Oxidative and inflammatory biomarkers of ischemia and reperfusion injuries

Ischemia-reperfusion injuries occur when the blood supply to an organ or tissue is temporarily cut-off and then restored. Even though the restoration of blood flow is absolutely essential in preventing tissue death, the reperfusion of oxygenated blood to the oxygen-deprived areas may in itself augment the tissue damage in excess of that produced by the ischemia alone. The process of ischemia-reperfusion is multifactorial and there are several mechanisms involved in the pathogenesis. Ample evidence shows that the injury is in part caused by an excessive generation of reactive oxygen species or free radicals. The free radicals consequently initiate an inflammatory response, which in some cases may affect distant organs, thus causing remote organ injuries. Ischemia-reperfusion injuries are a common complication in many diseases (acute myocardial infarctions, stroke) or surgical settings (transplantations, tourniquet-related surgery) and they have potential detrimental and disabling consequences. The tolerance of ischemia-reperfusion has proven to be time-of-day-dependent and the size of myocardial infarctions has proven to be significantly higher when occurring in the dark-to-light period. This period is characterized by and coincides with a rapid decrease in the plasma levels of the hormone melatonin. Melatonin is the body's most potent antioxidant and is capable of both direct free radical scavenging and indirect optimization of other anti-oxidant enzymes. It also possesses anti-inflammatory properties and is known to inhibit the mitochondrial permeability transition pore during reperfusion. This inhibiting property has been shown to be of great importance in reducing ischemia-reperfusion injuries. Furthermore, melatonin is a relatively non-toxic molecule, which has proven to be safe for use in clinical trials. Thus, there is compelling evidence of melatonin's effect in reducing ischemia-reperfusion injuries in many experimental studies, but the number of human clinical trials is very limited. In this PhD thesis we set out to explore the oxidative and inflammatory biochemical markers of ischemia and reperfusion injuries and the possible effect of melatonin on these markers. We have reviewed the literature on the tourniquet-related oxidative damage and found that ischemic preconditioning and the use of propofol could significantly reduce the release of such markers. However, the relevance of this reduction in terms of clinical outcomes is still to be investigated (paper 1). We undertook the characterization of a human ischemia-reperfusion model without the influencing factors of surgery and anesthesia, and subsequently found ways to improve this model (paper 2). In order to apply an intracoronary melatonin administration, we investigated whether melatonin could be dissolved in non-ethanol based buffers and still activate the melatonin receptors (paper 3). We found this to be possible, and in a porcine closed-chest model of acute myocardial infarction (AMI) we randomized the pigs to intracoronary and systemic melatonin or placebo in order to test whether melatonin could attenuate the oxidative and inflammatory biomarkers following reperfusion (paper 4). The outcomes were not optimal for this model, and the effect of melatonin still remains to be explored in a large animal model. We are currently still awaiting the results of the IMPACT-trial - a randomized, placebo-controlled, clinical trial exploring the effect of intracoronary and systemic melatonin given to patients suffering from AMI and undergoing primary percutaneous coronary intervention (pPCI) (paper 5). Though pPCI is undisputedly life-saving, it holds a built-in consequence of aggravating the ischemic injury, paradoxically due to the reperfusion. The optimization of existing treatments and the exploring of new suitable interventions, such as melatonin, for minimizing the ischemia-reperfusion injuries is therefore of great interest.