ICPP 11 - Intracellular Signalling Pathways

Question 1

Where is 99% of the human bodies 1kg of calcium found?
What is the serum levels of Ca2+ in the body?
How is whole body Ca2+ homeostasis regulated?

Answer 1

• In bone
• 1.9-2.3mM of which 50% is free Ca2+
• by intestinal Ca2+ uptake (from diet), Ca2+ reabsorption in kidneys and bone calcium regulation - these are under endocrine control (PTH increases, calcitonin decreases)

Question 2

What kind of processes are (small) changes in IC Ca2+ responsible for?

Answer 2

Muscle contraction, neurotransmission, fertilisation, cell death, learning and memory.

Question 3

By what mechanisms are Ca2+ gradients set up and maintained?

Answer 3

• Relative impermeability of PM to Ca2+
• Pumps and transporters that move Ca2+ out of the cytoplasm
• Ca2+ buffer proteins.
• -> PMCA and SERCA move Ca2+ out of the cytoplasm
• -> NCX moves Ca2+ out of the cell in exchange for Na+

Question 4

Which mechanisms increase IC Ca2+?

Answer 4

• Ca2+ influx via ligand gates ion channels or voltage operated ion channels.
• Ca2+ movement out of the SER through RyR’s or IP3 receptors (CICR)

Question 5

Describe the signal pathway that occurs when Gs is activated.

Answer 5

• Gs activated, GDP swapped for GTP
• alpha and beta/gamma subunits dissociate
• Adenylyl cyclase activated, converts ATP into cAMP
• cAMP activates PKA which phosphorylates target proteins within cell.

Question 6

Give 3 examples of receptors that are coupled to Gs and their associated ligands.

Answer 6

1) B-adrenoreceptors (adrenaline/noradrenaline)
2) D1-dopamine receptors (dopamine)
3) H2-histamine receptors (histamine)

Question 7

Describe the signal pathway that occurs when Gi is activated.

Answer 7

• Gi activated, GDP swapped for GTP
• Inhibits the activation of adenylyl cyclase
• Less/no cAMP synthesised
• Less/no activation of PKA, and therefore intracellular effects lacking.

Question 8

Give 4 examples of receptors coupled to Gi and their associated ligands.

Answer 8

1) a2-adrenoreceptors - adrenaline
2) D2-dopamine receptors (dopamine)
3) U-opioid
4) M2/M4 mAChR’s (ACh)

Question 9

Describe the structure of PKA and how cAMP levels regulate its activity.

Answer 9

• Heterotetramer with 2 regulatory subunits attached to 2 catalytic sub-units
• When cAMP is low, regulatory subunits and catalytic subunits have high affinity for each other, stay bound and inhibit activity of catalytic subunits
• When cAMP is high, cAMP binds to R sub-units and catalytic subunits dissociate, leading to phosphorylation of target proteins.

Question 10

Describe the signal pathway that occurs when Gq is activated.

Answer 10

• Gq activated, GDP swapped for GTP
• Activated PLC, causing cleavage of PIP2 into DAG and IP3
• IP3 acts on IP3 receptors in ER, activating Ca2+ channel and entry of Ca2+ into cytoplasm
• DAG remains in membrane, binds to PKC which PP’s key substrate proteins

Question 11

Give 3 examples of receptors coupled to Gq and their associated ligands

Answer 11

1) a1-adrenoreceptors (adrenaline)
2) M1/M3/M5 mAChR’s (ACh)
3) H1 histamine receptors (histamine)

Question 12

Why is only a few molecules of ligand required to create a massive cellular response in GPCR’s?

Answer 12

Due to the large amplification effect that occurs - the step from AC to cAMP is large amplification step, that means we don’t need to waste cellular energy providing large amounts of ligand for receptors.

Question 13

What is the effect of adrenaline binding to B1-adrenoreceptors in the heart on inotropy?

Answer 13

• Activated Gs, increases Ca2+ entry through cascade, increased contractility, and therefore a positive inotropic effect.

Question 14

What is the effect of noradrenaline binding to a1-adrenoreceptors in smooth muscle

Answer 14

• Vasocontriction via the Gq signalling pathway