The mechanism of the calcium signal and correlation with histamine release in 2H3 cells.

Rat basophil leukemic (2H3) cells ( Siraganian , R.P., McGivney , A., Barsumian , E. L., Crews, F. T., Hirata , F., and Axelrod , J. (1982) Fed. Proc. 41, 30-34) loaded with fluorescent Ca2+ indicator quin 2 ( Tsien , R. Y. (1980) Biochemistry 19, 2396-2404) showed a rapid increase in free cytosol calcium concentration ( Cai) when histamine release was induced. Intracellular quin 2 concentrations up to 7 mM did not affect release of histamine in response to antigen (aggregated ovalbumin) or concanavalin A with cells primed with antigen-specific monoclonal IgE, or in response to Ca2+ ionophores. The Cai increased from approximately 105 nM to a maximum of approximately 1200 nM within 2 to 3 min after antigenic stimulation and then declined slowly over 30 min toward the level in unstimulated cells. Histamine release was most rapid as Cai reached the maximum value and then decreased continuously with Cai over the subsequent 30 min. Neither the Ca signal nor histamine release was observed when the Ca2+ concentration in the medium ( Cao) was less than 50 microM, but both responses were restored on readdition of Ca2+ to 1 mM. The maximal Ca signal was obtained when Cao was approximately greater than 1 mM and was half-maximal at Cao congruent to 0.4 mM. In marked contrast Cai in unstimulated cells varied very little with Cao from 0.1 to 1 mM. Maintenance of the Ca signal required the continuous presence of stimulating ligand, external Ca2+, and the maintenance of cellular ATP; metabolic inhibitors blocked or reversed the Ca signal. La+ ions also caused a rapid and reversible block of the Ca signal and histamine release. The data are interpreted in a model in which the Ca signal is generated by a La3+-sensitive signal influx pathway that is functionally independent of the normal Ca2+ influx pathway in unstimulated cells, and that allows a 10-fold or greater increase in rate of Ca2+ entry. The Ca signal is maintained dynamically by the balance between the increased Ca2+ influx and active Ca2+ efflux across the plasma membrane.