Donna H' Korzick, Ph'D'

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Regulation of Cardiac EC-Coupling A
Cellular Update
Experimental Biology 2003 APS Refresher
Course San Diego, CA
Donna H. Korzick, Ph.D. Noll Physiological
Research Center and The Department of Kinesiology
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Muscle Physiology From Cellular to
IntegrativeRegulation of Cardiac Performance

• Microdomains and Local Control of EC Coupling
• Transarcolemmal Ca2 fluxes
• Ca2 sparks
• Ca2 release mechanisms
• Whats Relatively New in Adrenergic Signaling?
• Are ?1-ARs the only receptors responsible for
  changes in contractility?
• GPCRs and beyond
• What Should Our Students Know About Intracellular
  Signaling?
• Protein scaffolds
• Receptor Desensitization
• Relatively New Ideas
• Nitric Oxide as a regulator of cardiac
  performance

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Muscle Physiology From Cellular to
IntegrativeRegulation of Cardiac Performance

• If you only have 6-10 lectures, what do you
  teach?
• APS CV Objectives
• Is there a perfect textbook?
• No
• Supplement with good review articles

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• VO2 Cardiac Output x a-vO2 diff
• Cardiac Output Heart Rate (HR) x Stroke Volume
  (SV)
• Regulation of pacemaker activity (HR)
• 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
• 3. contractility is defined as cardiac
  performance independent of changes in preload
  and afterload

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Efflux
SR Ca2-ATPase NCX SL Ca2-ATPase Mitochondrial
Ca2 Uniporter
Influx
VGCCs NCX
Cooperative binding to myofilaments (Ca
sensitivity)
Typical twitch contractile force reaches 45
of max (requires 70 ?mol/L cytosol 600 nmol/L
Cai
Bers, Circ Res 87275-281,2000
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Point Mechanisms that alter cytosolic Ca2 or
myofilament Ca2 sensitivity alter LV developed
force

• Central Dogma for Cardiac 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 (DHPR)
• Influx of Ca2 via the Ca2I
• Ca2I activates Ca2 release channels on the
  SR (RYR)
• Contractile element activation

What underlies the Ca2I? The elementary event
of SR Ca2 release in cardiac muscle is the Ca2
spark Local Control of EC-Coupling The Ca2 i
arises as Ca2 sparks sum
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Normal EC coupling involves a well-ordered and
stereotyped sequence of events
Ca2 Sparks represent Ca2 passing thru
RYRs represent small local Ca2 release
events sparks can be evoked by depolarization
and APs
0.5 s
E
Guatimosim et al, J Mol Cell Cardiol, 34
941-950, 2002
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• Why Ca2 Sparks?
• Summation of sparks provides the microscopic
  basis of the Ca2i
• Provides an explanation for graded contractions
  which are modulated by local control
• Provides insight into microdomains and where they
  occur
• Provides insight into defects into pathological
  changes in EC Coupling
• Calcium waves and arrhythmias

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RyR2 as a Ca2 Release Channel

• RYR2 Ca2 release channel
• Phosphorylated by PKA, ? PKC, ? PKG, ? CamKII
• scaffolding protein that localizes numerous key
  regulatory proteins to the junctional complex

Bers, Nature, 415 198-205, 2002
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RyR2 as a Macromolecular Signaling Complex
RyR2 -tetrameric channel -4 565,000 dalton
subunits -gt2.3 million daltons -FKBP12.6 -PKA -AK
APs -PP1 (spinophilin) -PP2A (PR130)
Point Local control of RyR2 function by
macromolecular signaling complexes allow for a
graded physiological response to stress
Marks, J Mol Cell Cardiol, 33 615-624, 2001
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Role of FKBP12.6 on RyR2 Regulation

• Functions of FKBP12.6
• Stabilize the closed state of the RYR to prevent
  Ca2 leak
• Coupled Gating
• Regulate RyR2 sensitivity to Ca2 activation

Bers, Nature, 415 198-205, 2002
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Physiologic Regulation of the Inotropic State
Myocardial Performance
Adrenergic Stimulation
Frank-Starling Effect
1.
2.
Adrenergic Regulation
4.
Force-Frequency Relation
3.
Vascular Function
5.
Modified from Ross et al, Circulation 1995 92
2327-2332
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Contraction and Relaxation are Enhanced by SNS
Stimulation
Figure 24-24
Figure 24-18
Berne and Levy
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Signal Transduction of Myocardial Performance
Response
Signal ? Receptor ? Coupling Protein
Second Messengers ? Response
?1-ARs ?2-ARs ?1-ARs
cAMP (PKA) PDE/PP DAG (PKC) IP3
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G-Protein-Coupled Receptor Signaling
Rockman et al, Nature, 415 206-212, 2002
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Sympathetic Influences on Myocardial Contraction
NE
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Berne and Levy
(PKA)
4.
Reducing Na gradient (? Nai/? Nao)
Point Mechanisms that alter cytosolic Ca2
alter LV developed force
Point ?1-adrenergic stimulation increases both
cardiac inotropy and lusitropy.
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RyR2 is also a target of PKA phosphorylation
Bottom line increase Po of RyR2
Bers, Nature, 415 198-205, 2002
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Dual Coupling of ?2-ARs to Gs and Gi
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Gq-coupled GPRCs confer positive inotropic
effects and myocardial cell growth
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Important Seven-Transmembrane-Spanning Receptors
Modified from Rockman et al, Nature, 415
206-212, 2002
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Chemical Control of Myocardial Performance

• Epi (adrenal medulla)/ Norepi ?1, ?2, ?1 - ARs
• Ang II positive inotrope (Gq, IP3/PKC pathway)
• Opioids positive inotrope (Gq, IP3/PKC pathway)
• Endothelin I positive inotrope (Gq, IP3/PKC
  pathway)
• Thyroid Hormone positive inotrope/chronotrope
  (genomic and nongenomic effects)
• Insulin positive inotrope (PKC, PI3K)
• Glucagon positive inotrope/chronotrope (AC)

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Receptor Desensitization Agonist-Induced
Phosphorylation of ?ARs by GRK2
?1/2-AR
GRK2 ? Adrenergic Receptor Kinase 1 (?ARK1)
Rockman et al, Nature, 415 206-212, 2002
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Relatively New Ideas Nitric Oxide as a
Mediator of Cardiac Performance
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Relatively New Ideas Nitric Oxide as a
Mediator of Cardiac Performance

• NO as a positive inotrope
• NO as a negative inotrope
• NO modulates ?-AR effects on contractile
  performance
• eNOS production by cardiac myocytes in response
  to stretch
• Machinery for endogenous NO production resides in
  cardiac myocytes
• May subserve SR Ca2 release functions
• PI3K-dependent phosphorylation of Akt and eNOS ?
  increased NO production
• eNOS as a regulator of mitochondrial respiration
• iNOS as protective
• nNOS as a regulator of SERCA
• May subserve Ca2 reuptake mechanisms

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Thank You