Glucagon Suppression by Hyperglycemia in the Presence and Absence of Amino Acid Infusion

T2DM and prediabetes are characterized by abnormal post-prandial suppression of glucagon, which contributes to postprandial hyperglycemia by increasing endogenous glucose production (EGP). In rodents, altered glucagon signaling changes α-cell function and mass - an effect mediated by changes in circulating AA concentrations. Are the elevated concentrations of branched-chain AA and other AA metabolites in T2DM a cause or an effect of global α-cell dysfunction? To measure glucagon secretion, as well as glucagon action, we have developed a population model of glucagon kinetics allowing us to deconvolve secretion from glucagon concentrations in a manner similar to Van Cauter's model for insulin secretion from C-peptide. This enables better characterization of α-cell function in humans. In addition to our novel methodology, we can characterize α-cell responsiveness to a graded glucose infusion, by quantifying (G50) - the change in glucose concentration necessary to suppress glucagon secretion by 50%. These experiments will determine if the glucagon secretion in response to AA differs in obese individuals with T2DM from that observed in obese individuals without T2DM.

Inclusion criteria

- Obese Subjects with Type 2 Diabetes:

• HbA1c ≤ 8.5% (type 2 diabetic subjects).
• HbA1c ≤ 6.5% (obese and lean subjects).
• BMI ≥ 28 Kg/M^2 (Obese subjects with and without type 2 diabetes).
• BMI ≤ 25 Kg/M^2 (Lean subjects without type 2 diabetes).
• Use of sulfonylureas or metformin only (type 2 diabetec subjects).
• For female subjects: negative pregnancy test at the time of enrollment or study.
• No history of prior upper abdominal surgery such as adjustable gastric banding, pyloroplasty and vagotomy.
• No active systemic illness or malignancy.
• No symptomatic macrovascular or microvascular disease.
• No contraindications to MRI (e.g., metal implants, claustrophobia).
• Hematocrit > 35%.
• TSH > 0.4 or < 5.5.
• Consumption of < 2 alcohol drinks per day or < 14 per week or a negative AUDIT questionnaire.

Exclusion criteria

- Obese Subjects with Type 2 Diabetes:

• HbA1c ³ 8.5%
• BMI ≤ 28 Kg/M2
• Use of insulin or agents other than sulfonylureas or metformin.
• For female subjects: positive pregnancy test at the time of enrollment or study
• History of prior upper abdominal surgery such as adjustable gastric banding, pyloroplasty and vagotomy.
• Active systemic illness or malignancy.
• Symptomatic macrovascular or microvascular disease.
• Contraindications to MRI (e.g., metal implants, claustrophobia).
• Hematocrit < 35%
• TSH < 0.4 or > 5.5.
• Consumption of > 2 alcohol drinks per day or > 14 per week or a positive AUDIT questionnaire.

Inclusion criteria

- Obese Subjects without Type 2 Diabetes:

• BMI ≥ 28 Kg/M2.
• > 5% liver fat content, as determined by MRI using the proton density fat fraction (PDFF) technique.

Exclusion criteria

- Obese Subjects without Type 2 Diabetes:

• HbA1c ≥ 6.5%
• BMI ≤ 28 Kg/M2
• Use of any glucose-lowering agents including metformin or sulfonylureas.
• For female subjects: positive pregnancy test at the time of enrollment or study
• History of prior upper abdominal surgery such as adjustable gastric banding, pyloroplasty and vagotomy.
• Active systemic illness or malignancy.
• Symptomatic macrovascular or microvascular disease.
• Contraindications to MRI (e.g., metal implants, claustrophobia).
• Hematocrit < 35%
• TSH < 0.4 or > 5.5.
• Consumption of > 2 alcohol drinks per day or > 14 per week or a positive AUDIT questionnaire.

Inclusion criteria

- Lean subjects without Diabetes:

• BMI ≤ 25 Kg/M^2).

Exclusion criteria

- Lean Subjects without Diabetes:

• HbA1c ≥ 6.5%.
• BMI ≥ 25 Kg/M^2.
• Use of any glucose-lowering agents including metformin or sulfonylureas.
• For female subjects: positive pregnancy test at the time of enrollment or study.
• History of prior upper abdominal surgery such as adjustable gastric banding, pyloroplasty and vagotomy.
• Active systemic illness or malignancy.
• Symptomatic macrovascular or microvascular disease.
• Contraindications to MRI (e.g., metal implants, claustrophobia).
• Hematocrit < 35%.
• TSH < 0.4 or > 5.5.
• Consumption of > 2 alcohol drinks per day or > 14 per week or a positive AUDIT questionnaire.
• Liver fat content ≥ 5% as determined by MRI using the proton density fat fraction (PDFF) technique.