Excitation/contraction Coupling

Front 1

What is E-C coupling?

Back 1

*E-C coupling refers to the mechanism by which action potentials cause muscle myofibrils to contract.

*It describes a process, in which electrochemical signals are transduced into mechanical force.

Front 2

Cardiac Myocyte Ultrastructure:

Back 2

*Don't make up the majority of the cells in the heart--fibroblasts are!
*Myofibrils, lost of mitochondria, sarcolemma, t-tubules which bring sarcolemma close to the SR.

Front 3

Calcium Movements During E-C Coupling:

Back 3

1) AP
2) LTCC
3) CICR from RyR
4) Ca binds to Tn-C
5) Contraction
6) Ca comes off myofilaments, SERCA & PL bring to SR.

Front 4

Clinical Significance of E-C Coupling:

Back 4

*Alterations in E-C coupling play a key role in the development of heart disease (e.g., heart failure and arrhythmias).

*Novel molecular approaches to improve E-C coupling are currently developed that could become useful therapeutic strategies in the future.

Front 5

Structure of the L-Type Calcium Channel:

Back 5

*LTCC is comprised of a pore-forming a1c subunit plus three accessory subunits (a2, d and b). 4 transmembrane domains, with 6 subunits each.
*alpha2 and delta subunits are disulfide linked.

Front 6

Structure of the Ryanodine Receptor:

Back 6

*Four RyR2 monomers (along with associated proteins) form a functional, Ca2+-releasing channel in the SR membrane.
*Isoform 2 is the one in the heart.
*N terminal domain makes up the bulk of the protein.
*Lots of "extramembrane real estate"-- can bind many things like protein kinases, calmodulin kinases, phosphatases--> critical for regulation of Ca2+.
*Closed in diastole; open in systole.

Front 7

Regulation of SR Calcium Reuptake:

Question: What is the effect of increased PLN phosphorylation on contraction?

Back 7

*Phospholamban (PL) is a major regulator of SERCA (inhibitory when dephosphorylated).
*PL gets phosphorylated, moves away and forms a pentamer, and allows Ca2+ to flow back into SR.

Answer: You'd increase contraction--more and faster SR reuptake of Ca2+.

Front 8

What is Excitation-Contraction (E-C) Coupling Gain?

Question: What are potential causes for a decrease in E-C coupling gain as seen in heart failure?

Back 8

*E-C Coupling Gain describes the efficiency of ICa (Ca current through LTCC) at triggering Ca2+ release from the SR.

1) Functional defects of LTCC.
2) Increased space b/t LTCC and RyR2.
3) Abnormalities (mutations) in the RyR2.
4) Decrease in SR content due to:
-Reduced re-uptake of Ca into SR.
-Increased Ca extrusion from the cell.
- Ca leak from the SR during diastole.

Front 9

Examples of Mutations in E-C Coupling Proteins Associated with Human Muscle Diseases:

Back 9

Front 10

Organization of the Sarcomere:

Back 10

*Sarcomere- Contractile unit between the Z-bands
*Thin Filaments- Actin and its regulatory proteins
*Thick Filaments- Myosin and a few associated proteins
*Z-Line- Location where the thin filaments meet
*M-Line- Center of the thick filaments
*A-Band- Length of thick filaments (anisotropic band)
*H-Band- Regions of myosin without overlapping actin
*I-Band- Regions of actin without overlapping myosin (isotropic band); Spans neighboring sarcomeres.

**She breezed thru this**

Front 11

Organization and Regulation of Myofilaments:

Back 11

*Thin Actin Filaments:
1) Actin proteins: Provide scaffolding for myosin binding.
2) Tropomyosin (Tm): Coils around actin, prevents actin-myosin binding at rest.
3) Troponin Complex: Regulates actin-myosin binding.
*Troponin T (Tn-T): Holds troponin complex to tropomyosin.
*Troponin I (Tn-I): Inhibits actin-myosin binding at rest.
*Troponin C (Tn-C): Binds Ca2+, displaces Tn-I from actin-myosin binding site.

*Thick Myosin Filaments:
1) Myosin: Two heavy and four light chains; Head & neck important for contraction.

**More important than previous slide to know**

Front 12

Actin-Myosin Crossbridge Cycling:

Back 12

*Breezed through.
*ATP required for power stroke AND relaxation.

Front 13

Mutations in Myofilament Proteins Can Cause Familial (Inherited) Forms of Cardiomyopathy. Discuss:

Back 13

*Mutations in genes that encode sarcomeric proteins
(mostly myofilament proteins and a few sarcomere-associated and Z disk proteins) account for ~75% of familial hypertrophic cardiomyopathy.
*Most occur in myosin heavy chain.

Front 14

Discuss Isometric Contractions:

Back 14

*Force transducer records the isometric force response to a single stimulus.
*Muscle length adjustable at rest; held constant during contraction.

*Both ends of the muscle are fixed, so that it cannot shorten.

Front 15

Discuss Isotonic Contractions:

Back 15

*Weight attached to muscle that must be lifted.
*Platform beneath weight prevents overstretching.

*One end of muscle is free, and the muscle is compelled to lift a weight.

Front 16

How do heart muscle contractions relate to P-V loops?

[Discuss this in context of Isometric Contractions (Fixed Length): Effect of Muscle Length on Resting & Active Tension]

Back 16

*Note transition from 1 to 3 to 5. Increased resting tension. Active tension increases at 3, and decreases again at 5.

*Resting Tension: Force required to stretch a resting muscle to different length.

*Active Tension: Tension developed upon stimulation when length is held constant. Depends on muscle length at which contractions occurs. Max. active tension is developed at intermediate length (Lmax).

*Total Tension: Sum of “Resting Tension” plus “Active Tension.”

Front 17

How do heart muscle contractions relate to P-V loops?

[Discuss this in context of Isotonic Contractions (Afterloaded): Relationship to the Cardiac Muscle Length-Tension Diagram]

Back 17

*Muscle can freely move, so it changes in length but the tension stays the same.

Isotonic:
*Weight placed on resting muscle (preload) increases length.
*Muscle length decreases at constant tension.

Afterloaded:
*Load on the muscle at rest (preload) determines muscle length at rest (same as above).
*Additional load on the muscle when it begins to contract (afterload).
*Muscle shortens only after tension > total load.

Front 18

Discuss Preload and Afterload:

Back 18

*The time during contraction when muscle first encounters load:

*Preload: Stretches a muscle before it contracts.
*Afterload: Not evident to the muscle during the resting state but when it begins to contract.

Front 19

Relating P-V loop and Tension-Length diagrams:

Back 19

*Cardiac Muscle Strip: Tension - Length Diagram
*Cardiac Ventricle (LV): Pressure -Volume Loop
*Be able to make sense of how these relate.

Front 20

Back 20

A: Cardiac cycle. LV pressure and volume are plotted against time.
B: P-V loop. LV pressure and volume plotted against each other.

Front 21

Effect of Increased Preload on Afterloaded Contractions (A) and Ventricular Stroke Volume (B):

Back 21

*PV loop gets shifted to right --> higher SV.
*Resting tension curve shifted to right as well.

Front 22

Effect of Increased Afterload on Afterloaded Contractions (A) and Ventricular Stroke Volume (B):

Back 22

*Increased afterload, AV opens later, less SV.
*Must overcome more load before muscle can shorten. Overall shortening of muscle is much less.

Front 23

Effect of Increased Contractility on Afterloaded Contractions (A) and Ventricular Stroke Volume (B):

Back 23

*No ∆ in preload or afterload.
*Intrinsic ability to contract is enhanced.

Front 24

Summary of tension-length and P-V loops in increased preload, increased afterload, and increased contractility:

Back 24

Front 25

Regulation of Myocyte Contractility by the Autonomic Nervous System:

Back 25

*NE activating kinases (sympathetic).
*Countered by ACh and M receptors (PS).
*cAMP-mediated PKA activity is key here!
*Note change in contraction curve on bottom right.

Front 26

Sites of action of established and experimental therapies in heart failure:

Back 26

*Increase cAMP with ß agonists (acutely! Not long term therapy.).
*ß-receptor blockers are useful to lead to an increase in ß-receptor density (over a long period in chronic HF...not acutely!).
*Experimental therapies seek to circumvent the up and downregulation issues by increasing SERCA, sensitizing myofilaments, etc.

Front 27

What happens to ß-receptors in the heart in HF?

Back 27

*Over the long term, continuous catecholamine release causes ß-receptors to get desensitized and/or downregulated in HF. You see less ß-receptors in HF.
*You also could have upregulation of M receptors.

Front 28

Summary of strategies in HF treatment (current and experimental):

Back 28

1) Increase cAMP.
2) Increase Ca available for contraction.
3) Target myofilaments.
4) Mitigate neurohormonal storm the heart is exposed to (ß-blocker, ACEi, ARB).
5) Device therapy
6) Cardiac remodeling therapy with stem cells or fibroblasts.

Front 29

Contractility Regulation by LTCC blockers:

Back 29

*Don't use acutely!

Front 30

Contractility Regulation by cardiac glycosides:

Back 30

Front 31

Nice summary diagram of myofibril structure, Ca cycling, and contraction:

Back 31