Fig. 5. Effects of the extracellular Ca²⁺ concentration on Ca²⁺-dependent degranulation. Mast cells were stimulated with PMA and A23187 at various extracellular Ca²⁺ concentrations. (A) Degranulation activity at 20 minutes after stimulation is plotted against extracellular Ca²⁺ concentration. Values are expressed as the percentage of total ß-hexosaminidase as shown in Fig. 3, but ß-hexosaminidase at [Ca²⁺]ex=0 was subtracted. Each point was obtained from four independent preparations of wild-type cells () and four independent knockdown clones () [mean±s.e.m. (n=4)]. (B) Degranulation activity is re-plotted against intracellular Ca²⁺ concentration, which was estimated from Ca²⁺ concentration at plateau phase after stimulation using Fura-2 as shown in the inset. (Inset) Timecourses of intracellular Ca²⁺ concentration in wild-type cells induced by PMA and A23187 at various extracellular Ca²⁺ concentrations, indicated on each line. Cells were stimulated at the time indicated by the arrow. Ca²⁺ concentration was converted from ratio values and shown as a second ordinate.