Flashcards in Cardiac Muscle Contraction Deck (72)
Is the basic contractile machinery in cardiac and skeletal muscle similar?
Yes, their cell structure is basically the same
Describe the steps involved in excitation-coupling in cardiac muscle
1) The cardiac action potential is initiated in the myocardial cell membrane, and the depolarization spreads to the interior of the cell via the T tubules which results in an inward Ca2+ current
2) Entry of Ca2+ into the myocardial cell produces an increase in intracellular Ca2+ concentration which triggers the release of more Ca2+ from the endoplasmic reticulum
3) Ca2+ binds to troponin C, moving tropomyosin out of the way allowing actin and myosin to interact
4) Cross-bridges form and then break, resulting in the thin and thick filaments to move past each other to produce tension
When does relaxation occur?
When Ca2+ is reaccummulated in the sarcoplasmic reticulum by the action of Ca2+ ATPase
What is contractility, or iontropism?
The intrinsic ability of myocardial cells to develop force at a given muscle cell length
Agents that produce an increase in contractility are said to have _____ inotropic effects
What do positive inotropic effects do?
They increase both the rate of tension development and the peak tension
Agents that produce an decrease in contractility are said to have _____ inotropic effects
What do negative inotropic effects do?
They decrease both the rate of tension development and the peak tension
What is contractility directly related to?
intracellular calcium concentration
What are the 2 factors in which the amount of Ca2+ released from the sarcoplasmic reticulum depend on?
- size of the inward Ca2+ current during the plateau of the myocardial action potential
- the amount of Ca2+ previously stored in the sarcoplasmic reticulum for release
Stimulation of the sympathetic nervous system has a ____ inotropic effect
What are the 3 important features of the positive inotropic effect of the sympathetic nervous system?
- increased peak tension
- increased rate of tension development
- faster rate of relaxation
Faster relaxation means that the contraction is _____.
What are the 2 proteins that are phosphorylated to produce an increase in contractility?
- sarcolemmal Ca2+ channels
How does phosphorylation of the sarcolemmal Ca2+ channels produce an increase in contractility?
There is increased inward Ca2+ current during the plateau and increased trigger Ca2+, which increases the amount of Ca2+ released from the sarcoplasmic reticulum
What is phospholamban?
A protein that regulates Ca2+ ATPase in the sarcoplasmic reticulum
How does phosphorylation of phospholamban increase contractility?
It stimulates the Ca2+ ATPase, resulting in greater uptake and storage of Ca2+ by the sarcoplasmic reticulum
What are the 2 effects of increased Ca2+ uptake by the sarcoplasmic reticulum?
- it causes faster relaxation
- it increases the amount of stored Ca2+ for release on subsequent beats
Stimulation of the parasympathetic nervous system has a negative inotropic effect on what?
The G protein that is coupled to the muscarinic receptors of the parasympathetic nervous system causes a _____ in contractility. Explain why...
Because it is inhibitory
What are the 2 factors that are responsible for the decrease in atrial contractility caused by the parasympathetic nervous system?
- ACh decreases inward Ca2+ current during the plateau of the action potential
- ACh shortens the duration of action potential and, indirectly, decreases the inward Ca2+ current (by shortening the plateau phase)
What are cardiac glycosides?
a class of drugs that act as positive inotropic agents
What is the action of cardiac glycosides?
inhibition of Na+ - K+ ATPase
Inhibition of Na+ - K+ ATPase results in ____ inotropic effects
Describe the mechanism of the positive inotropic effect of cardiac glycosides
1) cardiac glycosides inhibit Na+ - K+ ATPase at the extracellular K+ binding site
2) less Na+ is pumped out of the cell, increasing the intracellular Na+ concentration
3) a Ca2+ - Na+ exchanger pumps Ca2+ out of the cell against an electrochemical gradient in exchange for Na+ moving into the cell
4) less Ca2+ is pumped out of the cell and intracellular Ca2+ concentration increases
5) tension increases due to an increase in intracellular Ca2+ concentration
What are cardiac glycosides typically used for therapeutically?
in the treatment of congestive heart failure
What does the maximal tension that can be developed by a myocardial cell depend on?
its resting length
At what length do cardiac cells develop maximal tension?
At this point there is maximal overlap thick and thin filaments
What are the 2 other length-dependent mechanisms in cardiac muscle that alter tension developed?
- increasing muscle length increases the Ca2+ sensitivity to troponin C
- increasing muscle length increases Ca2+ release form the sarcoplasmic reticulum