At least six types of endocrine cells have been identified by light and electron microscopy in the gastrointestinal mucosa of the rabbit. Among them are two types of amine producing cells, the enterochromaffin and enterochromaffin-like cells, and four types of polypeptide producing cells. The pyloric G cells, very likely producing the hormone gastrin, and the functionally undefined X cells, mainly scattered in the fundic mucosa, have been characterized as gastric endocrine cells of the polypeptide type. Small granule cells (S cells), perhaps related to the hormone secretin, and large granule cells (L cells) of unknown function have been found scattered along the intestinal mucosa.
Reserpine (2.5-5mg/kg intraperitoneally) depleted the 5-hydroxytryptamine stores of the duodenal enterochromaffin cells, reduced the osmiophilia of their granules without evidently affecting their protein matrix and activated the Golgi complex. Enterochromaffin-like cells of the fundic mucosa were not markedly altered by the drug.