Recent research has suggested that intranasally administered insulin can reach the brain quickly without passing through circulation and evoke increased insulin sensitivity and tissue glucose consumption during insulin stimulation (low-dose hyperinsulinemic, euglycemic clamp). It is still not known what mechanism causes these changes or what tissues are involved in this.

In this study, the changes in tissue-specific insulin sensitivity and glucose uptake will be investigated by using glucose-analogue radiotracer ([18F]-fluorodeoxyglucose) with positron emission tomography (PET) imaging during insulin stimulation. Ten healthy males are studied, each receiving nasal sprays containing insulin or placebo in a randomized order on two separate days. After spray administration, glucose uptake in skeletal muscle, liver, subcutaneous and visceral adipose tissue, myocardium, intestines, brown adipose tissue and brain assessed by PET imaging and glucose uptake in these tissues is analyzed. Endogenous glucose production is calculated facilitating the measurements glucose and radiotracer uptake in tissues and tracer loss into urine.

As skeletal muscle consumes most of the glucose available, it is likely that administration of insulin sprays will result in an increased uptake in this tissue. Some increase in glucose uptake might also be seen in other tissue types after insulin spray versus placebo spray administration.

The objective of this study is to investigate the effects of intranasally administered insulin versus placebo on tissue specific glucose uptake of the key metabolic organs as well as on endogenous glucose production.

SAMPLE SIZE:

A total of ten (n=10) healthy males are planned to be included, and the study is conducted with a cross-over design with each subject being investigated twice, once using intranasal insulin and once using intranasal placebo in single-blinded randomized order.

SCREENING VISIT/ELIGIBILITY:

Main inclusion and exclusion criteria will be evaluated at a screening visit. This visit will include a frequently-sampled 75 g oral glucose tolerance test as well as anthropometric measurements and clinical examination.

PET STUDY VISITS:

On the PET study days two cannulas will be inserted, one in a radial or an antecubital vein for tracer injection and infusion of glucose and insulin, another in the opposite radial or antecubital vein for blood sampling. The arm where the samples are taken will be kept warm to obtain arterialized venous blood. The subjects will rest in supine position on the PET/CT scanner bed during the study.

HYPERINSULINEMIC EUGLYCEMIC CLAMP:

Whole-body insulin sensitivity will be assessed by hyperinsulinemic euglycemic clamp with an primed insulin infusion rate of 0.25 mU/kg/min. To maintain the glucose level at 5.0 mmol/l intravenous glucose infusion is administered individually as needed to maintain euglycemia.

NASAL SPRAY ADMINISTRATION:

After 30 minutes of intravenous insulin infusion, the subjects will receive either 160 IU human insulin or placebo intranasally. The small amount of insulin known to be absorbed into circulation after the insulin nasal spray will be mimicked on placebo day. Therefore subjects will be administered 2.5 mU/kg of additional intravenous insulin over 15 minutes together with placebo spray administration.

LABORATORY SAMPLES:

To verify comparable insulin levels in circulation during the study, serum insulin and C-peptide levels will be determined in fasting state, and repeatedly throughout the experiment.

BLINDING:

The clinician and the study nurse are aware which spray is given, but subjects are blinded and additional insulin infusion on placebo day is done in a manner that the subjects remain blinded. The sprays are given on a randomized order. The randomization code was generated by a statistician using SAS, version 9.3 for Windows. The randomization block size was four and the method was permuted block randomization. The randomization was done for 16 subjects in case of discontinuations.

HEART RATE VARIABILITY:

Heart rate variability is recorded to evaluate changes in autonomous nervous system activity after spray administration on both visits. Polar RS800CX heart rate monitor (Polar Electro Ltd., Kempele, Finland) with two electrodes is used.

FDG-PET-IMAGING:

70 minutes after the start of clamp and 40 minutes after the application of nasal sprays the positron emission tomography (PET) scanning is started with the injection of glucose-analogue tracer labeled with positron-emitting fluorine-18 ([18F]-fluorodeoxyglucose, [18F]-FDG). PET/CT, GE DiscoveryTM ST System (General Electric Medical Systems, Milwaukee, WI, USA) with final resolution of 3.75 mm in PET images is used. All data will be corrected for dead-time, decay and measured photon attenuation. Dynamic PET-scans will be reconstructed with iterative reconstruction method. Radiotracer [18F]-FDG is produced in the PET Centre radiochemistry laboratory. Plasma radioactivity is measured with an automatic gamma counter by a medical technologist in the laboratory of the Turku PET Centre (Wizard 1480 3", Wallac, Turku, Finland).

PET-IMAGE ANALYSIS:

To determine tissue-specific glucose uptake during insulin stimulation from PET data, time-activity-curves of tissue activity and activity measured from plasma during the scanning are analysed graphically to define tracer's fractional uptake (Ki). To calculate glucose uptake rate (reported as µmol/kg/min), Ki is multiplied with plasma glucose level, divided with a constant that accounts for the differences in transport and dephosphorylation rates between D-glucose and FDG (lumped constant, LC) and divided with tissue specific gravity. Based on previous studies the LC is 1,2 for skeletal muscle; 1,0 for heart and liver; 1,14 for adipose tissue; 1,1 for intestines and 0,8 for brain. After the PET/CT an urine sample will be collected to quantify endogenous glucose production (EGP). This will be determined by subtracting urine loss and glucose infusion rate from rate of glucose disappearance.

Inclusion Criteria:

1. BMI 18,5-25 kg/m2
2. Fasting plasma glucose less than 6.1 mmol/l
3. Normal 2-hour oral glucose tolerance test (OGTT)

Exclusion Criteria:

1. Any chronic disease or medication that could affect glucose metabolism
2. History of anorexia nervosa or bulimia
3. Smoking of tobacco, taking of snuffs, or use of narcotics
4. Abusive use of alcohol
5. Any other condition that in the opinion of the investigator could create a hazard to the subject safety, endanger the study procedures or interfere with the interpretation of study results