An improved measurement of the Ca-binding affinity of fluorescent Ca indicators.

Fluorescent Ca indicators are widely used to measure the intracellular Ca concentration ([Ca]) in living cells, including neurons. By calibrating an indicator in solutions that mimic the main ionic constituents of the actual cytoplasm, [Ca] can be determined from the measured fluorescence intensity. However, different studies have reported considerably different Ca-binding affinities (K) for the same indicator, even though they used calibrating solutions with similar compositions.