Extracellular effect of calcium on compound 48/80 stimulated mast cells

The effects of changes in the extracellular concentration of calcium on activation of rat mast cells by compound 48/80 were studied. The intracellular exchangeable Ca2+ pools at various concentrations of extracellular Ca2+ were determined by equilibration of the cells with 45Ca2+. The cells stimulated by compound 48/80 in the presence of 2.5 μM and 1.6 mM extracellular Ca2+ released comparable amounts of histamine. However, the intracellular Ca2+ pool was doubled in 2.5 μM Ca2+ and was increased sixfold in 1.6 mM Ca2+. In 14.4 mM extracellular Ca2+, there was neither release of histamine nor uptake of Ca2+ which suggested an impairment in activation. The kinetics of Ca2+ influx in the presence of 2.5 μM Ca2+ did not reveal intracellular mobilization of calcium. The cells activated in 1.6 mM Ca2+ at 0 °C when allowed to stand in 14.4 mM extracellular Ca2+ released decreased amounts of histamine upon warming to 37 °C. The inhibition of the release progressively increased with time of standing at 0 °C. The decrease in histamine release was not seen with the cells standing in 1.6 mM Ca2+ at 0 °C. The effect of 14.4 mM Ca2+ added prior to the challenge with compound 48/80 did not depend on the time of incubation. The data presented in this paper suggest that the high concentration of Ca2+ inhibits the histamine release from mast cells by interfering with membrane-associated phenomena.