Functional Expression of the Rat Glucagon-like Peptide-I Receptor, Evidence for Coupling to Both Adenylyl Cyclase and Phospholipase-C

In man, glucagon-like peptide-I-(7-37) [GLP-I-(7-37)] is the most potent endogenous insulin-stimulating hormone. Although GLP-I-(7-37)-stimulated insulin secretion from the beta-cell is associated with an increase in cAMP accumulation, little is known about the signal transduction pathways used by this peptide. Using a cDNA encoding a high affinity rat GLP-I-(7-37) receptor [Kd = 4.1 nM for GLP-I-(7-37); Kd = 1 microM for GLP-I-(1-36) amide] expressed in a monkey kidney cell line (COS-7), we have demonstrated that the receptor is not only coupled to adenylyl cyclase, but is associated with an increase in the free cytosolic calcium level ([Ca2+]i). GLP-I-(7-37) increased both cAMP and [Ca2+]i in a dose-dependent manner and with equal potency (ED50 = 2.0 nM). The major source of the increased [Ca2+]i was found to be through the release of intracellular pools of Ca2+ associated with an increase in phosphoinositol turnover. Northern blot hybridization studies demonstrated that the GLP-I-(7-37) receptor gene was expressed in relatively high abundance in pancreatic islets and lung, but was also expressed at lower levels in the brain, liver, kidney, and skeletal muscle. This study establishes that a single GLP-I receptor species can mediate the effects of GLP-I-(7-37) through multiple G-protein-coupled signaling pathways, including the adenylyl cyclase system, phospholipase-C, and changes in [Ca2+]i.