GIP-like immunoreactivity in glucagon cells. Interactions between GIP and glucagon on insulin release

In the present study the cellular and subcellular distribution of immunoreactive glucagon and GIP (gastric inhibitory polypeptide) were studied in the mouse. Furthermore, the effects of pure GIP and glucagon on basal and stimulated insulin secretion were investigated. Immunohistochemistry revealed that immunoreactive GIP occurred in the pancreatic glucagon cells and in endocrine cells, also displaying glucagon immunoreactivity, scattered along the small and large intestines. Electron immunocytochemistry revealed that the GIP-like material and glucagon coexisted in the secretory granules of the pancreatic glucagon cells. Pure porcine GIP and glucagon both stimulated basal insulin release. When equipotent doses of the peptides were given together, the two peptides antagonized each other's effect. Both peptides potentiated glucose- and carbachol-induced insulin release. When equipotent doses of the two peptides were given together prior to the administration of each of these secretagogues their effects on insulin release were additive.