Glucagon Regulation of Glucose Metabolism
Condition: Insulin Secretion
Study type: Interventional
Phase: Phase 1

Summary: 
      Glucagon is a 30 amino acid peptide hormone that is produced exclusively in alpha-cells of
      the pancreatic islets. Glucagon binds to a G-protein coupled receptor and activates
      intracellular signaling by increasing the synthesis of cyclic AMP by adenylate cyclase. The
      glucagon receptor is most prominently expressed by hepatocytes and the cardinal action of
      glucagon is to stimulate hepatic glucose output by increasing glycogenolysis and
      gluconeogenesis. A deep body of literature supports physiologic actions of glucagon to
      maintain fasting blood glucose and counter-regulate hypoglycemia, and the current view of
      glucose metabolism is that insulin and glucagon have opposing and mutually balancing effects
      on glycemia. However, it has long been appreciated that glucagon actually stimulates insulin
      secretion and islet β-cells express the glucagon receptor and respond to its activation by
      increasing cAMP.

      The most potent stimulus for glucagon release is hypoglycemia and both low glucose per sé, as
      well as sympathetic nervous system activity are potent activators of the alpha-cell. However,
      glucagon is also stimulated by elevations of circulating amino acids, including after protein
      containing meals; this setting is one in which the release of glucagon during a period of
      elevated glycemia could contribute to postprandial insulin secretion. In fact, we have
      demonstrated that normal mice injected with glucagon while fasting (BG 75 mg/dl) have a
      prompt rise in blood glucose, whereas mice given glucagon while feeding (BG 150 mg/dl)
      increase insulin output 3 fold and have a decrease in glycemia. Moreover, in studies with
      isolated mouse and human islets we have demonstrated that glucagon stimulates insulin release
      by activating both the glucagon and GLP-1 receptors. This counter-intuitive observation has
      been reported by several other groups as well as ours.

      In the studies proposed herein we wish to extend our novel observations to humans. The
      possibility that glucagon acts in the fed state to promote insulin secretion and glucose
      disposal would change current views of physiology in both healthy and diabetic persons.
      Moreover, since one of the more promising area of drug development is the creation of
      peptides that activate multiple receptors (GLP-1 + glucagon, GLP-1 + GIP + glucagon) the
      results of our studies have potential implications for therapeutics as well.
    

Detailed description: 
      Subjects will have a screening visit for history, medication usage, and blood work; those who
      qualify will be offered participation. Subjects will be instructed to consume their usual
      diet, including at least 200 g carbohydrate, and not to engage in strenuous physical activity
      for the 3 days prior to a study. After an overnight fast they will present to the CRU at the
      Stedman Building on the Center for Living campus and have intravenous catheters placed in
      both forearms, one for infusion of test substances and the other for blood sampling; the
      sampling arm will be warmed with a heating pad to improve venous blood flow. All studies will
      start following withdrawal of several basal samples over an extended period:

        1. Glucagon infusion in fasting and hyperglycemic subjects: dose finding. Subjects receive
           graded doses of intravenous glucagon after a 12-14 hour fast. Doses will start at 10
           ng/kg/min and be increased to 50 and 100 ng/kg/min at 30 minute intervals. Glucose
           concentrations will be measured at the bedside using a YSI glucose analyzer. Plasma
           samples will be collected at 10 minute intervals for assay of insulin, C-peptide, and
           glucagon. These pilot studies will provide insight into the relative sensitivity of
           hepatic glucose production (fasting study) and insulin secretion (glucose infusion
           study) to glucagon. These studies will include up to 10 volunteers each.

        2. Effects of glycemia to mediate glucagon-stimulated hepatic glucose production (HGP) and
           insulin secretion. Following placement of intravenous catheters subjects will have an
           infusion of saline with a tracer dose of deuterated glucose for the remainder of the 300
           minute protocol. [6, 6]2H2 glucose will be started as a 4 mg/kg bolus over 5 minutes
           followed by a continuous infusion of 0.020-0.4 mg/kg/min. After a 2 hour equilibration
           period to label the glucose pool, subjects will have A) saline infusion, B) initiation
           of a hyperglycemic clamp, C) infusion of exendin-(9-39) 750 pmol/kg/min and a
           hyperglycemic clamp. The clamp will be generated with the infusion of a 20% solution of
           dextrose enriched to 2% with deuterated glucose. The infusion rate will be started at 30
           mg/kg/h and adjusted every 5 minutes until the blood glucose reaches ~150 mg/dl; the
           infusion will be adjusted thereafter to maintain this level of glycemia. Blood samples
           will be taken at 10 minute intervals throughout the study to measure: enrichment with
           [6, 6]2H2 glucose, insulin, glucagon and C-peptide. After 120 minutes of A) tracer
           alone, B) hyperglycemia, or C) exendin-9 plus hyperglycemia, subjects will receive
           glucagon as an infusion of 10-100 ng/kg/min for 30 or 60 minutes.

      The primary outcome variables from these experiment will be HGP and insulin secretion. The
      hypothesis to be tested is that glucagon given at fasting glucose levels will cause a rapid
      rise in HGP and blood glucose (50-150 mg/dl over basal) with a secondary rise of insulin
      secretion that follows the change in glycemia; and that glucagon given at mild hyperglycemia
      will promptly stimulate insulin secretion and limit the response of HGP. The protocol and
      predictions of results are depicted below.
    

Intervention name(s): Glucagon
Intervention type(s): Drug
Intervention description(s): Intravenous infusion of glucagon at fasting or elevated glycemia. Blockade of the GLP-1 receptor during hyperglycemia with and without glucagon.
Other intervention name(s): exendin-(9-39); hyperglycemia
Intervention MeSH terms: Glucagon
Allocation: Randomized
Intervention model: Crossover Assignment
Intervention model description: Within-subject design with each subject receiving 3 separate single day interventions.
Masking: None (Open Label)
Primary purpose: Basic Science
Keywords: glucagon; GLP-1 receptor; insulin secretion; glucose tolerance