Preliminary data suggest the presence of the delta-to-beta cell endocrine axis within the islet in which intraislet somatostatin inhibits insulin secretion. The purpose of this competitive renewal project is to prove the following hypotheses: 1) intraislet somatostatin inhibits secretion via a delta-to-beta cell endocrine axis in the human, rat and mouse pancreas and that the effect is glucose-dependent, 2) the somatostatin receptor subtype responsible for the inhibition of insulin is species-specific, and 3) genetic ablation of the somatostatin receptor subtype 5 will alter insulin secretion and glucose homeostasis in the mouse. The effect of intraislet somatostatin will be determined by examining the insulin response to immunoneutralization of intraislet somatostatin with antibodies and FAb fragments of antibodies directed against somatostatin in isolated perfused human, rat and mouse pancreas models. Electron microscopy will help to determine the compartment of immunoneutralization. The somatostatin receptor subtype responsible for the inhibition of insulin will be determined by examining the response of insulin secretion to infusions of specific somatostatin receptor subtype agonists in these models. Immunohistochemistry will be performed using polyclonal antibodies directed against SSTR 1-5 to determine which receptor subtypes are present in the human, rat and mouse pancreas. It appears that the somatostatin receptor subtype 5 is responsible for inhibition of mouse insulin secretion, therefore two models will be developed using state-of-the-art transgenic techniques: the first is a total somatostatin receptor subtype 5 gene ablation model and the second is a beta cell-specific somatostatin receptor subtype 5 gene ablation model. In vivo and in vitro physiology studies will be performed in these mice to determine the effect of genetically altering the delta-to-beta cell endocrine axis on insulin secretion and glucose homeostasis. The pancreas of the gene-ablated mice will be studied using immunohistochemistry with antibodies directed against the somatostatin receptor subtypes to determine if the somatostatin receptor subtypes are altered in the islets of the gene ablated mice. These studies will help elucidate physiologic mechanisms regulating insulin secretion and will determine whether there are species differences in this regulation. Furthermore, it will be determined whether there are pathophysiologic consequences to genetically altering these mechanisms.
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