Calcium Regulation

Question 0

Outline the release of calcium during muscle contraction.

Answer 0

Spread of depolarisation down T-tubule opens voltage-operated calcium channel (VOCC) which causes some calcium to enter the cell

This small amount of calcium opens the ryanodine receptor on the SR to release a large amount of calcium through SERCA (calcium-induced-calcium release)

Calcium stores refilled by recycling released calcium (cardiac myocyte) and using voltage-operated calcium channels/capacitative calcium entry

Question 1

How is the calcium gradient set up and maintained?

Answer 1

Relative impermeability of plasma membrane to calcium (therefore cell’s ability to expel Ca2+ across plasma membrane determines calcium gradient e.g. Ca2+-ATPase, NCX)

Increase in [Ca2+]i -> Calcium-calmodulin -> Calcium-calmodulin-Calcium-ATPase -> reduction in [Ca2+]i

Calcium buffers limit diffusion of calcium (ATP & calcium binding proteins)

Intracellular calcium stores:
- rapid: uptake & release of Ca2+ into SR by SERCA (phospholipase C -> IP3 -> release of Ca2+) AND Calcium-induced-calcium release (ryanodine receptor)

• slow: uptake of calcium into mitochondria by uniporter (protective buffer)
  note: stimulates mitochondrial metabolism/apoptosis

Question 2

How are calcium stores refilled, and why is this needed?

Answer 2

Signal terminated
Calcium removed from cell by reversing the NCX (cytosolic calcium recycled)
Capacitive calcium entry (VOCC)

Needed after repetitive signalling