Glucagon-like peptide-1 stimulates insulin secretion but not phosphoinositide hydrolysis from islets desensitized by prior exposure to high glucose or the muscarinic agonist carbachol

Metabolism. 1996 Feb;45(2):273-8. doi: 10.1016/s0026-0495(96)90066-9.

Abstract

In the present series of experiments, the ability of the postulated incretin factor, glucagon-like peptide-1 (GLP-1), to stimulate insulin release from desensitized islets was determined. Compared with responses observed from control islets incubated for 3.5 hours with 5.6 mmol/L glucose alone, prior exposure to 10 mmol/L glucose, 20 mmol/L glucose, or 10 micromol/L carbachol reduced peak second-phase insulin release rates to a subsequent 20-mmol/L glucose stimulus by 63%, 81%, or 70%, respectively. Efflux of 3H-inositol from prior high-glucose- or carbachol-exposed islets was abolished and accumulation of inositol phosphates (IPs) in response to 20 mmol/L glucose was reduced. Further addition of 10 nmol/L GLP-1 together with 20 mmol/L glucose significantly increased insulin output from desensitized islets. Carbachol (10 micromol/L) preexposure also abolished the subsequent insulin secretory and 3H-inositol efflux responses to 8 mmol/L glucose plus 10 micromol/L carbachol. Inclusion of 10 nmol/L GLP-1 together with 8 mmol/L glucose plus 10 micromol/L carbachol improved but did not normalize secretion from these islets. These improvements in secretory responsiveness from high-glucose- or carbachol- desensitized islets occurred despite the lack of any apparent restorative effect of GLP-1 on agonist-induced increases in phosphoinositide (PI) hydrolysis. Finally, unlike the situation observed with carbachol or high-glucose preexposure, chronic exposure of islets to GLP-1 (100 nmol/L) did not desensitized islets to a subsequent 20 mmol/L glucose stimulus. We conclude from these studies that the incretin factor GLP-1 may play an important role in maintaining insulin output from islets in which phospholipase C (PLC)-mediated hydrolysis of islet PI pools in impaired. GLP-1 may prevent a further decline in beta-cell function and the associated deterioration in glucose tolerance that accompanies chronic exposure of islets to one of several agonists, including high glucose.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Carbachol / administration & dosage*
  • Carbachol / pharmacology
  • Glucagon / pharmacology*
  • Glucagon-Like Peptide 1
  • Glucose / administration & dosage*
  • Glucose / pharmacology
  • Hydrolysis
  • Insulin / metabolism*
  • Insulin Secretion
  • Islets of Langerhans / drug effects
  • Islets of Langerhans / metabolism*
  • Male
  • Muscarinic Agonists / administration & dosage*
  • Muscarinic Agonists / pharmacology
  • Peptide Fragments / pharmacology*
  • Phosphatidylinositols / metabolism*
  • Protein Precursors / pharmacology*
  • Rats
  • Rats, Sprague-Dawley

Substances

  • Insulin
  • Muscarinic Agonists
  • Peptide Fragments
  • Phosphatidylinositols
  • Protein Precursors
  • Glucagon-Like Peptide 1
  • Carbachol
  • Glucagon
  • Glucose