To examine whether 12 weeks of treatment with a dipeptidyl peptidase-4 ( DPP-4 ) inhibitor , sitagliptin , influences the insulin secretion induced by glucose , glucose-dependent insulinotropic polypeptide ( GIP ) and glucagon-like peptide-1 ( GLP-1 ) during a hyperglycaemic clamp in patients with type 2 diabetes ( T2DM ) .