Ischaemia reperfusion injury is important to anaesthetists and intensivists because many patients in the operating theatre or intensive care unit undergo overt or occult ischaemia reperfusion events. Through diverse mechanisms, ischaemia reperfusion results in oxygen free radical-mediated injury. Oxygen free radicals such as OH•, O•−2, H2O2 and HOCl are highly reactive species with one or more unpaired electrons and the capacity to oxidize compounds. These radicals have been implicated in the pathophysiology of many human diseases, including atherosclerosis, cancer and neurodegenerative disorders [1-3]. Free radical-induced peroxidation of cell membrane lipid macromolecules is an important element of ischaemia reperfusion injury .
Arachidonic acid is a complex lipid macromolecule and the precursor for the prostaglandin (PG)/leukotriene/thromboxane families of biologically active mediators, pivotal to inflammatory and coagulation cascades. In 1990, it was reported that free radicals catalyzed the peroxidation of arachidonic acid to form a series of PG-like compounds [5,6]. The compounds are now termed isoprostanes.
Isoprostanes are assuming increasing clinical importance for several reasons. The measurement of isoprostanes, e.g. in urine or plasma, provides the most reliable non-invasive approach to assess oxidative stress in vivo in human beings. Serial measurement of isoprostanes in biological fluids and tissues makes possible investigation of the role of free radicals in the pathophysiology of ischaemia reperfusion injury. The measurement of isoprostanes provides a means to assess the effects of prophylactic and therapeutic interventions, such as antioxidants, to attenuate free radical/ischaemia reperfusion injury. Changes in isoprostane concentration may have the potential to predict organ function following ischaemia reperfusion injury, such as those that occur following renal transplantation. Finally, the future development of agents that antagonize the deleterious effects of isoprostanes may enhance our ability to prevent and treat oxidant dependent injury.
The objective of this review is to illustrate the importance of isoprostanes as investigative tools that increase our understanding of the mechanism underlying common diseases, as sensitive indices of organ dysfunction and as (non-invasive) measures of the response to therapy.
What are isoprostanes?
Isoprostanes are a group of PG-like compounds that are primarily, but not exclusively, formed by the free radical-catalyzed peroxidation of arachidonic acid independent of cyclo-oxygenase activity [7,8]. An important structural distinction between isoprostanes and cyclo-oxygenase-derived PGs is that the former contain side-chains that are predominantly oriented cis to the prostane ring and the latter possess exclusively trans side-chains [5,9]. Isoprostanes have a similar nomenclature to PGs. They can be classified based on structure into four groups: F2-isoprostanes, D2/E2-isoprostanes, A2/J2-isoprostanes and isothromboxanes (Fig. 1). F2-isoprostanes are characterized by F-type prostane rings, are analogous to PG F2, and are the most important isoprostanes because they are most abundant and are most commonly used as markers of lipid peroxidation in vivo.
Lipid peroxidation is caused by an attack on a lipid by a reactive species that abstracts a hydrogen atom from a methylene (-CH2-) group. An adjacent double bond weakens the energy of attachment of the hydrogen atoms on the next carbon atom, especially if it is present on both sides of the -CH2- group (i.e. = CH-CH2-CH=). After abstraction of the hydrogen atom, carbon radicals (C•) are likely to combine with O2 (C• + O2COO•). Peroxyl radicals (COO•) are also capable of abstracting a hydrogen atom from the lipid molecule (COO• + CHCOOH + C•). Then the carbon radical (C•) formed can react with O2 to form another peroxyl radical (COO•) and so the chain reaction of lipid peroxidation can continue .
Figure 2 shows the formation of F2-isoprostanes. Arachidonic acid, the isoprostane precursor, is shown at the top of Figure 2. After abstraction of a hydrogen atom (stage 1), formation of F2-isoprostanes proceeds through intermediates comprised of four positional peroxyl radical isomers of arachidonic acid (stage 2), which undergo endocyclization of the radicals (stage 3), followed by the addition of another molecule of oxygen to yield four endoperoxide (PGG2-like) regioisomers (stage 4). Then F2-isoprostanes derive from the reduction of these endoperoxides (stage 5) . Compounds comprising regioisomers III and VI predominate because regioisomers IV and V derive from the same arachidonyl radical precursor (stage 1) . The intermediate (PGG2-like) endoperoxides, if not efficiently reduced to F2-isoprostanes, may rearrange to form D- and E-ring isoprostanes (D2/E2-isoprostanes). Isoprostanes endoperoxides can also rearrange to form isothromboxane compounds. A2/J2-isoprostanes are produced in vivo as dehydration products of D2/E2-isoprostanes (Fig. 1)[13,14].
In terms of tissue concentration, F2-isoprostanes are the most abundant among these isoprostanes. These are approximately four times greater than D2/E2-isoprostanes and isothromboxanes. In contrast to F2-isoprostanes, D2/E2-isoprostanes cannot be detected in the circulation of human beings or rats under normal conditions . Even following lipid peroxidation, A2/J2-isoprostanes are detected only in the tissues, not in the circulation .
Unlike PGs, F2-isoprostanes are generated initially in cell membranes at the site of free radical attack from which they are cleaved, presumably by phospholipases, circulate and are excreted in the urine [15,16]. As a steady-state plasma concentration of F2-isoprostanes is maintained, they must be continuously generated because they are rapidly metabolized and excreted in the urine . Some of isoprostane F2α-III are metabolized into 2,3-dinor-5,6-dihydro-isoprostane F2α-III through β-oxidation and double bond reduction. 2,3-dinor-5,6-dihydro-isoprostane F2α-III is the major urinary metabolite in human beings [18-20].
How are isoprostanes measured?
F2-isoprostanes can only be quantified as free compounds in biological fluids in vivo, not in an esterified state in the tissue. Therefore, to quantify the concentration of F2-isoprostanes esterified to phospholipids, the phospholipids are first extracted from the tissue sample and then subjected to alkaline hydrolysis to release free F2-isoprostanes . Isoprostane F2α-III (8-iso-PGF2α or 8-epi-PGF2α) was initially studied as an index of lipid peroxidation in vivo. Isoprostane F2α-VI has recently been targeted for this purpose because unlike isoprostane F2α-III , it exhibits no capacity for cyclo-oxygenase-dependent formation and it is more abundant in human urine. Isoprostane F2α-VI also forms a cyclic lactone that provides a means of separating them from class III to V F2-isoprostanes [21,22]. More recently, a metabolite of isoprostane F2α-III, 2,3-dinor-5,6-dihydro-isoprostane F2α-III, also has been targeted as a potential marker of ischaemia reperfusion injury [23,24].
Mass spectrometric assays (GC-MS assays or GC-MS-MS assays)
Mass spectrometry is an analytical technique for identifying a compound by ionizing it and separating the resultant stream of charged particles according to their mass and thereby providing a quantitative measure of material. Gas chromatography is a process for separating a target material from various gaseous chemicals according to their different affinities for a standard absorbent. A combination of these techniques, termed gas chromatography-mass spectrometry (GC-MS), is the method most commonly employed for measurement of F2-isoprostanes . GC-MS assays are both highly sensitive and accurate (precision ± 6%, accuracy 96%), but are time-consuming, labour-intensive and require a technically demanding purification process [16,22].
The recent development of new techniques, namely electrospray ionization (ESI) and tandem mass spectrometry (MS-MS), has markedly improved detection sensitivity for isoprostanes . The most reliable, sensitive and specific method for the measurement of F2-isoprostanes is gas chromatography-tandem mass spectrometry assays (GC-MS-MS assays) [25,26]. The sensitivity of the mass spectrometric method appears sufficient to quantify concentration of F2-isoprostanes in small biopsies of human tissue, which can permit a direct assessment of oxidant injury in key tissues of interest.
High performance liquid chromatography-mass spectrometry assays (LC-MS assays)
Recently, high performance liquid chromatography-mass spectrometry assays have become more sensitive. The principle of high performance liquid chromatography is that a sample injected into a moving stream of solvent passes through a column in which separation of mixtures of substances occurs by absorption, partition, ion exchange or size exclusion. A combination of high performance liquid chromatography and mass spectrometric techniques, termed an LC-MS assay, has been developed that is capable of measuring isoprostane F2α-III in human plasma and urine . This method circumvents laborious and time-consuming pretreatments required for GC-MS assays by means of a very simple and rapid pretreatment method using a membrane filter-type solid-phase extraction column for human urine extracts or intact plasma .
Immunoassays are quick and technically simple compared with GC-MS techniques. Owing to non-specific reactivity, immunoassays for isoprostanes in urine require some degree of purification before quantification, introducing the possibility of uncontrolled loss of the target isoprostanes. Antibodies raised against specific F2-isoprostanes may cross-react with other isomers or their largely uncharacterized metabolites .
Compared with GC-MS, radioimmunoassays frequently produce a greater value , possibly because the autoformation of isoprostanes may occur during the radioimmunoassay procedure since samples are frequently incubated for several hours to permit antibody binding.
Comparison with malondialdehyde, an alternative marker of lipid peroxidation
The traditional method of quantification of lipid peroxidation has been the measurement of malondialdehyde , a highly toxic byproduct formed in part by lipid oxidation-derived free radicals . Measurement of malondialdehyde is commonly performed using the thiobarbituric acid reacting substances test because it is easy to perform and is inexpensive . The thiobarbituric acid reacting substances test works reasonably well when applied to defined systems such as liposomes and microsomes, but it is subject to many problems related to artefactual and non-specific reactions when applied to more complex biological systems in vitro or in vivo[31-33]. Malondialdehyde is not a specific product of lipid peroxidation . On the other hand, isoprostanes have some advantages compared with malondialdehyde. F2-isoprostanes are stable, specific products of lipid peroxidation . They are detectable in all normal biological fluids and tissues , and the normal concentration of F2-isoprostanes in plasma (35 ± 6 pg mL−1) and urine (1.6 ± 0.6 ng mg−1 creatinine) have been defined . Longmire and colleagues demonstrated that following administration of CCl4 to rats, the concentration of F2-isoprostanes esterified to lipids increased >80-fold, whereas the concentration of malondialdehyde in the liver increased only 2.7-fold . Thus, F2-isoprostane estimation is a more sensitive index of lipid peroxidation in vivo. It is important to recognize that urinary F2-isoprostane concentrations reflect 'whole-body' rather than renal lipid peroxidation in some clinical settings  because systemically produced F2-isoprostanes are excreted in the urine.
What are the biological effects of isoprostanes?
F2-isoprostanes are not simply markers of lipid per-oxidation but are key mediators of the cellular effects of oxidant injury . The formation of isoprostanes from membrane-derived arachidonic acid disrupts cell membrane function and results indirectly in cellular and organ dysfunction. The availability of synthetic isoprostanes, particularly isoprostane F2α-III, has yielded multiple insights into their potential for direct biological activity, and sheds further light on the pathways leading to oxidant injury .
Several F2-isoprostanes produce vasoconstriction induced by either the thromboxane A2/PG H2 receptor or a unique receptor [39-44]. In the rat kidney, isoprostane F2α-III (8-iso-PG F2α) causes intense vasoconstriction of the afferent renal arterioles and decreases glomerular filtration and renal blood flow without affecting systemic blood pressure . In the rat lung, isoprostane F2α-III causes pulmonary vasoconstriction and bronchoconstriction [19,42,43]. Intratracheal instillation of isoprostane F2α-III induces a dose-dependent obstruction of airflow and plasma exudation in the guinea pig . Isoprostane F2α-III also causes porcine and bovine coronary artery constriction . In human platelets, isoprostane F2α-III can induce a change of shape and amplify the aggregation response [45,46]. Isoprostane F2α-III promotes platelet plug formation by decreasing the antiadhesive and antiaggregatory effects of nitric oxide .
At the cellular level, isoprostane F2α-III stimulates inositol 1,4,5-triphosphate production and DNA synthesis in rat aortic smooth muscle cells . Although some isoprostanes may act as incidental ligands for prostanoid receptors, their effects differ from those of the cognate ligand. For example, compared with PG F2α, 8,12-iso-isoprostane F2α-III, which ligates the PG F2α receptor and induces cardiomyocyte hypertrophy, activates different downstream signalling pathways .
Clinical and animal studies using isoprostanes
Isoprostanes are currently the best available indices of (oxygen-dependent) free radical injury. The measurement of F2-isoprostanes in biological fluids has radically altered the way in which oxidative stress is quantified in vivo. Measurement of isoprostanes has enhanced our understanding of the mechanisms underlying ischaemia reperfusion injury itself, and the clarified role of ischaemia reperfusion injury in common disease processes. In addition, serial plasma and urine isoprostane measurements provide a useful non-invasive tool to evaluate injury severity and assess prognosis during and after ischaemia reperfusion events. Of increasing importance in the clinical context, isoprostane measurements have been used to quantify the response to therapeutic strategies to prevent or treat oxidant-mediated injury processes. We shall examine the role of isoprostanes in the context of the major organ systems.
Oxidant stress is thought to mediate myocardial ischaemia reperfusion injury and 'stunning' that may follow cardiopulmonary bypass or thrombolytic therapy [50-52]. Isoprostane formation is increased in a canine model of coronary thrombolysis, with urinary isoprostane F2α-III unchanged after circumflex artery occlusion, but increased immediately after reperfusion . This is consistent with the evidence that it is reperfusion rather than ischaemia per se that causes endothelial injury [19,52,54]. Urinary excretion of isoprostane F2α-III was increased in patients with acute myocardial infarction given thrombolytic therapy when compared with healthy patients with stable coronary disease . First, this study indicated that the measurement of urinary isoprostane F2α-III is a useful method to assess oxidant stress in cardiac patients. An increase in urinary isoprostane F2α-III was also observed following elective coronary artery bypass graft surgery, which peaked 15 min after global myocardial reperfusion and decreased to the normal levels 24 h later . Similarly, urinary isoprostane F2α-III and isoprostane F2α-VI concentrations were greater in patients undergoing percutaneous transluminal coronary angioplasty (PTCA) for acute myocardial infarction compared with patients undergoing elective PTCA . Pratico indicated that the urinary concentration of isoprostane F2α-III increased following carotid reperfusion in patients undergoing carotid endarterectomy . These data exemplify that isoprostane measurement makes it possible to detect small changes in the severity of lipid peroxidation and to characterize more accurately the time profile of ischaemia reperfusion injury.
More recently, Iuliano and colleagues demonstrated that coronary sinus isoprostane F2α-III and isoprostane F2α-VI concentrations increased following elective PTCA. Isoprostane concentrations in coronary sinus following PTCA were similar to those shown to induce coronary artery constriction in animals and amplified the platelet aggregation response in human beings. Isoprostanes may play an important role in the PTCA-related sequelae such as vasospasm .
Furthermore, the measurement of isoprostane concentration is currently making possible the investigation of the efficacy of therapeutic interventions. There have been several studies of the effects of antioxidants such as vitamins C  and E [58-60] on cardiac injury. Guan and colleagues examined the effect of vitamin C on free radical production after PTCA for acute myocardial infarction. No difference in urinary concentration of isoprostane F2α-III between the patients pretreated with vitamin C and controls. The authors confidently concluded that vitamin C was not effective in this setting .
Free radical-induced lipid peroxidation, as assessed by isoprostane formation in tissue, plasma and bronchoalveolar lavage fluid, plays a key role in the pathogenesis of lung diseases such as chronic obstructive pulmonary disease [61-64], cystic fibrosis [65-67], adult respiratory distress syndrome (ARDS) [68,69] and asthma [70-72]. The importance of ischaemia reperfusion in the pathogenesis of critical illness is illustrated by the correlation between oxygen requirement and urinary isoprostane F2α-III in patients with acute-phase ARDS [19,73].
Reperfusion effects are important determinants of outcome in critically ill patients who have suffered pulmonary ischaemia (e.g. by transplantation or cardiopulmonary bypass). Because the lung is not always hypoxic when ischaemic, if ventilation of the lungs is maintained while pulmonary blood flow is impaired, mechanisms of pulmonary ischaemia reperfusion injury are likely to differ from systemic organs, where reactive oxygen species generated during reperfusion mediate organ dysfunction . This contention is supported by the finding that after 45 min 'ventilated ischaemia' in isolated ferret lungs, tissue F2-isoprostanes increased by only 60% , a relatively small increase compared with that which occurs in non-ventilated ischaemic organs.
Laffey and colleagues demonstrated that therapeutic hypercapnia, i.e. the administration of CO2 for therapeutic effect, protects against the pulmonary and systemic effects of lung ischaemia reperfusion injury . Lung tissue isoprostane concentrations were less in rabbits with hypercapnic acidosis compared with controls, supporting the hypothesis that hypercapnic acidosis attenuates free radical-mediated organ injury . This is consistent with a previously documented reduction of lipid peroxidation by hypercapnia in homogenized tissue .
Estimation of isoprostane concentration has been employed in a number of clinical and experimental settings relevant to renal disease and injury. These include haemodialysis , renal transplantation [38,78,79], drug effects [80-82] and intrinsic renal disease [83-85]. While renal reperfusion injury has traditionally been studied primarily by assessing renal function or measuring the concentration of another marker of lipid peroxidation, malondialdehyde, measurement of urinary isoprostane concentration is increasingly used in this context.
Urinary isoprostane F2α-III concentration in renal transplant recipients tends to correlate with recovery of kidney function. Recipients of kidneys from live donors with immediate graft function demonstrate a small transient increase in urinary isoprostane F2α-III concentration compared with the five- to sevenfold increase observed in cadaveric kidney recipients with delayed graft function . This is consistent with the concept that isoprostane F2α-III concentration is an index of the severity of organ damage due to ischaemia reperfusion injury and potentially could be used to evaluate prognosis. Urinary isoprostane F2α-III concentration showed no significant correlation with cold ischaemia time , implying that cold ischaemia did not enhance lipid peroxidation. In another study of renal transplantation, following administration of high doses of corticosteroids, urinary excretion of isoprostane F2α-III did not differ between healthy controls and kidney transplant recipients in the first 5 days following transplantation. This suggests that corticosteroid (immunosuppressive therapy) attenuates oxidative stress due to ischaemia reperfusion injury .
Initial work in vivo with the isoprostanes employed two models of liver injury in rats in which lipid per-oxidation had been implicated as an important factor: administration of CCl4 to the normal rat  and diquat to the selenium-deficient rat . These studies demonstrate that quantification of F2-isoprostanes in animal models of oxidant injury is an accurate method to assess lipid peroxidation in vivo. Morrow and colleagues demonstrated a marked overproduction of 'total' F2-isoprostanes in patients with the hepatorenal syndrome . In contrast to urinary fluids (which may serve as a 'whole-body' product of F2-isoprostanes), quantification of F2-isoprostanes excretion in bile provides a sensitive and quantitative index of hepatic lipid peroxidation .
The anaesthetic volatile agent halothane can induce liver injury, especially under hypoxic conditions, which is thought to involve the production of free radicals via the reductive metabolism of halothane . Indeed, following administration of halothane to rats, the concentration of F2-isoprostanes in both liver tissue and plasma increased fivefold compared with the non-halothane control rat, even under normoxic conditions . In a rat model of liver ischaemia reperfusion, a 60-250% increase of F2-isoprostanes concentration in plasma was identified during reperfusion. Compared with ischaemia reperfusion injury, the tert-butyl hydroperoxide (tBHP) infusion rats demonstrate a clearly greater increase of F2-isoprostanes concentration with less severe parenchymal cell injury. These facts suggest that lipid peroxidation may not be the primary mechanism of reperfusion injury in liver parenchymal cell injury and most of the lipid peroxidation occurs locally in the vascular space (e.g. endothelial cell) . This is consistent with the hypothesis that Kupffer cells induce oxidant stress and early reperfusion injury, which causes neutrophil infiltration afterwards [92-94].
Oxidant stress plays a role in neurodegenerative diseases  (particularly Alzheimer's disease [96-98]), ischaemia reperfusion injury  and head trauma [100,101]. Recently, a technique for measurement of isoprostane concentration in cerebrospinal fluid has been described .
Measurement of isoprostane concentration has been used to measure the response to novel neuroprotective agents in animal models. In a rat model of cerebral ischaemia reperfusion, a 27-fold increase of isoprostane F2α-III (8-epi PGF2α) in tissue was observed in the ischaemic hemisphere compared with the corresponding hemisphere of the sham-operated rat. Treatment with the neuroprotective agent BN 80933 significantly decreased isoprostane F2α-III (8-epi PGF2α) elevation in tissue , clearly illustrating the potential utility of this agent.
Isoprostanes - future direction
The application of isoprostane measurement is still in its infancy and many important questions remain unanswered. The development of rapid, simple assays of isoprostane formation offers the potential to investigate further the mechanisms underlying ischaemia reperfusion injury, and other injury processes in which oxidant injury plays a role. The early diagnosis of occult ischaemia reperfusion injury should allow for the rapid institution of therapeutic strategies designed to minimize its deleterious consequences. These assays may allow for the evaluation of the severity of cellular injury and permit assessment of the impact on organ function during and after unavoidable ischaemia reperfusion events such as occur during organ transplantation.
Further investigation is required to elucidate the cellular actions of isoprostanes. In particular, the pathophysiological relevance of potentially deleterious actions of isoprostanes remains to be clarified. This is particularly important because the demonstration that isoprostanes are directly deleterious raises the potential to attenuate the injury by prevention of their effects. One potential strategy would be to inhibit their production using free-radical scavengers. A more novel strategy might be to alter isoprostane signalling at the receptor level. The demonstration of a novel isoprostane receptor might lead to the development of agents that specifically block isoprostanes actions, and thus directly attenuate oxidant mediated injury.
The contribution of ischaemia reperfusion injury to the pathogenesis of multiple organ dysfunction and critical illness is increasingly recognized. Traditional methods of quantifying ischaemia reperfusion injury, including measurement of malondialdehyde and assessment of organ function, lack specificity and sensitivity. The isoprostane concentration in biological fluids and tissue is sensitive and specific as a measure of ischaemia reperfusion injury. As the techniques and methods have improved, measurement of isoprostanes in various biological fluids and tissues has made it possible to characterize further the oxidant-mediated injury and to evaluate the response to prophylactic and therapeutic interventions. In the future, the development of rapid, simple assays of isoprostane formation offers the potential to assess the prognosis during and after ischaemia reperfusion events. Further insights into the biological actions of isoprostanes offer the intriguing possibility of developing a novel class of agents that directly attenuates free radical-mediated injury.
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