Aequorin measurements of free calcium in single mammalian cells

The calcium-triggered photoprotein aequorin has been used as a sensitive indicator of cytoplasmic free calcium in cells of relatively large size. We describe here several novel modifications to the technique that make it possible to detect resting levels of free calcium in single mammalian oocytes, fibroblasts and granulosa cells. Concentrations of free calcium as low as 1 X 10(-7) M have been measured in single fibroblasts, corresponding to a detection limit of 10(-18)mol.