Light Activation of Calcein Inhibits Vesicle Release of Catecholamines

Calcein, a fluorescent fluid phase marker, has been used to track and visualize cellular processes such as synaptic vesicle fusion. It is also the fluorophore for live cells in the commonly used Live/Dead viability assay. In pilot studies designed to determine fusion pore open size and vesicle movement in secretory cells, imaging analysis revealed that calcein reduced the number of vesicles released from the cells when stimulated with nicotine. Using amperometry to detect individual vesicle release events, we show that when calcein is present in the media, the number of vesicles that fuse with the cellular membrane is reduced when cells are stimulated with either nicotine or high K⁺. Experimentally, amperometric electrodes are not undergoing fouling in the presence of calcein. We hypothesized that calcein, when activated by light, releases reactive oxygen species that cause a reduction in secreted vesicles. We show that when calcein is protected from light during experimentation, little to no reduction of vesicle secretion occurred. Therefore, photoactivated calcein can cause deleterious results for measurements of cellular processes, likely to be the result of release of reactive oxygen species.