What are the differences between calcium ion indicators: Cal 520, Cal 520FF, and Cal 520N?

All three of these Ca²⁺ indicators differ mainly by their dissociation constant (K_d):

• Cal 520^® AM has a K_d of 320 nM
• Cal 520FF^™ AM has a K_d of 9.8 μM
• Cal 520N^™ AM has a K_d of 90 μM

Ca²⁺ indicators that have a very low K_d value are high-affinity chelators and have a greater tendency to bind Ca²⁺. When the K_d value is high, it is a low-affinity chelator.

A variety of ion indicators are available for detecting intracellular Ca²⁺ over a broad range (e.g., less than 50 nM to more than 50 μM). Higher affinity indicators, such as Cal 520^® AM, are designed to quantify Ca²⁺ levels in the cytosol. Whereas, lower affinity indicators can be optimized for measuring Ca²⁺ in subcellular compartments with higher Ca²⁺ concentrations (e.g., ER).