Exogenous and endogenous ghrelin counteracts GLP-1 action to stimulate cAMP signaling and insulin secretion in islet β-cells

► Ghrelin inhibits glucose-dependent insulinotropic action of GLP-1 in rat islets. ► Ghrelin inhibits GLP-1-induced increases in cAMP and [Ca2+]i in islet β-cells. ► Ghrelin antagonist enhances cAMP production and GLP-1-induced insulin release. ► Exogenous and endogenous ghrelin modulates ability of GLP-1 and related medicines.