GLP-1(9-36) mediates the glucagonostatic effect of GLP-1 by promiscuous activation of the glucagon receptor

The incretin hormone glucagon-like peptide 1(7-36) (GLP-1(7-36)) stimulates insulin and inhibits glucagon secretion. The mechanisms by which GLP-1 suppresses glucagon release are unclear as glucagon-secreting α-cells express GLP-1 receptors (GLP-1Rs) at very low levels. Here, we examine the underlying mechanisms. We find that both GLP-1(7-36) and its degradation product GLP-1(9-36) inhibit glucagon secretion at physiological (pM) concentrations. Whereas the effect of GLP-1(7-36) is sensitive to PKA inhibition, GLP-1(9-36) exerts its effect by a PKA-independent mechanism sensitive to pretreatment with pertussis. The glucagonostatic effects of both GLP-1(7-36) and (9-36) are retained in islets from Glp1r knockout mice but only GLP-1(9-36) remains glucagonostatic in the presence of the DPP-4 (the peptidase catalyzing the formation of GLP-1(9-36)) inhibitor sitagliptin. Glucagon receptor (GCGR) antagonism specifically prevents the inhibitory effects of GLP-1(9-36) whilst not affecting that of GLP-1(7-36). We conclude that GLP-1(7-36) and GLP-1(9-36) regulate glucagon secretion via interaction with GLP-1R and GCGR, respectively.