Lec 7 Excitation-Contraction Coupling

Question 1

What is CICR?

Answer 1

Calcium Induced Calcium Release

Question 2

What does RyRs stand for? What are they?

Answer 2

• Ryanodine receptors
• ligand-gated channel
• on SR membrane
• activated by binding Ca on cytosolic side
• in response to activation, release Ca from SR into cytosol

Question 3

What is LTCC?

Answer 3

L-type Ca channel

Question 4

What are rest concentrations of Ca in extracellular, cytosol, and SR?

Answer 4

extracellular: 2 mM [10x-3]
cytosol: 100nM [10x-7]
SR: 1mM[10x-3]

Question 5

What is magnitude of concentration difference Ca between SR and cytosol?

Answer 5

10,000 fold

Question 6

Experiment to test that contraction requires CICR? [3 experimental conditions and outcomes]

Answer 6

1. control conditions
   - - have Ca entry from extracellular and CICR
   - - see huge increase Ca in systole
2. wash on caffeine
   - - caffeine depletes SR Ca so no CICR
   - - do have Ca entry from extracellular
   - - only see small increase Ca in systole
3. Replace extracellular solution with Ba2+
   - - no extracellular Ca so no Ca influx to cell
   - - theoretical possibility of Ca release from SR
   - - in reality so no change in Ca in systole

Question 7

5 steps in EC coupling

Answer 7

1. membrane depolarization opens L type Ca channel in cell membrane
2. Ca enters through L type Ca channels
3. Ca binds Ryanodine receptors in Sr membrane
4. Ryanodine receptors release 3-4x more Ca than entered cell
5. Overall increase Ca initiates contraction pas binds myofilaments]

Question 8

How is calcium recycled in EC coupling?

Answer 8

• the Ca that came in from extracellular leaves by NCX

- the Ca that came from SR is pumped back in via SERCA pump [an ATPase]

Question 9

Key difference between EC coupling in skeletal and cardiac muscle?

Answer 9

In skeletal:

• • Nicotinic Ach receptor initiates AP, AP activates L-type Ca channels [DHPRs]
• • L type Ca channels physically connected to RyRs so as soon as L type Ca channel is activated, RyRs starts to release Ca
• —- Ca doesn’t actually need to enter cell for this to happen

in cardiac:

• L type Ca channels not physically linked to RyRs
• Need Ca to enter through Ca channels in order to open RyRs

Question 10

In what types of action potentials does Ca need to enter cell in order to get muscle contraction? In which does it not need to enter cell?

Answer 10

• in cardiac muscle Ca needs to enter cell to get RyRs release of Ca from SR
• in skeletal muscle Ca does not need to enter cell because L type Ca channel is physically linked to RyRs

Question 11

How is sub-cellular structure organized to maximize CICR?

Answer 11

• T tubules = extension of membrane that penetrate into cell
• RyRs clustered next to T tubule membrane
• means you can get Ca taken up everywhere and not as much delay [all cell locations no more than 1 um from cell membrane]

Question 12

Breakdown of how Ca is decreased in relaxation

Answer 12

• 70% back into SR via SERCA
• 28% via NCX to extracellular
• 2% into mitochondria or other

Question 13

2 Principles of steady state Ca flux sources and sinks

Answer 13

1. Total Ca entering via L-type Ca = total Ca exiting via Na-Ca exchange
2. Total Ca released through RyRs = total Ca re-sequestered in SR via SERCA

Question 14

What is the primary mech to modulate strength of contraction?

Answer 14

• altering the amount of Ca stored in SR
• If more CA in Sr –> more Ca released –> stronger contraction
• nonlinear effect

Question 15

What happens if you increase amount Ca entering via L-type Ca channels above that amount exiting from NCX?

Answer 15

• you now have more Ca coming in
• on next beat some of that extra Ca goes into Sr so you have increased total cellular Ca [cyto + SR]
• then have increased SR Ca release and thus increased systolic Ca –> bigger contraction

Question 16

What happens if Ca within SR is extremely high?

Answer 16

• normally clusters of RyRs can randomly release small amount Ca but event is localized and harmless
• if high SR Ca –> local spontaneous release can trigger more releases –> triggers ceullular event
• end up with more Ca leaving via NCX which causes depolarizing current and can initiate inappropriate AP

Question 17

What is a DAD?

Answer 17

• delayed after depolarization

- it is an AP when you don’t want it

Question 18

What are findings of EC coupling in heart failure?

Answer 18

• smaller Ca movement
• slower relaxation
• weaker contraction
• less Ca pumped into SR and less Ca available to be released
• downregulation of SERCA

Question 19

What 2 things happen to structure of cell in heart failure

Answer 19

1. down-regulation of SERCA [ATPase in SR membrane]

2. get irregular T tubule structure, so increased distance to RyRs so more difficult to trigger SR Ca release

Question 20

How does organization of Ca release change in heart failure?

Answer 20

• less synchronous release of Ca from SR

- Ca changes disorganized spacialy, so less efficient contraction

Question 21

What does digitalis do?

Answer 21

inhibits Na-K pump

Question 22

6 steps in mech of digitalis

Answer 22

1. inhibits Na-K pump so get higher Na
2. increased Na makes it harder to remove Ca via NCX
3. Since can’t remove Ca via NCX, more Ca pumped out of cytosol into SR
4. Have higher Sr Ca
5. get more SR Ca release
6. Get stronger contractions [positive inotropy]

Question 23

What is potential toxicity of digitalis?

Answer 23

• have increased SR Ca

- might get unstable/spontaneous cellular Ca release leading to membrane depolarization by NCX and thus inappropriate AP