GLUCAGON ACTS THROUGH ITS OWN RECEPTORS IN THE PRESENCE OF FUNCTIONALGLUCAGON-LIKE PEPTIDE-1 RECEPTORS ON HAMSTER INSULINOMA

The observations that glucagon binds to glucagon-like peptide-1 (tGLP- 1) receptors have raised the question of whether glucagon receptors me diate the insulinotropic effect of glucagon. We have investigated the presence and selective activation of glucagon and tGLP-1 receptors on tumor-derived cells. Northern blot analysis detected either glucagon o r tGLP-1 receptor messenger RNA in hamster (HIT) and mouse (beta TCS) beta-cell lines, respectively, whereas both receptor messenger RNA wer e revealed in Syrian hamster insulinoma. Their expression in insulinom a plasma membranes was confirmed by specific covalent labeling with ei ther [I-125]glucagon or [I-125]tGLP-1. Both glucagon and tGLP-1 recept ors showed a single class of high affinity, binding sites with respect ive K-d values of 1.11 +/- 0.11 and 0.82 +/- 0.11 nM. [I-125]tGLP bind ing was dose dependently inhibited with a hierarchy of exendin-4 > tGL P-1 > exendin-(9-39) > oxyntomodulin > glucagon. [I-125]Glucagon bindi ng was only inhibited by glucagon and oxyntomodulin. Both glucagon and tGLP-1 increased cAMP formation in insulinoma plasma membranes in a d ose-dependent manner, with ED(50) values of 170.0 +/- 25.0 and 3.1 +/- 0.4 pM, respectively. Exendin-(9-39), a tGLP-1 receptor antagonist, i nhibited tGLP-1-induced, but not glucagon-induced, cAMP formation.Our data demonstrate on hamster insulinoma the presence of high affinity g lucagon and tGLP-1 receptors selectively coupled to adenylyl cyclase. The observed low affinity of tGLP-1 receptors for glucagon sustains th e idea that each hormone has a direct insulinotropic effect.