ACTIVATION OF A CAMP-REGULATED CA2-SIGNALING PATHWAY IN PANCREATIC BETA-CELLS BY THE INSULINOTROPIC HORMONE GLUCAGON-LIKE PEPTIDE-1()

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotro pic hormone that is currently under investigation for use in the treat ment 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 intra cellular Ca2+ ([Ca2+](i)) while simultaneously determining what action it exerts on ion channel function. Measurements of [Ca2+](i) were obt ained from single rat beta-cells and from beta TC6 and HIT-T15 insulin oma cells loaded with the Ca2+ indicator fura-2, and changes in membra ne potential and current were monitored using the perforated patch cla mp technique. We report a previously undocumented action of GLP-1 and analogs of cAMP (8-bromo-cAMP, Sp- or Rp-adenosine 3',5'-cyclic monoph osphothionate triethylamine) to raise [Ca2+](i) that is attributable t o the activation of a prolonged inward current designated here as I-cA MP. Activation of I-cAMP is associated with an increased membrane cond uctance, membrane depolarization, and triggers large increases of [Ca2 +]. I-cAMP is primarily a Na+ current that is blocked by extracellular ly applied La3+ or by intracellular administration of Ca2+ chelators ( 1,2-bis(2- aminophenoxy)ethane N,N,N',N'-tetraacetic acid/acetoxy-meth yl EGTA) and which exhibits a reversal potential of about -26 mV. We p ropose that I-cAMP results from the opening of nonselective cation cha nnel that are activated by intracellular Ca2+ and cAMP and which might play an important role in the regulation of insulin secretion from pa ncreatic beta-cells.