Immunoblockade of endogenous glucagon-like peptide-1 by monoclonal antibodies in conscious rats: effect on the insulin response to intragastric glucose.

The physiological action of endogenous active forms of glucagon-like peptide-1 (GLP-1) on the insulin response to intragastric glucose was studied in conscious male Wistar rats by immunoblockade with two monoclonal antibodies directed against different epitopes of GLP-1(7-36)amide. Plasma concentrations of intraperitoneally injected monoclonal antibodies were determined before and during each experiment by an enzyme-linked immunosorbent assay (ELISA) specific for GLP-1-binding antibodies. Three hours after injection of the two monoclonal antibodies, the plasma insulin response (area under the curve) following intragastric glucose 1 g/kg was reduced to a mean level (mean +/- SEM) of 60%+/-8% (n = 11) of control responses previously determined in the same rats, and the time course of the response showed almost no increase in insulin during the first 10 minutes, reaching a maximum of 45.1+/-4.6 microU/mL at 30 minutes, in contrast to the rapid increase of the control response to a maximum of 64.5+/-5.1 microU/mL at 15 minutes. Total C-terminally amidated GLP-1 measured by radioimmunoassay (RIA) of acid ethanol-extracted plasma increased from a mean basal level of 10+/-2 pmol/Lto a peak of 31+/-5 pmol/L at 15 minutes in the control experiments, while basal and response levels greater than 100 pmol/L were recorded after antibody treatment. The increase of plasma glucose was reduced in the presence of the antibodies, peaking at a mean of 9.7+/-0.3 mmol/L at 30 minutes, compared with 11.8+/-0.5 mmol/L at 30 minutes in the control experiments. The action of GLP-1 appears particularly important for the early insulin response to ingested glucose, and the unexpected effect of the antibodies on the glucose response may point to a net promoting effect of GLP-1 on intestinal glucose absorption.