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From Syncitium to Regulated Pump:
A Cardiac Muscle Cellular Update
Experimental Biology 2010: APS Refresher Course in Cardiovascular Physiology
Anaheim, CA
Donna H. Korzick, Ph.D.
Program in Physiology and
Department of Kinesiology
Refresher Course in Cardiovascular Physiology
From Syncitium to Regulated Pump
I.
Fast Forward Seven Years…
• “The more things change, the more they stay the same”
• “The heart generates pressure, and pressure makes the blood go ‘round”
II.
Review Microdomains and Local Control of EC Coupling
• Transarcolemmal Ca2+ fluxes
• Ca2+ sparks
III. Microdomains and Local Control of Pacemaker Function?
• Is the pacemaker current (If) current solely responsible for pacemaker
regulation?
• Paradigm Shift: The Coupled Clock Pacemaker System
IV. References
• Medical Physiology (2nd ed.), Boron and Boulpaep (2009)
• Primary literature
Refresher Course in Cardiovascular Physiology
From Syncitium to Regulated Pump
I.
If you only have 6-10 lectures, what do you teach?
• APS CV Objectives
• http://www.the-aps.org/education/MedPhysObj/cardio.htm
II.
Is there a perfect textbook?
• No, but they’re getting better!
• Supplement with good review articles and classic papers
II.
Overarching themes
• The heart as a pump: “The Heart Develops Pressure, and Pressure
Makes the Blood Go ‘Round”
• Mechanisms that alter cytosolic Ca2+ or myofilament Ca2+ sensitivity
alter left ventricular (LV) developed pressure and cardiac inotropy
• A possible third theme: “Mechanisms that alter cytosolic
(subsarcolemmal) Ca2+ alter cardiac automaticity”?
An Elegant Design......“The Heart Develops Pressure, And
Pressure Makes the Blood Go ‘Round”
 Closed loop system
Systemic
Pulmonary
Figure 17-3. Circulatory beds
From Boron and Boulpaep.
Medical Physiology (2nd ed.).
2009.
 High pressure on arterial side
 Low pressure on venous side
 2 pumps in series – LV is huge!
Systemic organs arranged
in parallel – why?
VO2 = Cardiac Output x a-vO2 diff
Cardiac Output = Heart Rate (HR) x Stroke Volume (SV)
A.
Regulation of pacemaker activity (HR)
B.
Regulation of myocardial performance (SV)
C.
Major Points
1. principle control of HR is by the ANS (extrinsic)
2. both intrinsic and extrinsic mechanisms regulate SV
(VO2, net diffusion of oxygen; a-vO2 diff, arteriovenous difference of oxygen;
ANS, autonomic nervous system)
Cardiac Output = Heart Rate (HR) x Stroke Volume (SV)
I.
Regulation of pacemaker activity (HR)
A. Sympathetic Nervous System (SNS) input
B. Parasympathetic Nervous System (PNS) input
II.
Intrinsic Regulation of myocardial performance (SV)
A. Frank-Starling Mechanism
B. Rate-Induced Regulation
1.
Treppe/Staircase
2.
Post-extrasystolic Potentiation
III.
Extrinsic Regulation of myocardial performance (SV)
A.
β-adrenergic stimulation (SNS)
B.
Other (PKG, PKC, CAM-kinase, MAPK)
(PKG, protein kinase G; PKC, protein kinase C;, CAM-kinase, calcium/calmodulin-dependent
kinases; MAPK, mitogen-activated protein kinase)
Efflux
Influx
VGCC’s
NCX
Figure 1A
From Bers, Circ Res: 87:275281,2000
Freely available at
http://circres.ahajournals.org/cgi/co
ntent/full/87/4/275
Ca2+-ATPase
NCX
SL Ca2+-ATPase
Mitochondrial Ca2+ Uniporter
(NCX, Na-Ca exchanger; SL,
Sarcolemma; SR, sarcoplasmic
reticulum; VGCC, voltage-gated
calcium channel)
Cooperative binding to
myofilaments (Ca + sensitivity)
Typical twitch – contractile
force reaches ~45% of max
(requires 70 mol/L cytosol
= ~600 nmol/L [Ca]i
Point: Mechanisms that alter cytosolic Ca2+ or myofilament Ca2+
sensitivity alter LV developed pressure
Central Dogma for Cardiac Excitation-Contraction (EC) Coupling
• Ca2+-Induced Ca2+ Release
• Small Ca2+ increases in the vicinity of the SR
lead to much larger Ca2+ release from the SR
• Electrical excitation at the SL membrane activates VGCCs
[dihydropyridine receptors (DHPR)]
• Influx of Ca2+ via the intracellular Ca2+ transient ([Ca2+]i)
• [Ca2+]i activates Ca2+ release channels on the SR [ryanodine receptor
(RYR)]
• Contractile element activation
What underlies the [Ca2+]i?
The elementary event of SR Ca2+ release in cardiac muscle is the Ca2+ spark
“Local Control of EC-Coupling”
The [Ca2+ ]i rises as Ca2+ sparks sum
“Normal EC coupling involves a well-ordered and stereotyped sequence of events”
Figure showing image of Ca sparks, cartoon
of cardiac myocyte and line scan image.
From Guatimosim et al, J Mol Cell Cardiol,
34: 941-950, 2002
Available for fee at http://www.jmmconline.com/issues/contents?issue_key=S002
2-2828%2800%29X0111-3
Ca2+ Sparks:
*represent Ca2+ passing
thru RYRs
*represent small local Ca2+
release events
*sparks can be evoked by
depolarization and action
potentials
1. Summation of sparks provides the microscopic basis of the [Ca2+]i
2. Provides an explanation for graded contractions which are modulated by
local control
3. Provides insight into microdomains and where they occur
4. Provides insight into pathological changes and defects in EC Coupling
• Calcium waves and arrhythmias
A True Increase in Contractility Increases the Slope of the
ESPVR and thus Stroke Volume; cardiac performance
independent of preload and afterload
Figure 22-13B. Increased
contractility
From Boron and Boulpaep.
Medical Physiology (2nd ed.).
2009.
Contraction and Relaxation
are Enhanced by SNS
Stimulation
Figure 24-18.
From Berne and Levy Physiology
(6th ed.), 2009.
Figure 24-24.
From Berne and Levy Physiology
(6th ed.), 2009.
Sympathetic Influences on Myocardial Contraction:
NE
P
RyR Channels
Point: Mechanisms that alter cytosolic Ca2+ alter LV developed pressure
β1-AR, β1-adrenoceptor; Gs, Gs alpha subunit protein; NE, norepinephrine
Signal Transduction of Myocardial Performance:
Important Seven-Transmembrane-Spanning Receptors
Response
M2Rs:
Signal  Receptor  Coupling Protein
βγ subunit
directly regulates K+
channels; PDE effects
on adenylate cyclase
Second Messengers  Response
G Proteins
1-ARs
2-ARs
1-ARs
Gs
Gs/Gi
Gq
cAMP (PKA)
PDE/PP
DAG (PKC)
IP3
DAG, diacylglycerol; IP3, inositol 1,4,5-triphosphate; M2Rs, muscarinic acetylcholine receptor;
PDE, phosphodiesterase; PKA, protein kinase A; PP, protein phosphatase
Cardiac Output = Heart Rate (HR) x Stroke Volume (SV)
I.
Regulation of pacemaker activity (HR)
A. Sympathetic Nervous System (SNS) input
B. Parasympathetic Nervous System (PNS) input
II.
Intrinsic Regulation of myocardial performance (SV)
A. Frank-Starling Mechanism
B. Rate-Induced Regulation
1.
Treppe/Staircase
2.
Post-extrasystolic Potentiation
III.
Extrinsic Regulation of myocardial performance (SV)
A.
β-adrenergic stimulation (SNS)
B.
Other (PKG, PKC, CAM-kinase, MAPK)
The Magical Spontaneously Beating
Heart…..
SA Node Action Potential Dogma: If is
the pacemaker of the heart

0
3


4

SANCs
Automaticity
Does not maintain a steady Vm, but
instead depolarize
Slow (diastolic) depolarization (DD):
Pacemaker current (If)






Rise in Na+ conductance
Fall in K+ conductance
If activated at end of phase 3
Threshold for activation is -40 mV
Upstroke mediated by slow inward
Ca2+ channel (L-type)
Surface membrane ion channels
work as a “Membrane Clock” to
regulate automaticity
(PCa, PK, PNa, Ca, K, and Na potentials; SANCs,
sinoatrial nodal cells; Vm, membrane potential)
SA Node Action Potential Dogma: Modulation of
Pacemaker Activity by SNS and PNS
Parasympathetic stimulation through
AchR stimulation: 1) decreases If, 2)
shifts maximum diastolic potential and
3) reduces ICa and shifts threshold to
more positive values
Figure 21-6. Modulation of
pacemaker activity
From Boron and Boulpaep.
Medical Physiology (2nd ed.).
2009.
Sympathetic stimulation through β-AR
stimulation: 1) increase If and 2)
increases ICa and shifts threshold to
more negative values
(AchR, acetylcholine receptor; β-AR, βadrenergic receptor; ICa, Ca current)
A Paradigm Shift: Rhythmic RyR-generated
subsarcolemmal local Ca2+ releases (LARs) function
as a “Ca2+ Clock” and underlies the pacemaker
function of SANCs
Figure showing membrane
potential, various currents,
calcium dynamics, and
calcium clock phases vs.
cycle length.
From Lakatta et al., Circ Res,
106: 659-673, 2010
Available for fee at
http://circres.ahajournals.org/
cgi/content/full/106/4/659
• LCRs occur during DD and
activate INCX
• Drives MP to threshold
• ICaL opening induces local
CICR, SR Ca2+ release from
RyRs and depletion; AP
upstroke
• Stops LCRs, clock is reset!
• Ca2+ influx refills SR
• IK and Ca2+i contribute to
repolarization and Ca2+
efflux (early DD)
(CICR, calcium-induced calcium release; ICaL, IK, INCX , L-type
Ca, K, NCX currents; LAR, ; LCR, ligase chain reaction)
Preliminary Summary
1. There is a rhythmic spontaneous Ca oscillator or ‘clock’ that generates
LCRs within SANCs
2. LCRs (generated by the clock) emerge late in the DD
3. The intracellular clock requires recurring APs to provide Ca2+ to sustain its
rhythmic oscillator function
4. LCRs generated by the Ca2+ clock interact with the surface membrane and,
via activation of the INCX, generate miniature curent and voltage oscillations
that confer the exponential increase to the late phase of the DD
5. This late DD acceleration by INCX is required for sufficient ICaL activation to
generate the AP upstroke
6. **In the absence of the LCR-activated INCX ‘prompt’ to the membrane,
rhythmic APs do not occur.
7. Rise in [Ca2+] i comes from RyRs (triggered by the AP)
8. SR calcium depletion suspends LCRs
A Paradigm Shift: The Coupled-Clock Pacemaker
System
Figure showing pacemaker
system within cell.
From Lakatta et al., Circ
Res, 106: 659-673, 2010
Available for fee at
http://circres.ahajournals.or
g/cgi/content/full/106/4/659
• LCRs activate adenylate
cylase (types 1 and 8)
• high basal cAMP and PKA
phosphorylate Ca2+
pumps/channels
• sustains basal LCRs and
LCR period
• LCRs activate CaMKII
• sustains basal LCRs
• high basal cAMP/PKA and
CamKII balanced by high
basal PDE, PP activity
• Autonomic rate modulation
of M and Ca2+ clock events
(CaMKII, Ca2+/calmodulin-dependent kinase)
CaMKII, SERCA, RyR and NCX Immunolabeling in
SANCs
A. Figure 7
From Vinogradova et al., Circ Res. 87: 760–767, 2000.
Freely available at http://circres.ahajournals.org/cgi/content/abstract/87/9/760
B. Figure S-1 A and C
C, D, E. Figure 2
From Lyashkov et al. Circ. Res. 100:1723-1731, 2007.
Freely available at
http://circres.ahajournals.org/cgi/content/full/100/12/1723
LCRs appear during late DD in SANCs and βAdrenergic stimulation Reduces the LCR Period
Figure 3A
From Lakatta E.G. et al., Circ.
Res. 106: 659-673, 2010
(Data from Bogdanov et al. 2006)
Available at
http://circres.ahajournals.org/cgi/c
ontent/full/106/4/659
Data from Joung et al. 2009
Line-scan image of LCRs with
superimposed spontaneous APs
LCR period determines the
“ticking speed” of the coupled
clock system
RyR blockade
negates effects of
β-Adrenergic
stimulation
Summary
1. Rate regulation is governed by intracellular (SR) Ca2+ cycling and
its coupling to the surface membrane and represents the ‘clock’
that controls SANC normal automaticity, leading to mutual
functional entrainment.
2. Rate and amplitude of SR Ca2+ cycling is controlled by the
amount of free Ca2+ in the system, the SR pumping rate and the
number of activated RyRs.
3. LCR period and amplitude determine the time and amplitude of
the late exponential DD phase and thus whether the membrane
achieves excitation threshold to generate the next rhythmic AP
via activation of INCX.
4. Pacemaker flexibility and rate regulation is ensured by coupling
SR Ca2+ cycling to surface membrane G protein coupled receptor
(GPRC) signaling.
5. A possible third theme for your lectures: “Mechanisms that alter
cytosolic (subsarcolemmal) Ca2+ alter cardiac automaticity”?