Five healthy volunteers will be asked to attend the clinical research facility for 2 consecutive days. Subjects 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 commencing a 24-h subcutaneous infusion of (t=0; 08.00) of 10 mg (Pyr1)apelin-13 dissolved in 10 mL water for injection. Further venous sampling will take place 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.