Measurement of cytosolic calcium: fluorescent calcium indicators.

The invention of intracellularly trappable fluorescent indicators of [Ca2+i] has provided an important new way to answer many key questions about the role of Ca2+ and of other messengers in cell activation. Perhaps equally important, the results of these investigations have thrown up many interesting new questions. The suitability of this, or any other method of measuring [Ca2+]i, naturally depends on the tissue, the experimenter, the facilities available and the particular question to be asked. For simply measuring [Ca2+]i, fura-2 and indo-1 are likely to be preferable to quin2, although right now more is known about quin2, its behaviour and its calibration in cells. Anecdotal evidence suggests that indo-1 may have advantages over fura-2 although many more people are presently using fura-2. For manipulating [Ca2+]i by deliberately adding Ca2+ buffering power, quin2 may have advantages and is certainly the tried and tested method. For epithelial cell work, where the relation of the cells to the rest of the tissue and their polarity are critical, suspensions and even monolayer measurements will seldom be adequate. Calcium microelectrodes or single-cell fluorescence techniques will surely be required and rewarding.