Cardiovascular System - Heart Flashcards
What are the secondary messengers for the two signalling pathways in the heart?
cAMP and Ca2+
Briefly summerise how the heart contracts on a molecular level:
- After depolarisation Ca2+ enters through dihydropyridine receptors
- It then binds to the Ryanodine receptors to stimulate Ca2+ release from the sarcoplasmic reticulum
- It then stimulates contraction by binding to troponin in the thin filament
What happens to the calcium after it has stimulated contraction in the cardiomyocyte?
- either:*
- Removed from cell by Na+/Ca2+ exchangers or Plasma Membrane Calcium ATPase (PMCA)
- or*
- Taken back into the sarcoplasmic reticulum by Sarco-endoplasmic reticulum calcium ATPase (SERCA2a)
Which enzyme is responsible for the removal of >70% of myoplasmic Ca2+?
SERCA2a
How is the activity of SERCA2a regulated?
Phospholamban (PLN) - the target of phosphorylation by Protein Kinase A via the adrenergic receptor pathway
Dephosphorylated form: PLN inhibits SERCA2a
Phosphorylated form: PLN dissociates from SERCA2a activating it
What happens to the activity of the Myocardium when beta-adrenergic activity increases, and by which pathway?
(explain each step)
- More cAMP
- More Protein Kinase A
- More Phospholamban is phosphorylated and therefore dissociates from SERCA2a
- More Ca2+ is pumped back into the sarcoplasmic reticulum
Therefore the rate of cardiac relaxation is increased
Contractility of subsequent beats is increased - due to increased size of SR Ca2+ store
What are the four main ion channels involved in regulating the sinoatrial action potential?
If - Funny Channel
ICa (T) - Transient T-type Ca2+ channel
ICa (L) - Long-lasting L-type Ca2+ channel
IK - K+ channel
What is the If Channel?
What is its role in regulating the Sinoatrial action potential?
What ion does it transfer?
Funny channel:
Hyperpolarisation-activated cyclic nucleotide-gated channel
(HCN Channels)
Na+
Responsible for allowing the action potential to propagate
When does the If channel open?
What does it do?
Opens at the most negative potential
Propagates the action potential by causing a certain amount of depolarisation before the calcium channels open
Recall what a graph of the action potential of the Sinoatrial node looks like:
(Including which ion channels are open at specific periods)
What is the function of the Calcium channels in regulating the SAN AP?
They do the majority of the depolarisation
Transient first (T-type)
then
Long-lasting (L-type)
for the longest time obvs
What is the role of Potassium Channels in regulating the AP of the SAN?
They are responsible for repolarisation
How does sympathetic stimulation effect the Action potentials of the Sinoatrial node?
Beta adrenoceptors are coupled with adenylate cyclase which increases cAMP
This is important in opening the If channel so with more cAMP the HR is increased
(cAMP is also responsible for increasing Calcium entry so also increases contractility)
How does Parasympathetic stimulation effect the Action Potential of the Sinoatrial node?
It is negatively coupled with Adenylate cyclase so reduces cAMP
It therefore promotes Potassium channel opening and prolongs repolarisation slowing the HR
What four factors affect Myocardial Oxygen demand?
Heart rate
Preload
Afterload
Contractility
Why does an increased preload increase myocardial oxygen demand?
Preload is linked to venous return
Higher venous return increases cardiac contractility
How does an increase in Afterload increase myocardial oxygen demand?
An increase in Afterload is associated with an increase in TPR so the heart has to work harder against the increased TPR
What are three classes of drugs that reduce HR to lower the Myocardial Oxygen demand?
What are their targets?
Beta blockers: If + ICa
Calcium channel antagonists: ICa
Ivabradine: If
How can myocardial Oxygen demand be reduced?
By lowering the Heart rate and reducing contractility
How do calcium antagonists reduce cardiac contractility?
They block Plasma membrane Calcium channels so the influx of external calcium is reduced
This also lowers the amount of Calcium-induced calcium-release