Session 5 - Control of cytosolic Ca Flashcards
What does establishing and maintaining the Ca gradient rely on?
- Relative impermeability of the PM
- Dependant upon the cells ability to expel Ca2+
What determines a cells impermeability to Ca?
-Open/closed state of ion channels
What is PMCA?
-A primary active transporter in cell membranes which uses ATP hydrolysis to expel Ca
What feedback system does PMCA work on?
-[Ca]i increases ->Ca binds to CaM-> Ca-CaM binds to PMCA-> Ca removed
What is the affinity and capacity of PMCA?
- High affinity
- Low capacity
- Removes residual Ca
What is NCX?
- Na-Ca-exchanger
- Secondary active transporter in plasma membrane which uses the Na concentration gradient to drive Ca out of the cell
- 3Na in for every Ca out (antiport)
Describe the affinity and capacity of NCX
-Low affinity
-High capacity
Main remover of high [Ca]i
What are Ca buffers?
-Buffers in the body contain Ca-binding proteins which cause Ca to diffuse more slowly across PM by binding to Ca and preventing it diffusing across
What are the 4 main factors of cells which provide the ability to control Ca?
- PMCA
- NCX
- Ca buffers
- Intracellular stores
In what two ways can Ca influx across the PM?
- Voltage-gated Ca channels
- Ligand-gated Ca channels
What is SERCA?
-Primary active transporter which uses energy from ATP hydrolysis to pump Ca against its concentration gradient into SR when [Ca]i are high
Describe the affinity and capacity of SERCA
- High affinity
- Low capacity
How is Ca released from the SR?
- Via IP3 ligand-gated Ca channels
- Via Ryanodine receptors (CIRC)
Describe how Ca is released from SR via a1 in vascular smooth muscle cells
- NA binds to a1- adrenoreceptors, Gaq dissociates and activates PLC
- PLC cleaves PIP2 to IP3 and DAG
- DAG regulates PKC
- IP3 binds to IP3R on SR, opens the ion channel
- Ca influx into cell
Describe Gas pathway
- Gas dissociates and stimulates AC
- AC converts ATP to cAMP
- cAMP increases the activation of PKA
Describe Gai pathway
- Gai dissociates and inhibits AC
- cAMP decreases
- Decreased activation of PKA
What do Gbg subunits do?
-Open/close ion channels
Describe Ca-induced Ca-release
- Intracellular Ca rises
- Ca binds to ryanodine receptors on SR
- Ligand-gated Ca channel opens rapidly releasing Ca into cell
Where is the main site of CIRC? Why?
-Cardiac myocytes in response to Ca entry through voltage-gated Ca channels to maintain plateau by releasing intracellular calcium -> contraction
When does NCX reverse directions?
- During depolarisation as intracellular Na becomes high
- 3Na out 1Ca in
What is the minor contributor of Ca handling in cardiac myocytes?
-NCX
What is the major contributor to Ca handling in cardiac myocytes?
-SERCA
Where are non-rapidly releasing stores of Ca within cells?
-Mitochondria
When will mitochondria take up Ca?
-When [Ca]i is high
How do mitochondria participate in normal Ca signalling?
-They are in a cellular arrangement where they will be exposed to high levels of Ca
Describe mitochondrial uptake of Ca
-Uniporter which is low affinity and high capacity
How is [Ca]i returned to normal?
- Ca removal
- Intracellular store refil
How are intracellular stores of Ca refilled?
- SERCA and mitochondria to recycle cytosolic Ca
- In excitable cells there can be voltage-gates Ca2+ sequestering whereby voltage-gated channels open and the Ca influx is sequestered
- Capacitative Ca2+ entry
What is capacitative Ca2+ entry?
- Capacitative/store operated channels
- SR signals depletion of Ca and SOC open to allow Ca2+ utake
- Regulated by specific proteins STIM (an ER located Ca2+ sensor) and ORAI( a PM channel) which interact to activate SOC
Why is it important to have mechanisms to move Ca in and out of cells?
- Ca cannot be metabolised and high doses are toxic
- Ca needs to be at a low concentration within cells so there is a large concentration gradient -> movement of small amounts of Ca cause big changes in Ca intracellulaar concentration
Describe Gaq pathway
- Gaq activates PLC
- PLC cleaves PIP2 into IP3 and DAG
- IP3 acts on IP3R and is a second messenger
- DAG activates PKC
