A novel method of estimating real [Ca2+]i dynamics from fluorescence signals

To detect changes in the concentration of intracellular calcium ions ([Ca2+]i), fluorescence measurement techniques are used in experimental studies. When the calcium concentration is calculated from fluorescence signals, an equilibrium between calcium ions and the fluorescent dye is assumed in conventional methods. However, using this assumption, calculated calcium concentrations would not be an accurate representation of [Ca2+]i if the dynamics of [Ca2+]i are so fast that the reaction between calcium ions and the dye cannot reach equilibrium. In this report, we propose a new method of estimating [Ca2+]i in dye-free conditions from fluorescence signals of the dye using differential equations expressing reactions between calcium ions and the dye without assuming equilibrium. We compared our method with a conventional method with respect to calculations of [Ca2+]i using a model of a dendritic spine. Simulation results showed that our method gave a better estimation of the changes in [Ca2+]i than did the conventional method.