Measurement of cytoplasmic calcium concentration in cell suspensions: correction for extracellular Fura-2 through use of Mn2+ and probenecid.

1321N1 astrocytoma cells loaded with Fura-2 were found to continuously transport Fura-2 to the extracellular medium. To correct for extracellular Fura-2 fluorescence a protocol was developed in which Mn2+ was added to duplicate cuvettes of cells to quench extracellular Fura-2 at the beginning and end of the experimental time course. Since the export of Fura-2 was linear with time, two separate quench determinations allowed the amount of fluorescence from extracellular Fura-2 fluorescence to be estimated at every point in the time course and subtracted from the data. The uncorrected and Mn2+-corrected basal cytoplasmic calcium concentrations averaged 153 nM and 72 nM, respectively. The peak intracellular calcium concentrations following muscarinic stimulation with 300 microM carbachol averaged 1159 nM (uncorrected) and 889 nM (Mn2+-corrected). Probenecid (2.5 mM) was found to block the export of Fura-2 from these cells and did not change the basal calcium concentration or the muscarinic calcium response.