In vivo inhibition of dipeptidyl peptidase 4 allows measurement of GLP-1 secretion in mice.

Dipeptidyl peptidase (DPP)-4 and neprilysin (NEP) rapidly degrade glucagon-like peptide 1 (GLP-1) in mice. Commercially available sandwich ELISA kits may not accurately detect the degradation products, leading to potentially misleading results. We aimed to stabilize GLP-1 in mice allowing reliable measurement with sensitive commercially available ELISA kits. Non-anesthetized male C57Bl/6JRj mice were subjected to an oral glucose tolerance test (OGTT; 2 g/kg glucose), and plasma total and intact GLP-1 were measured (Mercodia and Alpco ELISA kits, respectively). No GLP-1 increases were seen in samples taken beyond 15 minutes after the glucose load. Samples taken at 5 and 10 minutes after the OGTT showed a minor increase in total, but not intact GLP-1. We then administered saline (control), or a DPP-4 inhibitor (valine pyrrolidide or sitagliptin) with or without a NEP-inhibitor (sacubitril) 30 minutes before the OGTT. In the inhibitor groups only, intact GLP-1 increased significantly during the OGTT. After injecting male C57Bl/6JRj mice with a known dose of GLP-1(7-36)NH2, peak GLP-1 levels were barely detectable after saline, but 5-10-fold higher during sitagliptin and the combination of sitagliptin/sacubitril. The half-life of the GLP-1 plasma disappearance increased up to 7-fold during inhibitor treatment. We conclude that reliable measurement of GLP-1 secretion is not possible in mice in vivo with commercially available sandwich ELISA kits, unless degradation is prevented by inhibition of DPP-4 and perhaps neprilysin. The described approach allows improved estimates of GLP-1 secretion for future studies, although it is a limitation that these inhibitors additionally influence levels of insulin and glucagon.