The purpose of the present proposal is to 1) examine the role of dipeptidyl peptidase (DPP-4) inhibition on insulin release during a hyperglycemic clamp while GLP-1 is being infused and, 2) further elucidate the role of the metabolite of GLP-1, that is GLP-1 9-36 amide (GLP-1m). During stable and very reproducible elevated plasma glucose levels the effect of increased active incretin levels with DPP-4 inhibitors should result in increased plasma insulin levels. Therefore the aim of this protocol is to document whether plasma insulin levels are increased following GLP-1 infusion in the presence or absence of DPP-4 inhibitors. Additionally, the investigators have shown that some improvement in glucose homeostasis during GLP-1 administration is due in part to the metabolite of GLP-1, i.e. GLP-1 (9-36) amide (GLP-1m). Therefore, the investigators will also test the role of the latter by infusing GLP-1m when the volunteers are being treated with DPP-4 inhibitors.