Effect of Ca2+ on isolated mast cell granules

Isolated rat mast cell granules were used to study a possible direct effect of Ca2+ on membrane fusion. More than 60% were intact, membrane-limited granules providing a suitable model for investigating in vitro membrane fusion. Various concentrations of Ca2+ were investigated (10(-2) to 10(-5) M) and no sign of membrane fusion was found. When the granules had been fixed after incubation with Ca2+, only the highest Ca2+ concentration proved to cause the granules to aggregate. Without fixation, the aggregating effect of Ca2+ was more expressed, even in lower concentrations. In the granule aggregates, membrane areas of mutual contact were free of intramembrane particles. These attachment sites correspond to pentalaminar structures, as seen in thin sections.