The islet beta-cell is the critical regulatory element in the glucose homeostasis system. Changes in insulin sensitivity and/or beta-cell mass elicit precise functional adaptions of the remaining beta-cells, thereby maintaining normoglycemia. How is that accomplished? The paradox is the normal driving force for insulin secretion and synthesis is glycemia, which is unchanged. The current application explores the mechanistic basis for the beta-cell adaptive changes that accompany a 60% pancreatectomy (Px) in rats. These rats maintain normoglycemia through a multiplicity of beta-cell adaptive responses: insulin secretion is upregulated to the """"""""normal"""""""" level because of an enhanced beta-cell glucose responsiveness, partial regeneration of the excised beta-cells, and maintenance of normal beta-cell insulin stores despite the augmented insulin secretion. This application proposes a model based on extensive preliminary data which integrates these diverse beta-cell adaptions into a defined sequence of events. The core element is a posttranslational upregulation of glucokinase catalytic activity; the increased flux through glycolysis augments insulin secretion and also """"""""tricks"""""""" the beta-cells to act as if in an hyperglycemic environment with a key response being to increase the islet Insulin Receptor Substrate-2 level (IRS-2) which recent information has suggested is an important regulator of proinsulin biosynthesis and the beta-cell replication rate. The result is a coordinated augmentation of insulin secretion and proinsulin synthesis which maintains a normoglycemic environment. This application uses biochemical, microscopic, and functional techniques to validate this model by fully characterizing the beta-cell adaptive responses in 60% Px rats and testing predicted perturbations with targeted pharmacologic agents by 1) Performing a comprehensive biochemical analysis of glycolysis in Px islets to identify the basis for the enhanced beta-cell glucose responsiveness; 2) Determining the regulation of islet IRS-2 in 60% Px rats; 3) Determining the role of hyperglycemia in initiating the enhanced beta-cell glucose sensing/responsiveness and increased IRS-2 level using phloridzin; 4) Determining the regulation of the beta-cell mass post 60% Px correlated to indices of beta-cell growth (both hyperplasia and hypertrophy) and apoptotic cell death; 5) Determining the regulation of the pancreas insulin content by focusing on proinsulin synthesis. These studies will provide fundamental mechanistic information regarding beta-cell adaptive mechanisms and how they regulate glucose homeostasis.
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