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

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Abstract
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 pancreaticor 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 the 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.