The effect of concentration on the binding of compound 48/80 to rat mast cells: a fluorescence microscopy study

A fluorescent analog of the chemical histamine liberator, compound 48/80, has been synthesized by the covalent attachment of rhodamine to the 48/80 polymer (R-48/80). The histamine liberating characteristics of this analog were similar to those of the parent compound. The binding characteristics of R-48/80 to rat peritoneal mast cells were then studied using fluorescence microscopy. At concentrations that caused minimal secretory stimulation (less than 1.0 microgram/ml), R-48/80 bound to the mast cell surface in a diffuse manner, with no indication of patching or capping. When the cells were incubated at higher concentrations, where non-cytotoxic histamine secretion was stimulated, the drug bound heavily to the exposed granules, but not to unexposed granules or other cell organelles. At cytotoxic concentrations, R-48/80 caused extensive cell clumping, with the drug bound to masses of cell debris and released granules. Therefore, although R-48/80 binds initially to the cell membrane, its primary binding site at concentrations that induce secretion becomes the mast cell granule. The properties of these granules should thus be considered when studying the binding of compound 48/80 or other cationic drugs to rat peritoneal mast cells.