Lecture 21 : G-Protein Coupled Receptors

Question 1

How is noradrenaline released

Answer 1

Sympathetic nerves
Adrenal medulla

Question 2

How is adrenaline released

Answer 2

Adrenal medulla

Question 3

What are adrenoreceptors

Answer 3

Alpha and Beta

G protein coupled receptors

Membrane protein

Respond to catechoamides (adrenaline and noradrenaline)

Question 4

α₁-Adrenoreceptors

Answer 4

Vasoconstriction (↑ BP)

Pupil dilation (mydriasis)

Bladder sphincter contraction

Glycogenolysis in liver

Mechanism: Coupled to Gq → activates PLC → ↑ IP₃ & DAG → ↑ intracellular Ca²⁺

Question 5

α₂-Adrenoreceptors

Answer 5

Location: Presynaptic nerve terminals, platelets, pancreas
Function:

Inhibits neurotransmitter (noradrenaline) release

Platelet aggregation

Inhibits insulin secretion

Mechanism: Coupled to Gi → inhibits adenylate cyclase → ↓ cAMP

Question 6

β₁-Adrenoreceptors

Answer 6

Location: Heart (mainly), kidneys
Function:

↑ Heart rate (positive chronotropy)

↑ Contractility (positive inotropy)

↑ Renin release (from JG cells in kidney)

Mechanism: Coupled to Gs → activates adenylate cyclase → ↑ cAMP → ↑ PKA

Question 7

β₂-Adrenoreceptors

Answer 7

Location: Bronchi, vascular smooth muscle (skeletal muscle), uterus, liver
Function:

Bronchodilation

Vasodilation

Uterine relaxation

Glycogenolysis, gluconeogenesis

Mechanism: Coupled to Gs → ↑ cAMP

Question 8

β₃-Adrenoreceptors

Answer 8

Location: Adipose tissue, bladder
Function:

Lipolysis

Relaxation of detrusor muscle (in bladder)

Mechanism: Coupled to Gs → ↑ cAMP

Question 9

Affinity of Catecholamines

Answer 9

Adrenaline: High affinity for β > α
Noradrenaline: High affinity for α > β₁ (minimal β₂)
Isoprenaline (synthetic): Pure β-agonist