Drug interactions with receptors and ion channels: Signalling mechanisms

Question 1

Adenylyl cyclase

Answer 1

Generates cAMP which controls cell functions through protein kinase A.

Activated by e.g β1 adrenoceptor via Gs
Inhibited by e.g α2 adrenoceptor via Gi

NB βγ complex can itself be inhibitory, through the mopping up of free αs.

Question 2

Cholera toxin

Answer 2

Causes ADP-ribosylation of αs

- inhibits GTPase activity of αs, causes sustained activation of adenylate cyclase.

Question 3

Pertussis toxin

Answer 3

Causes ADP-ribosylation of αi.

Prevents activation of αi in response to receptor stimulation.

Question 4

ALF4-

Answer 4

Mimics γ phosphate of GTP

Causes persistant activation of G proteins

Question 5

Phospholipase C (PLC)

Answer 5

Activated via Gq and G11, by e.g ACh at muscarinic receptors, noradrenaline at α1 receptors, substance P.

Cleaves PIP[2]- both cleavage products (DAG & IP[3] ) act as second messengers.

Question 6

Diacylglycerol (DAG)

Answer 6

Stimulates membrane-bound protein kinase C -> which modifies properties of various proteins, such as ion channels, by phosphorylation.

Question 7

Inositol 1,4,5-triphosphate ( IP[3] )

Answer 7

Causes release of Ca2+ from endoplasmic reticulum
Rapidly metabolized
- by phosphorylation to IP[4]
- by dephosphorylation successively to IP[2], IP[1] and inositol.

Question 8

Lithium

Answer 8

Uncompetitively inhibits the phosphatase responsible for converting IP[1] to inositol.
- thereby blocks recycling of inositol
Brain cannot take up plasma inositol (cannot cross BBB) and so depends heavily on inositol recycling to allow signalling through receptors that use IP3.

Used in treatment of manic depression (schizophrenia)

Question 9

Tyrphostins

Uses?

Answer 9

Inhibitors of tyrosine kinases.
May prove useful for controlling neoplastic growth

• RTKs regulate cell proliferation & differentiation
• v-erbB (viral oncogene) encodes a truncated, constitutively activated EGF receptor
• v-sis (viral oncogene) encoded a PDGF chain

Question 10

Desensitization of the β2 adrenoceptor

3 main processes

Answer 10

1) Uncoupling of receptor from its G protein. Seconds to minutes. Involves 2 separate types of phosphorylation resulting in heterologous and homologous desensitisation.
2) Sequestration of receptors. Receptors removed from plasma membrane by endocytosis, transferred to vesicles in cytoplasm. Can be shuttled back to plasma membrane or destroyed in lysosomes. Minutes
3) Down regulation. Possible due to reduced receptor synthesis by an effect of PKA on mRNA stability. Longer term, minutes to hours.

Question 11

Heterologous desensitiztion

Answer 11

Receptor disproportionately stimulated -> large rise activity of PKA which is able to phosphorylate the β2 receptor itself, at sites:
- on the third cytoplasmic loop
- on the first part of the C terminal domain.
Phosphorylation uncouples the receptor from the α subunit of Gs.

Takes place with low concentrations of ligand.
Heterologous as PKA can also phosphorylate other receptors with similar amino acid sequences to β2- other receptors than β2 can be desensitized without first being stimulated by ligand binding.

Question 12

Homologous desensitization

Answer 12

β-adrenoceptor kinase (βARK) can phosphorylate agonist occupied β2 receptors at the end of the C terminal domain.
Phosphorylation increases the affinity of the β2 receptor for the protein β-arrestin.
Binding of β-arrestin uncouples the receptor from the α subunit of Gs.

Takes place with high concentrations of ligand.
Homologous as βARK only works on agonist-occupied receptors- so this desensitisation does not affect any other receptors that might be present.