The catecholamine-forming mast cells in rat intestinal mucosa have been studied by means of a histochemical fluorescence method and quantitative fluorometric analysis. Following the administration of l-DOPA, an accumulation of dopamine occurred in these cells which could be prevented by pretreating the animals with a DOPA decarboxylase inhibitor. Evidence is presented which indicates a decarboxylation of DOPA by an intracellularly located enzyme. d-DOPA was less effective in producing catecholamine fluorescence in the mucosal mast cells, thus indicating a stereo-specific uptake or decarboxylation of DOPA. No evidence for an enzymatic degradation of dopamine in the mast cells was obtained, as judged by the effect of monoamine oxidase inhibitors or a catechol-O-methyl transferase inhibitor on the fluorescence intensity. The weak, naturally occurring fluorescence in the mucosal mast cells may be due to dopamine but the properties of the fluorescence are also compatible with a low concentration of 5-hydroxytryptamine.
