Week 1 - Channels and Transporters Flashcards
(36 cards)
what is the voltage patch-clamp method? what does it tell us about channels and gating?
detects currents flowing through single membrane channels due to depolarization
- separate Na+ and K+ channels
- voltage sensors in channels
- high conductance of channels to specific ions
- while channels open and close in all-or-none fashion with a fast switch between open/closed states in stochastic manner, gatings transition between open and closed states and involve temporary conformational change in channel’s structure
macroscopic and microscopic methods
macroscopic (voltage-clamp) currents: due to flow through many channels (whole-cell recording; strong pulse of suction so cytoplasm is continues with pipette interior; the pipette is still attached to the membrane)
microscopic (patch clamp) currents: due to current flow through one channel (inside-out recording; exposed to air and cytoplasmic domain is accessible within the pipette)
patch clamp measurements of ionic currents through single Na+ channels, both macroscopically and microscopically
depolarization increases probability of a channel being open, and hyperpolarization decreases it
- K+ channels were blocked via TEA
- comparing the macro and micro, it showed that the sum of many trials microscopically was correlated with the macroscopic trial
sustained response for K+ channels
on average, K+ channels tend to be an open state while the membrane is depolarized
K+ compared to Na+ channels
K+ are in the opposite direction, with longer latency for activation and long duration of activation
summary of patch clamp (microscopic) and voltage-clamp (macroscopic) Na+ and K+ channels
Na+ opening is voltage-dependent, near beginning of depolarization pulse
- they inactivate, current reverses at E Na
- blocked by TTX
K+ opening is voltage-dependent, and open later
- many don’t inactivate, but merely close
- blocked by TEA or Cs+
what do multiple potassium channel types do?
add diversity
- most CNS neurons have multiple K+ channels with different characteristics
- voltage dependence of activation (low-voltage VS high voltage activation)
- rate of activation (how fast population reaches max conductance)
- inactivation properties (creates diversity of spike waveforms and spike patterns for different cells)
functional roles of fast after hyperpolarization
2-5 ms; shortens AP by quickly repolarizing membrane
- only affects early spike frequency at very high frequencies
- big K+ channels activation by Ca++ and depolarization, then rapid inactivation
functional roles of medium after hyperpolarization
10-100 ms; controls early interspike interval
- contributes to early spike-frequency adaptation by slowly activating Ca++ entry
- controls late spike-frequency adaptation
- intermediate and small K+ channels are non-inactivating
functional roles of slow after hyperpolarization
100-3000 ms; limits firing frequency by unknown channel
channel timings in neurons
- Na+ channels open
- Na+ channels inactivate, Ka+ and Kdr+ channels open
- Kbk+ channels open
- Ca++ channels open
- other known and unknown K+ channels open
how many types of ions pass through voltage gated and ligand gated channels?
voltage gated channels allow only a single type of ion to pass through the channel, though there are exceptions
ligand gated ion channels usually allow 2+ types of ions to pass through the channel
how are neurons with diverse electrical properties created?
large numbers of ion channel genes
- 10+ for Na+ and Ca++ channels, 100+ for K+
- splicing variations produce different characteristics
how are ionic channels organized?
based on sequence homology
-voltage-dependent ion channels differ in cellular expression and subcellular localization impacting their relative contribution to brain function
what do Kv4.1 channels do?
play a positive role in tumorigenic human mammary cells
difference between Kv1.4 and Kv2.1
- 4 - axons; in terminal fields of medial perforant path in middle molecular layer of dentate gyrus, and mossy fiber axons and terminals
- 1 - soma and proximal dendrites; most prominent in pryamidal cell CA-1 layer
why are there so many genes encoding K+ channels?
so the channels can differ in: -activation -gating -inactivation and shape complex electrical responses -influence duration of AP and resting membrane potential
relationship between Kv2.1, Kv4.1, inward rectifier channels, and Ca++ activated K+ channels
Kv2.1 - little inactivation, and are related to channels involving in repolarization
Kv4.1 - inactivate rapidly to depolarization
IRC - allow more current flow during hyperpolarization than during depolarization
CAKC - open in response to increased intracellular Ca++ and sometimes to membrane depolarization
summary of ion channel properties
- encoded by large and diverse families of homologous genes
- differ widely in cellular expression and subcellular localization
- different voltage-gated channels differ in functional properties (activation, inactivation, gating)
- contribute to complex (rich) electrical responses
- their diversity is key to developing new therapeutics for CNS disorders
what are channelopathies?
genetic diseases due to mutations in channel genes
what are channelopathies from mutations in voltage-gated Ca++ channels?
- congenital stationary night blindness
- familial hemiplegic migrane
- episodic ataxia type 2
what are channelopathies from mutations in Na+ channel?
generalized epilepsy with febrile seizures
what are channelopathies from mutations in K+ channels?
benign familial neonatal convulsion
what are ion channel target sites for toxins?
extracellular domains and pore regions
