GLP-1 action in L6 myotubes is via a receptor different from the pancreatic GLP-1 receptor

The incretin hormone glucagon-like peptide-1 (GLP-1)-(7-36) amide is best known for its antidiabetogenic actions mediated via a GLP-1 receptor present on pancreatic endocrine cells. To investigate the molecular mechanisms of GLP-1 action in muscle, we used cultured L6 myotubes. In L6 myotubes, GLP-1 enhanced insulin-stimulated glycogen synthesis by 140% while stimulating CO2 production and lactate formation by 150%. In the presence of IBMX, GLP-1 diminished cAMP levels to 83% of IBMX alone. In L6 myotubes transfected with pancreatic GLP-1 receptor, GLP-1 increased cAMP levels and inhibited glycogen synthesis by 60%. An antagonist of pancreatic GLP-1 receptor, exendin-4-(9-39), inhibited GLP-1-mediated glycogen synthesis in GLP-1 receptor-transfected L6 myotubes. However, in parental L6 myotubes, exendin-4-(9-39) and GLP-1-(1-36) amide, an inactive peptide on pancreatic GLP-1 receptor, displaced I-125-labeled GLP-1 binding and stimulated glycogen synthesis by 186 and 130%, respectively. These results suggest that the insulinomimetic effects of GLP-1 in L6 cells are likely to be mediated by a receptor that is different from the GLP-1 receptor found in the pancreas.