Preload, Afterload & Contractability Flashcards
What is the cardiac myocyte basic unit of contractions?
1) contractile cells
2) nodal cells
3) purkinje fibers
How is contraction elaborated and what is the key player in this process?
Elaborated via the conduction system in response to an AP the delivers Ca to contractile proteins
-Calcium is the key player
What does contractile force depend on?
Ca
How does calcium react to the wave of depolarization from the AP?
- increase Ca concentration follows wave of depoarization
- Ca moves down steep conc gradient from outside to inside cell during propagating AP
What are T-tubules?
- are extensions of sarcolemma (PM) that convey excitement deep into the interior of the cell
- allows deep muscles to contract
- associated with Sarcoplasmic reticulum
What is the sarcoplasmic reticulum?
- Ca storage site
- form an intricate web of intracellular tubules that cover the sarcomeres
- closley associated with T-tubules
what is SERCA?
- pumps that cover the sarcoplasmic reticulum
- suck Ca from cytoplasm back into SR during diastole (relaxation)
what is Calcium Induced Calcium Release?
- AP causes T-tubule depolarization, opens L-type Ca channels, small amount of external Ca enters
- causes major internal Ca release by binding to Ca release channels (ryanodine receptors) on the SR
- Ca binds and activates contraction
- then re-uptaken by Serca, #Na/Ca exchanger, and Ca PM pumps
- low Ca levels allows relaxation
Where does Ca for contraction come from? Why?
- 15% extracellular , 86% from intracellular SR storage
- diffusion of Ca from external–> internal takes too long
why use T-tubule and SR system?
-ensures uniform and synchronous delivery of internally stored Ca to DEEP muscle fibers
what are the 4 ways Ca is mobilized (2) & re-sequestered (x2)from cytoplasm ?
1) SERCA (main)
2) 3Na/Ca exchanger
3) L-type Ca channels
4) Ryanodine Receptor (Ca release channels)
what determines force of contraction?
-the amount of Ca and rate of arrival at myofilament
How does Ca alter the force of contraction?
-more Ca causes removal of more troponin from tropomyosin, allows more myosin cross bridge to bind and a greater contractile force as a result
How does skeletal muscle react to Ca release?
-skeletal muscle reacts maximally due to large Ca concentrations fully saturating myofilaments
How does cardiac muscle react to Ca release? what does this indicate?
- activates sub maximally
- under normal conditions AP results in only 50% of max contraction
- means force of cardiac muscle contraction is regulated by amount of Ca release due to AP
Can heart AP be summated?
-no, the cardiac myocytes are electrically coupled, when 1 myocyte activated all are activated
factors promoting large Ca release?
- norepinephrine
- epinephrine
- caffeine
- elevated serum Ca
factors that promote less Ca release
1) barbiturates
2) beta-blockers
3) hypoxia
How does T-tubule depolarization trigger SR Ca releasE?
1) T tubules and SR membrane have Didactic Cleft between them, when T tubule depolarized L type Ca channels release Ca into cleft
2) this activator Ca is the Ca+ spark indicated increase local Ca conc, causes Ryanodine Receptor to open, release Ca into cytoplasm(CICR)
How Ca re-sequestered and removed to allow relaxation?
1) SERCA is main determinant, regulating by inhibitor protein
2) 3Na/CA exchanger is next in line, (3Na in down gradient for 1 Ca out)
3) PM Ca Pump, very low yield
Amount of Ca in SR?
- is constant beat to beat so what is released must be re-sequestered
- amount fo Ca in SR effects contraction force
How are contractile force and relaxation interrelated?
- SERCA pumps 70-90% of released Ca back into SR so is major determinate of relaxation rate
- increased search enhances relaxation, greater SR filling and more Ca available for next beat
