Localization of xenin-immunoreactive cells in the duodenal mucosa of humans and various mammals

Xenin is a 25-amino-acid peptide extractable from mammalian tissue. This pe ptide is biologically active. It stimulates exocrine pancreatic secretion a nd intestinal motility and inhibits gastric secretion of acid and food inta ke. Xenin circulates in the human plasma after meals. In this study, the ce llular origin of xenin in the gastro-entero-pancreatic system of humans, Rh esus monkeys, and dogs was investigated by immunohistochemistry and immunoe lectron microscopy. Sequence-specific antibodies against xenin detected spe cific endocrine cells in the duodenal and jejunal mucosa of all three speci es. These xenin-immunoreactive cells were distinct from enterochromaffin, s omatostatin, motilin, cholecystokinin, neurotensin, and secretin cells, and comprised 8.8% of the chromogranin A-positive cells in the dog duodenum an d 4.6% of the chromogranin A-positive cells in human duodenum. In all three species, co-localization of xenin was found with a subpopulation of gastri c inhibitory polypeptide (GIP)-immunoreactive cells. Immunoelectron microsc opy in the canine duodenal mucosa demonstrated accumulation of gold particl es in round, homogeneous, and osmiophilic secretory granules with a closely adhering membrane of 187 +/- 19 nm diameter (mean +/- SEM). This cell type was found to be identical to the previously described canine CIP cell. Imm unocytochemical expression of the peptide xenin in a subpopulation of chrom ogranin A-positive cells as wetI as the localization of xenin immunoreactiv ity in ultrastructurally characterized secretory granules permitted the ide ntification of a novel endocrine cell type as the cellular source of circul ating xenin.