Ca increase in secretory granules of stimulated mast cells

The elemental content of rat peritoneal mast-cell secretory granules has been measured by X-ray micro-analysis. Two distinct categories of granules were analyzed: intact granules, seen in control samples, and spumous granules, corresponding to exocytosed granule matrices. The average Ca content of intact granules was found to be approximately equal to cytosolic concentration, and to increase up to 40-fold in spumous granules. A significant increase was also observed for Na and Cl. These changes were not observed (for Ca) or weaker (for Na and Cl) if the cells had been challenged in the absence of nominal extracellular Ca; in this case, there was also a significant decrease in the sulphur content, suggesting a partial dispersion of the organic matrix components. In exocytosed granule matrices, in the presence but not in the absence of extracellular Ca, a slow and long-lasting increase of intragranular free Ca was monitored by changes in the fluorescence of the Ca-sensitive probes Fluo-3 and Calcium Green-5N, accumulated within rat mast-cell secretory granules. These findings are discussed along two lines: It is proposed that the calcium uptake by the exocytosed mast-cell granule matrices can have a physiological relevance for the surrounding tissue. Mast-cell granules do not disperse after exocytosis. The major uptake of Ca which is seen after opening of the exocytotic pore could be responsible for the exceptional stability of the externalized matrices.