Immunocytochemical localization of glucagonlike and gastric inhibitory polypeptidelike peptides in the pancreatic islets and gastrointestinal tract.

The distribution of cells displaying glucagonlike or gastric inhibitory polypeptide (GIP)-like immunoreactivity was examined in the pancreatic islets and gastrointestinal tracts of rats, dogs, and humans. A-cells in the pancreatic islets in all three species were stained by antisera having regional specificity for pancreatic-type glucagon, gut-type glucagon (glicentin), or GIP. Oxyntic A-cells of the gastric mucosa in dogs and humans also were stained comparably by these three antisera. In contrast, the K- and L-cells in the intestinal mucosa of those species were stained only by antisera capable of reacting with GIP or gut-type glucagon, respectively. Tests of antibody specificity showed that the GIP antiserum did not cross-react with either pancreatic- or gut-type glucagon. Likewise, the glucagon antisera showed no cross-reactivity with GIP. Hence, these findings suggest that pancreatic and gastric A-cells contain a peptide with GIP-like immunoreactivity distinct from glucagon or glicentin per se. Although the exact basis of th GIP-like immunostaining of A-cells is unknown, it may be due to the presence of a glucagon precursor sharing certain amino-acid sequences with GIP. This hypothesis is consistent with several recent investigations showing that the processing of proglucagon molecules differs between the A- and L-cells.