Determining zinc with commonly used calcium and zinc fluorescent indicators, a question on calcium signals

Investigations into the roles of Ca2+ and Zn2+ in cell biology have been facilitated by the development of sensitive fluorometric probes that have enabled the measurement of Ca2+ or Zn2+ in both extracellular and intracellular environments. It is critical to be aware of the specificity and relative selectivity of a probe for the targeted ion. Here, we investigated metal-ion responses by screening nominally Zn2+- or Ca2+-selective fluorophores in solutions containing various concentrations of Ca2+, as a potential interferent for Zn2+, or Zn2+, as a potential interferent for Ca2+. The results suggested that Zn2+-sensitive dyes were more specific for their targeted ion than dyes that targeted Ca2+. Ca2+-sensitive dyes such as Calcium Green-1, Fura-2, and Fluo-3 showed a wide range of interaction with Zn2+, even responding to Zn2+ in the presence of high concentrations of Ca2+. We demonstrate that these Ca2+ indicators can effectively measure dynamic changes of cytosolic Zn2+. Our results appeal for a new generation of Ca2+ fluorophores that are more specific for Ca2+ over Zn2+. One implication of these results is that data obtained using Ca2+-sensitive dyes may need to be re-examined to determine if results previously attributed to Ca2+ could, in part, be due to Zn2+.