Signal Transduction in the Heart: Calcium Signalling Flashcards
Who and when realised the the importance of calcium within in heart?
Sydney Ringer in 1883
What did Sydney Ringer do?
Developed a physiological saline that maintained cardiac contraction-of which it only worked when it contained calcium. This was not the case for skeletal muscle (it contracted even without the calcium). This solution is now called ‘Ringer’s Solution’
This solution highlighted the necessity of extracellular calcium in cardiac muscle contraction
How is calcium involved in muscle contraction?
Calcium binds to Troponin C, a complex on the thin Actin filaments.
The binding of calcium to troponin causes a contortion of the Actin which exposes myosin binding sites on the thick filament
Exposure of binding sites means that actin is able to ‘walk’ down the myosin filament with binding sites forming cross bridges that enable contraction
What is excitation-contraction coupling?
The mechanism by which electrical activity is converted to mechanical activity
What is the most important ion in E-C Coupling?
Calcium
During which phase of action potential does calcium enter a cardiac myocyte?
During Phase 2-the Plateau phase
What is an L-type VGCC?
An L-type Voltage Gated Calcium Channel
A type of channel on the t-tubules of cardiac myocytes that allow the movement of calcium into the myocyte once a certain electrical threshold has been reached
What happens when L-type VGCCs are depolarised?
It is voltage dependent so at around +20mV when non-pacemaker cells have repolarised slightly, they undergo a conformational change that allows calcium to flow through it from the extracellular fluid to the inside of the cell
What is a Ryanodine Receptor? (RyR2)
A type of receptor located on the membrane of the sarcoplasmic reticulum that enables the release of calcium from the SR
What other ion does the RyR2 allow to pass and why?
POTASSIUM-if only calcium moved out of the SR, the SR would become very negative and so the movement of calcium out of the SR would stop (it would no longer have a concentration gradient-there would be no electrical potential of the SR), so as calcium moves out, potassium moves IN to the SR to ensure that the SR remains positive
How could you describe potassium in terms of the RyR2?
Potassium is the COUNTER ION of the RyR2
What proteins modulate the RyR2?
- FK-Binding Protein
- Calmodulin
How are RyR2s arranged?
In highly structured assays on the Sarcoplasmic Reticulum and are connected to one another by FK-Binding Proteins
The SR and RyR2 are clustered around L-Type VGCCs on the t-tubules
What is a Calcium Spark?
It is a spatio-temporally restricted increase in calcium concentration i.e a group of 20/30 receptors spontaneously release a large quantity of calcium
The calcium sparks arise close to the t-tubules because this is where RyR2s are clustered
What is the concentration of extracellular calcium?
1.5mM
What is the concentration of intracellular calcium?
100uM
Why is it important that the concentrations of calcium extracellularly and intracellularly are different?
The concentration of calcium extracellularly is much higher than intracellular which is important to maintain a movement gradient of calcium IN TO the cell-if intracellular calcium was higher, calcium would move out of the cell and this would prove detrimental to myocyte contraction
What is a calcium sparklet?
An increase in calcium concentration localised around the luminal side of the L-type VGCC
Why are calcium sparklets important?
Calcium sparklets are necessary as they allow Calcium Induced Calcium Release from the RyR2- i.e the influx of calcium through the L-type VGCC triggers the RyR2 channel to release calcium too
What is a calcium spark?
The increase in calcium concentration around the SR following calcium release by the RyR2
What does a calcium spark do?
The increase in concentration of calcium is big enough in calcium to trigger a contraction, so if enough all happen at once then the cell will contract
What is a calcium skrap?
When calcium is released from the SR by RyR2, the intra-SR levels of calcium fall and this is known as a calcium skrap- it is the opposite of a calcium spark
How do we know about sparklets, sparks and skraps?
They can all be visualised using fluorescent microscopy
Why do the RyR2 channels need to be autoregulated?
If the channels weren’t regulated, then just one calcium ion could result in complete SR calcium depletion due to Calcium Induced Calcium Release- the cycle would just keep going-every time another calcium was released, it would cause the release of further calcium until there was no calcium left. this would be bad as it would eventually stop contraction
How are the RyR2 channels autoregulated?
There is a feedback system from luminal SR calcium.Once the SR reaches a certain reduced level of calcium, it switches the channel off
Why does calcium sensitivity not spread from cluster to cluster?
The RyR2s are sensitive only to the calcium released from its own SR
What are spontaneous sparks?
RyR2s can release spontaneous calcium sparks, but individual sparks are insufficient to cause a contraction. Multiple sparks must all occur in a synchronised fashion across the cell to induce contraction. This prevents the occurrence of arrhythmias
How are the L-type VGCCs and RyR2s related spatially?
The L-type VGCCs are located on the T-tubules and the RyR2s on the Sarcoplasmic reticulum are clustered around the L-type VGCCS-so the diffusion distance of calcium is very short and maximal effect can be obtained
What factors ensure synchronisation of SR Calcium Release/Calcium Spark generation?
- t-tubule integrity
2. L-type VGCC and RyR2 positioning
What did ‘Brette, Physiology, 2007’ say?
remodelling of t-tubules and desynchronisation of l-type VGCCs and RyR2s is seen in some heart failure models
how does a myocyte contraction stop?
The calcium must be removed in order to reduce the calcium concentration
By what mechanisms are calcium removed from the cytosol of the myocyte?
- ATP-calcium pumps on the myocyte membrane
- Mitochondrial calcium uptake
- SERCA/SR Calcium pump
- Na/Ca exchanger on t-tubule membrane
Which mechanism is the most important?
SERCA-it is responsible for removing 68% of cytosolic calcium
What type of SERCA molecule is found in cardiac myocytes?
SERCA2a
How does SERCA2a work?
2 calcium and 1 ATP molecules bind the the cytosolic side of the SERCA with high affinity. The phosphate from ATP causes phosphorylation of SERCA which causes occlusion of SERCA to stop calcium being pumped out and then calcium is carried back in to the SR by several domains
What is Phospholamban?
It is an endogenous inhibitor of SERCA
When does phospholamban not work?
When it is phosphorylated by cAMP protein kinases
So what happens when phospholamban is phosphorylated?
It can no longer inhibit SERCA so the re-uptake of calcium back into the SR from cytosol increases and therefore the myocyte can relax quicker and the heart rate can increase
What are catecholamines?
A group of drugs such as adrenaline and noradrenaline that can increase production of cAMP and therefore result in phosphorylation of phospholamban and thus increase heart rate
What is the stoichiometry of the Na/Ca exchanger?
3 Sodium ions for 1 calcium ion-this process is electrogenic and creates a gradient because for each 3 sodiums moved in, it creates 1 positive charge moved in the direction of sodium
Na/CA exchanger is bidirectional?
at -80mv resting potential, sodium is pumped in and calcium is pumped out.
at depolarisation, this process is reversed and calcium flows IN
What is heart failure?
When the heart fails to produce a cardiac output that adequate to meet the metabolic demands of the body
how many people in the UK suffer from heart failure?
approximately 750,000
How is excitation-contraction coupling linked to heart failure?
defective calcium handling is a central cause in both contractile dysfunction and arrhythmia development in heart failure
What are the main defects of EC-Coupling seen in heart failure?
- Sarcoplasmic reticulum calcium stores are significantly reduced
- uncoupling of l-type GVCCs and their adjacent RyR2s
- hyperphosphorylation of RyR2
Why do SR calcium stores decrease in HF?
SERCA is downregulated for some reason which reduces the amount of calcium put back into the SR during diastole.
This is accompanied by an upregulation of the Na/Ca exchanger which pumps excess calcium out of the myocyte so calcium is no longer available for reuptake by SERCA
Why are l-type VGCCs and RyR2s uncoupled?
canine models have shown shrinking of the t-tubules which separates L-type VGCCs from their adjacent RyR2-this results in late or asynchronous calcium induced calcium release because the distance for calcium diffusion is randomly increased
Why is RyR2 hyperphosphorylated?
Compensatory mechanisms try to repair heart failure-sympathetic innervation of adrenergic receptors increases which produces cAMP so RyR2s become hyperphosphorylated-this allows excessive release of calcium and can cause random release that causes arrhythmia
What is force of contraction dependent on?
Calcium concentration-the bigger the calcium concentration, the bigger the force of contraction
How can inotropy be increased?
through the use of mechanisms that increase calcium concentration e.g:
- increasing calcium influx through l-type VGCCs
- Increasing calcium release from the SR via RyR2
- Increasing troponin affinity for calcium
- Increasing myosin ATPase activity
- increasing SERCA activity to speed up heart relaxtion and put more Calcium back into the SR ready for contraction
- Inhibiting calcium efflux across the sarcolemma
What does stimulation of b-adrenergic receptors do?
causes an increase in the production of cAMP which enables the phosphorylation of multiple targets
What does b-adrenergic stimulation result in?
- Increase in L-type VGCC permeability to calcium
- Increase calcium influx during action potentials
- increase calcium release from the SR (phosphorylation of RyR2)
- increase the removal of calcium from the cytosol by SERCA (phosphorylation of phospholamban(
What happens to b-adrenergic receptors during heart failure?
They become hyper-stimulated in an attempt to compensate for heart failure.
Is b-adrenergic compensation successful?
No, it is at first but then eventually results in a failure
What potential treatments are there for heart failure?
Studies are currently trialling SERCA2a gene therapy e.g CUPID trial-serca gene therapy may restore some electrical stability
What have SERCA2a gene trials shown?
Currently have been unsuccessful but there is still promise for the future
What is CPVT?
Catecholaminergic Polymorphic Ventricular Tachycardia
What causes CPVT?
Mutations to the RyR2 channel
What happens in CPVT?
mutations cause the RyR2 to behave abnormally and the release of calcium from the SR doesn’t happen properly. This means that the SR calcium content becomes increased
Calcium release is slow meaning that calcium sparks propagate to form waves which are slower than standard sparks. These waves also become non-uniform and occur spontaneously so arrhythmias occur
