M&R S5 - Effects of Electrical Signals, Ligand Gated Channels

Question 1

How does an action potential open Ca2+ channels in a cell membrane?

Where might this happen?

Answer 1

Might happen at a synapse

Action potential arrives at presynaptic axon terminal

Opens voltage gated Ca2+ channels causing an influx of Ca2+ into the cell down the concentration gradient

This increase in Ca2+ leads to the release of neurotransmitter from the axon terminal

Question 2

Why can influx of Ca2+ into the cell have such a large effect on internal concentration of Ca2+?

Answer 2

Ca2+ concentration in cells is so low that any influx can have a large effect on it

Question 3

What is the key difference between Na+ and Ca2+ channels?

Explain this difference in terms of channel blockers

Answer 3

Ca2+ channels have structural diversity

This is well shown through the action of channel blockers, a blocker of one type of channel may not block the others

Question 4

How can calcium channel blockers have a localised effect in the body?

Answer 4

Different types of calcium channel have different primary locations, so selectively blocking one type of channel will have a localised effect

Question 5

Give an example of a calcium channel type and a blocker of that channel type

Answer 5

L type channels

Blocked by DHPs (Dihydropyridines)

For example, Nifedipine

Question 6

What is fast synaptic transmission?

Answer 6

The receptor protein is also an ion channel, binding of the transmitter causes it to open

This effect is relatively fast

Question 7

How does Ca2+ influx into a cell result in ACh release?

Answer 7

Ca2+ enters through Ca2+ channels (found in high density at the axon terminal)

Ca2+ binds to synaptotagmin

This leads tot he formation of a snare complex

The snare complex forms a fusion pore which allows the release of ACh into the synaptic cleft from vesicles that bind to the snare complex

Question 8

How does the release of ACh lead to an end plate potential being generated in muscle?

Answer 8

ACh crosses the synaptic cleft

Ach binds to Nicitonic Ach receptors (nAChR) (ligand gated ion channels) on the post junctional membrane

End plate potential is produced and the depolarisation will raise the muscle above threshold so that an action potential is produced

Question 9

What are the two types of blockers for nicitonic receptors?

Explain a bit about each and provide examples

Answer 9

Competitive blockers:

Bind at the ACh binding site
E.g. Tubocurarine

Depolarising blockers:

Cause a maintained depolarisation at the post junctional membrane, adjacent Na+ channels will not be activated due to accommodation
E.g. Succinylcholine (used to induce paralysis)

Question 10

Describe myasthenia gravis (TOB Semester 1)

Hint: Causes, Symptoms, biochemistry behind the symptoms

Answer 10

Autoimmune disease targeting nAChR

Patients suffer:
Drooping eyelids
Profound weakness (increasing with exercise)
Fatigue

Caused by antibodies directed at nAChR on post-synaptic membranes in skeletal muscle

Leads to loss of functional nAChR by complement mediated lysis and receptor degradation

Endplate potentials reduced in amplitude leading to the muscle weakness and fatigue

Question 11

Why is control of intracellular Ca2+ important?

In VERY broad terms, how does it do this?

Answer 11

Many cellular process are calcium sensitive

E.g.
Fertilisation
Secretion
Neurotransmission
Metabolism
Contraction
Learning and memory
Apoptosis
Necrosis

As Ca2+ can’t be metabolised, the cell must regulate Ca2+ concentration by moving Ca2+ in and out of the cell

Question 12

What is the at rest intra and extracellular Ca2+ concentration?

This gradient means thar movement of calcium out of the cell is…?

Answer 12

Intra - 100nM

Extra - 1-2mM

… Energy expensive

Question 13

What are the major advantages and disadvantages of a high Ca2+ concentration gradient?

Answer 13

Advantage - Changes in intracellular Ca2+ occur rapidly with little movement

Disadvantage - Rapid Ca2+ overload can lead to loss of regulation and cell death

Question 14

What does the Ca2+ gradient rely on?

Answer 14

Impermeability of the cell membrane

Ability to expel Ca2+ (Ca2+ ATPase / NCX)

Ca2+ Buffers

Intracellular stores (Rapid and slow release)

Question 15

What regulates membrane permeability to Ca2+?

Answer 15

The open/closed state of the Ca2+ ion channels

Question 16

Describe the action of Ca2+ ATPase

Answer 16

High affinity, low capacity

Responds to intracellular [Ca2+] increase

Ca2+ binds to calmodulin - A binding trigger protein

Calmodulin-Ca2+ binds to Ca2+ ATPase

Ca2+ is expelled from the cell

This process requires ATP

Question 17

Describe the action of Na+/Ca2+ Exchanger (NCX)

Answer 17

Low affinity, high capacity

Na+ gradient used as driving force

Transports 3Na+ out and 1Ca2+ in (Antiport)

Electrogenic

Works best at resting membrane potential

Question 18

What is the function of Ca2+ buffers?

Give examples of Ca2+ buffers

Answer 18

Ca2+ buffers limit diffusion of Ca2+ through ATP and Ca2+ binding proteins

Example binding proteins are:

Parvalbumin
Calsequestrin
Calbindin
Calreticulin

Question 19

What does rate of Ca2+ diffusion depend on?

Answer 19

Concentration of binding molecules and their level of saturation

Question 20

To what level can intracellular [Ca2+] rise when being used to regulate cell activity?

Some intracellular process appear to require even higher concentrations than this, how is this achieved?

Answer 20

From 100nM to ~1uM

Microdomains are areas of localised high [Ca2+] intracellularly (higher than the global concentration of Ca2+ in the cell)

E.g. These might appear around open Ca2+ ion channels

Question 21

What are the major mechanisms for changing intracellular [Ca2+]?

Answer 21

Ca2+ influx across membrane (altered permeability):

• Voltage gated calcium channels (VGCC)
• Receptor operated ion channels (ionotropic receptors)

Ca2+ release from rapidly-releasable stores:

• G-protein coupled receptors (GPCR)
• Ca2+ induced Ca2+ release (CICR)

Ca2+ release from non-rapidly releasing stores

Question 22

Briefly explain the action of Voltage gated Ca2+ channels

Answer 22

Open in response to membrane depolarisation allowing calcium to flow into the cell

Question 23

Briefly explain the action of receptor operated ion channels

Answer 23

Open in response to ligand/agonist binding to the channel allowing Ca2+ to flow into the cell

Question 24

Describe where the major rapidly releasable store of Ca2+ is found in the cell and how it is

Answer 24

Found in the Sarco/endoplasmic reticulum

Set up by the SERCA protein which moves Ca2+ into the S/ER using ATP where it binds to proteins such as calsequestrin

Question 25

Explain how G-protein coupled receptors being activated leads to rapid Ca2+ release from the sarco/endoplasmic reticulum

Answer 25

Ligand binds to GPCR on cell membrane

G-alpha-q subunit is activated, which then binds to the membrane phospholipid PIP2

G-alpha-q binding to PIP2 releases IP3

IP3 binds to its receptor on the sarco/endoplasmic reticulum, triggering the release of calcium into the cell

Question 26

Explain how Ca2+ can induce the release of Ca2+ from the sarco/endoplasmic reticulum (CICR)

Answer 26

Ca2+ binds to the ryanodine receptor on the sarco/endoplasmic reticulum

This triggers the release of Ca2+ into the cell

Question 27

Give an example of an important physiological process that Ca2+ induced Ca2+ release (CICR) is involved in

Answer 27

In the cardiac myocyte

Causes explosive release of Ca2+ from intracellular stores into the cell, initiating strong, coordinated contraction

Question 28

Describe how Ca2+ is handled by cardiomyocytes

Answer 28

During the early part of depolarisation NCX will reverse, leading to Ca2+ influx into the cell

Ca2+ influx leads to rapid release of Ca2+ from the SR

Ca2+ will initiate contraction

AS Ca2+ increases and repolarisation begins NCX will revert to normal resulting in Ca2+ efflux

Ca2+ will also be returned to the SR via SERCA

Question 29

Explain how elongation of the cardiac action potential comes about

Answer 29

After initial depolarisation Ca2+ channels will open (voltage sensitive) and prolong the depolarisation during Ca2+ influx

Low K+ conductance through the membrane during depolarisation also helps elongate this action potential

Question 30

What is the main function(s) of mitochondria in relation to Ca2+?

Hint: Include the channel involved

Answer 30

Ca2+ uptake by Ca2+ uniporters (driven by respiration) aids in buffering (protective when [Ca2+] is high), regulates signalling and stimulates ATP production

Participates in Ca2+ signalling via microdomains

Question 31

How is intracellular Ca2+ returned to basal state following signalling?

Why is this necessary?

Answer 31

Return to basal levels requires:

• Termination of signal
• Ca2+ removal
• Ca2+ store refilling

Necessary because repeated signalling requires return to basal state and high Ca2+ is toxic to the cell

Question 32

How are Ca2+ stores in the sarcoplasmic reticulum replenished?

Answer 32

Replenished from Cytosolic Ca2+

Also from Ca2+ stores in the mitochondria by store-operated Ca2+ channel (SOC)