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B. Gallwitz, W. E. Schmidt, J. M. Conlon, and W. Creutzfeldt

ABSTRACT

Glucagon-like peptide-1(7–36)amide (GLP-1(7–36)amide) is a potent stimulator of insulin secretion. Receptors for this hormone have been found on different insulinoma-derived cell lines, e.g. the RINm5F cell line which is derived from a radiation-induced rat insulinoma. To characterize the part of the GLP-1(7–36)amide molecule that is responsible for binding to its receptor on RINm5F cells, binding studies with synthetic C-terminal (GLP-1(21–36)amide) and synthetic N-terminal (GLP-1(7–25)) GLP-1 fragments were carried out. GLP-1(21–36)amide showed dose-dependent binding to the GLP-1(7–36)amide receptor but was approximately 1500 times less potent in inhibiting binding of 125I-labelled GLP-1(7–36)amide than the intact hormone. GLP-1(7–25) at concentrations up to 10 μmol/l did not inhibit binding of label. Neither fragment changed intracellular cyclic AMP concentrations, in contrast to GLP-1(7–36)amide which increased intracellular cyclic AMP. GLP-1(21–36)amide, however, acted as a weak partial antagonist of GLP-1(7–36)amide with respect to GLP-1(7–36)amide-dependent stimulation of cyclic AMP production.

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B Gallwitz, M Witt, U R Fölsch, W Creutzfeldt, and W E Schmidt

ABSTRACT

Glucagon-like peptide-1(7–36)amide (GLP-1(7–36) amide) and gastric inhibitory polypeptide (GIP), peptides of the glucagon family, stimulate insulin secretion in vitro and in vivo. They possess high N-terminal sequence homology. Binding studies with 125I-labelled GIP and 125I-labelled GLP-1(7– 36)amide were performed in RINm5F insulinoma cells to investigate receptor specificity and to compare both receptors directly. Both binding sites were highly ligand-specific: GIP did not bind to the GLP-1(7–36)amide receptor and vice versa. Both peptides increased intracellular cyclic AMP levels; GLP-1(7– 36)amide was 100-fold more potent in stimulating cyclic AMP production when compared with GIP. At ranges of 1–10 nmol GLP-1(7–36)amide/1 and 0·1–10 GIP/1, corresponding to submaximal binding concentrations, the hormones showed an additive effect on cyclic AMP production. The N-terminal portion of GIP was important for binding, as GIP(1–30) showed almost full binding and biological activity. GIP(17–42) bound in a concentration-dependent manner with approximately 500-fold lower potency than GIP. At concentrations of up to 10 μmol GIP(17–42)/1 no stimulation of cyclic AMP was observed.