Glucagon-like peptide-1 (GLP-1), composed of 30 amino acids (7–36), is secreted from enteroendocrine cells of the distal small intestine in response to food ingestion (1). GLP-1 plays an important role in glucose metabolism and homeostasis. It inhibits gastric emptying, glucagon secretion, and glucose production (2); it also induces insulin release from the pancreatic β-cells as well as their proliferation. The GLP-1 receptor has been identified in normal tissues, such as the pancreas, stomach, brain, and lung, and it has been shown to be highly overexpressed in human insulinomas and gastrinomas (3). In insulinomas, GLP-1 receptor density is considerably higher and is expressed more often than somatostatin receptors.
Exendin, isolated from the venom of the lizard Heloderma suspectum (4), is a specific and competitive antagonist of GLP-1. Exendin(9-39) shares 53% sequence homology with GLP-1. Exendin(9-39) has been labeled with [125I]-Bolton-Hunter reagent (N-succinimidyl-3-(4-hydroxy-3-[125I]iodophenyl)propionate) at the Lys residues for single-photon emission computed tomography imaging of the GLP-1 receptor (5).