Cell homeostasis - Na and Ca (flipped)

Question 1

What is the value of Extracellular and Intracellular Na+ under normal conditions? (2)

Answer 1

Extracellular [Na+] = 145 mM
Intracellular [Na+] = 15 mM

Question 2

Why is it important to keep intracellular Na low?

Answer 2

To maintain gradient for sodium entry

Question 3

What is the function of the sodium potassium pump?

Answer 3

keeps intracellular Na+ low

Question 4

What is the function of NKCC (takes in Na+, K+, 2Cl-)?

Answer 4

Reabsorption of NaCl

Question 5

What happens to NKCC if intracellular Na+ is increased?
What does this mean for body?

Answer 5

-If intracellular Na is increased, then driving force for NKCC (Na+) is less so less NaCl reabsorbed

-Means loss of water in urine and more NaCl

Question 6

Describe does the NNKCC take in?

How does this effect Na+ and K+? (2)

Answer 6

-Takes in Na+, K+, 2Cl-

-Na+ leaves via sodium pump
-K+ is recycled across apical membrane to allow maximum uptake of sodium chloride

Question 7

Why is it important to regulate intracellular Na+ in terms of Action Potentials? (2)

Answer 7

-Decrease in inward chemical gradient
-Slower conduction of action potential

Question 8

What is Sodium potassium ATPase (Sodium Potassium pump) made up of?

Answer 8

Made up of A and B subunits

Question 9

What does Sodium potassium ATPase (Sodium Potassium pump) use to work?

Answer 9

ATP

Question 10

Describe the steps of Na+/K+ pump action? (8)

Answer 10

1 – No iron bound to pump, ATP bound to protein, gate closed, and no ions bounds

2- Na ions from intracellular domain go into pore and bind to sodium binding regions (3 of them)

3- Hydrolysis of ATP, pump is phosphorylated, Causes conformational change, closes to intracellular domain

4 – Further conformational change, pore opens to extracellular domain, which causes affinity of sodium to decreases, sodium leaves binding site and goes to extracellular domain

5 – Then change that increases affinity for potassium ions

6 – Potassium ions go from extracellular domain and bind to potassium binding sites

7 – This causes dephosphorylation, leads to conformational change closing outer gate and trapping potassium

8– Next molecule of ATP goes and binds to binding pocket, leads to conformational change which opens gate on inside of pore and decreases affinity of potassium binding and potassium is released into intracellular domain

(Circular mechanism, can go through another cycle now)

Question 11

What is the Na+/K+ pump inhibited by?

Answer 11

The cardiac glycosides – ouabain and digoxin
(e.g at step 5 Ouabain can causes blockage and inhibit action of pump)

Question 12

What is transport rate limited by in Na+/K+ pump?

Answer 12

ATP

Question 13

What are the roles of the Na+/K+ Pump in the membrane potential? (2)

Answer 13

-Electrogenic transport of 3 +ve charges out, two +ve charges in – net effect is to make inside of the cell more negative

-More importantly – accumulation of K+ inside the cell creates the driving force for K+ to leave the cell through K+ channels – making the cell negative

Question 14

What is sodium gradient needed for?

Answer 14

Gradient is used for physiological process rather than recycling sodium

(passive entry of Na+ into cell is greatly favoured)

Question 15

What is the Extracellular and Intracellular Ca2+ under normal conditions?

Answer 15

Extracellular Ca2+ ~ 1mM = 1000000nM
Intracellular Ca2+ ~ 100nM

(very large gradient)

Question 16

Why is Ca regulation important?

Answer 16

Ca2+ is an important second messenger involved in many signalling pathways

Question 17

How is intracellular Ca2+ kept low? (despite favourable conditions for Ca2+ entry) (2)

Answer 17

2 main mechanisms
Na+/Ca2+ exchanger
Ca2+ ATPase

Question 18

What gene family is the Na+/Ca2+ exchanger a part of?

Answer 18

SLC8 gene family
(in mammals NCX(1-3))

Question 19

How does the Na+/Ca2+ exchanger keep Ca2+ so low? (3)

Answer 19

The Na+/Ca2+ exchanger is electrogenic

-Stoichiometry is 3Na+:1Ca2+
-Affect of tenfold gradient is cubed due to stoichiometry 3:1

-This means the effect of Na+ gradient is magnified
-Creates 10000-fold conc gradient for calcium moving form outside to in

Question 20

What ATPase family are Ca2+ ATPases a part of?

Answer 20

Members of the P-type ATPase family – also includes the Na+/K+ pump

Question 21

What are the 3 types of Ca2+ ATPases (Ca2+ pumps)? (3)

Answer 21

-SERCA – Ca pumps found on the sarcoplasmic (SR) or endoplasmic (ER) reticulum membranes
-Act to pump Ca2+ out of the cytoplasm into the organelles which act as calcium stores

-PMCA – plasma membrane calcium pumps
-Act to pump Ca2+ across the plasma membrane out of the cell

-SPCA – Ca2+ pumps found on the Golgi Apparatus
-Also transport Mn2+

Question 22

What Is the role of PMCA?

Answer 22

-Increases in Ca2+i – activation of Calmodulin.

-Removal of autoinhibition and activation of PMCA.

-At resting levels is the major mechanism for controlling Ca2+i

Question 23

What is the role of NA+/Ca2+ exchanger?

Answer 23

-Major role when Ca2+i rises above resting levels

-Important when there are large influxes of Ca2+

Question 24

Describe the two classes of calcium channels in store membranes? (4)

Answer 24

I-P3 receptors
Channel is activated following binding of IP3. This type of receptor is expressed in most cell types.

-Ryanodine receptors
Low concentrations of ryanodine activate the channel, higher concentrations inhibit. The channel is also stimulated by caffeine. The natural activator is cADP ribose. These channels tend be found in excitable cells.

Question 25

Describe the mechanism of calcium replenished via store operated calcium channels?

Answer 25

These channels involved in refilling stores in calcium channels
PLC breaks down PIP2 into DAG and IP3
IP3 activates IP3 receptors in store membranes, opens calcium channels so calcium form store leaves calcium channels
Decreases in calcium in store causes replenishing of calcium

Question 26

Describe how calcium is replenished? (6)

Answer 26

(When calcium stores are depleted)
-Calcium no longer binds to STIM1, so this activates STIM and forms larger complexes
-Fully activated STIM1 oligomer can bind and interact with the Orai1 channel
-Activates Orai1 channel so calcium can then move across from extracellular domain into cytoplasm
-STIM1 complex also inhibits PMCA slightly and SERCA pumps are stimulated
-This favors entry of calcium into plasma membrane and inhibiting exit pathway, we favor replenishing of calcium
-As calcium increases, process is reversed and complexes break apart, normal processes resumed