The enterochromaffin cells of the guinea pig duodenum and 5-hydroxytryptamine incorporated into gelatin models gave, after formalin fixation, nearly identical reactions with methenamine silver, ferric-ferricyanide, and diazotized o-aminoazotoluene. Following formaldehyde fixation the indole reaction with p-dimethylaminobenzaldehyde was negative in tissue enterochromaffin cells; similarly the indole reaction was negative after formaldehyde fixation of gelatin models.

In the duodenum of the guinea pig 5-hydroxytryptamine appears to be largely, if not entirely, contained in the enterochromaffin cells. Its intracellular concentration estimated from relative cell volume and assay of concentration in the whole tissue, was not less than 5 mg./gm. of cells, and as judged by the intensity of staining reaction may have been as high as 10 to 20 mg./gm. of cells.

Reserpine administered parenterally caused release of the chromogenic material of duodenal enterochromaffin cells and a parallel reduction in the amount of extractable 5-hydroxytryptamine.

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