Glp-1 pathway (simplified)

Glp-1 receptor is expressed in β-cells of the pancreas. After the ingested nutrient is digested and reaches the intestines, specialized cells secrete the incretin Glp-1 (glucacon like peptide-1). Glp-1 binds and activates the Glp-1 receptor (a G-protein coupled receptor). The resulting activation results in a signalling cascade that promotes insulin synthesis and secretion from the pancreatic β-cells.

The G protein (attached to the G-protein coupled receptor)  is a heterotrimer composed of α,β, and γ subunits. The alpha subunit detaches and reattaches like a switch based on its interaction with GTP and GDP respectively.