Activation of a cAMP-regulated Ca2+-Signaling Pathway in Pancreatic β-Cells by the Insulinotropic Hormone Glucagon-like Peptide-1

Abstract

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone that is currently under investigation for use in the treatment of diabetes mellitus. To investigate the Ca2+ signaling pathways by which GLP-1 may stimulate the secretion of insulin from pancreatic beta-cells, we examined its effects on the concentration of free intracellular Ca2+ ([Ca2+]i) while simultaneously determining what action it exerts on ion channel function. Measurements of [Ca2+]i were obtained from single rat beta-cells and from beta TC6 and HIT-T15 insulinoma cells loaded with the Ca2+ indicator fura-2, and changes in membrane potential and current were monitored using the perforated patch clamp technique. We report a previously undocumented action of GLP-1 and analogs of cAMP (8-bromo-cAMP, Sp- or Rp-adenosine 3',5'-cyclic monophosphothionate triethylamine) to raise [Ca2+]i that is attributable to the activation of a prolonged inward current designated here as IcAMP. Activation of IcAMP is associated with an increased membrane conductance, membrane depolarization, and triggers large increases of [Ca2+]i. IcAMP is primarily a Na+ current that is blocked by extracellularly applied La3+ or by intracellular administration of Ca2+ chelators (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/acetoxymethyl, EGTA) and which exhibits a reversal potential of about -26 mV. We propose that IcAMP results from the opening of nonselective cation channels that are activated by intracellular Ca2+ and cAMP and which might play an important role in the regulation of insulin secretion from pancreatic beta-cells.