Characteristics of histamine release from rat mast cells induced by a bracken toxin, braxin A1

The effect of braxin A1, a new bracken glucoside, on histamine release from isolated rat peritoneal mast cells was studied. Braxin A1 caused the release of histamine in a dose-dependent manner; the release was slow and increased gradually with time, finally reaching a maximum release of 100%. The action of braxin A1 depended on the incubation temperature in the range from 4 degrees C to 49 degrees C, while it was almost abolished at 0 degree C. The action of braxin A1 was unaffected by removing calcium or any inorganic ions from the incubation medium and by the addition of 2,4-dinitrophenol or theophylline. The mast cells exposed to braxin A1 were vitally stained with trypan blue and swelled greatly. The cell swelling was characterized by the protrusion of swollen cytoplasmic granules. The present results for braxin A1 were similar to those for the ionophore X537A except for the extracellular inorganic ion dependency, but they were different from those observed with compound 48/80. These results suggest that braxin A1 releases histamine from mast cells without both exocytosis and membrane lysis, but with a cytotoxic action on cytoplasmic membranes by a different mode of action from that of X537A.