Direct measurements of the dextran‐dependent calcium uptake by rat peritoneal mast cells

The metallochromic indicator murexide has been used to monitor calcium concentration changes during the dextran‐induced, phosphatidylserine‐dependent degranulation of rat peritoneal mast cells. The dextran‐induced Ca2+‐uptake showed an absolute dependence on the presence of phosphatidylserine. The extent of Ca2+‐uptake increased with phosphatidylserine in a concentration‐dependent manner. At 25°C the half‐life of the uptake process equalled 35 ± 5 s. Exposure of the mast cells to dextran in the presence of Ca2+, but in the absence of phosphatidylserine, desensitized the cells. The subsequent addition of phosphatidylserine failed to restore the Ca2+‐uptake activity. However, the Ca2+‐ionophore A23187 did promote Ca2+uptake by the cells without PS.