8_25 Excitation-Contraction Coupling (Mende)

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What is happening (high level) APs are turned into contraction?

electrochemical signals are transduced into mechanical force

How does calcium get distributed to the SR?

T-tubules (at right angles to the terminal cisternae)

What is the specialized plasma membrane surrounding each myocyte?

sarcolemma

What is the functional unit of the myocyte?

sarcomere

This receptor is activated from transient Ca++ entering the cell from L-Type channels, and promotes release of Ca++ from the SR that leads to contraction:

Ryanodine

After contraction, this receptor takes up free Ca++, what is it and where?

SERCA (on SR) ... note ATP dependent process

Describe the structure of the L-Type Calcium channel:

comprised of a pore-forming subunit plus
three accessory subunits that are required for proper function

What are the monomers in the Ryanodine receptor? how many are there and what is their function?

RyR2 (four of them)

form Ca++ releasing channel in SR --> release during systole

What is a major regulator of SERCA? what is it's active state?

Phospholamban (PL): allows Ca++ in/active when phosphorylated (inhibitory when dephosphorylated)

*What is the effect of increased PL phosphorylation on contraction?

beta-adrenergic stimulation increases the force and shortening velocity of contraction (i.e., positive inotropy), and increases the rate of relaxation (i.e., positive lusitropy).

What is meant by "E-C Coupling Gain"?

describes the efficiency of I-Ca at triggering Ca2+ release from the SR (I-Ca is the current of Ca++ entering cell via L-Type from the AP)

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

functional defects in the L type calcium channel.
low extracellular calcium
increased space between the L type channel and ryanodine receptor
abnormalities in the ryanodine receptor

there are three mechanisms to lead to a decrease in SR calcium content:
1. One was reduced uptake in the calcium and to the SR
2. if we have more extrusion from the cell, and leaving less calcium that can be taken up into the SR
3. ryanodine receptors that are not fully closed in diastole so they actually are leaking calcium in diastole and therefore, there's less calcium in the SR that can be released when it's really needed.

What is a mutation in the RyR2 associated with?
in Phospholamban?

general: human muscle diseases

Arrhythmias and sudden death
Dilated cardiomyopathy

What delineates sarcomeres?

Z-lines

What are the roles of actin and myosin in myofilaments?

Actin (thin filaments) provide a scaffolding for myosin binding

Myosin (thick filaments) head and neck important for contraction
recall: myosin is "cocked" in the relaxed and energized (ADP) state

What is the role of Tropomyosin?

Coils around actin, prevents actin-myosin binding at rest

What is the troponin complex? and what is the role of each component?

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

Notes: T=tropomyosin holding
I=inhibits binding
C=Ca++ binding

What in the sarcomere binds Ca++?

Troponin C

What are the three general groupings of cardiomyopathies? (note: limited to myocardium, LV)

hypertrophic, dilated and restrictive.

*Describe the difference between isometric and isotonic contraction:

Isometric (constant length): Both ends of the muscle are fixed, so that it cannot shorten

Isotonic (constant force/resistance): One end of muscle is free, and the muscle is compelled to lift a weight

What is the preload in the intact heart?

The volume of blood in the chamber, *muscle length

(Stretches a muscle before it contracts)

What is the afterload? (in the intact heart)?

Afterload can also be described as the pressure that the chambers of the heart must generate in order to eject blood out of the heart and thus is a consequence of the aortic pressure (for the left ventricle).

Afterload is the tension or stress developed in the wall of the left ventricle during ejection.

In isometric contraction, at what length is maximum active tension generated?

intermediate length

In isotonic contraction, what happens to tension as muscle length decreases?

stays the same

What does increased preload do to afterload contraction? increased afterload to afterload contraction?

more preload = more change in length (more SV)

more afterload = less change in length, but with more total load (less SV)

What is is the inherent capacity of myocardial cells to develop force independent of the initial muscle length (preload) or afterload?

Contractility

*What role to protein kinases play in myocyte contractility? how does the autonomic nervous system influence this?

All Increase:
L-Type Ca++ Channels allow more in
RyR allow more Ca++ release
PL promotes more Ca++ uptake by SR
TnI promotes more actin/myosin binding

--> enhance rate of both contraction and relaxation

Sympathetic: NE --> B1 receptor --> increases cAMP conversion (increases kinase activity)
Parasympathetic: ACh --> muscarinic --> decreases cAMP conversion

what role does Phosphodiesterase (PDE) play in myocyte contraction?

decreases: PDE reduces cAMP, thus decreases kinase activity and decreases contractility

--> PDE inhibitors, thus increase contractility

what happens to contractility control in heart failure?

b1 adrenergics decrease effect
muscarinic increase effect

cAMP levels and overall contractility falls

Note:

current heart failure therapies involve b1 agonists and PDE inhibitors...

experimental therapies target the RyR, PL and SERCA function of the SR

How does a Ca++ channel blocker impact contractility?

Decrease

What do cardiac glycosides (digitalis) do? how do this impact contractility?

inhibit Na+/K+ ATPase

increases contractility - less Na gradient, decreases effect of Na/Ca antiport, so more Ca in the cell results in greater contraction

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