Establishment of novel specific assay for short-form glucose-dependent insulinotropic polypeptide and evaluation of its secretion in nondiabetic subjects.

The short-form glucose-dependent insulinotropic polypeptide (GIP) (1-30) is released from islet alpha cells and promotes insulin secretion in a paracrine manner in vitro. However, it is not well elucidated how GIP (1-30) is involved in glucose metabolism in vivo, since a specific assay system for GIP (1-30) has not yet been established. We first developed a sandwich enzyme-linked immunosorbent assay (ELISA) specific for GIP (1-30) by combining a novel antibody specific to the GIP (1-30) C terminus with the common antibody against GIP N terminus. Then, we explored cross-reactivities with incretins and glucagon-related peptides in this ELISA. GIP (1-30) amide, but not GIP (1-42), GLP-1, or glucagon increased absorbance in a dose-dependent manner. We next measured plasma GIP (1-30) concentrations in nondiabetic participants (ND) during a 75-g oral glucose tolerance test or cookie meal test (carbohydrates 75 g, lipids 28.5 g, proteins 8.5 g). Both glucose and cookie load increased GIP (1-30) concentrations in ND, but the increases were much lower than those of GIP (1-42). Furthermore, the DPP-4 inhibitor significantly increased GIP (1-30) concentrations similarly to GIP (1-42) in ND. In conclusion, we for the first time developed an ELISA specific for GIP (1-30) and revealed its secretion in ND.