Intensive glycemic control reduces the onset and slows the progression of microvascular complications in patients with type 1 diabetes (T1DM). Unfortunately, the major obstacle preventing the goal of euglycemia in T1DM is the associated three-fold increase in severe hypoglycemia. Many studies have identified acquired failure of neuroendocrine and in particular autonomic nervous system (ANS), counterregulatory responses as a cause of the increased prevalence of hypoglycemia occurring in intensively treated T1DM. Central to the development of the acquired ANS failure are prior episodes of hypoglycemia. The mechanisms responsible for prior hypoglycemia resulting in acquired ANS and neuroendocrine failure during subsequent hypoglycemia need to be determined. During the last several years, there has been a growing interest in the role played by cerebral mechanisms in regulating ANS and neuroendocrine responses during hypoglycemia. Brain corticosteroid and gamma-aminobutyric acid (A) (GABA (A)) receptors are known to exert inhibitory influences on ANS and neuroendocrine function. We hypothesize that activation of brain corticosteroid (type 1 MR and/or type 2 GR) and/or GABA (A) receptors by prior hypoglycemia may be important mechanisms responsible for the blunted ANS and neuroendocrine counterregulatory responses occurring in intensively treated T1DM patients. Currently, there are no readily available adjunct treatments that protect the ANS from the deleterious effects of repeated hypoglycemia. In this proposal, we will conduct experiments to determine whether the adrenal hormone dihydroepiandrostenedione-sulfate (DHEAS) which has recognized anticorticosteroid and anti GABA (A) receptor activity may preserve ANS function during prior episodes of hypoglycemia in T1DM. Both antecedent episodes of hypoglycemia and exercise can blunt epinephrine responses during subsequent episodes of hypoglycemia. However, it is unknown whether episodes of hypoglycemia and/or exercise will also reduce the physiologic actions of this key hormone in intensively treated T1DM patients. This information is needed as treatment strategies, aimed at increasing epinephrine responses during hypoglycemia in T1DM, will have limited effectiveness if target organs are unresponsive. The studies outlined in this proposal are therefore focused at determining the in-vivo mechanisms regulating ANS counterregulatory failure during hypoglycemia in T1DM humans. Experiments will use the glucose clamp technique. ANS responses to hypoglycemia will be assessed by measuring circulating catecholamines, pancreatic polypeptide, muscle sympathetic nerve activity, symptom scores, and heart rate variability. Neuroendocrine responses will be determined by measuring growth hormone, cortisol and ACTH. Metabolic counterregulatory mechanisms will be determined by measuring glucose and glycerol turnover, skeletal muscle metabolism with muscle biopsies, substrate levels, and substrate oxidation via indirect calorimetry.
The specific aims of this proposal are to determine: 1) the roles played by gamma aminobutyric acid (GABA (A)) and corticosteroid receptors in the development of hypoglycemia associated autonomic failure in type 1 diabetes, 2) if dihydroepiandrostenedione-sulfate can prevent hypoglycemia associated autonomic failure in type 1 diabetic man, 3) if prior hypoglycemia and/or exercise blunts subsequent metabolic/cardiovascular responses to epinephrine in type 1 diabetic man, and 4) if selective serotonin reuptake inhibitors blunt counterregulatory responses to hypoglycemia in type 1 DM man.
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