Melatonin protects against lipid peroxidation and membrane rigidity in erythrocytes from patients undergoing cardiopulmonary bypass surgery Introduction An increasing number of patients are undergoing cardiac surgery [1] and in most cases a cardiopulmonary bypass (CPB) is required. This technique is aggressive and the patients are subjected to a high degree of surgical risk [2, 3]. In addition to the unphysiological hemodynamic condi- tions, one significant complication arising from CPB is the ischemia–reperfusion sequence syndrome, in which a large number of free radicals are generated, among other causes, because of the metabolism of xanthine oxidase, the activa- tion of neutrophils, and the oxidation of catecholamines and prostaglandins [4–9]. These radicals give rise to a considerable degree of cell injury, both structural and functional, involving among other effects, damage to membrane lipids [10, 11]. Several authors have shown a correlation between membrane rigidity and the degree of lipid peroxidation [11–13]. As intactness and structural integrity of biological membranes are prerequisites for the proper maintenance of cellular functions, changes in membrane fluidity contribute substantially to alterations in cellular functions [12–14]. Thus, in the case of cardiac surgery the increase in the generation of free radicals after starting CPB could alter the fluidity of cellular membrane and, thereby, increase the possibility of postoperative complications and mortality in these patients. It is therefore important to identify antioxidative molecules which are able to stabilize membranes and protect against lipid peroxidation. In the last decade, melatonin has been shown to be a highly effective antioxidant and free radical scavenger [15–18]. This molecule also has a regulatory effect on the activities of antioxidant enzymes [19, 20], on enzymes involved in the generation of free radicals and on micro- somal membrane fluidity in situations of elevated oxidative stress [21–24]. These functions, in conjunction with its solubility in lipid and aqueous media, by which it is able to cross morphophysiological barriers and enter subcellular compartments [25], permit melatonin to function as a highly effective inhibitor of oxidative damage. The aim of this study was twofold. First, to determine whether there are changes in erythrocyte membrane fluidity during cardiac surgery involving CPB bypass and, sec- ondly, to examine whether melatonin would suppress Abstract: The first aim of this study was to test whether there are changes in erythrocyte lipid peroxidation products and membrane fluidity during cardiac surgery involving cardiopulmonary bypass. Secondly, in vitro tests were performed to examine whether melatonin alters induced lipid peroxidation and reduced membrane fluidity in erythrocytes from these patients. Fifteen patients undergoing cardiac surgery involving cardiopulmonary bypass (CPB) were selected. Five blood samples were taken at different times during surgery for analysis of thiobarbituric acid reactive substances (TBARS) content and membrane fluidity of the erythrocytes. TBARS are an index of the level of lipid peroxidation. The results revealed an increase in TBARS levels and a parallel decrease in erythrocyte membrane fluidity (increased membrane rigidity) after the onset of CPB with respect to the reference sample. Likewise, in vitro induction of lipid peroxidation in the erythrocyte samples from CPB patients followed a similar pattern. The changes in TBARS levels and membrane fluidity were suppressed by pre- incubation of erythrocytes with melatonin prior to the induction (by 70 lm Fe 2+ + 250 lm ascorbate) of lipid peroxidation in a concentration- dependent manner. The results constitute a persuasive argument for the use of melatonin for preventive and therapeutic purposes during CPB. Julio J. Ochoa 1 , Marı ´a J. Vı ´lchez 2 , Miguel A. Palacios 2 , Joaquı ´n J. Garcı ´a 3 , Russel J. Reiter 4 and Antonio Mun ˜ oz-Hoyos 5 1 Department of Physiology, Institute of Nutrition and Food Technology, University of Granada, Granada; 2 Department of Anaesthesia, ÔSan CecilioÕ Universitary Hospital, Granada; 3 Department of Pharmacology and Physiology, University of Zaragoza, Zaragoza, Spain; 4 Department of Cellular and Structural Biology, University of Texas Health Science Center, San Antonio, TX, USA; 5 Department of Pediatrics, ÔSan CecilioÕ Universiary Hospital, Granada, Spain Key words: cardiopulmonary bypass, erythrocyte, lipid peroxidation, melatonin, membrane fluidity, thiobarbituric acid reactive substances (TBARS) Address reprint requests to Dr A. Mun ˜oz- Hoyos, Department of Pediatrics, ÔSan CecilioÕ Universitary Hospital, 18012, Granada, Spain. E-mail: amunozh@goliat.ugr.es. Received January 6, 2003; accepted April 1, 2003. J. Pineal Res. 2003; 35:104–108 Copyright Ó Blackwell Munksgaard, 2003 Journal of Pineal Research ISSN 0742-3098 104