Cardiac Physiology IV:

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Cardiac Physiology IV Molecular Contractility
and Excitation-Contraction
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The Steric-Blocking or 2-state Model of
Myofilament Activation
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Current 3-Step Model of Myofilament Activation
Off
Closed
Open
Something happens here
Ca2
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More on the current 3-state models
A crucial issue remains how to explain the highly
cooperative dependence of force on Ca2 given
the weakly cooperative binding of Ca2 to cTnC.
An important point is that crossbridge binding
potentiates thin-filament activation beyond that
produced by Ca2 alone.
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Basic Question How do you link electrical
activity (action potential) to mechanical work
(cardiac contraction) at the cellular level?
?
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Myocardial E-C Coupling
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• Key Issues to Keep in Mind
• Structure Function Relationships
• Where is the flexibility in the system?
• Where are potential control points are there
• opportunities for intervention?

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Ca2 is the central messenger in ECC
RyR2 SERCA2/PB Calsequestrin
L-Type Ca2 Channel Na Ca Exchanger
cTnC
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At the cellular level, the goal is to modulate
Ca2i Increase DHPR ? RyR2 ? cTnC
ICa ?
cTnC Decrease cTnC ? SERCA2/PB ? SR
? Na/CaX ? Extracellular




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• Proteins involved with increasing cytoplasmic
  Ca2
• Dihydropyridine-sensitive (L-Type) Ca2 channels
  (DHPR)
• Ryanodine Receptor 2 (RyR2)
• Transient (T-Type) Ca2 Channel
• Na - Ca2 Exchanger (NCX)
• Various and sundry bit players (? IP3R2)

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ECC Coupling Skeletal vs Cardiac
Cellular Physiology Sourcebook, 2001
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Organization of the Sarcolemma
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Cardiac Myocyte
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(No Transcript)
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• Cardiac L-Type Ca2 Channel (DHPR)
• Voltage gated, widespread distribution
• Evolutionarily related to Na and K channels
• Ca2 binds at selectivity filter glutamic
  residues on pore
• Dihydropyridines bind at IIIS5, IIIS6 and IVS6,
  Phenylalkamines
• bind at IIIS6 and IVS6, Benzothiazepines bind
  nearby

Ca2 - dependent inactivation
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• Ryanodine Receptor (RyR)
• Three known isoforms RyR1, RyR2 and RyR3
• Acts as the main Ca2 release channel in the
  cardiac myocyte
• Exists largely as a homotetramer (total of
  2260 kDa!)
• Forms the junctional foot process which spans
  the gap between the SR and
• sarcolemmal membranes at their junctions
• First identified over 30 years ago via EM, 28
  nm along each side, 14 nm tall

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Alignment of Proteins in the Junctional Space
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Properties of Mammalian RyRs
Cellular Physiology Sourcebook, 2001
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3D Surface Representation of RyR
RyR2 RyR1
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• Calcium-Induced Calcium Release (CICR)
• Calcium influx across the sarcolemma is required
• Does not play a significant role in skeletal
  muscle ECC
• Fundamental release unit Ca2 spark
• End result is a very large amplification of the
  initial ICa
• Both ICa and SR Ca2 release contribute to
  Ca2i

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• Local Control Theory
• Small, local increases in Ca2 lead to a large
  release of
• Ca2 from the SR via multiple RyR2s acting as a
  functional unit.
• The end result is an amplification of the initial
  ICa and Ca2I
• Individual sparks are usually not
  distinguishable, the Ca2I is the result of
  multiple sparks which have been synchronized by
  the action potential and ICa,L
• The local decline in the Ca2 is a result of
  both diffusion and active SR uptake.

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Local Control Theory
(10 20 nm)
(10 20 nm)
Spark
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Measuring Sparks in Isolated Myocytes
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Calcium binds to cTnC and promotes the strong
binding state
And the end result is..
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Myocyte Contraction
Of course, this is only half of the story .
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Myocyte relaxation requires a decrease in Ca2i

• This occurs via three tightly regulated
  mechanisms
• SR Ca2 ATPase (SERCA).60 70 of Ca2 uptake
• Na Ca2 Exchange (NCX)
• Sarcolemmal Ca2 - ATPase

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Structure and Distribution of the Sarcoplasmic
Reticular Ca2 ATPase (SERCA) Isoforms
Cellular Physiology Sourcebook, 2001
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SERCA2a

• Represents 40 of SR
• proteins
• Ca2 translocation requires
• ATP
• There are 3 known binding
• domains
• Ultrastructure and protein
• protein interactions are not
• fully characterized

Cellular Physiology Sourcebook, 2001
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• SERCA2 function is tightly modulated
• Phosphorylation (by CAM kinase)
• ATP levels
• Phospholamban

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Phospholamban A strong inhibitor of SERCA

• 3 4 of SR protein
• No known isoforms
• Unclear stoichiometry
• with SERCA

Phosphorylated form is usually pentameric and is
inactive The dephosphorylated form is monomeric
and inhibits SR Ca2 uptake by SERCA2.
Cellular Physiology Sourcebook, 2001
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• Phospholamban is a key modulator of
  contractility
• Transgenic mice with ? PB had decreased SR Ca2
  uptake
• and markedly impaired LV function
• PLB knockout mice (both /- and -/-) had
  increased SR Ca2
• uptake and were hypercontractile
• Think about the mechanisms involved
• In all cases, these effects on contractility and
  function were
• virtually linear and these experiments have led
  to a great deal
• of interest in PLB as a possible therapeutic
  target.

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Modulation of Ca2i in the Cardiac Myocyte
Cellular Physiology Sourcebook, 2001