Physiology: Endocrine Systems and Receptors

Question 1

Describe the mechanism of action of a ligand-gated ion channel

Answer 1

1. Ligand binds to extracellular region of channel, changing the shape of the channel so ions e.g. Ca2+ can pass through
2. Ions induce a depolarisation or hyperpolarisation of the cell which changes the activity of other molecules e.g. ion-binding enzymes

Question 2

Describe the structure of a G-protein coupled receptor

Answer 2

Composed of a transmembrane region crossing the lipid bilayer seven times, which is coupled with a G-protein

Question 3

What are ligand-gated ion channels?

Answer 3

Channels that can open in response to the binding of a ligand

Question 4

Define endocrine regulation

Answer 4

Chemicals (i.e. hormones) released from secretory cells are usually transported via the circulatory system to target cells

Question 5

How long is the cellular response to a kinase-linked receptor?

Answer 5

Minutes-hours

Question 6

Name two examples of a kinase-linked receptor

Answer 6

Insulin, growth hormone

Question 7

Describe signal amplification with reference to GPCRs

Answer 7

The one ⍺-subunit can interact with several secondary messengers, which in turn activate multiple enzymes and catalyse many reactions - results in a cascade response

Question 8

Name the function of β2 adrenoceptors

Answer 8

Dilation of the bronchi

Question 9

Define autocrine regulation

Answer 9

‘Self-regulation’ - chemicals (i.e. signalling molecules) released from the cells bind to receptors on the cell that is releasing them

Question 10

Why are only a few ligand-gated ion channels needed to produce a response?

Answer 10

There is a huge influx of ions through each channel

Question 11

Name the function of β3 adrenoceptors

Answer 11

Thermogenesis in skeletal muscle, lipolysis

Question 12

Describe how G-protein coupled receptor activity is terminated

Answer 12

1. GTPase catalyses the hydrolysis of GTP on the α-subunit into GDP + Pi
2. GDP increases the α-subunit’s affinity for the βγ-subunit, allowing reformation of the G-protein complex
3. Ligand dissociates from the extracellular portion and G-protein reassociates with the receptor, reforming the GPCR

Question 13

How long is the cellular response to a ligand-gated ion channel?

Answer 13

Milliseconds

Question 14

How long is the cellular response to a G-protein coupled receptor?

Answer 14

Seconds

Question 15

Describe the mechanism of action of a G-protein coupled receptor

Answer 15

1. Ligand binds to G-portion of receptor which induces a conformational change
2. The change allows the G-protein subunit to bind to the receptor
3. GDP released from α-subunit of the G-protein and replaced with a GTP
4. This activates the G-protein, causing the α-subunit and bound GTP to dissociate from the transmembrane portion of the GPCR and βγ-subunit
5. α-subunit interacts with its relevant effectors and causes downstream effects e.g. ion channel opening

Question 16

What is a kinase-linked receptor?

Answer 16

A kinase is just a name for an enzyme that transfers phosphate groups to a protein or other target e.g. receptor tyrosine kinase transfers phosphate groups specifically to the amino acid tyrosine

Question 17

Describe the mechanism of action of a kinase-linked receptor

Answer 17

1. Ligand binds to the extracellular domains of two nearby receptor tyrosine kinases
2. The two neighboring receptors then come together (dimerize)
3. The receptors then attach phosphates to tyrosines in each others’ intracellular domains (autophosphorylation)
4. Relay proteins attach to the phosphorylated tyrosine residues
5. The relay proteins allow a varied cellular response - multifunctional

Question 18

Name the function of β1 adrenoceptors

Answer 18

Increase heart rate and cardiac muscle contraction

Question 19

Define paracrine regulation

Answer 19

Chemicals (i.e. signalling molecules) released from the cells bind to receptors on adjacent cells

Question 20

Name an example of a ligand-gated ion channel

Answer 20

Neurons have ligand-gated channels that are bound by neurotransmitters

Question 21

Name the function of ⍺1 adrenoceptors

Answer 21

Vasoconstriction of blood vessels

Question 22

What is the function of the hydrophilic core in a ligand-gated ion channel?

Answer 22

Lets ions to cross the membrane without having to touch the hydrophobic core of the phospholipid bilayer

Question 23

Describe the structure of a ligand-gated ion channel

Answer 23

Membrane-spanning region with a hydrophilic channel through the middle of it

Question 24

Give two examples of GPCRs

Answer 24

⍺ and β adrenoceptors

Question 25

Name the function of ⍺2 adrenoceptors

Answer 25

Presynaptic inhibition of norarenaline in the CNS

Relax GI tract