Insulin secretion from beta cells of islets of Langerhans is mediated by second messengers such as cyclic AMP, cyclic CMP, calcium, inositol trisphosphate, diacylglycerol, and fatty acids. Nitric oxide generated from L-arginine by nitric oxide synthase is a recently identified mediator of cell activity. Normally, glucose stimulates insulin secretion. However, continuous glucose stimulation leads to a desensitization of the beta cell secretory response. Nitric oxide may also support insulin release under normal conditions, but in high concentrations induces cytotoxicity. In this project, freshly isolated islets, and in vitro models of glucose- induced beta cell desensitization mimicking non-insulin dependent diabetes mellitus (NIDDH), and cytokine-induced nitric oxide toxicity mimicking insulin -dependent diabetes mellitus (IDDM) will be investigated for the role of specific second messengers and metabolic pathways in beta cell function. Nitric oxide synthases will be characterized and various pharmacological agents investigated for possible protective activity against nitric oxide cytotoxicity. These studies will also determine whether heterologous desensitization of adenylate cyclase accounts in part for desensitization of insulin release, glucagon sensitivity and glycogen metabolism. The role of adenylate cyclase in mediating the beta cell responses to atrial natriuretic factor (ANF) receptor stimulation will be explored in order to define the role of cyclic GMP and cyclic AMP. Subtypes of ANF receptors will be characterized. The role of myo-inositol transport, Na+,K+-ATPase activity, phospholipase C and inositol trisphosphate production in glucose desensitization will also be determined since these cell activities play an important role in mobilizing and maintaining cellular Ca2+ levels and secretion. Whether the polyol pathway affects phosphoinositide pathway function and secretion during desensitization will be determined. Increased understanding of the signal-transduction mechanisms which modulate insulin secretion under normal conditions and in models of diabetes mellitus, and of the pharmacological interventions which may defuse cytotoxic events, will increase our knowledge of and ability to overcome diabetes mellitus.
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