7.2 Effector mechanisms in intracellular signalling

Question 1

Which receptor does histamine work via?

Answer 1

H1-receptor

Question 2

what responses does Gq lead to in the body?

Answer 2

vascular
GI tract
airways smooth muscle contraction
mast cell degranulation
platelet aggregation

Question 3

What is the 2nd messenger in G protein pathway?

Answer 3

IP3

Question 4

What does Gt (transducin) activate? When?

Answer 4

following excitation of rhodopsin by a photon of light
activation of cyclic GMP phosphodiesterase leading to
breakdown of second messenger cGMP (decrease in cGMP)
leading to channel closure and membrane hyperpolarisation (more negative as Ca2+ and Na+ can’t enter)
altering signal output to CNS

Question 5

What happens with cyclic GMP in the dark? What does it cause?

Answer 5

levels of cGMP (of Gt pathway) are sufficient to open a second messenger-operated ion channel
allowing Ca2+ and Na+ to enter the cytoplasm

Question 6

what does increase in cAMP, cGMP, DAG, Ca2+ exert their actions via?

Answer 6

interactions with specific threonine/serine protein kinases

these protein kinases posphorylate specific serine / threonine AA residues within target protein

Question 7

Which protein kinase does cAMP target?

Answer 7

cAMP-dependent protein kinase (PKA)

Question 8

which protein kinase does cGMP target?

Answer 8

cGMP-dependent protein kinase (PKG)

Question 9

which protein kinase does DAG target?

Answer 9

PKC

Question 10

which protein kinase does Ca2+ target?

Answer 10

Ca2+ (CICR) / calmodulin-dependent protein kinase (Ca-M kinase)

Question 11

What are the 3 ways of signal amplification in signal transduction pathway

Answer 11

1. activated receptor can activate more than 1 G-protein (through GTP/GDP exchange)
2. activated Ga-GTP/free ßgamma can activate multiple effector molecules
3. effector molecules act catalytically e.g. adenylyl cyclase can convert 1000s of ATP –> cAMP, ion channel open by a-GTP - lots of ions move through channel
   ALL AMPLIFICATION CASCADE

Question 12

Name a deactivation pathway of amplification of signal (activation from G-protein)

Answer 12

once a receptor activates a G-protein, the bond between the ligand and the receptor weakens

Question 13

Name a deactivation pathway of amplification of signal (when lots of enzymes are active)

Answer 13

In cells, enzyme activity at the basal state are favoured, so if there are lots of active enzymes, mechanisms will be put in place for the enzyme activity to lower back to basal level
e.g. cGMP –> 5’-GMP (inactive) by phosphodiesterase

Question 14

Name a deactivation pathway of amplification of signal (second messenger)

Answer 14

downstream activation cascade of enzymes are inhibited by activities reversing the second messenger
e.g. phosphatase (prevent phosphorylate) reverses kinase

Question 15

Name a deactivation pathway of amplification of signal (GTP)

Answer 15

lifetime of a-GTP limited by intrinsic GTPase

Question 16

How does signal pathway bring about regulation of Chronotropy in the heart?

Answer 16

SAN firing rate can be affected by ACh release in paraNS
M2 (activation) –> opening K+ channels –> directly regulate ai-GTP
M2 (SAN, Gi) –> inhibit adenylyl cyclase
opening of K+ causes hyperpolarisation (more neg as K+ exit cell), slows down intrinsic firing rate at SAN –> negative CHRONOtropic effect

Question 17

How does signalling pathway regulate inotropy in the heart?

Answer 17

inotropy (force of contraction) can be affected by sympathetic innervation / circulating adrenaline
activating ß1-adrenoceptors increases both cAMP AND opening of VOCC –> increase in Ca2+ influx (indirect)
ß1 activate adenylyl cyclase (via alpha s-GTP)
increase in cAMP –> PKA –> interact directly with VOCC (direct)
indirect + direct increase in Ca2+ causes POSITIVE INOTROPIC effect

Question 18

How does signalling pathways cause arteriolar vasoconstriction?

Answer 18

sympathetic release of NA acts on a1-adrenoceptors –> stimulate PLC (via Gq)
generating IP3, releasing ER Ca2+ –> initiating contractile response
DAG –> PKC –> phosphorylation of target proteins important for sustaining vasoconstrictor response to NA

Question 19

How does signalling pathway allow for modulation of neurotransmitter release?

Answer 19

G-coupled protein receptors pre-synaptically influence the release of neurotransmitters at the synapse
e.g. pre-synaptic µ-opioid receptors can be stimulated to couple to Gai proteins
Gßgamma can interact with VOCC, reducing entry of Ca2+, decreasing Ca2+ influx –> inhibiting release of neurotransmitter (as neurotransmitter release is Ca2+ dependent)

Question 20

Which receptors do dopamine receptors act on?

Answer 20

D1 (stimulatory)

D2 (inhibitory)