The influence of calcium on morphology and histamine content of isolated intact rat mast cell granules

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Abstract
Histamine is released by “sequential exocytosis” in mast cells. The exocytosis involves fusion of the plasma membrane with the perigranular membrane and further fusions of adjacent perigranular membranes. To study a possible direct effect of Ca2+on granule membrane fusions, mast cell granule suspensions were prepared from sonicated rat mast cells. With the sonication method used, more than 60% of the granules obtained were found to be homogeneous, electron dense and surrounded by a perigranular membrane, when observed in the electron microscope. These granules correspond to normal, histamine‐containing granules found in untreated mast cells and are therefore named “intact” granules. The other granules were swollen, less electron dense and without a perigranular membrane. These “changed” granules are formed during the histamine release process. Aliquots of the granule suspension were incubated in 0.34 M sucrose buffered with 10 mM HEPES, pH 7.0, containing different concentrations of CaCl2, MgCl2(10 mM, 1 mM, 100 μM, 10 μM) or NaCl (10 mM). Only with the highest concentration (10 mM) of Ca2+or Mg2+was it possible to visualize an apposition of the perigranular membranes of “intact”, normal granules. No elimination of the individual membrane structures could be observed at the place of membrane contact. Thus, we found no signs of membrane fusions. The histamine content was lower in the suspensions incubated with lower concentrations of these ions or with 10 mM NaCl. Ca2+and Mg2+in high concentrations seemed to stabilize the perigranular membranes instead of initiating histamine release. Therefore, changes in the Ca2+‐ion concentration per se do not explain the membrane fusions seen in mast cells during “sequential exocytosis”.