Glucagon secretion from α-cells has long been viewed as primarily a counterregulatory mechanism - e.g. an agent with a role to prevent blood sugar from decreasing to levels that compromise function. Our group, along with other researchers, have begun to identify a much more complex role for α-cells, raising questions about when and how glucagon may influence blood glucose levels. This proposal looks to detail proglucagon peptide secretion from α-cells and the impact this has on β-cell function and glucose tolerance, in preclinical studies of human islets and translational studies in human subjects.

This protocol registration describes Aim 2 from this NIH grant which involves 2 study populations and separate protocols but addresses a common question. Aim 3 in the grant is focused on a separate hypothesis and will be conducted and published separately from Aim 2.

Subjects will undergo screening for medical history, medication usage, and blood work; those who qualify will be offered participation. Study participation will last approximately 4-5 weeks depending on appointment availability.

Aim 2A: Each participant will have two 5-hour hyperglycemic clamp procedures to test the effect of fasting glucagon-like peptide 1 (GLP-1) action before and after experimental insulin resistance. The effect of endogenous proglucagon peptides (glucagon and GLP-1) to stimulate insulin secretion will be determined by blockade of the GLP-1 receptor with the antagonist exendin-9 (Ex-9) during glucose infusions. Insulin secretion experiments will be repeated before and after induction of insulin resistance. To induce insulin resistance, subjects will take dexamethasone, a synthetic glucocorticoid that has been shown in published studies and in a pilot study by our group to reduce insulin sensitivity by ~30%.

Aim 2B: This study will recruit non-diabetic subjects with obesity. They will be studied on two occasions using a 4.5-hour procedure with a hyperglycemic clamp to measure insulin secretion, followed by a hyperinsulinemic, euglycemic clamp to measure insulin sensitivity. This procedure will be done 2 times, once with saline infused during hyperglycemia as a control, and once with exendin-9 given during hyperglycemia to determine the role of GLP-1 receptor action.

Aim 2A: Inclusion Criteria:

• Age 18-45
• Body Mass Index (BMI) < 27.0
• Fasting plasma glucose of ≤ 95 mg/dL or HbA1c value ≤ 5.8% as measured at screening visit

Exclusion Criteria:

• Active medical disease: e.g. active infectious, inflammatory, neurodegenerative or mental health disorders
• Personal history of diabetes or pancreatitis
• Personal history of cardiac, gastrointestinal, renal or liver disease
• Immediate family history of diabetes
• Renal insufficiency (eGFR < 60 mL/kg/min)
• Anemia (hematocrit < 34%) as measured at screening visit
• Pregnant females
• Poor vein access
• Consumption of daily medications that alter glucose metabolism of GI function (glucocorticoids, psychotropics, narcotics, metoclopramide)
• Apparent sensitivity to the study peptide as determined by the skin test

Aim 2B: Inclusion Criteria:

• Age 35-60
• Body Mass Index (BMI) ≥ 27.0
• Fasting plasma glucose of < 126 mg/dL or HbA1c value < 6.5% as measured at screening visit

Exclusion Criteria:

• Active medical disease: e.g. active infectious, inflammatory, neurodegenerative or mental health disorders
• Personal history of diabetes or pancreatitis
• Personal history of cardiac, gastrointestinal, renal or liver disease
• Immediate family history of diabetes
• Renal insufficiency (eGFR < 60 mL/kg/min)
• AST and/or ALT levels > 3x the upper limit of the normal range
• Anemia (hematocrit < 34%) as measured at screening visit
• Pregnant females
• Poor vein access
• Consumption of daily medications that alter glucose metabolism of GI function (glucocorticoids, psychotropics, narcotics, metoclopramide)
• Apparent sensitivity to the study peptide as determined by the skin test