Visit 1 Screening: All subjects will be performed indirect calorimetry to determine rates of energy expenditure and glucose/lipid oxidation for 40 minutes prior to all other procedures to obtain a fasting calorimetry. Subjects then will have a 75 gram oral glucose tolerance test to document the presence of normal glucose tolerance. Blood will be drawn at -30, -15, 0 and at 15, 30, 45, 60, 90 and 120 minutes thereafter for the determination of plasma glucose, insulin, C-peptide, glucagon, FFA, glycerol, GLP-1 and GIP concentrations. The total amount of blood to be taken during the OGTT is 148 ml or about 10 tablespoons. Participants will also undergo a total body fat (DXA) and hepatic fat content (MRS). Subjects will also have a routine physical exam, an ECG, weight, height, vitals and collection of prior medical history. The study team will also check blood glucose (sugar) and blood lipid (fat) levels. The total amount of blood that we will take for the screening blood tests, HbA1c, lipid, and other blood tests is 34 ml or about 2 1/3 tablespoons.
Visit 2 Adipose Tissue Metabolism Measurement: Subjects (n=12) will be admitted to the CRC at 4 PM on the day prior to study and received a standardized, weight maintaining diet containing 50% CHO: 30% fat: 20% protein. At 6 PM subjects will receive a continuous glucagon (3 ng/kg.min, N=6) or glucagon (6 ng/kg.min, N=6) for 12 hours. Subjects will remain fasting after 10 PM. Samples for glucagon, FFA, insulin and C-peptide will be collected at 1700, 1730, 1800, 1900, 2100, 2300, 0100, 0300, and 0500 hours.
At 6 AM on the following morning subjects will receive prime-continuous tracer infusions of the following for 4 hours (see figure 1):
(i) 14-C-glycerol (prime = 30 µCi; continuous infusion = 0.3 µCi/min) for determination of total body rate of lipolysis.
(ii) 3-3H-glucose (prime = 40 µCi; continuous infusion = 0.4 µCi/min) for determination of rates of total body glucose appearance and disappearance.
(iii) D2O (5 g/kg fat free mass at 9 PM orally) to determine the rate of de novo lipogenesis.
At 6 AM a surgical biopsy of subcutaneous abdominal adipose tissue will be performed for measurement of hormone sensitive (HSL) lipase activity, AMPK activity, and serine phosphorylation of HSL (see Figure 2), FGF-21, FAP, Beta-klotho. Insulin signaling (IR/IRS-1 tyrosine phosphorylation, PI-3 kinase activity, Akt) molecules, GLUT4, markers of inflammation (IkB, NF-kB, T4R4TLR4, JNK, p38 MAPK, M2/M1 macrophages, IL-1, IL-6, TNF alpha, MCP-1) also will be measured on the fat biopsy. Lipidomic analysis to quantitate the amount of and type of fat in the adipose tissue biopsy also will be performed. After surgical biopsy of adipose tissue, starting the tracer equilibration (6AM) blood will be drawn every 15-30 minutes for 4 hours (6-10 AM) for substrate (glucose, FFA, glycerol, triglycerides), hormone (glucagon, insulin, C-peptide, GLP-1, GIP, adiponectin, leptin, FGF21 \[active and total\], FAP \[degrading enzyme\]), radioisotope (14-C-glycerol and 3-3H glucose) specific activity. Participants will place their hand in a transparent plastic thermo-regulated box heated to 50-60°C (122-140°F) to ensure good blood flow to the hand. Pasma samples for radioisotope activity (background) will be obtained at 6 AM. At 9 AM continuous indirect calorimetry will be performed to determine rates of energy expenditure and glucose/lipid oxidation for 40 minutes.
The glucagon infusion will be continued through the 4-hour tracer infusion (i.e. 6AM-10AM). At the end of the glucagon infusion (\~10 AM) a repeat measurement of hepatic fat content will be obtained. A total of 366 ml of blood will be drawn.
At 8 AM the insulin sensitivity test will be done during the last two hours of the tracer infusion. During the 2 hours of the insulin sensitivity test, the insulin (60 mU/m2 ⋅ min) and glucose will be given through the catheter in the vein in the subject's arm to determine rates of whole-body insulin sensitivity. There are no expect adverse reactions to be experienced by the subjects during both infusions.
