The action of Ca2+ in a second messenger scheme?

Question

The action of Ca2+ in a second messenger scheme

a) involves the calcium-binding protein calmodulin as the actual second messenger

b) relies entirely on intercellular reservoirs of Ca2+ in the

endoplasmic reticulum

c) does not involve the G protein

d) produces sustained responses by controlling the flow of Ca2+ into the cell

Answer 1

The answer is "a".  Ligands bind to G-protein receptors, which activate phospholipase C.  PLC in turn activates IP3, which initiates release of Ca2+ from the ER (however, ER is not the only source of Ca2+ ions).  The increased levels of Ca2+ in the cell then bind to calmodulin, which is responsible for mediating the effects of Ca2+ in the cell.  Since it only takes very small changes in the concentration of Ca2+ in the cell to start having effects, there is no need to have a steady influx of Ca2+ into the cell.