What are the three common chemical forms of calcium indicators? What is the difference between salts, dextran conjugates, and acetoxymethyl (AM) esters?

The 3 common chemical forms of calcium indicators are salts, dextran conjugates, and acetoxymethyl (AM) esters.

Salt forms  are hydrophilic, they are membrane impermeable and require invasive loading procedures. They can be introduced into cells by multiple techniques including microinjection, diffusion from patch clamp pipettes, electroporation and lipo transfer using liposomes. Once introduced into the cell, the salt form of Ca²⁺indicators rapidly equilibrates and can be used for Ca²⁺ imaging measurements within minutes.

Dextran conjugates were specifically engineered to address the problem of compartmentalization. Dextrans have high water solubility, low toxicity, and exhibit essentially no compartmentalization over very long recording periods up to days in length. However, these indicators are membrane impermeable like salts form and must be invasively introduced into the cell.

Ca²⁺ indicator dyes with AM esters are membrane permeable and can be passively loaded into cells simply by adding them to the extracellular medium. Intracellular esterases then cleave the AM group and trap the dye inside cells. Another advantage of using AM-linked Ca²⁺ dyes is that subcellular compartments can be labeled. For example, low affinity Ca²⁺ indicators can be used to monitor Ca²⁺ levels in the endoplasmic reticulum.