Exendin-4 (9-39) amide is a potent and selective glucagon-like peptide-1 (GLP-1) receptor antagonist with a Kd of 1.7 nM at cloned human GLP-1 receptors. Exendin-4 (9-39) amide inhibits cAMP production and insulin release caused by GLP-1 (7-36) and exend