MnR S5 - Effects of electrical signals, ligand gated channels and control of intracellular calcium concentrations

Question 1

How does an action potential result in the opening of Ca2+

Answer 1

• Depolarisation occurs as an action potential reaches the nerve terminal
• Voltage-gated Ca2+ channels open
• Ca2+ influx doen concentration gradient

Question 2

Give an example of one type of Ca2+ channel and its blocking agent

Answer 2

L-Type channel - Dihydropyridines (nifedipine, amlodipine)

N-Type channel - Ziconotide

Question 3

How does Ca2+ stimulate the release of neurotransmitter?

Answer 3

• Vesicles are loosely docked in the active zones via SNARE protein interactions
• Ca2+ enters through calcium channels following action potential and depolarisation
• Ca2+ binds to synaptotagmin
• SNARE complex undergoes conformational change
• Fusion pore created by SNARE
• Transmitter molecule released through pore

Question 4

If nicotinic Ach receptor channels are equally permeable to K+ and Na+, what effect does acetylcholine binding have on membrane potential and why?

Answer 4

• Causes depolarisation
• Resting membrane potential is closer to Ek than ENa so opening the channel causes membrane potential to move towards ENa

Question 5

Could chlorine ions pass through the nAChR channels?

Answer 5

No - they are cationic specific

Question 6

Describe the sequence of events that occur during neuromuscular transmission

Answer 6

• Brief depolarisation creates end plate potential
• Activation of nicotinic ACh receptors by ACh binding
• Activates adjacent Na+ channels by local spread of charge
• Triggers muscle action potential

Question 7

Outline the differences between the ways in which competitive and depolarising nicotine ACh receptors drugs work

Answer 7

Competitive - bind to ACh receptors and prevents ACh from binding, channel remains closed (tubocurarine)

Depolarising - bind to ACh receptors and cause channel to open, generate slow and maintained depolarisation and fail to activate adjacent sodium channels as they become inactivated/accommodated (succinylcholine)

Question 8

Give an example of each type of nAChr blocker

Answer 8

Competitive - tubocurarine

Depolarising - succinylcholine

Question 9

List some uses of calcium as the stimulating or regulatory fact involved in the process

Answer 9

Fertilisation, proliferation, secretion, secretion, neurotransmission, contraction, metabolism, learning and memory, apoptosis

Question 10

What are the advantages and disadvantages of a large inward calcium gradient?

Answer 10

Advantage - Changes in intracellular calcium concentration occur rapidly and with little movement of ions

Disadvantage - Calcium overload can cause loss of regulation and cell death

Question 11

List four factors that are involved in maintenance of cytosolic calcium concentration

Answer 11

• Relative impermeability of plasma membrane to Ca2+
• Cell’s ability to expel Ca2+ across the plasma membrane via Ca2+-ATPase and NCX
• Ca2+ Buffers
• Intracellular Ca2+ stores

Question 12

Outline how the Ca2+-ATPase works

Answer 12

• Intracellular Ca2+ increases
• Ca2+ binds to calmodulin, a trigger protein
• Ca2+-calmodulin binds to Ca2+-ATPase
• Release of Ca2+ via channel

Question 13

At which membrane potential does the NCX work optimally and why?

Answer 13

Resting membrane potential because it is electrogenic, involving movement of 3 sodium ions in and one calcium out

Question 14

How do calcium buffers work?

Answer 14

Limit diffusion by ATP and Ca2+ binding proteins so calcium diffuses more slowly than predicted

Question 15

What factors does the rate of calcium diffusion depend on?

Answer 15

Concentration of binding proteins and their level of saturation

Question 16

What are the two ways intracellular Ca2+ can be elevated by influx across the plasma membrane

Answer 16

• Voltage operated Ca2+ channels

- Receptor operated Ca2+ channels

Question 17

Give an example of the inotropic channel opened by glutamate binding

Answer 17

NMDA

AMPA

Question 18

Where is the rapidly releasable store of calcium ions found within cells?

Answer 18

Endoplasmic / Sarcoplasmic reticulum

Question 19

List the mechanisms by which Ca2+ release from rapidly releasable stores is mediated

Answer 19

• GPCR mediated
• Ligand gated ion channel
• CICR

Question 20

Outline how GPCRs can cause an increase in intracellular calcium

Answer 20

• Stimulus detected by receptor
• Conformation change
• Alpha subunit splits from beta and gamma subunits
• Alpha-q activates phospholipase C
• Activates IP3 and DAG
• IP3 binds to IP3 receptors on the SR which leads to release of calcium

Question 21

What are the effects of alpha-s and alpha-i?

Answer 21

Alpha-s stimulates adenylyl cyclase causing an increase in cAMP

Alpha-i inhibits adenylyl cyclase causing a decrease in cAMP

Question 22

What is the receptor involved in calcium induced calcium release?

Answer 22

Ryanodine

Question 23

Where is non-rapidly releasable store found within the cell?

Answer 23

Mitochondria

Question 24

What are microdomains?

Answer 24

Sites in the cells cytoplasm with localised high Ca2+ concentration

Question 25

List the roles of mitochondrial uptake of Ca2+?

Answer 25

• Ca2+ buffering
• Stimulates mitochondrial metabolism
• Role in cell death

Question 26

What does the restoration of basal Ca2+ intracellular concentration require?

Answer 26

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

Question 27

How can intracellular stores of Ca2+ be replenished?

Answer 27

• Recycling of released, cytosolic Ca2+

- VOCC or Capacitative Ca2+ entry / SOC

Question 28

List two key proteins involved in capacitative Ca2+ entry and explain their roles

Answer 28

• STIM - acts as a Ca2+ sensor on the ER membrane
• When stores of Ca2+ are depleted no Ca2+ binds to sensor region
• STIM interacts with ORAI
• ORAI acts as a plasma membrane membrane channel
• Enables Ca2+ to move in