The most prevalent monogenetic diabetic subtype is named maturity onset diabetes of the young type (MODY3) or hepatocyte nuclear factor 1α (HNF1A)-diabetes. The aim of this study is to evaluate the effects of supra-physiological levels of GIP and GLP-1, respectively, on insulin and glucagon secretion at fasting plasma glucose (FPG) and "post-prandial" PG levels (1.5 × FPG) in patients with HNF1A-diabetes and matched healthy controls treated with or without a low dose of glimepiride (sulphonylurea). In addition, we will evaluate the maximal insulin and glucagon secretory capacity in both groups.

A total of 6 experimental days will be performed. The following is an outline of an experimental day:

Participants will meet after a 10-hour fast. A tablet of glimepiride 1.0 mg or placebo will be administered 90 minutes before the initiation of the experiment (-90 minutes) The mean FPG will be calculated from blood samples -105, -100 and -90 minutes. Two intravenous cannulas will be inserted in a cubital vein of each arm. One intravenous cannula will be used for infusions of glucose, arginine and GIP and the other will be used to collect venous blood. The forearm from which blood samples are drawn will be placed in a heating pad (50°C) throughout the experiment for arterialisation of venous blood.

At time 0 minutes, a glucose clamp will be established at the FPG level for 60 minutes and hereafter a post-prandial clamp period of 1.5 × FPG for another 60 minutes. At time 120 minutes, a bolus of 5g of L-arginine (given as 50% arginine HCl) will be infused during 30 seconds. The post-prandial clamp will be maintained for another 10 minutes until time 130 minutes to prevent reactive hypoglycaemia. Throughout the experiment (0-130 minutes) a continuous infusion of either GIP (1.5 pmol/kg/min), GLP-1 (0.5 pmol/kg/min) or placebo (saline) will be administered.

During the experiment PG will be kept stable by a continuous 20%-glucose infusion. The rate of infusion will be regulated according to PG determined by bed-site measurements every 5 minutes. After 60 minutes, a post-prandial clamp will be established by a bolus infusion over one minute using 50%-glucose to target 1.5 × FPG (the amount of glucose to be administered will calculated as follows: (1.5 × FPG - FPG) × 35 mg glucose × weight in kilogram).

Participants

Ten patients with HNF1A-diabetes and ten matched healthy controls will be recruited. Different inclusion and exclusion criteria applies for the two groups:

Inclusion criteria for HNF1A-patients

• Patients with HNF1A-diabetes verified by genetic testing
• Patients treated with diet or sulphonylurea monotherapy
• Normal haemoglobin (males 8.3-10.5 mmol/l, females 7.3-9.5 mmol/l)
• Informed consent

Exclusion criteria for HNF1A-patients

• Nephropathy (estimated glomerular filtration rate (eGFR) <60 ml/min/1.73m2 and/or albuminuria)
• Liver disease (serum alanine aminotransferase (ALT) and/or serum aspartate aminotransferase (AST) above 2 × normal values)
• Pregnancy or breastfeeding

Inclusion criteria for healthy controls

• FPG ≤6 mmol/l and glycated haemoglobin (HbA1c) ≤43 mmol/mol
• Normal haemoglobin as defined above
• Age ≥18 years
• Informed consent

Exclusion criteria for healthy controls

• No family history of type 1 or type 2 diabetes
• Nephropathy (defined above)
• Liver disease (defined above)
• Pregnancy or breastfeeding