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

Abstract

Rat pancreatic a-and P-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 cAMP production in P-cells, but their relative importance for islet function is still unknown. The present study shows expression at the RNA level in p-cells of receptors for glucagon, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide 1(7-36) amide (GLP-I), while RNA from islet a-cells hybridized only with GIP receptor cDNA. Western blots confirmed that GLP-I receptors were expressed in P-cells and not in a-cells. Receptor activity, measured as cellular cAMP production after exposing islet P-cells for 15 min to a range of peptide concentrations, was already detected using 10 pmol/1 GLP-I and 50 pmol/1 GIP but required 1 nmol/1 glucagon. EC 50 values of GLP-I-and GIP-induced cAMP formation were comparable (0.2 nmol/1) and 45-fold lower than the EC g0 of glucagon (9 nmol/1). Maximal stimulation of cAMP production was comparable for the three peptides. In purified a-cells, 1 nmol/1 GLP-I failed to increase cAMP levels, while 10 pmol/1 to 10 nmol/1 GIP exerted similar stimulatory effects as in P-cells. In conclusion , these data show that stimulation of glucagon,

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…The earliest report in 1996 from the Habener laboratory used in situ hybridization on rat tissue sections (Bullock et al, 1996). Similar results were demonstrated a month later by Moens and co-workers who also performed western blot analysis on sorted islet α cells and did not see any GLP-1R protein levels (Moens et al, 1996). Heller, showed in 1997, while in the Habener laboratory, that 20 % of glucagon-positive cells and 76 % of somatostatin-positive cells costained for GLP-1R using a polyclonal rabbit antibody .…”

“…Heller and Aponte performed dose response analysis of GLP-1 treatment of whole islets and did not see any increase in glucagon secretion (Heller and Aponte, 1995). Moens and colleagues also failed to elicit cAMP production in rat α cells in response to 1 nM GLP-1 (Moens et al, 1996): but it should be remembered that it can be difficult to measure intracellular cAMP in small numbers of primary cells. Likewise when Franklin and co-workers measured glucagon secretion from fluorescence activated cell sorted (FACS) individual rat α cells GLP-1 was shown to have no effect on pyruvate-stimulated glucagon secretion (Franklin et al, 2005).…”

Alpha cell function in health and disease: influence of glucagon-like peptide-1

“…Although somewhat controversial [55], displaceable binding of GLP-1 to pancreatic cells that test positive for glucagon immunoreactivity has been reported [56], suggesting the existence of GLP-1 receptors on pancreatic alpha cells, and thus a direct effect of GLP-1 on glucagon secretion. Although GLP-1 augments Ca 2+ -dependent exocytosis in rat pancreatic alpha cells [57], GLP-1 has been shown to inhibit glucagon release from the perfused rat pancreas [58][59][60][61], isolated rat islets in static incubations [62], the perfused canine pancreas [60,63], the perfused porcine pancreas, the perfused porcine nonantral stomach [64] and cultured human islets [65].…”

G-protein-coupled receptors and islet function—Implications for treatment of type 2 diabetes

“…Both these hormones potently augment glucose-stimulated insulin secretion through increased cAMP (Mayo et al, 2003;Moens et al, 1996).…”

“…GIP receptors, which are linked to G s -protein, are predominantly expressed in β cells (Drucker, 2006). Furthermore, GIP has been shown to augment glucose-stimulated insulin secretion (Mayo et al, 2003;Moens et al, 1996) and to inhibit β cell apoptosis (Trümper et al 2001). …”