Potassium channels Flashcards
1
Q
What is the basic structure of a potassium channel?
A
- Four similar protein subunits come together to form a single por for potassium permeation
2
Q
Describe the nature, structure and role of “A-type” K+ channels
A
Nature of current:
- activated when membrane of the cell starts to depolarise
- after activation the current rapidly declines because channel inactivates quicky due to ball and chain mechnaism
- Some experiments now suggest that although the ball region interacts with the channel pore, it may induce a channel conformational change, rather than having a plugging mechanism
- In either case, residues forming part of the channel pore interact with the protein’s N-terminal region
- Peak of current gets bigger as potential inside cell increases as more channels open and there is also a greater force driving potassium out of the cell
Structure:
- each subunt contains six helical membrane-spanning regions (S1-6)
- Fourth transmembrane element contains positive charges –> voltage sensor
- Pore formed by S5 and S6 (P-loop connects them)
- P-loop contains signature sequence (TXGYGD) –> glycine residues always conserved but there are a number of variations in this sequence
Function
- Translates intensity to frequency
- At relatively negative potentials type A become activated (after repolarisation and closure of delayed rectifiers)
- i.e. they open in response to the stimulus before the cell reaches its action potential threshold :.exerts hyperpolarising effect
- effect transient due to rapid inactivation
- the bigger the stimulus the faster it overcomes the ‘A’ current and hence more frequent action potential
- ‘A’ type channels recover from their inactivation when the cell membrane potential becomes strongly hyperpolarised again (by delayed rectifier channels)
3
Q
Describe the nature, structure and function of delayed rectifiers
A
Nature
- activates more slowly than ‘A’ type channels and only inactivates after depolarisation after many seconds
Structure
- S1-S6 with P loop and voltage sensor
Function
- helps reset the membrane potential following an action potential
- current is always larger at positive potentials than at negative potentials
- there is a delay following membrane depolarisation before the current appears
4
Q
Describe the nature, structure and function of inward rectifiying channels (Kir)
A
Nature
- Increased with hyperpolarisation
- inward current is higher than outward current because the channel is being blocked by Mg and positively charged intracellular polyamines such as spermine and spermidine
- Strong rectifiers; will not affect AP much but can set rest potential
- Weak rectifiers; can set resting potential also decreases the length of an AP as more potassium can come out
Function
- help set resting potential
- some inhibited by ATP (KATP) and associated with a second type of protein called sulfonylurea (SURs)
- others coupled to G proteins and responsible for slowing heart rate (KACh)
- More recently: Kir KO in endothelial cell decreased endothelial dependent vasodilation
Structure
- Two membrane spanning elements (M1 and M2)
- Pore between them
5
Q
Describe the nature, structure and funcition of two pore potassium channnels
A
Nature:
- Leak type currents
- Did not activate or inactive with changing membrane potential
- Able to open at all potentials
- More current flows in the outward direction and recently this has been shown to be due to the ion permeation pathway itself
- Weakly outward rectifying
- Activated by pH changes and membrane stretch following cell volume changes
- Also activated by volatile anaesthetics
- TRESK –> mutated in inherited form of migraine
Structure
- Look like two inward rectifiers sewn together
- Four membrane spanning domains and two pore loops (M1-M2, M3-M4)
- There is also a cap region which forms the above channel pore so K+ ions have to enter under the cap from the sid
- Both P loops contribute to functional pore :.only two subunits needed to form this channel
6
Q
Describe the nature, structure and function of BKCa channels
A
Nature
- becomes more active as the membrane is depolarised or as intracellular calcium rises
- Often acts as a negative feedback element in reponse to raised intracellular calcium
- E.g. at certain synaptic terminals the channel is activated by the membrane depolarisation and subsequent influx of calcium which trigger NT release
- The activation of BKCa channels thus helps to repolarise the membrane and turn off calcium channels, ending a cycle of exocytosis
Structure
- Six TM
- Additional S0 element
- Very extended C region
- C region contains two RCK domains
- RCK2 has string of AA containining many negative charges forming a “calcium bowl” which cradles a calcium ion in the crystal structure of this region
7
Q
Describe the nature, structure and function of SKCa channels
A
Nature
- NOT activated by voltage (unlike BKCa)
- Start to open only in response to calcium
Structure
- 6 TM
- S4 NOT have all the positive charges associated with S4 region of voltage gated channels
Function
- Sometimes provide “after-hyperpolarisation” following influx of calcium that occurs during action potential
- NMDA receptor activity regulation and hence formation of memory –> Ca2+ influx through synaptic NMDA receptors can activate SK channels which then hyperpolarise the cell and thus restore Mg block of the NMDA receptor
- SK channels also appear to have very important roles in endothelial cells and in cardiac tissue (where they regulate blood pressure and cardiac rhythm)
8
Q
Describe the structure, function and nature of IKCa channels
A
Nature
- relative of SKCa (6TM with few charged residues in S4)
Function
- Causes reduction in RBC volume as potassium ions are lost from the cell along with chloride ions and water
9
Q
What is the calcium activating mechanism for IKCa and SKCa?
A
- Mediated by a calmodulin molecule constituvely bound to the intracellular C-terminal segment of the protein
- Sometimes referred as beta subunit
10
Q
Describe the nature, structure and function of sodium activaed potassium channels
A
Nature
- Slack and Slick form subunit for this channel
- Activated by increased intracellular levels of sodium
- Closest relative is BKCa
- Large conductance
Structure
- Two RCK domains at C terminal
- NO calcium bowl
- S1-S6
Function
- Protective role in event of ischaemia
- Intracellular sodium concentration increases as a result of inhibition of the Na/K ATPase
- Current produced tends to be inwardly rectifying
11
Q
A
12
Q
Name and describe KATP channel openers
A
- Diazoxide - hypertension
- Nicorandil - prevention and treatment of angina
- Glibenclamide - target KNDP in smooth muscle
- Cromakalim - target KNDP in smooth muscle - lowers blood pressure w/o big effect on blood glucose (unlike diazoxide)
- Minoxidil - hypertension –> potent, long-lasting vasodilator –> causes reflex tachycardia and fluid retention :. administered with beta blocker and diuretic :.used in patients with severe hypertension resistant to other drugs –> used in male pattern baldness
SE: headache
13
Q
Describe KATP channel blockers
A
- Glibenclamide and tolbutamide are SURs and act via the suplhonylurea receptor subunt of KATP channels
- Block pancreatic B-cell and produce similar effects of raised intracellular ATP
- Leads to increase in cell excitability and promotes secretionof insulin
- Treatment for mild forms of type 2 diabetes
14
Q
Describe some BKCa openers
A
- Soyasaponins activate BKCa channels
- Dehyrdrosoyasaponins I require presence of BKCa channel beta subunits for activity
- Leads to increased channel open probability
- Other BKCa openers: MaxiKdiol
- Developed with intention of treating conditions such as urinary incontinence, overactive bladder, asthma and stroke
- None have reached beyond phase III
15
Q
Describe some SKCa enhancers
A
In some rat models of epilepsy drugs that enhance the activity of SKCa channels have proved effective in stopping epileptic discharge patterns
- they do not open channels
- prolong/enhance activation by calcium
- appear to bind at the interface between calmodulin and the channels :. difficulty in getting a selective drug due to sequence similiarity of this region
Treatment: hypertension as modulation of SKCa expression level in mice affects both arterial tone and blood pressure
16
Q
Describe some SKCa blockers
A
- Firing rate of dopaminergic neurones controlled by SKCa channels
- SKCa channels blockers can enhance firing rate :.increase dopamine release –> useful in treatment for Parkinson’s
- SK channels are present in a number of neurones and blocking them could risk inducing epileptic behaviour
- Some SK channels blockers can prevent or reduce arrhythmic activity in the heart
- genome wide association studies have linked a type of atrial fibtrillation (lone atrial fibrillation) to SK channels
- SK channels also modulate artial tone and blood pressure –> SK based treatment for arrhytmia can lead to hypertension
17
Q
Describe some Eag channel blockers
A
- Kv10.1 channels –> expressued in human tumours e.g. colon, breast, renal, etc whilst healthy tissue show much reduced expression levels
- Blocking Eag channel :.lead to apoptosis
- Overexpression of Eag can lead to cancer progression while inhibition reduces progression
- Eag2 –> upregulated in medulloblastoma
- Eag channels are translocated to the trailing edge of tumour cells where they facilitate migration and metastasis
- Antipsychotic thioridazine: inhibits Eag2 and slows medulloblastoma growth and metatasisin in a mouse model
- In a single human study –> treatment with thioridazine reduced size of original brain tumour
- Adverse side effects
18
Q
Why are Kv1.3 channels important?
A
- along with IKCa control efflux of K+ from human T-lymphocytes
- overexpression of Kv1.3 or IK channels restored anti-tumour activity
- shown to increase survival of tumour bearing mice
- Kv1.3 –> mitochondria in cancer cells
- mitochondria are involved in apoptotic pathway
- mitochondria are negatively charged :. accumulate inhibitors of mitoKv1.3 (TPP+) due to +ve charge
- Inhibition of mitoKv1.3 has been shown to selectively kill cancerous cells that overexpress mitoKv1.3 :. sparing non-cancerous cells
19
Q
Describe M-current openers
A
- M channels belong to Kv family (7.x) and are encoded by KCNQ2/3 genes
- Flupirtine: M-channel opener :. used as anelgesia
- Retigabine: opener :. epilepsy and neuropathic pain
–> was evaluated for postherpic neuralgia
–> binds to site formed at interface beween M-channel subunits –>acts to stabilise the open-channel conformation
20
Q
Describe M current blockers
A
- Cognitive enhancers that may act via inhibition of potassim channels
- In rodent lesion models of Alzheimer’s disease these drugs improve performance in animal water maze tests
- In vitro - increase NT release
- May interact with M-channel blockers
21
Q
What can be used to treat multiple sclerosis?
A
- Dalfampridine reported sig. improvements in ability of patients suffering MS to walk
- active ingredients: 4-aminopyridine (4-AP)
- Blocks many potassium channels
22
Q
Class III antidysrhymtics (hERG blockers)
A
- K+ channels subtypes expressed in cardiac muscle
- Together with Na+ and Ca2+ channels determine heart rate and AP shape
- Methanesulphonanilide group of III:
- block Kv11.1 channels (HERG)
- leads to AP prolongation, an increased refractory period and to a reduced likelihood of re-entrant arrhytmias
- Increase in AP by allowin more entry of more calcium can also have usefu inotropic action
- SE: cardiac toxicity of antihistamines terfenidine/astemizole has been attributed to block of thee cehhanl
- Block of inward rectifier channels –> important in controlling resting potential could play a part in antidysrhytmic action of class III drugs
- Proposed that K+ channel openers (acting on cardiac KATP) might have a useful role in cardiac dysrhytmia –> reduce AP duration :. treat long QT syndrome
- In other cases, potassium channel openers might be expected to be pro-arrhytmicy by reducing refractor period
23
Q
Immunomodulation
A
- Human T cells express Kv1.2 potassium channel
- Mice blockers of this channel inhibit T cell proliferation
- However mouse T cell may rely on a different repertoire of K+ channels
24
Q
Obesity, insulin resistance and more on pain
A
- Sh6K-186, a blocker of Kv1.3 channels –> reduced weight gain and adiposity, decreased blood levels of cholesterol –> also enhances peripheral insulin sensivity
- Changes are similar to effects seen when knocking out Kv1.3 gene
- However treatment of obesity with this type of blocker in man could have immunosupressive effects
25
Q
K2P channel openers
A
- Volatile anaesthetics activate members of the two pore potassium channel family
- Halothane is a potent activator of the TREK1 and TASK1 potassium channel while other more unusual anaesthetics such as xenon, NOS and cyclopropane activate only TREK1
- TREK1 KO mice challenged with some anaesthetic agents have been shown to have reduced senstivity (while there is no change in sensitivty to IV)
- TRESK mutation produced reduced channel activity and linked with familial form of migraina with aura
- suggests the possibility that activation of TRESK channels could be beneficial for migraine sufferes
- Loss of function mutations in TASK1 channel have been shown to cause human pulmonary hypertension
26
Q
K2P channel blockers
A
- Transgenic mice lacking gene responsible for TREK1 channels show depression resistant phenotype similar to mice treated with Prozac
- Fluoxetine –> SSRI –> blocks TREK1 channel but not at clinically relevant ranges
- TREK1 involved in suppressing the firing in serotonergic neurons, block or elimination of TREK1 current would upregulate serotonin release
27
Q
Sodium activated potassium channels
A
- High expression levels seen in sensory pain neurones so targeting these channels may be useful
