The effect of various carbohydrates on glucagon-like peptide-1 (GLP-1) release was studied in the in vivo perfused rat ileum. GLP-1 concentrations in the mesenteric venous effluent increased significantly after luminal perfusion with substrates of a sodium/glucose co-transporter (D-glucose, D-galactose, methyl-alpha D-glucoside, and 3-O-methyl-D-glucose). D-Fructose induced a sodium-independent release of GLP-1. Carbohydrates like 2-deoxy-D-glucose and N-acetyl-D-glucosamine, which are not substrates of a luminal sodium/glucose or fructose transporter, did not affect GLP-1 release. Since methyl-alpha D-glucoside is not a substrate of the basolateral glucose transport mechanism and 3-O-methyl-D-glucose is not metabolized within intestinal cells, it is concluded that intracellular metabolism of carbohydrates and intracellular removal are not essential to induce GLP-1 secretion in rats.

Glucagon-like peptide-1 (GLP-1) is the most potent endogenous insulin-stimulating hormone. In the present study the plasma stability and biological activity of a GLP-1 analog, [Ser 8 ]GLP-1(7-36)amide, in which the second N-terminal amino acid alanine was replaced by serine, was evaluated in vitro and in vivo. Incubation of GLP-1 with human or rat plasma resulted in degradation of native GLP-1(7-36)amide to GLP-1(9-36)amide, while [Ser 8 ]GLP-1(7-36)amide was not significantly degraded by plasma enzymes. Using glucose-responsive HIT-T15 cells, [Ser 8 ]GLP-1(7-36)amide showed strong insulinotropic activity, which was inhibited by the specific GLP-1 receptor antagonist exendin-4(9-39)amide. Simultaneous i.v. injection of [Ser 8 ]GLP-1(7-36)amide and glucose in rats induced a twofold higher increase in plasma insulin levels than unmodified GLP-1(7-36)amide with glucose and a fivefold higher increase than glucose alone.[Ser 8 ]GLP-1(7-36)amide induced a 1·5-fold higher increase in plasma insulin than GLP-1(7-36)amide when given 1 h before i.v. application of glucose. The insulinotropic effect of [Ser 8 ]GLP-1(7-36)amide was suppressed by i.v. application of exendin-4(9-39)amide. The present data demonstrate that replacement of the second N-terminal amino acid alanine by serine improves the plasma stability of GLP-1(7-36)amide. The insulinotropic action in vitro and in vivo was not impaired significantly by this modification.