Excitation contraction coupling in cardiac muscle Flashcards
What ion concentration is involved in cardiac muscle contraction?
Ca2+
What are the similarities between cardiac and skeletal muscle?
Both muscle types are striated
Both contain T tubules
Action potentials raise Ca2+ levels
Ca2+ is crucial for contraction
What are the difference s between cardiac and skeletal muscle?
Cardiac muscle has a less developed T-tubule and sarcoplasmic reticulum system
Heart contain specialised excitatory tissues
The heart is electrically connected at intercalated discs (gap junctions)
Ventricular action potential is 100x longer than skeletal muscle
What is excitation-contraction coupling?
The process whereby action potential triggers myocyte to contract
What is the route of electrical activity of the heart?
SA node AV node Bundle of His Bundle branches Purkinje fibres
What regulates calcium concentration in the cytoplasm?
Ryanodine receptors
Plasma membrane calcium L-type channels
SR calcium pump
Na/Ca exchangers
What are L-type voltage gated calcium channels?
Slow
Located in the plasma membrane
Found in close junction of SR
Requires a membrane potential greater than 30mV for activation
Essential for initiation and regulation of EC coupling
Smooth muscle depolarising opens them
Rapid influx of Ca2+
What are ryanodine receptors?
Mediate the release of calcium ions from the SR
Essential in contraction
Regulated by protein-protein interactions with the L type channel
What is the Na/Ca exchanger?
Plasma membrane enzyme that exchanges 3 moles of Na for 1 mole of Ca
What is the sarcoplasmic reticulum?
Organelle
Membrane bound system
Encases each myofibril within a muscle cell
Contains calcium pump
What is the SERCA pump?
Sarcoendoplasmic reticulum Ca-ATPase pump
Resides in the SR plasma membrane
Couple to ATP hydrolysis to transport calcium from cytosolic to luminal space
What is phospholamban?
Major phosphoprotein component of the SR
Help regulate SR Ca2+ uptake
Phosphorylation of phospholamban activates it
Describe phosphorylation of phospholamban
Can be phosphorylated at 3 different sites
Serine 10 by protein kinase C
Serine 16 by protein kinase A
Threonine 17 by calcium calmodulin dependent protein kinase
When is phospholamban active and inactive?
Dephosphorylated version is an inhibitor of the SERCA pump
Phosphorylated form dissociates from SERCA and activates the calcium pump
What links excitation and contraction?
Increased cytosolic Ca2+
What increases Ca2+ levels in the ventricular action potential?
Ca2+ influx through L-type Ca2+ channels in the sarcolemma
What stimulates ryanodine receptors?
“Trigger calcium”
What do ryanodine receptors do?
Release stored Ca2+ from the SR
What is the first step of the cardiac contraction cycle?
Cardiac myocyte contraction begins with hydrolysis of ATP to ADP by myosin
What is the second step of the cardiac contraction cycle?
Ca2+ released from the SR binds to TN-C
Conformational change with tropomyosin allows myosin to form an active complex with actin
What is the third step of the cardiac contraction cycle?
Dissociation of ADP from myosin allows the myosin head toned
Pulls Z lines closer together and shortens the band
What is the fourth step of the cardiac contraction cycle?
Binding of a new ATP molecule to myosin allows the actin-myosin complex to dissociate
Ca2+ dissociates from TN-C
What influences cytosolic calcium concentrations?
Beta-adrenoreceptor coupled mechanism
Which nervous system regulates the heart rate?
Autonomic
What are chronotropic agents?
Factors influencing the slope of the pacemaker and therefore the heart rate
What is the effect of sympathetic stimulation?
Increases cardiac muscle contractility
What is the beta adrenergic receptor signalling pathway?
Noradrenaline/adrenaline B1 adrenergic receptor Activation of GTP binding protein Stimulates adenylyl cyclase cAMP PKA Phosphorylates proteins related to EC coupling
Which two proteins together result in signal transduction?
Gs and Gi
What does inotropic mean?
Something that increases the developed contractions
What does lusitropic mean?
Something that affects cardiac relaxation
How does the sympathetic pathway modulate calcium?
PKA has a lusitropic effect
Speeds up reuptake of calcium
PKA has an inotropic effect
