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SGUL - Fundamentals of Pharmacology and Physiology > Cholinergic and Adrenergic Effects > Flashcards

Cholinergic and Adrenergic Effects Flashcards

1
Q

TRUE OR FALSE - Adrenoreceptors are examples of GPCRs

A

TRUE

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2
Q

What enzymes do the following adrenoreceptors activate?
- α1
- α2
- β1
- β2
- β3

A
  • α1 ADRENORECEPTOR
    Enzyme: STIMULATE Phospholipase C (Gαq/11)
  • α2 ADRENORECEPTOR
    Enzyme: INHIBIT Adenylate Cyclase (Gαi)
  • β1/2/3 ADRENORECEPTOR
    Enzyme: STIMULATE Adenylate Cyclase (Gαs)
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3
Q

Describe the effects α1 adrenoreceptors have, what tissue they affect and their mechanism of action.

A
  • α1-adrenoreceptors contract vascular smooth muscle (e.g iris muscles for dilation). They also cause vasoconstriction of blood vessels, causing arteries to narrow through the contraction of smooth muscles. They also cause pupils to widen through contraction of the radial muscles in the eyes, as well as piloerection.
  • Sympathetic nerves synthesize and release noradrenaline which binds to the α1-adrenoreceptor on vascular smooth muscles.
  • The α1-adrenoreceptor is linked to a Gαq/11 subunit. This activates and stimulates Phospholipase C, which catalyzes the cleavage of PIP2 to IP3 and DAG, both of which behave as secondary messengers. The IP3 stimulates the release of Ca2+ from the SR, leading to increased muscle contraction.
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4
Q

Describe how the α2 adrenoreceptor works and what its effect is.

A
  • Sympathetic nerves synthesize and release noradrenaline which binds to the α2 adrenoreceptor
  • The α2 adrenoreceptor is linked to a Gαi subunit and inhibits adenylate cyclase. This means that less ATP is converted to cAMP so less PKA will be activated.
  • Reduced sensitization of calcium ion channels by PKA so reduced influx of calcium ions (calcium influx required for transmitter release)
  • α2 adrenoreceptors inhibit transmitter release from sympathetic nerves
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5
Q

Describe the β1 adrenoreceptor cardiac effects.

A

When activated:
- Heart rate is increased (positive chronotropy)
- Conduction velocity is increased (positive dromotropy)
- Cardiac contraction force is increased (positive inotropy)
- Rate of relaxation is faster (positive lusitropy)

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6
Q

Describe how β1 adrenoreceptors work

A
  • Sympathetic nerves synthesize and release noradrenaline which binds to the β1 adrenoreceptor
  • The β1 adrenoreceptor is linked to a Gαs subunit, so it stimulates adenylate cyclase.
  • This leads to an increase in intracellular cAMP concentration, due to increased conversion of ATP to cAMP which leads to an increase in activated PKA.
  • PKA causes sensitisation of calcium ion channels leading to increased calcium ion influx
  • Muscle contraction is increased i.e the contractions of the heart are much more forceful
  • Contraction occurs more frequently due to increased calcium influx but is also more briefer due to greater and more rapid uptake of calcium
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7
Q

Outline the order of potency for α and β adrenoreceptors.

A

For α receptors
- order of potency is noradrenaline>adrenaline>isoprenaline

For β receptors
- order of potency is isoprenaline>adrenaline> noradrenaline

where > means more potent than

ISOPRENALINE - synthetic chemical designed for research purposes

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8
Q

What effect do β1/2 adrenoreceptors have on smooth muscle? **

A
  • Smooth muscle of the GI, uterus and bladder relax
  • Airway widening due to relaxation of smooth muscle in the airway - hence why β2 agonists such as salbutamol are used for asthma to allow the bronchi to relax
  • Increased vasodilation of the blood vessels e.g the smaller coronary arteries leading to skeletal muscle (due to relaxation of the smooth muscle cells)
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9
Q

Describe the role of β1-adrenoreceptors in the kidneys.

A
  • Affects the Juxtaglomerular Apparatus (JGA) which is located between the afferent and efferent renal arteries.
  • Increased renin release. Renin is used to convert inactive Angiotensinogen into its active form, Angiotensin I
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9
Q

Outline the mechanism of action of β2 adrenoreceptors on smooth muscle.

A
  • Sympathetic nerve synthesises and releases noradrenaline which binds to the β2 adrenoreceptor
  • β2 adrenoreceptor is linked to a Gαs subunit, so it stimulates adenylate cyclase
  • Increased conversion of ATP to cAMP
  • Increased activation of protein kinase A
  • Increased potassium ion channel activity
  • Action of myosin light chain kinase is inhibited by protein kinase A due to phosphorylation of MLCK enzymes
  • Reduced phosphorylation of myosin - needed for cross-bridge formation
  • Reduced smooth muscle contraction
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10
Q

Outline the mechanism of action of β2/3-adrenoreceptors in metabolism during sympathetic responses

A
  • Sympathetic nerve synthesises and releases noradrenaline which binds to the β3-adrenoreceptor
  • β3 adrenoreceptor is linked to a Gαs subunit, so it stimulates adenylate cyclase
  • Increased conversion of ATP to cAMP
  • Increased activation of protein kinase A
  • Modulation of enzymes involved in metabolism is altered - namely those enzymes involved in lipolysis(in adipocytes) and glycogenolysis (i.e lipases and glycogen phosphorylase) are activated and stimulated
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11
Q

What type or nerves are sympathetic nerves?
What type of nerves are parasympathetic nerves?

A

Adrenergic - SYMPATHETIC
Cholinergic - PARASYMPATHETIC

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12
Q

TRUE OF FALSE - Sweating is mediated by the release of noradrenaline during adrenergic responses

A

FALSE - mediated by acetylcholine but it IS adrenergic

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13
Q

How many types of receptors does smooth muscle express?

A

2
- It expresses both α and β-adrenoreceptors
- β-receptors seen as more ‘potent’ - biological response elicited by lower concentrations of noradrenaline compared to α-receptors which require higher concentrations

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14
Q

Outline how acetylcholine is synthesised in the parasympathetic nervous system

A
  • Choline taken up by the parasympathetic nerve
  • Acetylated using choline acetyl transferase (in process acetyl CoA deacetylated to Coenzyme A)
  • Acetylcholine is packaged and kept within vesicles
  • Depolarisation of the membrane causes the opening of voltage gated calcium ion channels
  • These channels trigger the movement of the vesicles towards the membrane, and acetylcholine is released by exocytosis
  • They bind to nAChRs at the ganglia as well as muscarinic receptors at the effectors
  • Response is brief- hydrolysis of acetylcholine by cholinesterases
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15
Q

Name the two types of cholinoceptors. Give examples and if possible, state where they are found.

A

Nicotinic AChRs - ligand-gated ion channel receptors
- N1/NN: present on skeletal muscle
- N2/NM: present on all autonomic ganglia and CNS

Muscarinic receptors - GPCRs
-M1: present in the CNS, peripheral neurons, and gastric parietal cells
-M2: on the atria, AVN and SAN
-M3:on visceral smooth muscle, secretory glands, and endothelial cells
-M4: on the CNS
-M5: on the CNS

16
Q

RECAP - What occurs upon stimulation of nicotinic ACh receptors?

A
  • Acetylcholine binds to the nAChR
  • This induces a conformational change, allowing an aqueous pathway to immediately form through which ions can pass through
  • Influx of cations e.g Na+ ions occur
  • Depolarisation causes intracellular changes and effects
  • Terminated by dissociation or hydrolysis of ACh
17
Q

What are the main effects of the parasympathetic nervous system?

A
  • Slower heart rate and less forceful cardiac contractions
  • Contraction of the airways, GI tract, bladder, and iris circular muscles(pupil constriction)
  • Increased saliva, gastric acid, airway mucus, and tear production
  • Increased sweat production (usually a sympathetic response)
  • Visceral smooth muscle contraction
  • Artery widening i.e vasodilation
18
Q

Describe the mechanism of action at a adrenergic synapse e.g between a post-ganglionic fibre and the heart

A
  • Noradrenaline is synthesised and packaged into vesicles.
  • The vesicles translocate to the membrane which causes the release of neurotransmitter into the synaptic cleft.
  • This then bonds to a receptor (adrenoreceptor) which induces a biological response.
  • If too much noradrenaline is released it can bind to an α2 adrenoreceptor on the presynaptic terminal – a negative feedback mechanism so less noradrenaline is released.
  • It can also be taken up by the presynaptic terminal – recycled for further use (taken into vesicles or broken down by enzymes – monoamine oxidase).
19
Q

Describe direct and indirect regulation of adrenergic transmission

A
  • Direct – Drugs that act at adrenoceptors – mimics the noradrenaline.
  • Indirect – Drugs that act at altering release/termination of transmission – modulate/inhibit the system.
20
Q

What is the difference between nicotinic and muscarinic receptors?

A
  • Nicotinic receptors are ligand-gated ion channels that mediate a fast synaptic transmission of the neurotransmitter.
  • Muscarinic receptors are G-protein coupled receptors (GPCRs) that mediate a slow metabolic response via second messenger cascades.
21
Q

Describe the cardiac effects that follow the stimulation of the M2 receptor.

A
  • Limited parasympathetic innervation to the heart - innervations only go to atria and pacemaker cells
  • When the M2 receptor is stimulated (vagus nerve stimulated), it leads to a decrease in heart rate. Vagus affects the SAN and atria.
  • This occurs because the M2 receptor is linked to a Gαi subunit, which inhibits adenylate cyclase and reduces the conversion of ATP to cAMP made. This means that less PKA is made and activated. Reduced sensitization of calcium ion channels by PKA so reduced influx of calcium ions needed for cardiac muscle contraction.
22
Q

Describe the vascular effects that follow the stimulation of the M3 receptor.

A

When stimulated, the acetylcholine binds and activates it. The Phospholipase C then activates NO Synthase, which synthesises NO from Arginine.
Since there are no receptors on smooth muscle cells (only endothelial cells), to synthesise NO. There is the rapid diffusion of NO into smooth muscle cells. There, they bind to guanylyl cyclase, which converts GTP to cGMP. This causes the rapid relaxation of smooth muscle cells.

23
Q

How do the parasympathetic and sympathetic systems affect the eye?

A

The parasympathetic system causes the contraction of circular muscle via M3 receptors, leading to pupil constriction.

Stimulation of cholinergic receptors causes
* Contraction of ciliary muscle
* Relaxes suspensory ligaments
- Allows lens to bulge

The sympathetic system causes the contraction of radial muscle via α1 receptors, leading to pupil dilation.

24
Q

How does the parasympathetic nervous system affect secretory cells and how?

A
  • Acetylcholine from vagus augments acid production(M1)
  • Salivary glands(M3)
  • Sweat glands(M3)
    Sweat glands are under sympathetic control

Both M3 and M1 are linked to a Gαq/11 subunit. This activates and stimulates Phospholipase C, which catalyzes the cleavage of PIP2 to IP3 and DAG, both of which behave as secondary messengers. The IP3 stimulates the release of Ca2+ from the SR, leading to increased contraction

25
Q

FINAL RECAP

What responses do the following mediate?

  • α1
  • α2
  • β1
  • β2
  • β3
  • M1
    -M2:
    -M3
    -M4
    -M5:
A
  • α1: Arterial narrowing, seminal vesical contraction, radial muscle contraction leading to wide pupils
  • α2: Reduction of transmitter release from sympathetic nerves
  • β1: Increased heart rate and force of contraction
  • β2: Airway and artery relaxation. Bladder and uterus relaxation
  • β3: Sugar metabolism. Bladder and uterus relaxation
  • M1: Gut acid production and other secretions
    -M2: Decreased cardiac activity
    -M3: Saliva production, release of NO from endothelium, contraction of visceral smooth muscles, pupil narrowing
    -M4: CNS
    -M5: CNS

SGUL - Fundamentals of Pharmacology and Physiology (15 decks)

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