Mast Cells and 5‐HT. Intracellular Release of 5‐Hydroxytryptamine (5‐HT) from Storage Granules during Anaphylaxis or Treatment with Compound 48/80

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Abstract
The release of 5‐hydroxytryptamine (5‐HT) and granules from peritoneal mast cells during in vitro and in vivo anaphylaxis and after treatment with compound 48/80 in vitro was investigated by quantitative cytochemical methods. Sensitization was achieved by s.c. injections of horse serum with H. pertussis, and anaphylaxis was observed 3–30 weeks later. The 5‐HT content of mast cells was assayed by microfluorimetry, the cellular content of granules by microinterferometric measurements of cellular dry mass. Incubation of peritoneal mast cells with antigen in vitro resulted in release of 7–32% of 5‐HT, compared to 20–50% release following i.v. injection of specific antigen to the same rat. Incubation of non‐sensitized mast cells with compound 48/80 (1.6 μg/ml) caused a release of 45% of the 5‐HT. Microfluorimetric and microinterferometric measurements of the same individual mast cells revealed a proportionally greater release of 5‐HT than lowering of dry mass values. Since only a small part of the cytoplasmic dry mass is non‐granular material, the results indicate that under the present experimental conditions about 40% of the total 5‐HT release was not accompanied by a simultaneous extrusion of granules. It could also be demonstrated by fluorescence microscopy that a varying proportion of the granules had released their 5‐HT while still intracellularly located. This demonstration of intracelular release of 5‐HT from storage granules may be relevant also for other types of secretory cells.