Evidence for uptake of vital dye by activated rat peritoneal mast cells: an in vitro imaging study.

Uptake of material from surrounding medium by activated rat peritoneal mast cells (PMCs) was studied using in vitro peritoneal eluate cells, the vital fluorescent dye sulforhodamine B (SFRM-B), secretagogue compound 48/80, and an imaging technique. PMCs, which undergo different states of degranulation, are shown to possess the ability to take up (by endocytosis) SFRM-B in an activity-dependent manner. The endocytosed dye is incorporated in the granules and can be discharged into the medium when the cells are reactivated. Both the uptake and the discharge processes are calcium-dependent. The reactivity of mast cells to secretagogue is not altered by the application of the dye. SFRM-B, a negatively charged, nonspecific protein stain, displays greater photostability and less leakage than the positively charged acridine orange, and its fluorescence persists for hours, whereas acridine orange fluorescence fades within 1 min when exposed to ultraviolet illumination. The fluorescent image of the dye-loaded mast cells can be preserved overnight in a container at room temperature. SFRM-B elicits no detectable damaging influence on the activated afferent discharge of splanchnic afferent nerve fibers with mesenteric terminals. This enables the use of SFRM-B for studying the interactions between mesenteric afferent terminals and their surrounding mast cells.