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

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