Total-body Glucose Utilization in Obesity

The study consists of two separate parts: the dose escalation study and the part studying the effects of emotions and insulin on blood flow.

All participants will first undergo a screening visit starting with signing an informed consent form. Then they will undergo a physical examination, are interviewed about their medical history. Finally, some routine laboratory tests are collected, and a 75 g oral glucose tolerance test is performed. The participants who are eligibile for the study will continue to the PET studies.

The dose escalation study will be completed first. In this 8 participants will be studied once with Siemens Biovision Quadra PET/CT scanner. They will receive eight doses of oxygen-15 labelledradiowater (H2\[15O\]O) ranging from 25 to 700 MBq, with each dose repeated twice to evaluate repeatability of each dose. These participants will not undergo any further studies.

For the other subjects, there will be two more PET study visits.

On the first visit, the effects of emotions, evoked by short movie clips or cold exposure of one leg, on whole-body blood flow will studied with oxygen-15 labelled radiowater. Each participant will be studied 8 times in one session, with repeated H2\[15O\]O boluses of 400 MBq followed by 7.5 min dynamic PET scans with Siemens Biovision Quadra PET/CT scanner and a 5 min recovery and wash-out period.

On the other visit the effects of insulin on circulation will be studied by performing a hyperinsulinemic, euglycemic clamp during the PET scan. During the clamp, fast-acting insulin (NovoRapid, NovoNordisk) is administered intravenously at a fixed rate (40 mU/m2 body surface area/min). Plasma glucose is monitored every 5-10 minutes, and 20 % glucose is administered at a varying rate to maintain euglycemia (5.0 mmol/L).

At the scanner a fasting H2\[15O\]O PET/CT scan (400 MBq) will be performed first. After this, the hyperinsulinemic, euglycemic clamp is started, and the radiowater scans are repeated 10 minutes and 50 minutes into the insulin infusion. 60 minutes after the start of clamp, 100 MBq of glucose analogue tracer \[18F\]fluorodeoxyglucose is administered and a 50 min dynamic PET scan is started. After the scan, insulin infusion is discontinued and participants are monitored until plasma glucose levels are adequate for safe discharge.

For a subgroup of participants, skeletal muscle microvascular bloodflow will also be assessed with contrast-enhanced ultrasound performed during fasting and 30 minutes after the start of clamp. The participants will be administered intravenous contrast agent, and after steady plasma concentration is reached (2.5 min), the contrast agent micro bubbles are destroyed with a high-intensity ultrasound signal, and the rate at which small arterioles are refilled is measured.

Data will be analysed using in-house developed programs. Regions of interest will be drawn manually for peripheral tissues (skeletal muscle, visceral, brown and subcutaneous adipose tissue, liver, kidney, intestines) and using automated segmentation tools for the brain.

• Effects of insulin on skeletal muscle microvascular perfusion [Time frame: Change from fasting condition to 10 minutes and 50 minutes after the start of insulin infusion]
• Tissue insulin-stimulated glucose uptake [Time frame: Measured from 60 to 110 minutes after the start of insulin infusion]
• Effects of emotions on whole-body blood flow [Time frame: Measured during 60-second activation blocks (visual stimuli) or during 5 min cold-exposure]
• Range of effective and feasible radiowater doses with whole-body PET/CT [Time frame: During repetitive H2[15O]O scans (a total of 120 minutes)]