HYPO-RES

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Intensively-treated patients with type 1 diabetes suffer from an excess risk of hypoglycemia, due in part to defective hormonal and hepatic glucose counterregulation. An appropriate increase in endogenous glucose production (EGP) is critical for the recovery from hypoglycemia. Glycogen breakdown is thought to be the primary mechanism responsible for the increase in EGP during the initial phase of hypoglycemia (the first 1-3 hours), followed by an increase in gluconeogenesis. Due to alteration in hepatic glycogen content and breakdown, and due to abnormalities in the gluconeogenic pathway, patients with type 1 diabetes may respond abnormally to stimuli directed to increase EGP. Due to the inherent difficulties in acquiring meaningful information regarding in vivo hepatic glycogen content (and variations), no data currently exist on the relative contributions of glycogenolysis and gluconeogenesis during hypoglycemia counterregulation in humans. Furthermore, the specific alterations in these pathways in patients with type 1 diabetes are unknown. Thus, our goals are to quantify the changes in liver glycogen during hypoglycemia counterregulation using nuclear magnetic resonance (NMR) spectroscopy, in order to determine the relative contribution (and time frame) of net hepatic glycogenolysis to EGP. Aim 1. In order to examine the mechanisms responsible for the activation of EGP in response to hypoglycemia we will analyze the specific and independent contribution of the main counterregulatory hormones to the rates of hepatic glycogen metabolism and GNG in non-diabetic subjects and in intensively-treated T1DM patients. We will determine the specific contributions of epinephrine and glucagon (applying a pancreatic clamp and adrenergic blockade) to the increases in glycogenolysis and GNG to the rates and time course of EGP excursions during experimental hypoglycemia in non-diabetic subjects and in T1DM patients. We will use C NMR spectroscopy (to measure net glycogenolysis) coupled with the 2H2O methodology (to measure the rates of GNG and glycogenolysis). Thus, the 2H2O C NMR measurements will provide the necessary information regarding GNG and glycogen metabolism. The addition of the (3-3H) glucose tracer will provide an additional tool to estimate the rates of EGP, as well as another indirect but independent method to estimate the contribution of GNG to EGP. Thus, we will determine the net contribution of epinephrine and the additive/synergistic effect of glucagon to glycogenolysis and GNG to the counterregulatory EGP response in non-diabetic subjects and intensively-treated T1DM patients. Aim 2. Since autoregulation of EGP may play a critical role in counterregulation, we will examine glycogen and GNG fluxes responsible for non-hormonal stimulation of EGP and the possible defects associated with these defense mechanisms in T1DM. Thus, using the same methodology described in Aim 1 we will measure the contribution of glycogenolysis and GNG to EGP at low plasma glucose concentrations (~70 mg/dl) independent of the activation of counterregulatory hormone secretion. Aim 3. In order to examine the modulatory effects of insulin and GK manipulation on hypoglycemia counterregulation, we will analyze the counterregulatory hormonal responses and the rates of hepatic glycogen metabolism and GNG in non-diabetic subjects and in intensively-treated T1DM patients during experimental hypoglycemia. We will examine the specific contribution of high vs. low plasma insulin concentrations during clamped hypoglycemia on the counterregulatory hormonal response, glycogenolysis and GNG and will explore the abnormalities in glycogen metabolism and GNG that may affect this contributions in T1DM. Aim 4. We will examine the effects of fructose on hypoglycemia counterregulation in nondiabetic and intensively-treated T1DM patients. In order to determine the mechanisms by which fructose enhances the counterregulatory response to hypoglycemia, and whether these mechanisms are GK - dependent, we will examine the relationship of this effect with respect to the duration and dose of fructose infusion. In parallel, we will examine this modulatory effect of fructose on hypoglycemia counterregulation vis a vis the pathways (glycogenolysis and GNG) responsible for the increase in EGP in nondiabetic subjects and in T1DM patients using the methodology described in Aim 1.