Adrenergic signalling

Question 1

what do adrenergic receptors act through?

Answer 1

acts through Gs to activate adenyl cyclase

Question 2

what does adenylate cyclase do? what is the structure of adenylate cyclase and its location>

Answer 2

adenylate cyclase catalysises the conversion of ATP to 3’5’-cyclic AMP (cAMP) and pyrophosphate.

adenylate cyclase is a transmembrane protein, passes through the plasma membrane 12 times, its functionally important parts are in the cytoplasm.

Question 3

what does cAMP do and how??

Answer 3

cAMP activates PKA

4. Each PKA is a holoenzyme that consists of two regulatory and two catalytic subunits. Under low levels of cAMP, the holoenzyme remains intact and is catalytically inactive. 
5. When the concentration of cAMP rises, cAMP binds to the two binding sites on the regulatory subunits, which leads to the release of the catalytic subunits.
6. The free catalytic subunits can then catalyse the transfer of ATP terminal phosphates to protein substrates at serine, or threonine residues. This phosphorylation usually results in a change in activity of the substrate.

Question 4

what effects of adrenergic stimulation are mediated through cAMP-dependent PKA?

Answer 4

inotropy and lusitropy

Question 5

how does adrenergic stimulation affect chronotropy?

Answer 5

Adrenaline acts on pacemaker potential
cAMP induced by adrenaline increases inward Na+ current
Activated PKA increases inward Ca2+ current and outward K+ current
==> steeper rise in AP and faster repolarisation

Question 6

what are the effects of PKA?

Answer 6

1) Phosphorylation of sarcolemmal L-type calcium channel by PKA increases open probability, contributes to pacemaker depolarisation and prolongs plateau of ventricular AP
The increase in ICa,L is also expected to result in an increase in the Ca2+ content of the SR

2) PKA is found in a complex with RyR, mAKAP , phosphatase and FKBP12.6	
PKA phosphorylation of RyR2 enhances channel activity by sensitizing the channel to cytosolic calcium 


3)Phospholamban has a tonic inhibitory effect on the sarcoplasmic reticulum 	Ca2+-ATPase (SERCA2a). Phosphorylation by PKA relieves this inhibition, 	allowing greater Ca2+ uptake into the SR. This speeds relaxation, and enhances the subsequent contraction especially at high frequencies


4) PKA phosphorylates MyBP-C in the thick filaments and Troponin I in the thin filaments of the contractile apparatus
	PKA phosphorylates MyBP-C; the primary effect is an increase in the speed of stretch activation that enhances crossbridge kinetics and contractility at increased heart rates.

PKA phosphorylation of MyBP-C controls the rate of Stretch-activation
Cardiac myofibrils are stretched at 50% Ca2+-activation and held. There is a delayed tension response which plays a role in synchronising contraction in the following systole. The rate of this is increased by PKA phosphorylation of MyBP-C
	

5) The N terminus of Cardiac TnI interacts with TnC to increase Ca2+-sensitivity. Phosphorylation abolishes this interaction.
PKA phosphorylation of troponin I decreases Ca2+-sensitivity of contraction
The decrease in Ca2+-sensitivity is due to faster dissociation of Ca2+ from troponin C



The rate of Ca2+-release from thin filaments limits the rate of relaxation. Therefore faster dissociation of Ca2+ allows faster relaxation- Lusitropy and is essential for the inotropic response in whole heart
the primary effect of PKA phosphorylation of cTnI is reduced Ca2+-sensitivity of force, whereas phosphorylation of cMyBP-C accelerates the kinetics of force development.
These results predict that PKA phosphorylation of myofibrillar proteins in living myocardium contributes to both the accelerated relaxation in diastole and increased rates of force development in systole.

Question 7

what are the primary effects of PKA phosphorylating cTNI and cMyBP-C??

Answer 7

he primary effect of PKA phosphorylation of cTnI is reduced Ca2+-sensitivity of force, whereas phosphorylation of cMyBP-C accelerates the kinetics of force development.
These results predict that PKAq phosphorylation of myofibrillar proteins in living myocardium contributes to both the accelerated relaxation in diastole and increased rates of force development in systole.

Question 8

Regulation of cAMP- PKA system

Answer 8

BARK1 inactivates receptor and is activated by PKA phosphorylation (feedback mechanism)
Phosphodiesterases degrade cAMP and create microdomains
AKAPs anchor PKA to its target
Phosphatases reverse PKA phosphorylation. Activity is controlled by PI-1, a substrate of PKA and PKC
Muscarinic agonists antagonise adrenaline effects via phosphatase activation

Question 9

Cardiac pathology associated with the adrenergic system

Answer 9

Arrhythmia- Excessive adrenergic activation leads to Ca2+ overload and arrhythmia: importance of feedback control of adrenergic signalling
Heart failure- cells do not respond to adrenaline: receptor inactivation and internalisation: phosphorylation levels low
Familial dilated cardiomyopathy (sarcomeric proteins)- PKA activation normal; contractile apparatus does not respond to phosphorylation (uncoupling)
Stress (Takotsubo) cardiomyopathy- adrenaline surge causes β2AR to switch to Gi depressing contraction.