Activation of a cAMP-regulated Ca-Signaling Pathway in Pancreatic -Cells by the Insulinotropic Hormone Glucagon-like Peptide-1 (*)

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 Ca signaling pathways by which GLP-1 may stimulate the secretion of insulin from pancreatic β-cells, we examined its effects on the concentration of free intracellular Ca ([Ca]) while simultaneously determining what action it exerts on ion channel function. Measurements of [Ca] were obtained from single rat β-cells and from βTC6 and HIT-T15 insulinoma cells loaded with the Ca 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 [Ca] that is attributable to the activation of a prolonged inward current designated here as I. Activation of I is associated with an increased membrane conductance, membrane depolarization, and triggers large increases of [Ca]. I is primarily a Na current that is blocked by extracellularly applied La or by intracellular administration of Ca chelators (1,2-bis(2aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid/acetoxymethyl, EGTA) and which exhibits a reversal potential of about −26 mV. We propose that I results from the opening of nonselective cation channels that are activated by intracellular Ca and cAMP and which might play an important role in the regulation of insulin secretion from pancreatic β-cells.