What is the concentration of calcium inside cells?

When at rest, cells have an intracellular calcium concentration of 100 nM. Activation of signaling pathways which increase cytoplasmic calcium concentrations, such as the phospholipase C pathway, can increase calcium concentration up to 1000 nM. When choosing the appropriate fluorescent indicator to measure cellular calcium concentration, keep in mind to select a calcium indicator that has a K_d value within this range.