In vivo studies of brain glycogen in hypoglycemia

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Hypoglycemia unawareness is a complication of intensive insulin therapy often encountered after episodes of iatrogenic hypoglycemia. The associated blunting of counterregulation has been reported to occur in healthy humans after a single episode of hypoglycemia. "rhemechanisms by which the brain detects low blood sugar concentrations are uncertain. The brain contains approximately 3 mM glycogen that may serve as a fuel during moderate hypoglycemia. In most tissues, glycogen metabolism is insulin- and glucose-sensitive. Brain glycogen thus provides an aspect of cerebral carbohydrate metabolism that is sensitive to alterations in glucose homeostasis such as those seen in diabetic patients. The purpose of this project is to determine the effect of hypoglycemia on brain glycogen and glucose metabolism and a potential involvement of glycogen metabolism in mediating hypoglycemia unawareness. In addition, we will assess the relationship between glucose transport and cerebral blood flow during hypoglycemia. The hypotheses of this project are (a) That brain glycogen concentration and metabolism are modulated by plasma glucose and/or insulin concentrations in vivo. (b) That brain glycogen can serve as a reservoir of glucose equivalents that are used for extended time periods during hypoglycemia in vivo when glucose transport becomes rate limiting for metabolism and cerebral blood flow is increased. (c) That following a hypoglycemic episode, the brain stores more brain glycogen such that longer and deeper subsequent hypoglycemia is necessary to deplete brain glycogen, which may provide a mechanism leading to hypoglycemia unawareness with the following specific aims: (1) To separate the effect of acutely elevated brain glucose concentrations from elevated plasma insulin concentrations on in vivo brain glycogen metabolism. (2) To determine the degree of supercompensation of brain glycogen following graded hypoglycemia and to establish that brain glycogen serves as a significant reservoir of glucosyl units during hypoglycemia. (3) To determine that depletion of brain glycogen as well as the acute increase in CBF is triggered when brain glucose concentrations approach the low Km of hexokinase. These aims will be achieved in rat brain using localized 1H and 13C NMR spectroscopy and perfusion-based fMRI.