Measurement of [Ca2+]i in Cell Suspensions Using Indo-1

Intracellular calcium, in particular the cytosolic free ionized calcium concentration [Ca2+ ]i , is tightly regulated under physiological conditions. Stimulation of receptors, belonging to almost all the classes so far described, will result in changes in [Ca2+ ]i . These changes might be directly induced by either Ca2+ -influx or Ca2+ -mobilization from intracellular stores, or indirectly by a number of other mechanisms (1 –4 ). The development of fluorescent indicators of free [Ca2+ ] that could be loaded into intact cells has contributed enormously to the understanding of cellular Ca2+ homeostasis, especially dyes that respond to Ca2+ with shifts of excitation or emission spectra (5 –7 ). By measuring at two selected wavelengths (either dual emission or dual excitation), it is possible to calculate the proportion of dye in the Ca2+ -bound and Ca2+ -free forms.