Histamine distribution in human basophil secretory granules undergoing FMLP-stimulated secretion and recovery

We examined subcellular histamine localizations in purified human basophils that were stimulated to degranulate with FMLP using an ultrastructural enzyme-affinity technique. Basophils were collected at early (0, 20 seconds, 1 minute) and late (10 minutes to 6 hours) time points poststimulation and were prepared for routine ultrastructural and diamine oxidase-gold (DAO-gold) cytochemical analysis. Histamine was present in unaltered cytoplasmic secretory granules (30.77 gold particles per square micrometer; P < .001 compared with background); specificity controls (histamine absorption, diamine oxidase digestion) abrogated granule labeling for histamine. Altered granules in stimulated cells were not significantly labeled for histamine, as compared with background (P = not significant); unaltered granules in the same cells contained more histamine than altered granules (P < .05). During recovery times, spanning 10 minutes to g hours, granules again appeared to be electron-dense and contained histamine (33.49/μm²; P = not significant as compared with unaltered granules in 1- minute FMLP-stimulated cells, and P < .05 as compared with altered granules in 1-minute FMLP-stimulated samples). Other structures devoid of histamine in actively secreting cells included extruded granules and intragranular and extruded Charcot-Leyden crystals. Recovering basophils displayed morphologic evidence of material and membrane conservation, granule content condensation, and biosynthesis. Subcellular histamine-rich sites in actively recovering basophils included condensing granules and collections of cytoplasmic vesicles in three locations: beneath the plasma membrane, adjacent to granules, and in the Golgi region. These studies show that unaltered granules of actively releasing human basophils, as well as similar granules that are reconstituted after FMLP-stimulated degranulation, contain histamine, but that altered granules in stimulated cells undergoing degranulation are devoid of histamine. Reconstitution of histamine-rich granules is associated with DAO-gold-positive cytoplasmic vesicles, suggesting transport of histamine derived from either new synthesis, re-uptake of released histamine, or both, to reconstituted granules.