Mechanism of Cardiac Contraction & Relaxation and regulation of Cardiac cycle by DR Vaibhav Yawalkar, DM Cardiology

1. Mechanism of Cardiac
Contraction & Relaxation
Dr. Vaibhav Yawalkar
DM Cardiology

2. 1.Excitation – Electrophysiology
2.Microanatomy of Myocardium
3.Excitation – Contraction coupling
4.Actual Process of Contraction & Relaxation
5.Major regulators of Contraction – Relaxation
6.Interventions targeted at this process

3. Excitation of cardiac contractile unit occurs
because of “Voltage”
(opening of voltage gated Ca++ channels)
From where this “Voltage” comes ?

4. Action Potential Conducted from:
1.SA Node
2.Atrial Myocyte
3.AV Node
4.His-Purkinje Fibers
5.Ventricular Myocyte itself

5. Microanatomy of Myocardium

6. Myofibe
r
Myofilamen
ts

7.  Ventricular myocytes are roughly brick
shaped, typically 150 x 20 x 12 µm and are
connected at the long ends by specialized
junctions
 Atrial myocytes are smaller and more
spindle shaped (<10 µm in diameter and
<100 µm in length)

8. Sarcolemma & T tubules
 Myocyte is bounded by a complex cell
membrane, the sarcolemma.
 The sarcolemma invaginates to form an
extensive transverse tubular network (T tubules)
that extends the extracellular space into the
interior of the cell.
 Rows of mitochondria are located between the
myofibrils and also immediately beneath the
sarcolemma

10. Sarcoplasmic Reticulum
 Lipid membrane–bounded, fine interconnected
network spreading throughout the myocytes
 Terminal cisternae or the Junctional sarcoplasmic
reticulum (jSR), close to T - Tubules
 Longitudinal, free, or network sarcoplasmic
reticulum, consists of ramifying tubules that
surround the myofilaments

11. T tubule Junctional SR Network SR

12.  T - Tubules:
Contains Voltage gated L type calcium
channels. Conducts Action Potential
 Junctional SR:
Stores & Releases Calcium on excitation
 Network SR:
Reuptake of Calcium during relaxation

13. Contractile Proteins
 Two Myofilaments
Actin (Thin Filament) & Myosin (Thick Filament)
 One myosin filament is surrounded by 6 actin
filaments in a Hexagonal arrangement.
 Collection of these myofilaments arranged in
Hexagonal manner is called Myofibril

14. Inside of Myocyte

15. Actin
 Two helical intertwining actin polymers along with
tropomyosin and troponin complex form thin
filament
 Because of intertwining, grooves are formed
between two actin polymers
 Long Tropomyosin molecule runs through the
grooves, and in each groove spans 7 actin
monomers
 Tropomyosin so to speak covers myosin binding
sites on each actin monomer in relaxed state.

17.  At every seventh actin molecule (38.5 nm) there is
a three-protein regulatory troponin complex:
Troponin C (Ca++ binding)
Troponin I (Inhibitory)
Troponin T (Tropomyosin binding)

18.  Ca++ binding with troponin C causes
troponin C to bind more tightly to troponin I
 This causes Tropomyosin to roll deeper into
the thin filament groove, exposing myosin
binding sites on actin monomers.

20. Myosin
 Each Myosin molecule exists as Head, Neck
and Tail (Heavy chain)
 Two Myosin molecules exist as a pair in
which their tails intertwine as a coil , &
collection of such tails form thick filament.
 Heads of myosin (in 6 pairs) protrude out
from thick filament in six different directions

21. Pair of Two Myosin
molecules

22. Myosin dimer of two heavy chains and 4 light chains

23. Each Myosin head has an ATP-binding pocket
and a narrow cleft that extends from the base
of this pocket to the actin-binding face
Mechanical Flexion occurs at head
and neck region during power stroke.
Actin filament can be moved by
approximately 10 nm in each stroke.
Though Myosin exists as a dimer , at
any instance of contraction cycle only
one myosin head of a pair attaches to
actin binding site.
Actin
Monomer
ATP

24. The myosin molecules are oriented in reversed
longitudinal directions on either side of the M-
line (which itself contains only myosin tails),
such that each side is trying to pull the Z-lines
toward the center
M line

25. Sarcomere
 The structural & functional contractile unit
that is repeated through the filaments
 Limited on either sides by “Z” lines.
(Z for “Zuckung”, meaning Contraction in German)
 It’s length varies from 1.6 -2.2 µm
 Z lines are discs (when viewed in 3d) on
which molecules like Actin and Titin are
anchored.

27. H zone vanished

28. Titin & Length Sensing
 Titin is a giant molecule, the largest protein yet
described.
 It is long, slender and elastic
 It extends from the Z-line into the thick filament,
approaching the M-line, and connects the thick
filament to the Z-line.
 Titin has two distinct segments: an inextensible
anchoring segment and an extensible elastic
segment that stretches as sarcomere length
increases

29. Recoil Tendency prevents
excess stretch
Restore Tendency
Titin acts as a spring
Restores if sarcomere excessively shortened and prevents
excessive stretching by it’s recoiling capacity
Anchoring
segment
Elastic
segment

30. Functions of Titin
 It tethers myosin filaments to the Z-line, thereby
stabilizing sarcomeric structure.
 If excessively shortened , it helps to restore
sarcomere by it’s spring action, and aids in early
diastolic LV suction.
 Limits overstretching of sarcomeres and end-diastolic
volume and returns some potential energy during
systole
 Transduce mechanical stretch into growth signals
causing altered myocyte growth pattern (e.g. in
DCM)

31. Myosin Binding Protein C (MyBPC
 Traverses the myosin molecules in the A-band,
thereby potentially tethering the myosin molecules
,stabilizes the myosin heads. Also binds with Titin
and actin molecules
 Defects in myosin-binding protein C are genetically
linked to familial hypertrophic cardiomyopathy

33. Excitation – Contraction Coupling
Cascade of biological processes that begins with the
cardiac action potential and ends with myocyte
contraction

34. Overview
1. Action Potential reaches sarcolemma & then T tubules
2. Voltage gated L type calcium channels in T tubules gets
activated & small amount of Ca++ enters in sarcoplasmic
cleft
3. Ryanodine receptors in the vicinity get activated & release
large amount of Ca++ from junctional SR (called as Ca++
induced Ca++ release)
4. Ca++ reaches to Troponin C of Actin, and Troponin I –
Tropomyosin moves & Myosin binding sites are exposed
5. Power stroke of Myosin & sliding of actin on myosin
6. Ca++ reuptake by SERCA back to SR causing relaxation

36. Relatively small amounts of Ca2+ (trigger Ca2+) enter and
leave the cardiomyocyte during each cardiac cycle, with larger
amounts being released and taken back up by the SR
Ryanodine Receptors
• RyR channels that mediate Ca2+ release from SR are
mainly located in the jSR membrane at the junctions with
the T tubule
• Each junction has 50 to 250 RyR channels on the jSR that
are directly under a cluster of 20 to 40 sarcolemmal L-
type Ca2+ channels
• RyR2 (the cardiac isoform) functions both as a Ca2+
channel and as a scaffolding protein that localizes
numerous key regulatory proteins

37. Calmodulin

38. 1. When the T tubule is depolarized, one or more L-type Ca2+
channels open, and local cleft [Ca2+] increases sufficiently
to activate at least one local jSR RyR
2. Ca2+ released from these first openings recruit additional
RyRs in the junction through Ca2+-induced Ca2+ release
to amplify release of Ca2+ into the junctional space
3. Ca2+ diffuses out of this space throughout the sarcomere
to activate contraction.

39. Turning off Calcium Release
SR Ca2+ release turns off when [Ca]SR drops by approximately
50% from initial end diastolic value.
Role of Calmodulin (CaM)
• CaM is present on L-type Ca2+ channels, RyR2 channels as
well as many other channels.
• Binding of Ca2+ to CaM inactivates both L-type calcium
channels & RyR channels, turning of calcium release.
So the increasing sarcoplasmic
Ca2+ itself turns off further Ca2+
release

41. Role of CaMKII
(Ca2+ /Calmodulin Dependent Protein Kinase II)
• CaMKII limits the extent of Ca2+ dependent inactivation
and enhances Ca2+ current amplitude
• Increases the fraction of SR Ca2+ released from the RyR
• It phosphorylates PLB (Phospholamban) to enhance SR
Ca2+ uptake by SERCA (Sarco-endoplasmic reticulum
Calcium ATPase)
So it enhances Calcium release as
well as Calcium uptake back to SR

42. Calcium uptake into SR
SERCA (Sarco-endoplasmic reticulum Calcium ATPase)
• Ca2+ is transported into the SR by SERCA, which
constitutes almost 90% of the SR protein
• Three isoforms exist, in cardiac myocytes the dominant
form is SERCA2a
• For each molecule of ATP hydrolyzed by this enzyme, two
calcium ions are taken up into the SR
• SR Ca2+ uptake is the primary driver of cardiac myocyte
relaxation
• A reduction in SERCA expression or function is seen in
heart failure & results in slower rates of cardiac relaxation

43. SERCA

44. Phospholamban (PLB) = Phosphate Receiver
• PLB is a single-transmembrane pass protein that binds
directly to SERCA2a
• Under basal conditions, this reduces the affinity of
SERCA for cytosolic Ca2+ which results in weaker SR
Ca2+ uptake by SR
• However, when PLB is phosphorylated by either PKA or
CaMKII the inhibitory effect is relieved.
• Thereby resulting in
increased rates of Ca2+
uptake, cardiac relaxation
(lusitropic effect), and
increased SR Ca2+ content,
which drives stronger
contraction (inotropic effect)

45. Calsequestrin & Calreticulin
• The Ca2+ taken up into the
SR is stored within the SR
before further release.
• The highly charged, low-
affinity Ca2+ buffer
calsequestrin is found
primarily at the jSR &
enhances the local
availability of Ca2+ for release
by the nearby RyR.
• Calreticulin is another Ca2+
storing protein that is similar
in structure to calsequestrin

46. Calcium Transient
 Sarcoplasmic Ca2+ pool is formed by Ca2+ influx
from L-type Ca2+ Channels denoted as [Ca2+]i &
Ca2+ released by SR. (25% & 75% respectively)
 Because Ca2+ removal is slower than Ca2+ influx and
release from SR, a characteristic rise and fall in
[Ca2+]i called the “Ca2+ transient” takes place
 This parameter reflects the state of contractility
(inotropic state) of contractile system. Other
parameter is Ca2+ sensitivity of myofilaments.

47. Other channels for ion exchange
 Besides Ca2+ , the other ion which moves in & out of
myocyte is Na+
 To maintain steady-state Ca2+ and Na+ balance, the
amount of Ca2+ and Na+ entering during each action
potential must be exactly balanced by efflux before
the next beat
Channels across Plasma membrane
1. Na+/Ca2+ Exchanger (NCX)
2. Plasma membrane Ca2+ ATPase (PMCA)
3. Na+/K+ ATPase
4. Na+/H+ Pump (only during acidosis)

48. Na+/K+
ATPase
3 Na+
2 K+
H+
Na+ Ca2+ ATPase

49. Molecular Basis of Muscular Contraction
(Cross-bridge Cycle)
 During diastole, myosin heads normally have ATP
bound
 Hydrolysis of ATP to ADP & inorganic phosphate
charges the Myosin head and they are ready to bind
actin. Although at this stage ADP & inorganic
Phosphate are still bound to myosin and complete
energy has not yet been utilized.
 This interaction is permitted when Ca2+ arrives and
binds to troponin C, shifting the position of the
troponin-tropomyosin complex on the actin filament

50. Phosphate
Myosin Head ready to bind

51. When myosin binding sites on actin are exposed due to
arrival of Ca2+ , myosin head uses energy from ADP+Pi
complex.
Pi is released
Myosin head binds to actin monomer
Power stroke occurs
Myosin head rotates
Actin moved by 10 nm

54.  Release of ADP from strong binding state, causes state of
sustained contraction called as Rigor state.
 Unless new ATP molecule binds to now empty pocket in
myosin head, the Rigor state will continue, which explains
phenomenon of rigor mortis.
 As long as [Ca2+]i and [ATP] remain high, the cycle can
continue with myosin-ADP-Pi binding to a new actin
molecule
 If intracellular [ATP] declines too far (e.g., during ischemia),
ATP cannot bind and disrupt the rigor linkage, leaving cross
bridges locked in the strong binding state

56. Head
binds
To Actin
ATP

57. Adrenergic Regulation
The adrenergic response is a key physiologic mechanism for
increasing cardiac output
Beta 1
Receptor
G protein (Gs)
↑ cAMP
PKA activation
CaMKII
Phosphorylation at various sites
1. L – Type Ca2+ Channels ----- ↑ Inotropy ↑
chronotropy
2. Phospholamban ----- ↑ Inotropy ↑ Lusitropy
3. RyR ----- ↑ Inotropy
4. MyBPC ----- ↑ Inotropy
5. Troponin I ----- ↓ Inotropy ↓ Lucitropy

58. Cholinergic Regulation
 Cholinergic system antagonizes effect of adrenergic
regulation
 It acts by decreasing cAMP levels or by upregulating cGMP
 NO facilitates cholinergic signaling at two levels, the nerve
terminal and by increasing cGMP
 cGMP acts through PKG, mainly on L-type Ca2+ channels
 Cholinergic system has lesser affect on myocytes, but
prominent affect on conductive system

59. Inotropic agents & Mechanism
Levosimendan

60. Newer Inotropic agents
(Omecamtiv
mecarbil)
[ATOMIC-AHF]
[HORIZON-HF]

61. Determinants of Contractile Performance
1. Preload (Frank-Starling mechanism)
2. Afterload
3. Contractility (Ca2+ transient / Myosin Ca2+
Sensitivity)
4. Lusitropy (diastolic function)
5. Heart Rate

62. Physiologic Systole
• From the start of isovolumic contraction to the peak
of the ejection phase
• That is Physiologic systole ends when LV starts
Relaxing as Ca2+ is taken back to SR. At this stage
aortic valve has not closed yet.
Physiologic Diastole
• Starts before aortic valve closure and indicates LV
relaxation till the next contraction cycle starts

63. Cardiologic Systole
• Cardiologic systole is longer than physiologic systole
and is demarcated by the interval between the first
heart sound (M1) to the closure of the aortic valve
(A2)
• So it includes initial LV relaxation phase in which
ejection is maintained by Aortic elasticity
(Windkessel effect) till the aortic valve is closed
Cardiologic Diastole
• From the closure of the aortic valve (A2) to first heart
sound (M1)

64. Frank-Starling
law Diastolic stretch of the left ventricle (and increased
sarcomere length) increases the force of contraction
 More rapid the rate of rise the greater the peak pressure
reached, and the faster the rate of relaxation, so both a
positive inotropic effect and an increased lusitropic effect.
 Increase in the strength of contraction can generally be
categorized as either :
• A Frank-Starling effect (increased sarcomere length)
or
• An inotropic effect (altered Ca2+ transient or
myofilament Ca2+ sensitivity), although both effects
can occur simultaneously

65. Anrep Effect
 When the aortic pressure is elevated abruptly, it limits
ejection and tends to increase EDV, which acutely increases
force and pressure at the next beat by the Frank-Starling
effect, mechanism of which is “Increased myosin calcium
sensitivity”
 However, in a slower adaptation that takes seconds to
minutes, the inotropic state of the heart increases by
increment in “Calcium transients”
 This slower adaptation is called “Anrep effect” & is believed
to be due to stretch-induced activation of several autocrine
/paracrine myocyte signaling pathways

66. Wall stress , Preload & Afterload
 Wall stress =
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑥 𝑅𝑎𝑑𝑖𝑢𝑠
2 𝑥 𝑊𝑎𝑙𝑙 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠
 Preload = Wall stress at End diastole
(Measured as EDV or LVEDP or LV dimensions by 2DECHO )
 Afterload = Wall stress during Systole
(Measured as Aortic Impedance or Arterial Elastance)

67. Heart Rate and Force-Frequency Relationship
Relationship between Heart rate and force of contraction
Treppe or Bowditch Effect
• An increased heart rate progressively enhances the force
of ventricular muscle contraction
• However, at a very high heart rate, force progressively
decreases & diastolic stiffness occurs.
• These effects at the myocyte level are largely attributable
to changes in Na+ and Ca2+ in the myocyte

68. Mechanism of Treppe effect
Increased HR
More Na+ & Ca2+
entry
Less time to extrude
these ions
High Cellular & SR
Ca2+ & Cellular Na+
More Ca2+ released
for contraction
Increased Force of
Contraction
Still higher HR
Calcium Overload &
Failure of NCX
Diastolic Stiffness

69. Myocardial O2 Uptake
Increased Wall stress = Increased ATP requirement = Increased O2
uptake
Heart
Rate
Wall
Stress
• Preload
• Afterload
Contractilit
y
• Calcium Transient
• Calcium
sensitivity
O2
Uptake
Index of O2
Uptake
Double Product
= SBP x HR

70. Work of the Heart
 External work is done when Stoke volume is moved against
the arterial resistance. May account for 40% of total O2
uptake.
 Internal work or Potential energy is generated within each
contraction cycle, not used for external work but used in LV
relaxation plus to maintain ion fluxes.
 Both External & internal work can be traced in Pressure-
volume loop graph
 Minute work = SBP x SV x HR

71. Measurement of Contractile Function
 Vmax or V0 is defined as the maximal velocity of contraction
when there is no afterload to prevent maximal rates of
cardiac ejection. Vmax cannot be measured directly but
must be extrapolated from the force-velocity relationship
 Measurements of pressure-volume loops are among the
best of the current approaches for assessment of the
contractile function.
 End-systolic elastance (Ees)
When the loading conditions are changed, alterations in
the slope of this line joining the different Es points (the
end-systolic pressure-volume relationship) are a good load-
independent index of the contractile performance of the
heart

72. Effect of Afterload
Reduction (Vasodilator
therapy)