ryanodine receptors (RyRs)
    • Ca2+ release channels on SR membranes
      • RyR1: skeletal muscles
      • RyR2: cardiac muscle
      • RyR3: brain & other cells
    • In muscle, the close juxtaposition of Ca2+(L) channels to RyR2s accounts for the effective trigger of Ca2+ entering via the channels to raise local [Ca]i near the RyR
  • Phospholamban (PBL)
    • Interacts with Ca2+, Mg2+-ATPase or Ca2+ pumps to suppress Ca2+ uptake by SRs
    • cAMP-depenent phosphorylation of PLB reverses inhibition
      • Increases rate of Ca2+ uptake by the SR
      • Greater SR Ca2+ filling --> increased Ca2+ release next time

31

Excitation-Contraction Coupling

  • Excitation-contraciton coupling: AP firing at the cell membrane releases Ca2+ from the SR to produce contraction
  • APs propagate along cell membrane & in T-tubules
  • Depolarization of T-tubules reaches threshold potential to activate Ca2+(L) channels across the sarcolemma
  • Entry of Ca2+ at the triadic junction (T-tubule + terminal cisternae of the SR) triggers release of Ca2+ from the SR
  • Terminal cisternae of the SR contain Ca2+ release channels (ryanodine receptors, RyR2) that open when [Ca2+] rises, releaseing more Ca2+ from the SR ICa(L) channels
    • Ca2+-induced Ca2+-release: entry of low [Ca2+] elicits further relase of Ca2+ from the SR stores
  • [Ca2+]i activates myofibrils to generate force
  • Ca2+ is removed by the Na+-Ca2+ exchanger and ATP-dependent Ca2+ pumps on the SR

32

The Staircase (Bowditch) Effect

  • Cardiac muscle changes the AP duration as a function of HR
    • Allows AP & force of contractions to adapt to ranges of HRs
    • Explains how force of contractions adjusts to physiological conditions
  • Increase HR --> increase force of cardiac contractions
    • Increase HR --> gradual step-wise increase in force
    • The instant HR is increaesd, the force begins to increase by steps, levels-off, then remains steady but elevated at the new higher HR
  • If HR is increased...
    • & AP duration remains constant...
      • Resting phase decreases
    • & AP can't remain constant b/c HR is too high...
      • AP duration shortens
  • AP duration is dependent on the diastolic interval b/n repolarization of hte previous AP & depolarizaiton of the next AP

33

Experimental Record of Diastolic Interval & AP Duration

  • APD restitution curve: duration of AP vs. diastolic interval
    • Stimulus pulse (S1) is applied continuously to elicit APs
    • Second stimulus (S2) is applied w/ a shorter delay or diastolic interval to produce APs w/ shorter duration
  • APD restitution by pacing the heart at different rates or cycle lengths
    • Longer cycle lengths = longer AP durations = longer diastolic intervals
    • Shorter cycle lengths = shorter AP durations = shorter diastolic intervals
    • As cycle length decreases, AP durations continue to decrease until the stimulus fails to capture

34

Mechanisms involved in bringing about the shortening of AP w/ increasing HR

  • ICa(L) activation
  • ICa(L) inactivation
  • Positive vs. negative inotropic effect
  • Effect of increased HR on force

  • ICa(L) activates when the membrane potential is depolarized to its threshold (-45mV)
  • ICa(L) inactivates...
    • as a function of [Ca2+]i
      • High [Ca2+]i increases inactivation, limiting the influx of Ca2+ during an AP by shortening the AP duration
      • High HRs = more APs = elevated [Ca2+]i = accelerated ICa(L) inactivation = shorter AP duration
      • Low HRs = reduced force of contraction
    • as a funciton of membrane potential
  • Inotropic effects
    • Positive --> increased force of contraction
      • Higher HR = increase in Ca2+ load in cells b/c of increase of Ca2+ influx : Ca2+ efflux ratio
      • Ca2+ enters during AP duration and exits during diastole
      • As HR increases, diastole decrease more than AP duration decreases
      • Ca2+ entry is slightly reduced per AP but removal is greatly reduced so [Ca2+]i rises
      • Force of contraction increases for 12 beats until Ca2+ influx = Ca2+ efflux
    • Negative --> decreased force of contraction
  • Effect of increased HR on force
    • Increased HR --> increased diastolic & systolic Ca2+ --> increased force in contraction
      • Increase in Ca2+ during diastole has minimal effect b/c it remains below mechanical threshold
    • Increased force of contraction is due to increased force generated by myofibrils w/ increased [Ca2+]i
    • Increased in [Ca2+]i gets next cycle closer to threshold so the new influx of Ca2+ produces a greater force

35

Mechanisms underlying spontaneous pacemaker potential

  • Hyperpolarization-activated Cyclic-Nucleotide-gated (HCN) channel protein
  • Ca2+ clock

  • Hyperpolarization-activated Cyclic-Nucleotide-gated (HCN) channel protein
    • Channel in SA nodal cells that causes pacemaker potential
    • 4 isoforms
    • HCN2 & HCN4: dominant in the SA node
    • HCN4: responsible for slow inward funny current (If)
      • Cationic inward current carried by Na+ & Ca2+
      • Activated by negative membrane potentials
      • Modulated by cAMP-dependent phosphorylation
  • Ca2+ clock
    • Caused by the rhythmic, spontaneous releae of Ca2+ from the SR via RyR2
    • [Ca2+]i oscillations produce transients of Na/Ca exchange current which produces slow, diastolic potential or pacemaker potential
      • Elevation of [Ca2+]i increases the Na/Ca exchange current (3 Na+ in, 1 Ca2+ out)
      • Membrane depolarization triggers opening of ICa(L) to produce an AP
      • Speed of clock or [Ca2+]i oscillations increases w/ higher [Ca2+]i
    • Re-uptake of Ca2+ by SERCA2 Ca2+ pumps on the SR reduces [Ca2+]i
    • When the SR is reloaded w/ Ca2+, the cycle repeats

36

Parasympathetic stimulation

  • Vagus nerve stimulation
  • Ach release
  • Vagal stimulation or Ach release on SA node
  • Vagal stimulation or Ach release on atrial AP

  • Vagus nerve stimulation
    • Rate of diastolic depolarizaiton is suppressed --> membrane hyperpolarizes
    • Increases K+ permeability & membrane conductance for K+ (GK)
  • Ach release
    • Reduces pacemaker potential (diastolic depolarization) --> slows HR
    • Increased rate of loss of K+ from cells
  • Vagal stimulation or Ach release on SA node
    • Pacemaker activity slows
    • Resting potential becomes more negative
    • Rate of diastolic depolarization is reduced
  • Vagal stimulation or Ach release on atrial AP
    • Duration of AP shortens
    • Slow inward current is reduced
    • Outward potassium currents that cause repolarization are increased

37

Action of adrenalin on pacemaker cells (sympathetic effects)

  • Dominant action: increase slope of pacemaker potential --> increase firing frequency & HR
  • Decrease max diastolic potential --> decrease firing frequency
  • Increase overshoot
  • Threshold potential for upstroke doesn't change so doesn't affect firing frequency
  • Mechanism: activation of all ionic channels & ATP-driven transport pumps in the cell

38

Graphs: Changes of frequency of pacemaker firing & Effects of sympathetic stimulation on SA node

  • Mechanisms involved in changes of frequency of pacemaker firing
    • Reduction in slope of pacemaker potential diminishes frequency
    • Increase in threshold or increae in magnitude of resting potential diminishes frequency
  • Effects of catecholamines or sympathetic stimulation on the SA node
    • Increased rate of diastolic depolarization accelerates pacemaker activity due to increased inward current

39

Effects of adrenalin on ventricular cells

  • Sequence of events
  • At the level of the SR
  • At voltage-gated Na+ channels
  • At the Na/K pump
  • At the plateau phase
  • At K+ channels
  • At AP
  • In intact hearts, adrenalin causes...

  • Sequence of events
    • Noradrenalin/Adrenalin binds to beta1-adrenergic receptors
    • Ligand-receptor interaction stimulates adenylate cyclase
    • Increase in cAMP in the cytosol
    • Activation of protein kinases
    • Widespread signaling mechanism that activates virtually all channels & transporters
  • At the level of the SR
    • Phospholamban: protein that binds to Ca2+ pump on the SR membrane (SERCA2) --> inhibition of rate of Ca2+ uptake by SR
    • Adrenergic activation phosphorylates Phospholamban --> dissociation from SERCA2 --> increase in rate of Ca2+ uptake by SR
      • --> increase in rate of muscle relaxation
      • --> increaes in amount of SR Ca2+ release due to increased Ca2+ load in the SR
  • At voltage-gated Na+ channels
    • Increase in probability of channel opening upon membrane depolarization
    • Increased GNa --> increased INa --> increased upstroke velocity --> increased propagation of AP across ventricular muscle
  • At the Na/K pump
    • Phosphorylation increases rate of transport
    • Hyperpolarizes resting potential
  • At the plateau phase
    • Increaased ICa --> increased plateau potential --> increased Ca2+ entry
  • At K+ channels
    • Increases GK1
    • Increases delayed K+ current IKs
    • Accelerates repolarizatoin
    • Decreases AP duration
    • Force of contraction increases
    • Duration of contractions decreases
  • At AP
    • Increases rate of upstroke & rate of repolarization of downstroke
    • Raises plateau potential
    • Increases ICa(L)
    • Increases peak & relaxation rates
    • Increases HR by increasing slope of pacemaker potential
  • In intact hearts, adrenalin causes a positive inotropic effect by...
    • Direct action of adrenalin on cardiac cells
    • Action of adrenalin on the SA node --> increased HR --> additional positive inotropic change through the Staircase Effect

40

Effects of Ach on ventricular cells

  • Increases K+ conductance
    • Negligible b/c Ach receptors are sparse on mammalian ventricles
  • Negligible except when muscle has been exposed to adrenalin
    • Ach activates G inhibitory proteins to increase cGMP & inhibit cAMP
    • Exerts opposite effects to reduce the positive inotropic action of adrenalin
  • Inhibits adrenalin release from nerve terminals embedded in the ventricular tissue
    • Counteracts positive inotropic actions of adrenalin

41

Autonomic drugs

  • Beta-adrenoceptor agonists
  • Beta-adrenoceptor antagonists
  • Muscarinic receptor antagonists
  • Cardiac glycosides

Cardiac glycosides

  • Beta-adrenoceptor agonists
    • Adrenaline
    • Dobutamine
    • Isoproterenol
  • Beta-adrenoceptor antagonists
    • Propranolol
    • Atenolol
  • Muscarinic receptor antagonists
    • Atropine
  • Cardiac glycosides
    • Digoxin
    • Ouabain
    • Actions in increasing force of contraction & slowing HR (higher concentrations)

42

Drugs for arrhythmias that target ion channels

  • Class I
  • Class II
  • Class III
  • Class IV

  • Class I
    • Na+ channel blockers
    • Lidocaine
    • Flecainide
  • Class II
    • Beta-adrenoceptor antagonists
  • Class III
    • Drugs that prolong the refractory period of cardiac muscle
    • Amiodarone (non-selective block)
    • D-sotalol (IKr blocker)
  • Class IV
    • Ca2+ channel blockers
    • Verapamil
    • Diltiazem
    • Nifedipine