23-11-21 - Ion Channels Flashcards
Learning outcomes
- Identify the importance of studying ion channels
- Describe the key experimental techniques used to measure current flow through ion channels
- Interpret the types of information we can obtain from single channel measurements
- Describe examples of channel mutations that result in alterations in single channel function and cause pathologies
What are 4 examples of factors that opens ion channels?
What are examples of each?
- Voltage
- Changing membrane voltage in depolarizing or hyperpolarizing direction
- E.g voltage-gated Na+ or K+ channels
- Ligands
- Neurotransmitters binds to the extracellular part of the channel and causes a conformation shape change in the ion channel, causing it to activate and open
- E.g glutamate receptor
- A 2nd messenger can also bind at the cytosolic surface and activate the channel
- E.g Calcium binding to intracellular ryanodine receptor
- Stretch
* E.g mechanosensitive channels - Light
* R.g channel rhodopsin
What is channel gating?
What can be done with raw data from observation of channel gating?
What gating properties can we look into?
How can this be calculated?
- Channel gating refers to the opening or closing of ion channels
- With raw data of channel gating, a 50% threshold can be set, where if the current is above this point, the channel is considered open
- By establishing an open and closed state, we are idolizing the data, which allows us to look into the gating properties of channels
- We can look into how long the gate is in an open and closed state, the amplitude of opening current, and the probability of the channel being open
What is Ohms law?
What is single channel conductance?
How can it be calculated?
Why does this allow conductance to be calculated?
Why is calculating conductance important?
- Signal channel conductance is a measure of how good a channel is at letting current run through it
- Conductance can be calculated by measuring the current running across a channel at different voltages and plotting a graph of current against voltage, where resistance is equal to the gradient (V=IR – Ohms law)
- Resistance (Ohms) is equal to 1/g, with g being conductance (unit is siemens – S)
- This allows conductance to be calculated as channels act as resistors.
- When they are closed they prevent anything from moving through
- When they aare open, this allows for conductance as ions can move through from one side of the membrane to the other
- Calculating conductance is important as it gives us an important understanding of role of channels
What is macroscopic current?
How is it calculated?
- Macroscopic current (I) is the whole cell current
- It is calculated by finding the product of the number of channel in the membrane area (N), the unitary current (current through an ion channel - i) and the probability of the channel opening (Po)
What are ion channels selective to?
What are inhibitors of the Na+ and K+ ion channels?
- Ions channels are selective to the ions that pass through them (ion selectivity)
- Tetrodotoxin is an inhibitor the Na+ channels
- Tetraethylammonium is an inhibitor for K+ channels
What is the sarcoplasmic reticulum?
What is its purpose?
What 2 receptors are present on its surface?
What are the concentrations of calcium outside the cell, inside the cytoplasm, and inside the sarcoplasmic reticulum?
What is an example of another calcium channel in the cell?
What 2 things is homeostasis of calcium concentration important for?
What 2 things can dysregulation of calcium homeostasis cause?
- The Sarcoplasmic reticulum is a specialized form of endoplasmic reticulum that is found in most cells
- The sarcoplasmic reticulum acts as a calcium storage in muscle cells
- On its surface, there are Ryanodine receptor and IP3 calcium channels
- Outside the cell Ca2+ concentration is 1mM, in the cytoplasm it is 100nM, inside the SR it is 1mM (millimolar)
- There is also TPC2 calcium channel on the lysosome of the cell
- Homeostasis of calcium concentration is important for heartbeat and muscle contraction
- Dysregulation of homeostasis of calcium concentration can cause pathology, such as cardiac arrhythmias and malignant hyperthermia
Describe 8 the steps of the cardiac excitation-contraction coupling.
What occurs to return this cycle back to the start?
How can arrhythmias and heart failure be caused by this cardiac cycle?
- Cardiac action potential depolarised the plasma membrane
- This depolarisation activates voltage gated calcium channels on the plasma membrane
- Calcium flows down its concentration gradient from outside of the cardiac cell into the cell
- The calcium concentration is not high enough to cause a cardiac contraction by itself
- Calcium binds to Ryanodine 2 Receptor (RYR2 most common in cardiac muscle) on the sarcoplasmic reticulum as a ligand
- Through a process called calcium induced calcium release, the calcium activates the cardiac ryanodine receptor
- This causes calcium to flow down its concentration gradient from the sarcoplasmic reticulum into the cytoplasm of the cell
- Calcium can then bind to contractive machinery and stimulate contraction
- When we want the heart cells to relax, calcium is taken back up into the sarcoplasmic reticulum stores via a Calcium ATPase called SERCA, or it can be extruded from the cell by a calcium exchanger
- This returns calcium concentration to resting levels
- If there is inappropriate release of calcium from the stores in the sarcoplasmic reticulum, this can lead to arrhythmias
- If these stores become depleted, this can cause heart failure, due to weakened contraction
What is Ryanodine Receptor (RyR)?
What is The RyR regulated by?
How can disease be caused by the RyR?
- The RyR is an example of a macromolecular complex
- It is regulated by many different binding components e.g kinases, phosphatases, binding proteins
- It is found in diseases, there is change in these binding partners e.g excess phosphorylation, absence or altered function of binding partners
- This leads to altered function of the Ryanodine Receptor, which causes disease
What are channelopathies?
What is CPVT?
What are 3 symptoms of CPVT?
Why is CPVT difficult to diagnose?
How many mutations are associated with CPVT?
- Channelopathies are disease that develop because of defects in ion channels, which are caused by either genetic or acquired factors
- CPVT (catecholaminergic polymorphic ventricular tachycardia) is a an inheritable autosomal dominant channelopathy associated with the RyR
- Symptoms of CPVT include:
- dizzy spells
- fainting episodes
- sudden cardiac death
- Arrhythmias can be caused by dysregulation of the RyR, which causes excess calcium to be released.
- CPVT can be difficult to diagnose as arrhythmias displayed by CPVT is characterised as ventricular arrhythmia that only occurs during times of stress or exercise, which is when adrenaline is released.
- There are 69-point mutations identified in the RyR2 gene when patients were screened for CPVT1
Describe the 4 steps of the skeletal muscle excitation-contraction coupling
- Ryanodine receptor 1 (RyR1 most common in skeletal muscles) is activated by a mechanical coupling between the voltage gated calcium channel LTCC and the Ryanodine Receptor
- This results in a conformational shape change in the ryanodine receptor, which causes it to become active and release calcium
- Calcium flows down its concentration gradient from the sarcoplasmic reticulum into the cytoplasm of the skeletal muscle cell
- Here it can bind to contractive machinery and cause contraction
What is malignant hypothermia associated with?
What type of disorder is it?
What does it lead to a sever reaction to?
Where does it first manifest?
What is the underlying mechanism of MH?
What are 4 symptoms of MH?
What is administered prior to surgery to treat MH?
- Malignant hyperthermia is associated with Ryanodine receptor type 1 channel mutations
- Malignant hyperthermia is a pharmacogenetic disorder of the skeletal muscle
- MH leads to a severe reaction to commonly used anaesthetics and depolarising muscle relaxants
- MH first manifests in the operating room, and can be fatal if untreated
- Underlying mechanisms of MH are point mutations in the gene coding for RyR1, which causes the receptor to become very active and release sodium into the cell in response to anaesthetic
- Symptoms of MH:
- Muscle rigidity
- High fever
- Increased acid levels in blood and other tissues
- Rapid heart rate
- A postsynaptic muscle relaxant called dantrolene is administered prior to surgery, which inhibits ryanodine receptors and brings calcium levels back down to normal
