Insulinotropin: A Modulator Of B-Cell Glucose Signaling

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Glucagon-like peptide-1 (GLP-1; Insulinotropin) is an intestinally-derived blood glucose-lowering hormone that stimulates pancreatic insulin secretion and which is now under investigation for use as a therapeutic agent in treatment of type-2 diabetes mellitus. The Central Hypothesis presented here is that the beneficial insulinotropic action of GLP-1 at the islets of Langerhans results, in part, from an ability of GLP-1 to stimulate metabolism of D-glucose by the pancreatic beta- cells. Studies are presented demonstrating that GLP-1 augments the glucose-dependent production of ATP in beta-cells, as imaged by single photon counting of mitochondrially-targeted luciferase reporters. The action of GLP-1 is preceded by an increase of [Ca2+]i that reflects mobilization of Ca2+ from endoplasmic reticulum Ca2+ stores, and which is proposed to facilitate the enzymatic activity of mitochondrial dehydrogenases, thereby increasing [ATP]i. Subsequent inhibition of ATP-sensitive K+ channels (K-ATP) produces depolarization, oscillatory Ca2+ influx, and pulsatile exocytosis of insulin. To test our hypothesis, and to validate this model of GLP-1 signal transduction in human beta-cells, luminescence-based measurements of [ATP]i will be combined with fura-2 determinations of [Ca2+]i while monitoring K-ATP using the patch clamp technique. It will be determined: 1) if GLP-1 increases the potency and efficacy of glucose to stimulate production of ATP, 2) if stimulatory effects of GLP-1 on [Ca2+]i and [ATP]i explain how this hormone inhibits K-ATP, and 3) which second messengers and protein kinases mediate stimulatory effects of GLP-1 on beta-cell glucose metabolism. Our goal is to elucidate the complex cellular signal transduction properties of GLP-1 that explain its effectiveness for use in treatment of diabetes mellitus.