Etiology of Impaired Counterregulation in Glucose Homeostasis: Exploring the Roles of Glucagon, Somatostatin, Cortisol, and Epinephrine through Mathematical Modeling of Oral Glucose Tolerance Tests

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PROPOSAL SUMMARY Glucose homeostasis is tightly controlled in animals with plasma glucose levels maintained in a narrow range. While insulin, secreted by the beta cell, regulates by promoting efficient glucose disposal and suppressing glucose release from the liver when glucose levels rise, glucagon, secreted by the alpha cells, counterregulates by facilitating glucose release from the liver through glycogenolysis and gluconeogenesis when levels fall. Epinephrine, cortisol and growth hormone play supporting roles in glucose counterregulation. Impaired counterregulatory responses can lead to hypoglycemia, a potentially fatal condition. Hypoglycemia is frequently experienced by type 1 and late-stage type 2 diabetics, which limits the use of aggressive therapies in disease management. The etiology of this impaired counterregulation is not well understood. On the other hand, alpha cell dysfunction (impaired inhibition of glucagon secretion by glucose) leads to elevated fasting glucose levels and diminished early suppression of glucagon after glucose challenge. This form of impairment exacerbates type 2 diabetes and may contribute to its development and progression. I propose to study both forms of impairment. In oral glucose tolerance tests, diminished early suppression of glucagon followed by greater late glucagon suppression is observed in type 2 diabetics. This leads to worsened hyperglycemia followed by hypoglycemia. If the mechanism behind persistent glucagon suppression and delayed recovery is fully elucidated, it would be possible to protect against hypoglycemic events. I propose to explore the different mechanisms of somatostatin and GLP-1 mediated regulation of glucagon secretion by extending the parsimonious model of glucose-insulin- glucagon dynamics I developed. Reactive Hypoglycemia (RHG) occurs a few hours after ingesting a carbohydrate rich meal. Plasma glucose levels drop below 55 mg/dl and the patient displays neuroglycopenic symptoms which are relieved by glucose ingestion. There is currently no definitive explanation for this behavior. I will extend the glucose-insulin-glucagon minimal model I developed to include the other hormones and validate the model with data from OGTT studies of patients with RHG. I will also investigate the role of insulin in potentiating RHG. An overarching question in type 2 diabetes pathophysiology is the nature of disease progression from normal through prediabetic to overt diabetic state. The critical role of alpha cell dysfunction in disease development and progression has not yet been studied. I will interface the glucose-insulin-glucagon model I developed with existing disease progression models. This will identify the role of alpha cell dysfunction and glucagon action in development and acceleration of type 2 diabetes. I will study the etiology of hypoglycemia in aims 1 and 2 and the impact of alpha cell dysfunction in type 2 diabetes through longitudinal modeling of disease progression in aim 3.