CJH332H1 Lecture Notes - Lecture 3: Nicotinic Acetylcholine Receptor, Respiratory Failure, Ion Channel
Lecture 3: Electrophysiology
Channel selectivity
• Ion channels go all the way through the lipid bilayer – there must be a way to open/close it (gating
mechanism)Membrane channels differ considerably in their selectivity for ions
• While some are relatively unselecting (letting most ions pass), connexions for examples, most are selective for
one ions species
Probing individual channel function – TTX
• Open channel block:
- Channel is open but blocked
- Eg. tetrodotoxin (TTX) produced by Pufferfish blocks voltage-dependent Na+ channels, leading to death by
respiratory paralysis
- Bacteria within the fish generate a toxin called TTX
- TTX blocks Na+ channels paralyzing the victim (fugu most commonly consumed)
- Very specific in attacking type of channel – voltage gated sodium channels
• We need a way to block them if we have identified a channel
• When someone stops breathing after eating this, two options – paralysis of muscles or paralysis of nerves
- For fugu, it is the nerves that gets disrupted – we must look into how we block an AP
• We can block AP by blocking one channel
• AP relaying sensory information blocked to
inhibit sensation – local anesthetic
- Applying to nerve endings with voltage
gated sodium channels blocks sensory
information
• Cocaine (or –aine) is a local anesthetic of open
channel blockers that are lipid soluble
- We can inject this outside the nerve, cross
lipid membrane – unknown (how to these
local anesthetics BLOCK the channels?)
• These local anesthesia is lipophilic
Nicotinic acetylcholine receptors nAChR
• 1st channel to be studied in detail
- Ligand-activated channels = receptors (another mechanism of opening channel)
- Channel is nAChR and the ligand is Ach (a receptor with channel built in)
• Heavily localized in the postsynaptic membranes of skeletal muscle fibers
• Permeable to only cations (specific)
• 5 separate subunits arranged around a central core (known as receptor
stoichiometry – at least two different subunits)
• 2 a-subunits contain the binding sites for acetylcholine (Ach – the normal
physiologic ligand)
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nAChR
• Termed nicotinic because the actions of nicotine mimic the effect of Ach
• Ach can bind two receptors – ion channel nicotinic receptors and g-protein muscarinic receptor
- Muscarinic (g-protein) more associated with lipid rafts
• Term nicotinic distinguishes them from another class of receptors that bind Ach called
muscarinic AChRs (mAChRs) which are not channels but g-proteins
nAChR subunits
• Each subunit is composed of 4 membrane-spanning regions (M1-M4)
- Each subunit has this topology (whether a, B, y or delta)
- N terminal region that is extracellular – will bind a-subunit with Ach
- Transmembrane domain with M1-M4
- Cytoplasmic/intracellular (c-loop) between M3 and M4
- Small C terminal domain that is extracellular again
• M2 regions line the pore and form the gate where ions will flow thorugh
Proposed model of the nAChR structure
• Eah suuit has hads of M2 that lies the poe toads the iddle – closed
• 2 Ach will bind to 2 a-subunits, M2 regions rotate away
• An open pore is created by rotating each helix towards the channel wall
Ion Channel Summary
• Ion channels mediate the passage of ions across the plasma membrane
• Due to their electric charge, ions cannot enter the plasma membrane
- Channels strip the hydration shell off the ion allowing its passage
• Channels are passive transporters – do’t supply eegy fo io taspot
- Act by electrodiffusion which is solely driven by the electrochemical gradient across the membrane
- Electrochemical gradient – ionic gradient resulting from combined electrical + diffusional forces of that ion
• Have distinct regions to regulate the binding (n-terminal ECM), opening and
gating of the channel
Primary Active Transport: Na+/K+ exchanger
• These transporters (high affinity binding sites) are required to maintain the
concentration gradients of the ions
- Active transport mechanisms used
- Energy to move ions back across the membrane
- Na+ transported out and K+ needs to be transported in
• 3 affinity binding sites for Na+ when opened to intracellular surface
• Conformational change for ATP to bind and broken down → ADP
• Molecule phosphorylated and another conformational change to open to extracellular
• 2 affinity binding sites for K+
• Looses phosphorylation, conformational change open to intracellular, K+ released
• Primary active transport uses the hydrolysis of ATP
- 3 Na+ out and 2 K+ in for every ATP used – electrochemical gradient
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Document Summary
Probing individual channel function ttx: open channel block: Eg. tetrodotoxin (ttx) produced by pufferfish blocks voltage-dependent na+ channels, leading to death by respiratory paralysis. Bacteria within the fish generate a toxin called ttx. Ttx blocks na+ channels paralyzing the victim (fugu most commonly consumed) For fugu, it is the nerves that gets disrupted we must look into how we block an ap: we can block ap by blocking one channel, ap relaying sensory information blocked to inhibit sensation local anesthetic. Applying to nerve endings with voltage gated sodium channels blocks sensory information: cocaine (or aine) is a local anesthetic of open channel blockers that are lipid soluble. We can inject this outside the nerve, cross lipid membrane unknown (how to these local anesthetics block the channels?: these local anesthesia is lipophilic. Nicotinic acetylcholine receptors nachr: 1st channel to be studied in detail. Ligand-activated channels = receptors (another mechanism of opening channel)
