Visits 1 and 2 will occur over two consecutive days. Participants will attend at 07.30 and be given a light breakfast. An intravenous sampling cannula will be inserted into the antecubital vein of one arm. Blood samples (5 mL) will be taken immediately prior to subcutaneous injection (t=0; 08.00) of 5 mg (Pyr1)apelin-13 and at 5, 10, 15, 20, 25, 30, 45, 60, 90, 120, 180, 240, 300, 360, 480 and 600 min after injection. Subjects will then be discharged from the facility and reattend the following day at 08.00 for a single venous sample. Approximately 85 mL of blood will be sampled in total over this time.