Gastric inhibitory peptide-like immunoreactivity in glucagon and glicentin cells: properties and origin. An immunocytochemical study using several antisera.

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Abstract
Although gastric inhibitory peptide (GIP) has never been detected outside the upper small intestine by immunochemical methods, GIP-like immunoreactivity has been demonstrated by immunocytochemistry in the glucagon/glicentin cells of pancreas, and gut. In the present study several GIP antisera (five polyclonal and one monoclonal) were tested on specimens from pancreas and intestines of several mammalian species, including man. Two of the polyclonal antisera and the monoclonal one stained cells in the upper small intestine only, while the other three also stained cells in the pancreas, ileum, and colon. Monoclonal anti-GIP did not stain GIP cells in man. The immunostaining produced could not be abolished by pretreatment of the antisera with glucagon or glicentin in excess, whereas small amounts of synthetic or natural porcine GIP prevented the immunostaining. Thus, three of the antisera are specific for GIP, while the other three recognize not only GIP but also GIP-like peptides. The results suggest that the glucagon/glicentin cells contain peptides distinct from GIP but sharing an immunodeterminant with GIP. The GIP-like immunoreactivity in the glucagon cells of the rat pancreas was not altered by infusion of GIP or by elimination of the bulk of endogenous GIP by resection of the upper small intestine, indicating that the GIP-like peptide is produced in the glucagon cells rather than accumulated from the circulation. The nature of this GIP-like peptide is unknown. Conceivably, it represents the cryptic portion of the glucagon precursor molecule. In some species a proportion of the GIP cells in the proximal small intestine displayed glicentin-like immunoreactivity as well, emphasizing the relationship between GIP cells on the one hand and glucagon/glicentin cells on the other.