[Problems on the determination of intracellular free calcium concentration when measured by fura-2/AM in mast cells].

When rat peritoneal mast cells which had been loaded with fluorescent Ca2+ indicator fura-2/AM were exposed to compound 48/80, fura-2 fluorescence was increased in the presence of 1mM extracellular Ca2+, but remained unchanged in the Ca2(+)-eliminated medium. Only in the presence of 1mM extracellular Ca2+, both fura-2 fluorescence and histamine release were increased in response to compound 48/80, depending on the concentration of the stimulant. Both of these responses were attenuated in the same degree by such release inhibitors as disodium cromoglycate and HSR-6071. Fluorescent microscopic observation of fura-2/AM-loaded mast cells revealed that fura-2 fluorescence was densely localized in the granules and the nucleus, but less in the cytoplasm. All these results strongly suggest that increase in the fura-2 fluorescence might not result from the increase in cytoplasmic free Ca2+ concentration, but from binding of fura-2, which had been released together with chemical mediators from granules following stimulation, to extracellular Ca2+. Therefore, it seems likely that the fura-2/AM loading method is inadequate to investigate whether or not the increase in cytoplasmic free Ca2+ concentration triggers release of chemical mediators from mast cells.