4.1. Animal Preparations and Experimental Protocol

In the present investigation, we analyzed ECG data obtained in previous animal experiments described elsewhere [15]. In brief, experiments were performed in 25 Landrace pigs (30–45 kg body weight, both sexes). The study conformed to the ARRIVE guidelines (PLoS Bio 8(6), e1000412, 2010), the Guide for the Care and Use of Laboratory Animals, 8th Edition published by the National Academies Press (USA) 2011, the guidelines from Directive 2010/63/EU of the European Parliament on the protection of animals used for scientific purposes, and was approved by the ethical committee of the Institute of Physiology of the Komi Science Centre, Ural Branch of Russian Academy of Sciences (Russia). The animals were anesthetized with telazol (TELAZOL 100, Zoetis Inc., Parsippany, NJ, USA, 10–15 mg/kg, i.m.) and propofol (Norbrook Laboratories Ltd., Newry, Northern Ireland, UK, 1 mg/kg, i.v.), intubated, and mechanically ventilated.

The thorax was opened by a midsternal incision and the pericardium was cut for further ligature placement around the left anterior descending coronary artery (LAD) just distal to the first diagonal branch origin. Then, the ligature was tightened, and the animals in the melatonin group (n = 12) were given melatonin (4 mg/kg, intravenously, Sigma-Aldrich, St. Louis, MO, USA) at the first minute of ischemia. Control animals (n = 13) received saline in the amount matching the volume of fluid given to the animals in the intervention group. In the current study, melatonin was administered intravenously after coronary artery occlusion simulating a situation when medication is given after the onset of a heart attack. The chest was reclosed immediately after coronary occlusion. The duration of coronary occlusion was 40 min. The animals were euthanized by potassium chloride infusion under deep anesthesia at the end of the coronary occlusion period or immediately after VF development.