M&R 5.2 Control of intracellular Ca2+ Flashcards
Basal Ca2+ levels inside the cell are ~10,000 fold lower than the outside. Name 2 advantages and 2 disavantages of this
Advantages
- Inward movement of little Ca2+ causes rapid changes in [Ca2+]i
- Little Ca2+ has to be removed to re-establish resting conditions
Disadvantages
- Energy expensive to maintain
- Any inability to deal with Ca2+ can easily lead to Ca2+ overload and cell death
Name 4 factors that maintaining the Ca2+ gradient relies on
- relative impermeability of the plasma membrane
- Ability to expel Ca2+ across membrane (using Ca2+ ATPase and Na+ - Ca2+ exchanger)
- Ca2+ buffers
- Intracellular Ca2+ stores (both rapidly and non-rapidly releasable)
What regulates the membrane permeability to Ca2+?
The open/closed state of ion channels
How does the Ca2+ ATPase extrude calcium from the cell?
[Ca2+]i increases
Ca2+ binds to calmodulin
Ca2+-calmodulin complex binds Ca2+-ATPase
Ca2+-ATPase removes Ca2+
The Ca2+-ATPase has ______ affinity and ______ capacity
High affinity (can bind Ca2+ even when conc is low)
Low capacity (can’t remove lots of Ca2+ this way - better for when near basal levels to get last bits out)
How does the Na+/Ca2+ exchanger extrude calcium from the cell?
Antiporter
Uses inward concentration gradient of Na+ to move 3Na+ into cell in exchange for 1 Ca2+ out
Works best at RMP (because inside cell is -ve so Na+ is moving down both its electrical gradient as well at its conc gradient )
Less active in a depolarised membrane (because Na is moving down its conc gradient but up its electrical gradient)
The Na+/Ca2+ exchanger has _______ affinity and ________ capacity
Low affinity
High capacity
(so more helpful for initial process of moving lots of Ca2+ out)
What do Ca2+ buffers do?
Bind to Ca2+ to limit its diffusion (Ca2+ diffuses more slowly than would otherwise be predicted)
Therefore can help to buffer any changes in Ca2+
Name some Ca2+ buffers
Calsequestrin
Calbindin
Parvalbumin
Calreticulin
How are Ca2+ trigger proteins different from Ca2+ buffer proteins? List some examples.
Buffer proteins bind Ca2+ to buffer changes in [Ca2+]
Trigger proteins are proteins which bind Ca2+ to alter their own activity, e.g:
synaptotagmin (Ca2+ sensor involved in NT release)
calmodulin (activates Ca2+ ATPase in plasma membrane)
troponin (in muscle)
[Ca2+]i is elevated by which 3 main routes?
- Ca2+ influx across membrane
- Ca2+ release from ‘rapidly-releasable’ stores
- -> GPCRs
- -> Ca2+ induced Ca2+ release (CICR) - Ca2+ release from non-rapidly releasable stores (mitochondria)
By which 2 mechanisms can Ca2+ cross the plasma membrane?
- Voltage-gated Ca2+ channels (VGCCs)
2. Ionotropic receptors (ligand gated ion channels with selectivity for Ca2+ ions)
Name some examples of ionotropic (ligand-gated) Ca2+ channels
NMDA/AMPA receptors for glutamate (glutamate is major excitatory NT in brain)
Some nAChRs (certain ones have a particular combination of subunits that make them permeable to Ca2+ too)
Where is the rapidly-releasable intracellular store of calcium?
The endoplasmic reticulum (or sarcoplasmic reticulum in muscle)
Describe Ca2+ storage in the ER/SER
Free Ca2+ in the ER/SER is greater than in the cytoplasm
Therefore an active uptake process is required (=SERCA - sarcoendoplasmic reticulum Ca2+ ATPase)
ER/SER can store lots of Ca2+ by binding it to proteins with low affinity but high capacity (e.g. calsequestrin - just allows the store to hold onto Ca2+ better than if it was free)
Ca2+ release can be regulated by ion channels (large outwards gradient so Ca2+ can rapidly enter cytoplasm)
Name 2 methods by which Ca2+ release from rapidly-releasable stores is regulated, and which receptors are involved in each.
- Via GPCRs (by activation of IP3 receptors)
2. Via calcium-induced calcium release [CICR] (by activation of ryanodine receptors)
Describe GPCR mediated release of Ca2+ from rapidly releasable stores
Activation of G alpha q (Gq) G-protein via GPCR
Gq activates phospholipase C (PLC)
PLC cleaves a membrane lipid into DAG & IP3
IP3 acts on the IP3 receptor (a ligand-gated ion channel on the ER)
Binding of IP3 causes channel to open & Ca2+ to rush out of store down its conc gradient
(NB: DAG and increased [Ca2+]i as a result of IP3 also both work to activate protein kinase C)
Describe release of Ca2+ from rapidly-releasable stores by calcium-induced calcium release (CICR)
Ca2+ enters cell from another source (e.g. VGCCs, ionotropic receptors, or from intracellular stores)
The Ca2+ activates Ryanodine receptors on the ER/SER
Conformational change - pore opens and Ca2+ rushes out into cytoplasm
Where does CICR have a particularly important physiological role?
In cardiac myocytes
Describe CICR in cardiac myocytes
T-tubule allows depolarisation to get deep into cell
Change in MP detected by VGCCs - open - Ca2+ rushes into cell down conc gradient
Ca2+ activates ryanodine receptors on the SER
= Ca2+ release from SER
In cardiac myocytes, what proportion of the Ca2+ entering the cytoplasm comes from outside the cell and what proportion comes from intracellular stores?
15% comes from outside the cell (via VGCCs)
85% comes from intracellular stores (via ryanodine receptor RyR)
What is the advantage of the majority of the calcium released into cardiac myocytes coming from intracellular stores?
It allows an explosive release of Ca2+ right next to the contractile machinery
This ensures a strong, co-ordinated contractile event
How is cytoplasmic Ca2+ reduced again following a depolarisation?
- As [Ca2+]i increases and repolarisation starts, the Na+/Ca2+ exchanger reverts to Ca2+ extrusion from cell, lowering [Ca2+]i
- SERCA pumps Ca2+ back into the SR in preparation for another release event
(most of what is released is recycled ready for the next beat]
How does Ca2+ influx alter the cardiac myocyte AP compared to other APs?
VGCCs activate and inactivate similarly to Na+ channels but much slower
This means that once VGNCs have already been inactivated, VGCCs are still open and allowing Ca2+ influx
This allows more prolonged depolarisation and causes the long plateau of the cardiac myocyte AP
What else has a small contribution to Ca2+ entry in the cardiac myocyte AP?
When the membrane depolarises, VGNCs open and Na+ enters the cell down its concentration gradient
The alteration of the Na+ gradient allows reversal of the Na+/Ca2+ exchanger (NAX), so that a small amount of Ca2+ enters rather than being extruded
Describe the functional role of Ca2+ in muscle contraction
Increased [Ca2+]i in cytoplasm of myocyte
Ca2+ binds to troponin, initiating a conformation change
This causes tropmyosin to move and reveal actin binding sites
Myosin head groups can now bind to actin binding sites
Myosin undergoes cycles of attachment and detachment (using ATP) and head group moves, so that there is shortening of the sarcomere
=contraction of myocyte
Where are the non-rapidly releasable intracellular stores of calcium?
In the mitochondria
How do mitochondria uptake Ca2+?
Via a uniporter
Uses the driving force from respiratory chain proton production
(has low affinity but high capacity, so is better when cytoplasmic Ca2+ conc is high)
Mitochondrial stores of Ca2+ seem to be most important in what type of cells?
Neurons
Name 3 roles of mitochondrial Ca2+ stores
- Ca2+ buffering (to regulate pattern/extent of Ca2+ signalling)
- Stimulation of mitochondrial metabolism (to match energy demand with supply)
- Cell death (mitochondrial Ca2+ overload is key in triggering cell death)
Ca2+ stores can be refilled by:
- Recycling of released Ca2+ from cytosol back to store (especially in cardiac myocytes - other cells find it easier to send Ca2+ out across plasma membrane before they refill)
- Capacitive Ca2+ entry (particularly in non-excitable cells)
Describe capacitive Ca2+ entry
Store-operated channel (SOC) located on plasma membrane
When ER is depleted of Ca2+ a ‘depleted’ signal is sent to SOC
Ca2+ enters the cell via SOC
Once in the cell, the Ca2+ can enter the ER via SERCA
