Background
Acute mesenteric ischemia (AMI) is a life-threatening vascular emergency with an overall mortality of 60% to 80%, and besides its potential fatality, the incidence of this challenging surgical problem is reported to be increased [
1]. Appropriate management of AMI requires early diagnosis and rapid intervention with therapeutic methods that adequately restore mesenteric blood flow to prevent bowel necrosis [
2]
. The estimated proportion of common forms of acute mesenteric ischemia is approximately 50% for superior mesenteric artery (SMA) embolism, followed by SMA thrombosis (25%), non-occlusive mesenteric ischaemia (20%) and mesenteric venous thrombosis (5%) [
3]. Irrespective of the form of the mesenteric ischemia, the consequences are similar and include a range of intestinal tissue damage from mild disturbances in bowel function to transmural necrosis and gangrene [
1]. This tissue destruction due to occlusion of mesenteric blood flow is often the result of cellular injury related to reperfusion following therapeutic interventions [
4].
While short periods of mesenteric ischemia contribute to a mild increase in microvascular permeability, sustained ischemia and succeeding reperfusion results in disruption of the intestinal mucosal barrier, mainly due to generation of toxic oxygen free radicals, such as superoxide, peroxide, and hydroxyl radicals, which damage the cell membrane through lipid peroxidation [
2]. The injured intestinal mucosa loses its resistance to indigenous enteric microorganisms, which leads to translocation of bacteria to extraintestinal sites such as mesenteric lymph nodes, liver, spleen, and blood [
5]. This bacterial translocation may play an important role in the development of sepsis and multiple organ system failure [
1].
The role of oxygen free radicals in reperfusion injury is demonstrated by the reduction of tissue damage in the presence of antioxidants and free-radical scavenging substances such as N-acetylcysteine, selenium, vitamins E and C, superoxide dismutase, and catalase [
1]. Melatonin (
N-acetyl-5-methoxytryptamine), the primary hormone of the pineal gland, is a powerful scavenger of reactive oxygen species (ROS), including the hydroxyl and peroxyl radicals, as well as singlet oxygen, and nitric oxide [
6]. In addition to scavenging ROS, melatonin stimulates the antioxidant enzymes superoxide dismutase, glutathione peroxidase, and catalase [
7,
8]. Recently, several authors have described melatonin as one of the most effective antioxidants and scavengers of oxygen-free radicals after IR injury of the liver, lung, and intestine [
8,
9]. In this study, we aimed to investigate whether melatonin precludes bacterial translocation after intestinal ischemia-reperfusion injury in rats.
Discussion
According to the results of the present study, melatonin, the primary hormone of the pineal gland prevented intestinal bacterial translocation which was induced by mesenteric ischemia-reperfusion injury. This beneficial effect of melatonin seemed to be due to the ability of this agent to function as an intracellular scavenger of radical oxygen species (ROS) demonstrated by a reduction in the degree of lipid peroxidation, neutrophil accumulation and maintaining of GSH in ileal tissue specimens.
Physiologically, intestinal barrier function is composed of mucosal immunity and physical integrity [
16]. The main mission of the epithelium overlying mucosal surfaces of the intestinal tract is providing an effective barrier to the microorganisms present in the intestinal lumen. However, it has become obvious recently that the epithelial layer is much more than a simple physical barrier [
16]. The process of accomodating the microorganisms involves various host defence mechanisms which have evolved to regulate the composition of the them, and protection against infection and colonisation. The primary mediators of this activity are various anti-microbial peptides that are secreted by the epithelium and the mucus layer that coats the lumen. Below the mucus layer, the epithelial cells separated by junctions that represent binding of tight junction proteins [
16]. The most abundant constituents of epithelial cellular membranes are phospholipids. When phospholipids are dispersed in water, they spontaneously form lipid bilayers. The lipid bilayer serves as a matrix for embedded proteins, which function as transporters, ion channels, receptors for hormones and neurotransmitters, etc. After ischemia/reperfusion injury, the oxygen radicals which are the products of this oxidative stress, destroys these lipid bilayers via lipid peroxidation [
6]. Consequently, intestinal permeability increases and intestinal barrier function fails due to destruction of endothelial and epithelial cells resulting in breakdown of mucosal integrity [
17]. Finally, indigenous enteric bacteria translocate to extraintestinal sites such as mesenteric lymph nodes, liver, spleen, and blood, resulting in sepsis and multiple organ system failure [
17]. Since melatonin reduces lipid peroxidation in every cell and tissue, it was assumed that, in doing so, this hormone would also maintain cell membranes in a state of optimal fluidity and membrane rigidity [
6]. Moreover, it has recently reported that derivatives of melatonin that are formed when the indoleamine functions as a scavenger may actually be more effective than melatonin itself in neutralizing the peroxyl radicals [
18]. In our study, we observed a significantly reduced incidence of bacterial translocation to the blood, liver, spleen, and mesenteric lymph nodes in animals pretreated with melatonin in an acute mesenteric ischemia/reperfusion model. Furthermore, melatonin treatment provided a statistically significant reduction in ileal bacterial overgrowth. It was previously shown that mesenteric ischemia/reperfusion not only produces mucosal damage but also induces alterations of intestinal motor activity with a delay in gastrointestinal transit time [
19]. These alterations can be the result of structural and neuronal changes occurring within the enteric nervous system which may play a role in the progress of bacterial overgrowth with subsequent translocation due to inadequate bacterial clearance [
20]. In addition to its properties against I/R oxidative injury, melatonin has been suggested to act as a local regulator of gastrointestinal motility [
21].
The administration of melatonin significantly decreased the histological damage (Table
3) and PMNs infiltration in intestinal tissue specimens in our study when compared with the I/R only group of animals (Group II). In a previous study, melatonin has been found to reduce histological signs of intestinal injury in rats subjected to splanchnic artery occlusion shock [
22]. Melatonin also reduces the migration of PMNs into the inflammatory site [
22]. This effect of melatonin seems probably secondary to the protection of intestinal mucosa to endothelial oxidative injury and therefore conservation of endothelial barrier function. Myeloperoxidase activity, one of the markers of neutrophil accumulation, was found to be elevated in intestinal I/R injury [
23]. In our experiment, the reduction in neutrophil accumulation shown by a significant decrease in MPO activity within the ileal tissue of melatonin-treated group of animals is also seems to be secondary to the prevention of oxidative damage. The systemic activation of neutrophils after reperfusion appears to be secondary to mediators such as cytokines and ROS [
24]. This activated neutrophils further promotes inflammation and oxidative damage. It was reported that this vicious cycle among I/R injury, endothelial damage, and neutrophil infiltration causes additional ROS production [
25]. Previously, research has also demonstrated melatonin’s anti-inflammatory properties via down-regulation of proinflammatory cytokines [
26]. The inhibitory effect of melatonin against PMN infiltration was also observed in another study in which oxidative stress was induced by ischemia and reperfusion [
22]. When considered together with our findings, it seems that melatonin treatment effectively prevents I/R related intestinal injury by interfering with this vicious cycle.
Intestinal ischemia is a life-threatening abdominal emergency. The common clinical feature of the disease is caused by impaired blood perfusion of the intestine and the hypoxia associated sequelae, like bacterial translocation as well as local and systemic inflammation [
1]. Thus, rapid restoration of mesenteric blood flow with reoxygenation of the ischemic tissue is critical to its salvage, but it may paradoxically exacerbate tissue damage [
3]. During ischemia, there is an increase in microvascular permeability, release of lysosomal hydrolases and enhance in proteolysis [
19]. Those alterations are aggravated by reperfusion, since it triggers the accumulation of free radicals, which attack and damage the cellular membranes, attract neutrophils, and stimulate the release of inflammatory mediators [
17]. So in the clinical scenario of a patient suffering from acute mesenteric ischemia, a potentially therapeutic agent that is expected to block the deleterious effects of reperfusion such as release of free oxygen radicals, should be given before restoring the arterial flow. Therefore in our experimental model, melatonin was administered 30 minutes before the start of reperfusion in the treatment group (Group III), aiming that it is existed in circulation of the animal. Adverse effects of melatonin are few and it is generally regarded as safe in recommended dosages. There are isolated case reports of psychomotor disturbances (disorientation, fatigue, headache, dizziness, etc.), increased seizure risk, and blood clotting abnormalities associated with melatonin alone or in combination with other medications [
27]. In experimental studies, melatonin doses up to 800 mg/kg failed to cause death in mice [
6]. A lethal dose in 50% of mice, that is, LD50, has not been determined [
6]. In humans, for most non-sleep related disorders, doses from 10-50 mg daily have been used safely and effectively [
27]. We used a 10 mg/kg melatonin dosage in our study 30 minutes before reperfusion starts like other researchers did in experimental ischemia/reperfusion rat model investigations of melatonin [
7,
9,
26]. It seems to be a reasonable amount and timing when the relatively short serum half-life (30-60 minutes) and the total amount of melatonin in the gastrointestinal tractus (up 400 times more than in the pineal gland) taken into account [
27,
28].
Numerous mechanisms have been implicated in the evolution and development of intestinal I/R injury. These are overproduction of reactive oxygen species, increased expression and infiltration of leukocytes, and production of inflammatory mediators such as cytokines [
5]. Evidence has accumulated that melatonin is both a direct free radical scavenger and an indirect antioxidant because of its ability to promote the activities of a variety of antioxidative enzymes [
29]. While it is clearly a lipid soluble agent, it seems also capable of entering the aqueous environments of the cell which allows melatonin to be protective of membranes from free radical damage [
6]. ROS and other free oxygen radicals are believed to cause cellular injury and further necrosis via different mechanisms including especially peroxidation of cellular membrane lipids [
5]. It has been previously showed in a rat model of splanchnic artery occlusion and reperfusion that melatonin treatment abolished the increase in lipid peroxidation products, probably in part by scavenging the very reactive hydroxyl and peroxyl radicals [
22]. In this current experimental model, the occlusion of superior mesenteric artery followed by reperfusion resulted in a considerable increase in the ileal tissue levels of malondialdehyde, and melatonin treatment significantly prevented lipid peroxidation, which was verified by decreased MDA levels in the ileal tissues of animals.
GSH is an endogenous antioxidant found naturally in all animal cells. It has the potential of reacting with free radicals ensuing secondarily to I/R, and the provisions of glutathione precursors are protective for different types of free-radical-mediated cellular injury [
30]. Oxidants can upregulate the transcription of gamma-glutamylcysteine synthase genes, which is a rate-limiting enzyme for the synthesis of GSH [
31]. Melatonin’s high efficacy in reducing oxidative damage may involve both receptor-independent as well as receptor-mediated processes. In addition to direct free radical scavenging, antioxidative functions of melatonin may include synergistic actions with classic antioxidants and stimulation of the synthesis of the important intracellular antioxidant GSH [
32]. Melatonin stimulates glutathione peroxidase which converts hydroperoxides, including H2O2, to water and oxygen while oxidizing GSH [
29]. Once the oxidized form of glutathione is formed, it is recycled to GSH by glutathione reductase, another enzyme whose activity is enhanced by melatonin [
30]. In this present study, the reduction of GSH in the ileal tissue specimens seems to be a result of oxidant injury, and the restoration of GSH in ileal tissues of melatonin-treated animals can be attributed to these antioxidant features of melatonin.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MO coordinated and drafted the manuscript. CA conceived of the study, and participated in its design. NC carried out microbiological analyses. CY carried out the biochemical analyses. OB participated in the design of the study. GG carried out the pathological evaluation. BA performed the statistical analysis. IB help to design the study and execute the animal laboratory part. All authors read and approved the final manuscript.