Histamine release induced by histone and related morphological changes in mast cells

When isolated rat peritoneal mast cells were exposed to histone mixture, both histamine release and degranulation were evoked rapidly and dose-dependently at concentrations higher than 1 microgram/ml in a Ca-free medium. All of the histone subfractions (H1, H2A, H2B, H3 and H4) induced histamine release from mast cells in a Ca-free medium and, as in the case of histone mixture, the amount of histamine release was reduced by addition of Ca2+. By means of high-voltage electron microscopy, at least three types of degranulation were observed in mast cells exposed to histone mixture: (1) microfilament-associated degranulation, (2) extrusion of granules with the surrounding membrane, and (3) mass degranulation.