MR 5 Ligand gated channels and control of intracellular calcium conc Flashcards
What ion channels can be seen in the nerve terminal?
Na+, K+, Ca2+
What happens when an action potential reaches the nerve terminal?
Depolarisation opens voltage gated calcium channels
Ca2+ influx down conc grad
Increased intracellular [Ca2+] causes release of neurotransmitter
What is the basic structure of calcium channels similar to?
Sodium channels
What is a common calcium channel, where is it found and what blocks it?
L type
Muscle, neurones, lung
Dihydropyridines(DHP) e.g Nifedipine
What is fast synaptic transmission?
Where receptor is also ion channel
Describe transmitter release (e.g acetyl choline)
Ca2+ enters through Ca2+ channels Ca2+ binds to synaptotagmin Vesicle brought close to membrane Snare complex makes fusion pore Transmitter released through this pore
What are the two types of blockers of nicotinic receptors?
Competitive blcokers- bind at molecular recognition site for Ach
Depolarising blockers- cause a maintained depolarisation at post-junctional membrane. Adjacent Na+ channels will not be activated due to accomodation e.g Succinylcholine
What is succinylcholine used for?
To induce paralysis for operations
Describe Myasthenia Gravis
Autoimmune disease targeting nicotinic Ach receptors
Patients suffer drooping eyelids, profound weakness increasing with exercise.
Caused by antibodies directed against NAchrRs on postsynaptic membrane of skeletal muscle
Endplate potentials reduced in amplitude leading to muscle weakness and fatigue
What is Myasthenia gravis treated with?
Ach-esterase inhibitors so Ach spends longer in synaptic cleft
What causes the end plate potential?
Activation of nAchR by Ach activating adjacent Na+ channels causing depolarisation
What are minature end plate potentials caused be and what is their amplitude compared to end plate potentials?
Minature potentials caused by the spontaneous release of vesicles (about 1 per sec).
1mV as opposed to 10mV
Why do muscarinic Ach receptors create a slower response that nictotinic Ach receptors?
Because nAchR is a ligand channel while mAchR are coupled to G-proteins
Why is it important to control intracellular calcium levels?
Many cellular processes calcium sensitive
Ca2+ cant be metabolised so has to be regulated by moving Ca2+ in and out of cytoplasm
What cellular processes are calcium sensitive?
Fertilisation, secretion, neurotransmission, metabolism, contraction, learning and memory, apoptosis, necrosis
What are the resting calcium concentrations inside and outside cells?
Extracellular- 1-2mM
Intracellular- 0.1uM
the large gradient is energy expensive
What are the advantages and disadvantages of the large calcium gradient?
+ changes in [Ca2+]in occur rapidly with little movement
- Ca2+ overload can lead to loss of regulation and cell death
What does the calcium gradient rely on?
The relative impermeability of the plasma membrane
Ability to expel Ca2+ across membrane using Ca2+ATPase ans Na+Ca2+ exchanger
Ca2+ buffers
Both rapidly releasable and non-rapidly releasable intracellular Ca2+ stores
Describe the action of Ca2+ATPase
High affinity, low capacity Intracellular [Ca2+] rises Ca2+ binds to calmodulin (binding trigger protein) Calmodulin-Ca2+ binds to ATPase Ca2+ removed from cell ATP is hydrolysed in this process
Describe the action of Na+/Ca2+ exchanger
Low affinity, high capacity
Na+ grad set up by Na+/K+ATPase used
3Na+ in per 1Ca2+ out
Electrogenic and works best at RMP
What are Ca2+ buffers?
These limit diffusion through ATP and Ca2+ binding proteins like parvalbumin, calreticulin, calbindin and calsequestin. Diffusion depends on concentration of binding molecules and level of their saturation
Many other proteins also bind Ca2+, altering their function e.g calmodulin
How is intracellular calcium increased?
Influx across plasma membrane (VOCCs, receptor operated ion channels)
Release from rapidly releasable stores (GPCRs, CICR)
Release from non-rapidly releasable stores
Where is calcium stored within cells?
Sarco/Endoplasmic reticulum and mitochondria
How is calcium moved into the sarco/endoplasmic reticulum?
Via Sarco/Endoplasmic Reticulum ATPase (SERCA) using ATP then binds to proteins like calsequestrin
How do GPCRs facilitate calcium release from the SR/ER
Ligand binds to GPCR on cell membrane
Activates G-alpha-q sub unit
This binds to PIP2 triggering IP3 release
IP3 binds to IP3 receptor on SR/ER triggering calcium release down conc grad
What is Ca2+ induced Ca2+ release?
Ca2+ binds to ryanodine receptor on SR/ER triggering calcium release down conc grad
Where is CICR of particular importance?
Cardiac myocytes where Ca2+ influx through VOCCs after depolarisation and binds to ryanodine receptor causing explosive calcium release from stores
When is Ca2+ taken into mitochondria?
When Ca2+ conc high as a protective mechanism
Why do mitochondria take up Ca2+?
To aid in buffering, regulating signalling, stimulation of ATP production
How do mitochondria participate in normal Ca2+ signalling?
through microdomains- areas of cytoplasm with higher Ca2+ conc due to proximity to a channel
What do mitochondria use to take up calcium?
Ca2+ uniporter driven using respiration
What does a return to basal Ca2+ levels require?
Termination of signal
Ca2+ removal
Ca2+ store refiling
What channels increase calcium when intracellular sotres are depleted?
Store Operated Channels (SOCs) allow Ca2+ in through plasma membrane to then be taken into SR/ER via SERCA
