The proposed studies are important because they will clarify the mechanisms by which insulin regulates liver glucose uptake under feeding conditions, both when insulin is secreted by the pancreas as well as when it is injected during the treatment of people with diabetes. Consuming a diet high in fat and fructose severely impairs hepatic glucose uptake. We will determine the degree to which this is caused by the liver's defective response to insulin and glucose, we will define its molecular correlates, and we will explore whether this dysfunction can be effectively treated by interrupting sympathetic neural input or glucokinase activation. In addition, glucagon is implicated in diabetic hyperglycemia and we will investigate whether glucagon acting in the brain can affect hepatic glucose production.
The specific aims of this proposal are 1) to determine the impact of insulin's direct vs indirect effects on the regulation of hepatic glucose uptake and storage under hyperglycemic hyperinsulinemic conditions, 2) to characterize the source of impairment in hepatic glucose metabolism caused by a high fructose, high fat diet and to determine whether the withdrawal of sympathetic neural tone or glucokinase activation can overcome those defects, and 3) to determine the effect of brain glucagon action on the control of hepatic glucose production. Studies will be carried out in normal and diet induced glucose intolerant conscious dogs. Several weeks before study catheters will be inserted into the femoral artery, hepatic portal vein, and hepatic vein for measurement of hepatic glucose production and uptake, as well as in other vessels and sites as needed (splenic, jejunal, and jugular veins, carotid and vertebral arteries, the 3rd ventricle, etc.). The endocrine pancreas will be disabled with somatostatin, and insulin, glucagon and glucose will be infused as required by the study design. Liver glucose metabolism will be assessed using tracer and A-V difference techniques. Surgical and pharmacologic tools will also be used as needed to create the conditions required for the experiments. We will also assess alterations at the cellular level to better understand the physiologic phenotypes. The canine model is unique in that it allows for infusion (insulin, glucagon, glucose) into the hepatic portal vein, thus recreating real-life feeding conditions under steady state conditions which cannot be done in the human or rodent. In addition, the dog is unique in that it permits the measurement hepatic glucose uptake as well as output, which can only be done in a large animal. We believe that the knowledge gained from the proposed experiments will impact the development of new therapeutic approaches to the treatment of glucose intolerance and diabetes.
Insulin can inhibit hepatic glucose production (HGP) through both direct and indirect (adipose tissue, brain, alpha cell) mechanisms but much less is known about their role in controlling hepatic glucose uptake (HGU) because HGU cannot be directly measured in the human or rodent. It can, however, be studied in the dog and we will assess the impact of portal vein insulin delivery on HGU and the defects caused by insulin delivery through the peripheral route, as occurs in the treatment of diabetes. Likewise, we will explore the effect of a diet high in fat and fructose on the regulation of HGP and HGU with a particular focus on the role of the CNS, glucagon, and glucokinase.