The right answer was C, which I understand after the explanation. But, I chose A because of ryanodine receptors in the SR, and calcium-mediated calcium release. Was anyone else confused about this? And does anyone have any explanation why A is 100% wrong?
"Although Ca2+ is a major triggering ligand, it is not absolutely required to elicit channel opening. In the case of RyR1, the skeletal muscle isoform, it is widely assumed that there exists a physical link with the voltage-gated calcium channel CaV1.1 (17,–,20). In this case, voltage changes across the plasma membrane can cause opening of RyR1 in the absence of extracellular Ca2+. An intracellular loop of CaV1.1 that connects transmembrane repeats II and III (“II-III linker”) appears to be the primary interaction site for RyR1. However, not every RyR1 is near a CaV1.1 channel, and such channels are likely activated by Ca2+ released from neighboring channels."
