Expression and functional activity of glucagon, glucagon-like peptide I, and glucose-dependent insulinotropic peptide receptors in rat pancreatic islet cells

Rat pancreatic alpha- and beta-cells are critically dependent on hormonal signals generating cyclic AMP (cAMP) as a synergistic messenger for nutrient-induced hormone release, Several peptides of the glucagon-secretin family have been proposed as physiological Ligands for cAIMP production in beta-cells, but their relative importance for islet function is still unknown. The present study shows expression at the RNA level in beta-cells of receptors for glucagon, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide I(7-36) amide (GLP-I), while RNA from islet alpha-cells hybridized only with GIP receptor cDNA, Western blots confirmed that GLP-I receptors were expressed in beta-cells and not in or-cells, Receptor activity, measured as cellular cAIMP production after exposing islet beta-cells for 15 min to a range of peptide concentrations, was already detected using 10 pmol/l GLP-I and 50 pmol/l GIP but required 1 nmol/l glucagon, EC(50) values of GLP-I- and GIP-induced cAMP formation were comparable (0.2 nmol/l) and 45-fold lower than the EC(50) of glucagon (9 nmol/l), Maximal stimulation of cAMP production was comparable for the three peptides, In purified alpha-cells, 1 nmol/l GLP-I failed to increase cAMP levels, while 10 pmol/l to 10 nmol/l GIP exerted similar stimulatory effects as in beta-cells In conclusion, these data show that stimulation of glucagon, GLP-I, and GIP receptors in rat beta-cells causes cAMP production required for insulin release, while adenylate cyclase in alpha-cells is positively regulated by GIP.