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

Intracellular calcium, in particular the cytosolic free ionized calcium concentration [Ca
2+
]
i
, is tightly regulated under physiological conditions. Stimulation of receptors, belonging to almost all the classes so far described, will result in changes in [Ca
2+
]
i
. These changes might be directly induced by either Ca
2+
-influx or Ca
2+
-mobilization from intracellular stores, or indirectly by a number of other mechanisms (
1
–
4
). The development of fluorescent indicators of free [Ca
2+
] that could be loaded into intact cells has contributed enormously to the understanding of cellular Ca
2+
homeostasis, especially dyes that respond to Ca
2+
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 Ca
2+
-bound and Ca
2+
-free forms.