Regulation of Intracellular Calcium Levels

Question 1

What is the extracellular level of ca2+?

Answer 1

1 x 10-3 M (1 mM)

Question 2

What is the cytoplasmic level of ca2+?

Answer 2

100 nM or 1 x 10-7 M or 0.1 microM

Question 3

What is the difference between extracellular and cytoplasmic levels of ca2+?

Answer 3

Difference of 10,000

Question 4

What are the levels of ca2+ in the ER/SR?

Answer 4

2 x 10-4 M

Question 5

How are increases in cytoplasmic Ca2+ concentration mediated?

Answer 5

By movement of Ca2+ across the plasma membrane and release of Ca2+ from the ER/SR

Question 6

How are increases in cytoplasmic Ca2+ concentration opposed?

Answer 6

Relative impermeability of the plasma membrane to Ca2+, pumps and transporters that move Ca2+ out of the cytoplasm and Ca2+ buffer proteins (Ca2+ binding proteins)

Question 7

What receptors/pumps/transporters on the plasma membrane let calcium into the cell?

Answer 7

VGCC – lets calcium in when cell reaches a certain voltage
LGCC – lets calcium in when ligand (eg a hormone) binds to the receptor and opens it
NCX – (can work both ways) – 3 sodium in for 1 calcium out
SOC - comparatively very slow - importnt in accessing extracellular Ca2+ when the SER stores for Ca2 are depleted

Question 8

What receptors/pumps/transporters on the plasma membrane let calcium out of the cell?

Answer 8

Plasma Membrane Ca2+ ATPase (PMCA) – uses ATP to pump calcium against its concentration gradient out of the cell
NCX – (can work both ways) – 3 sodium out for 1 calcium in

Question 9

What receptors/pumps/transporters on the ER/SR decrease cytosolic Ca2+ levels?

Answer 9

S/ER Ca2+ ATPase (SERCA) - uses ATP to pump calcium into SR/ER

Question 10

What receptors/pumps/transporters on the ER/SR increase cytosolic Ca2+ levels?

Answer 10

Calcium induced calcium release (CICR) - Ryanodine receptors

IP3 receptors

Question 11

By how much can activation of IP3 R increase cytoplasmic concentration of calcium ions within a few seconds?

Answer 11

By 5-10 fold

Question 12

In which type of muscles are store operated channels important?

Answer 12

Smooth muscle where prolonged states of stable contraction are required

Question 13

Which channels are important in accessing extracellular Ca2+ when the SER stores for Ca2+ are depleted?

Answer 13

Store operated channels

Question 14

What are SOCs activated by?

Answer 14

Special Ca2+ sensing protein in the SER which detects when the Ca2+ concentration in the SER is low and interacts via close apposition to the SOC channel.

Question 15

How many ATP molecules does PMCA use to transfer one calcium ion out of the cytosol?

Answer 15

One

Question 16

How is the affinity of PMCA to calcium ions further optimised?

Answer 16

When it binds with Calmodulin, a cytoplasmic Ca2+ sensing protein

Question 17

What is the source of the energy used by NCX?

Answer 17

Does not use ATP directly, uses the electrochemical energy gradient provided by the large concentration of extracellular Na+ to exchange 3 Na+ in for every one Ca2+ pumped out

Question 18

Which is the primary pump of Ca2+ when levels are > 10 micromolar?

Answer 18

NCX

Question 19

Where is NCX especially active?

Answer 19

In excitable tissues such as nerve and muscle where continuous large movements of Ca2+ underpin physiological activity

Question 20

When does the NCX act in reverse?

Answer 20

In heavily depolarised cells if the intracellular sodium concentration increases significantly. Moves 3 Na+ out for every one Ca2+ in.

Question 21

Which type of GPCR contributes to regulation of Ca2+ efflux from the intracellular stores?

Answer 21

Gq

Question 22

What acts as the ligand on ryanodine receptors?

Answer 22

Ca2+

Question 23

Why are ryanodine receptors associated with calcium induced calcium release in smooth and cardiac muscle?

Answer 23

With depolarisation of the t-tubule, the VGCCs open and allow an influx of Ca2+. This Ca2+ then binds with the RyR which results in a very large synchronous outward low of SR Ca2+ into the sarcoplasm.

Question 24

Why are ryanodine receptors associated with calcium induced calcium release in skeletal muscle?

Answer 24

RyR still activated by increased concentration of intracellular calcium concentration however there is a structural modification whereby the T-tubule VGCCs are directly physically coupled to the RyR receptor. This channel coupling means that when the VGCCs open, the RyRs also open and release massive amounts of Ca2+ into the sarcoplasm.

Question 25

Why is the rate of diffusion slower than expected as Ca2+ moves through the cytosol?

Answer 25

This is because of the presence of buffer proteins that act to smooth out/damp down the very rapid entry of Ca2+ through the cell

Question 26

What are some examples of calcium buffers?

Answer 26

Parvalbumin, calbindin, calsequestrin, calreticulin (last 2 bind Ca2+ in the SR and SER respectively)

Question 27

How many calcium ions can calmodulin bind?

Answer 27

Up to 4

Question 28

What happens when calcium ions bind to calmodulin?

Answer 28

Induces a conformational change in calmodulin which enables it to interact with a wide range of proteins - important as these proteins are often unable to bind calcium themselves.

Question 29

What is one good example of the transducer activity of Calmodulin?

Answer 29

Its modulation of PMCA. When calmodulin binds with calcium it can bind with PMCA and increase its sensitivity to the intracellular concentration of calcium ions.

Question 30

By how much does the binding of calmodulin to PMCA increase its sensitivty?

Answer 30

By a factor of about 10 fold