Adrenergic Receptor Signalling

Question 1

What is cardiac reserve?

Answer 1

The potential ability of the heart to perform work beyond that necessary under basal conditions

Question 2

How is Cardiac output calculated?

Answer 2

C.O.= SV x HR

Question 3

What is the effect of sympathetic stimulation on cardiac contractibility?

Answer 3

• Norepinephrine, epinephrine
  
  • Positive inotropy
    
    • Increases contractile force
  • Positive Chronotropy
    
    • Increases HR
    • faster contraction and relaxation will accommodate more beats/minute
  • Positive lusitropy
    
    • increases rate of relaxation

Question 4

What is inotropy?

Answer 4

Changes in contractile force generation

Positive inotropic effect = increase in contractile force

Question 5

Define chronotropy

Answer 5

Changes in HR by affecting the SA node

Positive chronotropic effect = increases HR

Question 6

Define Lusitropy

Answer 6

Change in (rate of) relaxation

Positive lusitropy = increased rate of relaxation

Question 7

Define dromotropy

Answer 7

Change in AV conductance

Dromotrophic = increased AV node conductance = Increased HR

Question 8

What changes are collectively called “Cardiac Reserve”

Answer 8

Inotropy

Chronotropy

Lusitropy

Dromotropy

Question 9

What is the structure of G-protein coupled receptor?

Answer 9

• seven trans membrane segments
• Intracellular C-terminus
• Extracellular N-terminus

Question 10

What are G-proteins?

Answer 10

Guanine nucleotide binding proteins

Heterotrimeric protein complexes comprised of Galpha, Gbeta and G gamma subunits

Four classes of G alpha subunits:

• Galpha-s
• Galpha i/o
• G-alpha q/11
• G-alpha 12/13

Question 11

Which signaling pathway is triggered by the following G-protein alpha subunits?

• Galpha-s
• Galpha i/o
• G-alpha q/11
• G-alpha 12/13

Answer 11

• Galpha-s
  
  • PKA
  • Gs -> Adenylyl cyclase -> cAMP -> PKA
• Galpha i/o
  
  • Gi inhibits adenylyl cyclase
  • Go activates PLC which activates
    
    • IP3 to release Ca2+
    • DAG to release PKC
• G-alpha q/11
  
  • Gq -> PLC which activates
    
    • IP3 to release Ca2+
    • DAG to release PKC
• G-alpha 12/13
  
  • G? -> PLA2 -> AA
    
    • releases PKC and
    • Many AA metabolites
  • Gt -> PDE —> cGMP

Question 12

Gi and Gs have opposing effects on ______

Answer 12

Gi and Gs have opposing effects on Adenylyl Cyclase

Question 13

Draw a GPCR

Answer 13

Pay attention to localization of AC and PLC within the cell membrane

Question 14

What are the 7 steps of GPCR signaling?

Answer 14

1. Receptor activation
   
   • Ligand binding
2. Conformation change of GPCR
3. GDP is replaced with GTP on the alpha subunit
4. Conformational change of G-protein
5. Dissociation of GTP-bound G-alpha subunit from G beta gamma and from GPCR
   
   • dissociated Galpha-GTP goes on to activate the downstream effectors
6. Hydrolysis of GTP to GDP on G-alpha (G-alpha has GTPase activity)
   
   • ligand comes off
7. G-alpha reassociates with Gbeta-gamma ; G protein rebinds with receptor

Question 15

How are the following adrenergic receptors distributed within the cardiovascular system?

Beta1-adrenoreceptors

Beta2-adrenoreceptors

Alpha-adrenoreceptors

Answer 15

• Beta1-adrenoreceptors
  
  • Cardiomyocytes -> increase in contractility
  • SA nodes -> increase HR
• Beta2-adrenoreceptors
  
  • SMC -> relaxation -> Vasodilation in the vasculature of skeletal muscles, bronchioles
• Alpha-adrenoreceptors
  
  • SMC -> contraction

SMC = Smooth muscle cell

Question 16

Which adrenergic receptor is dominant in the human heart?

Answer 16

Beta 1 adrenoreceptors

• especially dense in SA nodal tissue

Question 17

Beta-1-AR (dominant AR in heart) is associated only with the ______ subunit to mediate which effects?

Answer 17

Beta-1-AR is associated only with the G-alpha-s subunit to mediate inotropic, chronotropic, lusitropic and dromotropic effects

Overall effect = increase heart performance

Question 18

Beta-2-AR activates _____ (subunit) to cause _______

Also associated with ______ to cause _______ in striated muscles

Minor contribution to increase in __________ (minor compared to beta1-ar)

Answer 18

Beta-2-AR activates Galpha-i (subunit) to cause relaxation of SMC and dilation of arterioles

Also associated with Galpha-s to cause increasing muscle contractility in striated muscles

Minor contribution to increase in cardiac output, contractility or HR (minor compared to beta1-ar)

Question 19

Alpha-1-AR

• Activated by ____
• Activates _____ subunit which activates _____

Answer 19

Alpha-1-AR

• Activated by adrenaline
• Activates G-alpha-q subunit which activates PLC
  
  • ​PLC cleaves PIP2 into IP3 and DAG
    
    • IP3 mediates release of Ca++ from ER/SR in SMCs triggering smooth muscle contraction
• Vasoconstriction in many organs
• contraction of uterus

Question 20

Which two signaling pathways can be activated by Beta-2-AR?

Answer 20

G-alpha-s and G-alpha-i

Question 21

How is activated beta-2-AR internalized?

Answer 21

Activated beta-2-AR can be phosphorylated (by PKA, PKC or GPCR-kinase (GRK)), coupled with arrestin and internalized (to be recycled back to the membrane or degraded)

Question 22

_______ decreases HR to normal sinus rhythm

Answer 22

Acetylcholine decreases HR to normal sinus rhythm

Question 23

Importance of Ach receptors in

• SA node
• AV node
• Atrium

Answer 23

Importance of Ach receptors in

• SA node
  
  • decrease heart rate to baseline sinus rhythm
• AV node
  
  • reduct conduction velocity
• Atrium
  
  • decrease atrial contration

Question 24

How can Ach inhibit an increase in HR?

Answer 24

By opposing activation of beta-AR

Question 25

Effect of cAMP on I_f

Answer 25

Increases HR by positively regulating I_f

Question 26

What catalyzes the conversion of ATP to cAMP?

Answer 26

Adenylate (adenylyl) cyclase (AC)

Question 27

What is the structure of Adenylate (adenylyl) cyclase (AC)?

Answer 27

* two six-transmembrane domains * Two cytoplasmic catalytic domains (C1a and C2a)

Question 28

What is protein kinase A (PKA)?

Answer 28

cAMP-dependent protein kinase * Holoenzyme (apoenzyme + cofactor) * 2 regulatory subunits and 2 catalytic subunits * Phosphorylates its substrates

Question 29

What are phosphodiesterases (PDEs)?

Answer 29

A class of enzymes that _degrade cyclic nucleotides_ (cAMP and cGMP)

Question 30

Of the 21 genes that encode PDEs, which 7 have been found in the heart

Answer 30

1. PDE1 2. PDE2 3. PDE3 4. PDE4 5. PDE5 6. PDE8 7. PDE9

Question 31

What are phosphatases?

Answer 31

Phosphatases undo the function of kinases ie phosphatases catalyze the **dephosphorylation** of proteins (at tyrosine or serine/threonine) or lipids

Question 32

What are the primary phosphatases coupled to GPCR

Answer 32

* PP1 * PP2A * PP2B (calcineurin)

Question 33

What are A-kinase anchor proteins (AKAPs)?

Answer 33

* Provide a scaffold for the regulatory proteins such as kinases, PDEs and phosphatases * Work as a targeting device * bind to regulatory of PKA and confine the holoenzyme to discrete locations within the cell * Each AKAP contains a unique subcellular targeting domain that restricts its location within the cell

Question 34

Which AKAPs are located in the circulatory system and where are they located?

Answer 34

* mAKAP * Cardiac myocytes * AKAP95 * heart * AKAP148 * heart * AKAP79 * plasma membrane

Question 35

Which proteins are in the mAKAP complex?

Answer 35

mAKAP in cardiac myocytes has: * PKA * PDE4D3 * protein phosphatase 2A * ERK5 * EPAC1

Question 36

Describe the image

Answer 36

* A) under basal condition: PKA is inactive and the PDE maintains low intracellular concentrations of cAMP * B) Upon hormonal stimulation, the generation of cAMP increases and overcomes the basal rate of PDE-mediated cAMP degradation -\> PKA activation and phosphorylation of local substrates * PKA phosphorylation of mAKAP-anchored PDE enhances PDE activity * increases cAMP degradation and results in decreased PKA activity