Indo-1 is a popular dye that is used as a ratiometric calcium indicator similar to Fura-2. In contrast to Fura-2, Indo-1 has a dual emissions peak and a single excitation. The main emission peak in calcium-free solution is 475 nm while in the presence of calcium the emission is shifted to 400 nm. It is widely used in flow cytometry and laser scanning microscopy, due to its single excitation property. However, its use for confocal microscopy is limited due to its photo-instability caused by photobleaching. Indo-1 is also able to keep possession of its ratiometric emission, dissimilar to Fura-2.[1] The penta potassium salt is commercially available and preferred to the free acid because of its higher solubility in water. While Indo-1 is not cell permeable the penta acetoxymethyl ester Indo-1 AM enters the cell where it is cleaved by intracellular esterases to Indo-1.The synthesis and properties of Indo-1 were presented in 1985 by the group of Roger Y Tsien.[2]
In intact heart muscle, Indo-1, in combination with bioluminescent protein aequorin, can be utilized as a tool to distinguish between the internal and exterior inotropic regulation processes.[3]