Multisite regulation of insulin secretion by cAMP-increasing agonists: evidence that glucagon-like peptide 1 and glucagon act via distinct receptors.
The mechanisms by which glucagon-like peptide 1(7-36)amide (GLP-1[7-36]amide) potentiates insulin secretion were investigated by measurements of whole-cell K+ and Ca2+ currents, membrane potential, the cytoplasmic Ca2+ concentration ([Ca2+]i) and exocytosis in mouse pancreatic B-cells. GLP-1(7-36)amide (10 nM) stimulated glucose-induced (10 mM) electrical activity in intact pancreatic islets. The effect was manifested as a 34% increase in the duration of the bursts of action potentials and a corresponding 28% shortening of the silent intervals. GLP-1(7-36)amide had no effect on the electrical activity at subthreshold glucose concentrations (< or = 6.5 mM). In cultured B-cells, GLP-1(7-36)amide produced a decrease of the whole-cell ATP-sensitive K+ (KATP) conductance remaining at 5 mM glucose by approximately 30%. This effect was associated with membrane depolarization and the initiation of electrical activity. GLP-1(7-36)amide produced a protein-kinase-A-(PKA-) and glucose-dependent fourfold potentiation of Ca(2+)-induced exocytosis whilst only increasing the Ca2+ current marginally. The stimulatory action of GLP-1(7-36)amide on exocytosis was mimicked by the pancreatic hormone glucagon and exendin-4, a GLP-1 receptor agonist. Whereas the stimulatory action of GLP-1(7-36)amide could be antagonized by exendin-(9-39), this peptide did not interfere with the ability of glucagon to stimulate exocytosis. We suggest that GLP-1(7-36)amide and glucagon stimulate insulin secretion by binding to distinct receptors. The GLP-1(7-36)amide-induced stimulation of electrical activity and Ca2+ influx can account for (maximally) a doubling of insulin secretion. The remainder of its stimulatory action results from a cAMP/PKA-dependent potentiation of Ca(2+)-dependent exocytosis exerted at a stage distal to the elevation of [Ca2+]i.
Gromada
J
J
Ding
W G
WG
Barg
S
S
Renström
E
E
Rorsman
P
P
D000818:Animals; D002118:Calcium / Q000502:physiology; D000242:Cyclic AMP / Q000819:agonists* / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004594:Electrophysiology; D004789:Enzyme Activation; D005089:Exocytosis / Q000187:drug effects; D005934:Glucagon / Q000502:physiology*; D052216:Glucagon-Like Peptide 1; D004763:Glucagon-Like Peptides; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000201:enzymology / Q000502:physiology; D051379:Mice; D008815:Mice, Inbred Strains; D018377:Neurotransmitter Agents / Q000494:pharmacology; D010446:Peptide Fragments / Q000494:pharmacology; D010455:Peptides / Q000494:pharmacology / Q000502:physiology*; D011956:Receptors, Cell Surface / Q000502:physiology*
D007328:Insulin; D018377:Neurotransmitter Agents; D010446:Peptide Fragments; D010455:Peptides; D011956:Receptors, Cell Surface; C055453:glucagon-like peptide 1 (7-36)amide; D004763:Glucagon-Like Peptides; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D002118:Calcium