Cell homeostasis Flashcards
What gradient is important to drive secondary active processes in the cell?
The Na+ gradient (low intracellular and high extracellular
What is the concentration of Na+ INSIDE the cell?
15mM
What is the concentration of Na+ OUTSIDE the cell?
150mM
Which transport protein keeps intracellular Na+ low?
Na/K ATPase (pump)
Describe the apical and basolateral membranes of the epithelial cell in the thick ascending limb
Basolateral membrane:
1) Na/K ATPase keeps intracellular Na low
2) K+ channel
3) CLCK (Cl- out)
Apical membrane:
1) NKCC (Na, K, Clx2 in)
2) ROMK (K out)
In the thick ascending limb, what produces the driving force for NKCC?
Na/K ATPase - keeping intracellular Na+ low
ROMK - recycles out K+
In the thick ascending limb, what happens if [Na+] is increased inside the cell?
1) NaCl reabsorption inhibited as NCKK can’t work
2) Osmotic gradient across the thick ascending limb decreases
3) Less water absorbed (follows Na)
4) Increased salt wasting and diuresis
What is Ena?
+60mV
What happens, with regards to the action potential, if [Na+] is increased INTRACELLULARILY?
1) Ena becomes ~30mV
2) Reduced electrochemical gradient and driving force for Na+ into the cell
3) Take LONGER for the action potential to develop and the PEAK will not be as high
4) Slower conduction of the action potential and problems with propagation
What is the extrusion of 3Na and uptake of 2K by Na/K ATPase coupled to?
The hydrolysis of ONE ATP
What is the model of Na/K pump action?
1) Na+ binds to cleft on the INTERNAL side of the pump
2) Hydrolysis ATP –> ADP
3) Phosphorylation of the pump - causing a conformational change
4) Binding cleft exposed to the OUTSIDE of the cell
5) Na+ leaves and K+ binds
6) Pump loses phosphate group - returns to its original confirmation
7) Cleft exposed to INTERNAL environment
8) K+ leaves cleft and Na+ bind
What does the rate of transport of the Na/K ATPase pump depend upon?
1) SATURATION - rate is saturable by [Na]i and [K]o
2) Metabolic rate of the cell - rate is saturable by [ATP]
What inhibits the Na/K pump?
Cardiac glycosides:
- Ouabain
- Digoxin
How does Na/K maintain a the inside of the cell to be NEGATIVE?
Which factor plays more of an importance?
1) Electrogenic transport: 3+ out, 2+ in
- Makes inside more negative
2) Accumulation of K+ inside the cell (by Na/K ATPase transporting them in)
- Driving force for K+ to leave through OTHER channels
- Make intercellular more negative
- MAJOR FACTOR
What allows directional transport in the collecting duct?
Amiloride sensitive channels (NHE1) are on the APICAL membrane
Na/K ATPase is on the basolateral membrane
What produces depolarisation in excitable cells?
What is this recycled by?
Na+ entry
Recycled by the Na/K ATPase
What does Na+ drive in the kidney?
The REABSORPTION of many OTHER ions
What is the concentration of INTRACELLULAR Ca2+?
100nM
What is the concentration of EXTRACELLULAR Ca2+
1mM (1,000,000nM)
What is the difference between intracellular and extracellular concentration of Ca2+?
10,000 fold difference
Extracellular concentration in 10,000 more than intracellular concentration
What is Eca?
+120mV
Double that of Na and +61mV
Why is Ca2+ important?
SECONDARY messenger in many signalling pathways
What does Ca2+ release from stores cause in the pancreatic acinar cell?
Fusion of vesicles to the membrane and release of enzymes
What 2 mechanisms help to keep Ca2+ concentration low?
1) Na/Ca Exchanger
2) Ca2+ ATPase
What does the Na/Ca exchanger do normally?
How can this differ?
Exchange 3 extracellular Na for 1 intracellular C (Na in, Ca out)
Differs depending on the gradients set up - eg. this is reversed in the heart (Na out Ca in)
How can a 10 fold Na+ gradient keep Ca2+ in the cell so low when the Ca2+ gradient it 10,000 fold?
- 3 Na+ for every 1 Ca+
- Means the effect of the Na+ gradient is CUBED - giving a 1,000 fold Na+ gradient
- Membrane potential gives another 10-fold Na+ gradient
- 1,000 x 10 = 10,000
- Matches the 10,000 fold Ca+ gradient
What family is the Na/Ca exchanger part of?
SLC8 family
CaCA SUPERFAMILY
How many forms of the Na/Ca exchanger exist in mammals and what are they called?
Three:
NCX1
NCX2
NCX3
What family is the Ca pump a family of?
What transport protein is also in this pump?
P-type ATPase
Na/K pump is also in this family
What do ALL the Ca pumps (Ca ATPase) act to do?
REMOVE Ca2+ from the cytoplasm
How many of the 3 Ca pumps can be present in a given cell?
3
What are the 3 types of Ca pump?
Where are they found?
1) PMCA (plasma membrane)
2) SERCA (smooth endoplasmic reticulum or sarcoplasm)
3) SPCA (golgi apparatus)
What does SERCA act to do?
Pump Ca2+ OUT of the CYTOPLASM
Into organelles which act as calcium STORES
What do calcium stores do?
Store Ca2+ and release it into the cell when there is a trigger event
What does PMCA act to do?
Pump Ca2+ OUT of the cytoplasm across the cell wall
Which Ca pump can also transport Mn (manganese)?
SPCA
What are the 4 plasma membrane pathways for Ca2+?
1) VOCC (voltage operated calcium channels)
2) ROCC (receptor operated calcium channels)
3) Mechanically activated Ca+ channels
4) SOCC (store operated calcium channels)
What activates VOCC?
Depolarisation of the membrane
What activates ROCC?
Binding of an agonist
Where are VOCC found?
In excitable cells
Where are ROCC found?
In nerve terminals and secretory cells
What activates mechanically activated Ca2+ channels?
Deformation of the membrane
Where are mechanically activated Ca2+ channels found?
In MANY cells
Where are SOCC found?
In the CELL membrane
What activates SOCC?
Depletion of Ca+ stores in the cytoplasm
What do SOCC act to do?
Replenish the Ca+ stores in the cytoplasm
What RECEPTORS are present in Ca2+ store membranes?
Which is the most common?
Where is the other receptor present?
1) IP3 receptors
- Most common
2) Ryanodine receptors
- Excitable cells
What is the function of the IP3 and Ryanodine receptors present in Ca2+ store membranes?
To get Ca2+ from the stores into the cytoplasm
Happens in response to a signal
What activates the IP3 channel in the store membrane?
Binding of IP3
What activates the Ryanodine channel in the store membrane?
What is the NATURAL activator
- LOW concentrations of Ryanodine
- Caffeine
- cADP ribose (natural activator)
What inhibits the Ryanodine channel in the store membrane?
HIGH concentrations of Ryanodine
What happens when PLC is activated by a receptor?
1) Splits PIP2 into IP3 and DAG
2) IP3 binds to IP3 receptors on the Ca2+ store membrane
3) Ca2+ is released from the store into the cytoplasm - emptying the store
What occurs when the Ca2+ store is empty?
1) Signalling between the empty store and SOCC in the cell membrane
2) STIM Ca2+ binding state changes - STIM undergoes conformational change (dimer –> multimeric)
3) Further conformational change allows the multimeric STIM to tether and bind to the PM membrane
4) STIM interacts with Orai Ca2+ channels (SOCC) in the PM membrane
5) Orai channels open and Ca2+ move into the store
6) Complex feedback prevents Ca2+ from leaving the store
7) Once store is replenished - signalling stops
What is STIM?
A transmembrane spanning domain
Spans the SER (Ca2+ store)
Normally binds Ca2+ inside of the store
Where are Orai Ca2+ channels present?
In the plasma membrane
How does an empty store communicate with SOCC in the membrane?
Thought to be through tethering
What is the function of the Orai Ca2+ channel?
To allow Ca2+ INTO the store - to replenish it
It is a SOCC
Where is Orai Ca2+ channel present?
In the PLASMA membrane
