Calcium release

Question 1

CIRC is regenerative, so how does it stop?

Answer 1

 RyR inactivation (Ca2+-dependent or time-dependent)
 Local depletion of SR Ca2+ (store emptying – nothing left to release) (loss of RyR activation by luminal Ca2+)
 Stochastic attrition (a mechanism to terminate release)

Question 2

 Stochastic attrition

Answer 2

Stochastic attrition means that if the L-type Ca channels and all RyRs in the junction happen to close simultaneously (as channels gate stochastically), then the local [Ca] will fall rapidly and interrupt the otherwise regenerative release. This might work for 1 L-type ca channel and 1-2 RyRs but with more realistic numbers this becomes unlikely that they will close at one,

The coupled gating of RyRs might overcome this limitaiton but this remains to be tested.

RyR : L-type Ca2+ channel stoichiometry is not 1:1
 Ligand binding methods indicate that many more RyR than L-type channels.
 Gain of e-c coupling suggests that the opening of a single L-type channel initiates release of SR Ca2+ from several RyR channels.
Stochastic attrition will still terminate release; it will just take a bit longer.

Question 3

 Local depletion of SR Ca2+

Answer 3

 Local depletion of SR Ca2+ (store emptying – nothing left to release) (loss of RyR activation by luminal Ca2+)
The release of Ca from the SR is electrogenic, it affect the electrical potential in the SR lumen, and efficient release of Ca via RyR requires charge compensation. It contains ,onovalent anion and cation pathways to prevent generation of trans-SR diffusion potential.
Local depletion cannot explain completely the turn-off of release because very long lasting Ca sparks are observed that do not decline with time (>200ms). Thus diffusion from other regions of SR can limit local depletion.
However, during global Ca transient the whole SR Ca content declines and because luminal [Ca] modulates RyR gating, whole SR [Ca] reduction might contribute to shutting-off the global SR Ca release during a twitch. But this still cannot explain fully the termination

Question 4

RyR inactivation to terminate CIRC?

Answer 4

Two types of inactivation have been reported.

1. One is where the inactivation renders the channels unable to open until it recovers (similar to how Na requires reactivation before being able to open again)
2. The second one is called 'adaptation': after opening, the RyR 'relaxes' into a lower open probability but can still be reactivated by higher Ca concentrations. There are still many questions regarding these mechanisms, however, studies have consistently idnetified the presence of refractoriness in RyR opening.

Question 5

Ca sparks - would they induce other RyR to release Ca?

Answer 5

 SL L-type Ca2+ channel is very close to the SR Ca2+ channel.
 Sites of release are separated.
 Ca2+-sensitivity of SR release channel is low.
 Ca2+ diffusion away from release site is very fast.

Ca sparks are usually independent signalling events and do not interact with neighbouring Ca release sites. Thus the release of ca and trhe size of subsequent contraction is regualted depending on how many L-type Ca channels are activated. This independece ensures that a stable graded responses of the EC coupling mechanism and occurs because:

1) RyRs are relatively insensitive to activating Ca
2) once Ca has leftthe microdomain near the release site, if diffuses avay rapidly so does not evoke release from the neighbouring cluster. 
* * ahving more than one L-type Ca cahnnels, 10-25 in fact, creates a safety amrgin so that if one channel does not activate  the cluster, another will.