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+).