Signal Transduction in the Heart: Calcium SIgnalling-Extra Reading

Question 1

Bers, Nature, 2002

Answer 1

• Half maximal contraction requires 70umol of Calcium per litre of cyotosol
• Stretched myofilaments can bind more calcium, which is why increased venous return results in positive inotropy
• Increasing the amplitude or duration of calcium transient increases the concentration of calcium available so inotropy increases
• Calmodulin binds calcium and switches off the L-type VGCCs to limit influx of calcium
• Release of calcium from the RyR2 also contributes to the calcium-dependent inactivation of L-type VGCCs to prevent excessive calcium influx THIS CONTRIBUTES TO 50% OF L-TYPE VGCC INACTIVATION-this means that l-type VGCC influx is thus a POSITIVE FEEDBACK MECHANISM
• RyR2 inactivation and luminal depletion of the SR contribute to inactivation of CICR
• Phosphorylation of phospholamban is by far the dominant mechanism in increasing lusitropy
• Phosphorylation of RyR2 allows increased release of Calcium, but in heart failure the RyR2 are HYPERPHOSPHORYLATED which allows diastolic leak of calcium which can cause arrhythmias

Question 2

Pogwizd, Circ Res, 2001

Answer 2

• Arrhythmias and contractile dysfunction are the main causes of death in HF
• Rabbit HF models show a 2-fold increase in Na/Ca X expression which resulted in an unloading of SR calcium stores
• B adrenergic receptors become progressively downregulated in HF, but initially excessive sympathetic innervation tries to compensate for this
• BUT, Na/Ca X are also increased, so the inward current of Ca is increased meaning spontaneous calcium release can occur thus resulting in Arrhytmias
• In HF, 50% less calcium is required to trigger a myocyte contraction
• Most ventricular tachycardias result from electrical re-entry, but 3D mapping studies have shown that most fatal HF arrhythmias result from non-reentrant mechanisms such as DELAYED AFTER-DEPOLARISATION or EARLY AFTER DEPOLARISATION
• DADs are enhanced by B-adrenergic stimulation so they occur when the heart is trying to compensate
• Pogwizd using rabbit models demonstrated that Na/Ca X plays a central role in mediating arrhythmogenesis and contractile dysfunction by lowering SR calcium load
• As HF progresses, the number of sudden cardiac deaths decreases because B-adrenergic responsiveness is reduced (lowered expression)

Question 3

Brette, Physiology, 2007

Answer 3

-T-tubules are important in ensuring rapid and synchronous contraction of the cardiac myocyte
-T-tubules are virtually absent in pacemaker cells
-Cardiac t-tubule diameter is 100-300nm which is much larger than skeletal muscle thus allowing faster propagation of electrical impulse (also need to be bigger to allow calcium influx as this isn’t necessary in skeletal muscle)
-the DYAD is there area where the sarcolemmal L-type VGCCs and RyR2s are closely opposed
-DYADs are 25% abundant at the surface membrane but 75% abundant on t-tubules, demonstrating just how important the t-tubules are for bringing L-type VGCCs and RyR2s together
-Immunocytochemistry has shown that many key components of many major signalling pathways are located predominantly on the t-tubules
-Canine HF models show MARKED LOSS of t-tubules in ventricular myocytes
-Loss of t-tubules is not entirely to blame for all Calcium mishandling but it most definitely exacerbates the problem
-Several studies have correlated t-tubule remodelling and desynchronisation of SR calcium release
-Changes to t-tubule organisation may alter the spatial distribution of l-type VGCCs whereas RyR2 distribution remains the same so some RyR2s become orphaned and are no longer activated, and others take longer to activate due to increased diffusion distances resulting in desynchronisation
^this data indicates that GEOMETRIC factors play an important role in the pathophysiology of HF

Question 4

Scoote, Heart, 2003

Answer 4

-Previous non-specific inotropes aimed at amplifying EC coupling have been discredited as therapies for HF as they cause many unwanted side effects, especially arrhythmias
-There are no issues regarding reduced activity of the RyR2, the problem lies with diastolic calcium leak that can induce DADs
-Beta blockers prevent hyperphosphorylation of RyR2 and so prevent diastolic leak thus preventing DADs
-New drugs that target the terminal effector mechanisms in EC coupling are needed to avoid the arrhythmogenic side effects
-LEVOSIMENDAN is a new class of calcium sensitising drug that increases the sensitivity of the myofilament to calcium-it has shown promise in clinical trials
Other studies have indicated that FK-Binding proteins that modulate RyR2 may be useful in preventing excessive RyR2 opening and diastolic leak
-pharmacological upregulation of SERCA shows promise, as does replacement using Gene therapy
-We could use anti sense mRNA to reduce phospholamban expression or gene therapy to restore SERCA expression

Question 5

Pleger, Circ Res, 2013
Lyon, 2011
Cuther, Circulation, 2012

Answer 5

-CUPID trial is investigating SERCA2a gene therapy as it may stabilise SR calcium load, reduce RyR2 phosphorylation, and decreases SR calcium leak-
but has shown disappointing results in clinical trial
-S100A1 is a protein that binds to both RyR2 and SERCA2a to improve their activities
-S100A1 knockout mice showed worsened Post-MI contractile performance whereas S100A1 overexpressing mice demonstrated improved outcomes-this means we could therapeutically increase S100A1