Uptake of 5-hydroxytryptamine by mast cells in vivo: a cytofluorometric study of mast cells and individual mast cell granules.

Uptake, distribution and turnover of 5-Hydroxytryptamine (5-HT) was studied by cytofluorometric analysis of whole mast cells and individual granules. Injection of 5-HT as well as 5-Hydroxytryptophan (5-HTP) intraperitoneally or subcutaneously resulted in a parallel uptake of 5-HT in cells and granules. Intraperitoneal injections of 5-HT in such small quantities that may be available under physiological conditions resulted in an increase in fluorescence intensity of the mast cells, indicating a very efficient uptake mechanism for 5-HT in vivo. Much larger doses of 5-HTP were required to obtain a corresponding uptake of 5-HT in the mast cells. The 5-HT was rather rapidly taken up in the granules and eliminated very slowly, at the same rate both from granules and mast cells. The low elimination rate confirms our previous findings that the turnover of 5-HT is much lower in mast cells than in other amine containing cell systems. The combination of an extremely efficient, rapid uptake of 5-HT with a slow elimination suggests a specific function for mast cells in the regulation of free amine concentrations in tissues.