Nitrite augments tolerance to ischemia/reperfusion injury via the modulation of mitochondrial electron transfer

Type 2 diabetes, mitochondrial biology and the heart

Diabetes is recognized as an independent risk factor for cardiovascular morbidity and mortality. This is due, in large part, to premature atherosclerosis, enhanced thrombogenicity and activation of systemic inflammatory programs with resultant vascular dysfunction. More enigmatic mechanisms underpinning diabetes-associated cardiac pathophysiology include the direct metabolic consequences of this disease on the myocardium. Nevertheless, a role for diabetes-associated disruption in cardiac contractile mechanics and in increasing cardiomyocyte susceptibility to ischemic-stress has been implicated independent of vascular pathology. This review will focus broadly on the direct effects of diabetes on the cardiac myocardium with more specific reference to the role of the modulation of cardiomyocyte mitochondrial function in these disease processes. This focus in part, stems from the growing recognition that in some instances mitochondrial dysfunction is central to the development of insulin resistance and diabetes, and in others, diabetes associated disruption in mitochondrial function exacerbates and accentuates the pathophysiology of diabetes.

Dietary sources of nitrite as a modulator of ischemia/reperfusion injury

The nitrite anion is an endogenous product of nitric oxide (NO) metabolism, a key intermediate in the nitrogen cycle in plants and bacteria, and a constituent of many foods. Research over the past 6 years has revealed a surprising biological and cytoprotective activity of this anion. Its ability to restore NO homeostasis throughout the physiological oxygen gradient in vivo has transformed this once-thought to be inert anion into a critical molecule in health and disease. Ischemia-reperfusion (I/R) injury is a major clinical problem worldwide. NO has been shown to be one of the most important molecules for the prevention of injury from I/R. Paradoxically, however, enzymatic NO formation from NO synthase (NOS) is inactive during conditions of inadequate oxygen and substrate delivery, such as in ischemia. Nitrite has emerged as a viable alternative source of NO under ischemic conditions. As nitrite is known to be derived not only from the oxidation of NO but also through diet, understanding nitrite metabolism and mechanisms of cytoprotection may offer novel and natural means to prevent disease or at least limit injury from an I/R event. Here, we review the current body of knowledge regarding dietary sources of nitrite and its modulation of cytoprotection in an I/R injury.

Effects of the NO donor sodium nitroprusside on oxygen consumption and energetics in rabbit myocardium

Nitric oxide (NO) has influence on various cellular functions. Little is known of the influence of NO on myocardial energetics. In the present study oxygen consumption and mechanical parameters of isometrically contracting rabbit papillary muscles (1 Hz stimulation frequency) were investigated at varying interventions while maintaining physiological conditions (37°C; 2.5 mM Ca²⁺) to study the effects of NO on energetics. The NO donor sodium nitroprusside (SNP) showed a negative inotropic effect. SNP decreased the maximal force in normal rabbit muscle strips by 30%, the force time integral (FTI) by 40% and the relaxation time by 20%. In addition the oxygen consumption decreased by 60%, a notably disproportional decrease compared to the mechanical parameters. Consequently, the economy as a ratio of FTI and oxygen consumption is significantly increased by SNP. In contrast the negative inotropic effect due to a reduction in extracellular Calcium (Ca²⁺) from 2.5 to 1.25 mM reduced FTI and oxygen consumption proportionally by 40% and did not change economy. The effect of NO on force and oxygen consumption could be reproduced by the application of the cyclic guanosine monophosphate (cGMP) analogue 8-bromo-cGMP. In summary, NO increased the economy of isometrically contracting papillary muscles. The improvement in contraction economy under NO seems to be mediated by cGMP as the secondary messenger and maybe due to alterations of the crossbridge cycle.

In vivo cardioprotection by S-nitroso-2-mercaptopropionyl glycine

The reversible S-nitrosation and inhibition of mitochondrial complex I is a potential mechanism of cardioprotection, recruited by ischemic preconditioning (IPC), S-nitrosothiols, and nitrite. Previously, to exploit this mechanism, the mitochondrial S-nitrosating agent S-nitroso-2-mercaptopropionyl glycine (SNO-MPG) was developed, and protected perfused hearts and isolated cardiomyocytes against ischemia-reperfusion (IR) injury. In the present study, the murine left anterior descending coronary artery (LAD) occlusion model of IR injury was employed, to determine the protective efficacy of SNO-MPG in vivo. Intraperitoneal administration of 1 mg/kg SNO-MPG, 30 min prior to occlusion, significantly reduced myocardial infarction and improved EKG parameters, following 30 min occlusion plus 2 or 24 h reperfusion. SNO-MPG protected to the same degree as IPC, and notably was also protective when administered at reperfusion. Cardioprotection was accompanied by increased mitochondrial protein S-nitrosothiol content, and inhibition of complex I, both of which were reversed after 2 h reperfusion. Finally, hearts from mice harboring a heterozygous mutation in the complex I NDUSF4 subunit were refractory to protection by either SNO-MPG or IPC, suggesting that a fully functional complex I, capable of reversible inhibition is critical for cardioprotection. Overall, these results are consistent with a role for mitochondrial S-nitrosation and complex I inhibition in the cardioprotective mechanism of IPC and SNO-MPG in vivo.

Myocardial protection by nitrite: evidence that this reperfusion therapeutic will not be lost in translation

The circulating anion nitrite (NO(2)(-)), previously thought to be an inert product of nitric oxide (NO) oxidation, has now been identified as an important storage reservoir of bioavailable NO in the blood and tissues. Reduction of NO(2)(-) to NO over the physiologic pH and oxygen gradient by deoxyhemoglobin, myoglobin, xanthine oxidoreductase, and by nonenzymatic acidic disproportionation has been demonstrated to confer cytoprotection against ischemia-reperfusion injury in the heart, liver, brain, and kidney. Here, we review the mechanisms that have been established to regulate hypoxic NO(2)(-) reduction to NO, analyze the preclinical and clinical evidence supporting NO(2)(-)-mediated cytoprotection after ischemia-reperfusion injury, and examine the therapeutic potential of NO(2)(-) for cardiovascular disease. Evidence is accumulating that suggests NO(2)(-) has surmounted many of the direct challenges to reperfusion therapeutics summarized by the National Heart, Lung, and Blood Institute Working Group in "Myocardial protection at a crossroads: the need for translation into clinical therapy." In this context, we discuss important considerations in designing human clinical trials to test the efficacy of NO(2)(-) in the setting of ischemia-reperfusion injury, with particular attention to the study of ST-segment elevation myocardial infarction.

Nitrite reductase activity of cytochrome c

Small increases in physiological nitrite concentrations have now been shown to mediate a number of biological responses, including hypoxic vasodilation, cytoprotection after ischemia/reperfusion, and regulation of gene and protein expression. Thus, while nitrite was until recently believed to be biologically inert, it is now recognized as a potentially important hypoxic signaling molecule and therapeutic agent. Nitrite mediates signaling through its reduction to nitric oxide, via reactions with several heme-containing proteins. In this report, we show for the first time that the mitochondrial electron carrier cytochrome c can also effectively reduce nitrite to NO. This nitrite reductase activity is highly regulated as it is dependent on pentacoordination of the heme iron in the protein and occurs under anoxic and acidic conditions. Further, we demonstrate that in the presence of nitrite, pentacoordinate cytochrome c generates bioavailable NO that is able to inhibit mitochondrial respiration. These data suggest an additional role for cytochrome c as a nitrite reductase that may play an important role in regulating mitochondrial function and contributing to hypoxic, redox, and apoptotic signaling within the cell.

Brief periods of nitric oxide inhalation protect against myocardial ischemia-reperfusion injury

BACKGROUND

Prolonged breathing of nitric oxide reduces myocardial ischemia-reperfusion injury, but the precise mechanisms responsible for the cardioprotective effects of inhaled nitric oxide are incompletely understood.

METHODS

The authors investigated the fate of inhaled nitric oxide (80 parts per million) in mice and quantified the formation of nitric oxide metabolites in blood and tissues. The authors tested whether the accumulation of nitric oxide metabolites correlated with the ability of inhaled nitric oxide to protect against cardiac ischemia-reperfusion injury.

RESULTS

Mice absorbed nitric oxide in a nearly linear fashion (0.19 +/- 0.02 micromol/g x h). Breathing nitric oxide rapidly increased a broad spectrum of nitric oxide metabolites. Levels of erythrocytic S-nitrosothiols, N-nitrosamines, and nitrosyl-hemes increased dramatically within 30 s of commencing nitric oxide inhalation. Marked increases of lung S-nitrosothiol and liver N-nitrosamine levels were measured, as well as elevated cardiac and brain nitric oxide metabolite levels. Breathing low oxygen concentrations potentiated the ability of inhaled nitric oxide to increase cardiac nitric oxide metabolite levels. Concentrations of each nitric oxide metabolite, except nitrate, rapidly reached a plateau and were similar after 5 and 60 min. In a murine cardiac ischemia-reperfusion injury model, breathing nitric oxide for either 5 or 60 min before reperfusion decreased myocardial infarction size as a fraction of myocardial area at risk by 31% or 32%, respectively.

CONCLUSIONS

Breathing nitric oxide leads to the rapid accumulation of a variety of nitric oxide metabolites in blood and tissues, contributing to the ability of brief periods of nitric oxide inhalation to provide cardioprotection against ischemia-reperfusion injury. The nitric oxide metabolite concentrations achieved in a target tissue may be more important than the absolute amounts of nitric oxide absorbed.

Tissue processing of nitrite in hypoxia: an intricate interplay of nitric oxide-generating and -scavenging systems

Although nitrite (NO(2)(-)) and nitrate (NO(3)(-)) have been considered traditionally inert byproducts of nitric oxide (NO) metabolism, recent studies indicate that NO(2)(-) represents an important source of NO for processes ranging from angiogenesis through hypoxic vasodilation to ischemic organ protection. Despite intense investigation, the mechanisms through which NO(2)(-) exerts its physiological/pharmacological effects remain incompletely understood. We sought to systematically investigate the fate of NO(2)(-) in hypoxia from cellular uptake in vitro to tissue utilization in vivo using the Wistar rat as a mammalian model. We find that most tissues (except erythrocytes) produce free NO at rates that are maximal under hypoxia and that correlate robustly with each tissue's capacity for mitochondrial oxygen consumption. By comparing the kinetics of NO release before and after ferricyanide addition in tissue homogenates to mathematical models of NO(2)(-) reduction/NO scavenging, we show that the amount of nitrosylated products formed greatly exceeds what can be accounted for by NO trapping. This difference suggests that such products are formed directly from NO(2)(-), without passing through the intermediacy of free NO. Inhibitor and subcellular fractionation studies indicate that NO(2)(-) reductase activity involves multiple redundant enzymatic systems (i.e. heme, iron-sulfur cluster, and molybdenum-based reductases) distributed throughout different cellular compartments and acting in concert to elicit NO signaling. These observations hint at conserved roles for the NO(2)(-)-NO pool in cellular processes such as oxygen-sensing and oxygen-dependent modulation of intermediary metabolism.

Nitrite-nitric oxide control of mitochondrial respiration at the frontier of anoxia

Actively respiring animal and plant tissues experience hypoxia because of mitochondrial O(2) consumption. Controlling oxygen balance is a critical issue that involves in mammals hypoxia-inducible factor (HIF) mediated transcriptional regulation, cytochrome oxidase (COX) subunit adjustment and nitric oxide (NO) as a mediator in vasodilatation and oxygen homeostasis. In plants, NO, mainly derived from nitrite, is also an important signalling molecule. We describe here a mechanism by which mitochondrial respiration is adjusted to prevent a tissue to reach anoxia. During pea seed germination, the internal atmosphere was strongly hypoxic due to very active mitochondrial respiration. There was no sign of fermentation, suggesting a down-regulation of O(2) consumption near anoxia. Mitochondria were found to finely regulate their surrounding O(2) level through a nitrite-dependent NO production, which was ascertained using electron paramagnetic resonance (EPR) spin trapping of NO within membranes. At low O(2), nitrite is reduced into NO, likely at complex III, and in turn reversibly inhibits COX, provoking a rise to a higher steady state level of oxygen. Since NO can be re-oxidized into nitrite chemically or by COX, a nitrite-NO pool is maintained, preventing mitochondrial anoxia. Such an evolutionarily conserved mechanism should have an important role for oxygen homeostasis in tissues undergoing hypoxia.

Reactive oxygen species production in energized cardiac mitochondria during hypoxia/reoxygenation: modulation by nitric oxide

Mitochondria are an important source of reactive oxygen species (ROS), implicated in ischemia/reperfusion injury. When isolated from ischemic myocardium, mitochondria demonstrate increased ROS production as a result of damage to electron transport complexes. To investigate the mechanisms, we studied effects of hypoxia/reoxygenation on ROS production by isolated energized heart mitochondria. ROS production, tracked using Fe(2+)-catalyzed, H(2)O(2)-dependent H(2)DCF oxidation or Amplex Red, was similar during normoxia and hypoxia but markedly increased during reoxygenation, in proportion to the duration of hypoxia. In contrast, if mitochondria were rapidly converted from normoxia to near-anoxia ([O(2)], <1 micromol/L), the increase in H(2)DCF oxidation rate during reoxygenation was markedly blunted. To elicit the robust increase in H(2)DCF oxidation rate during reoxygenation, hypoxia had to be severe enough to cause partial, but not complete, respiratory chain inhibition (as shown by partial dissipation of membrane potential and increased NADH autofluorescence). Consistent with its cardioprotective actions, nitric oxide ( O) abrogated increased H(2)DCF oxidation under these conditions, as well as attenuating ROS-induced increases in matrix [Fe(2+)] and aconitase inhibition caused by antimycin. Collectively, these results suggest that (1) hypoxia that is sufficient to cause partial respiratory inhibition is more damaging to mitochondria than near-anoxia; and (2) O suppresses ROS-induced damage to electron transport complexes, probably by forming O-Fe(2+) complexes in the presence of glutathione, which inhibit hydroxyl radical formation.

Dietary nitrite restores NO homeostasis and is cardioprotective in endothelial nitric oxide synthase-deficient mice

Endothelial production of nitric oxide (NO) is critical for vascular homeostasis. Nitrite and nitrate are formed endogenously by the stepwise oxidation of NO and have, for years, been regarded as inactive degradation products. As a result, both anions are routinely used as surrogate markers of NO production, with nitrite as a more sensitive marker. However, both nitrite and nitrate are derived from dietary sources. We sought to determine how exogenous nitrite affects steady-state concentrations of NO metabolites thought to originate from nitric oxide synthase (NOS)-derived NO as well as blood pressure and myocardial ischemia-reperfusion (I/R) injury. Mice deficient in endothelial nitric oxide synthase (eNOS-/-) demonstrated decreased blood and tissue nitrite, nitrate, and nitroso proteins, which were further reduced by low-nitrite (NOx) diet for 1 week. Nitrite supplementation (50 mg/L) in the drinking water for 1 week restored NO homeostasis in eNOS-/- mice and protected against I/R injury. Nitrite failed to alter heart rate or mean arterial blood pressure at the protective dose. These data demonstrate the significant influence of dietary nitrite intake on the maintenance of steady-state NO levels. Dietary nitrite and nitrate may serve as essential nutrients for optimal cardiovascular health and may provide a novel prevention/treatment modality for disease associated with NO insufficiency.

Molecular mediators of liver ischemia and reperfusion injury: a brief review

Ischemia and reperfusion injury is a dynamic process that involves multiple organ systems in various clinical states including transplantation, trauma, and surgery. Research into this field has identified key molecular and signaling players that mediate, modulate, or augment cellular, tissue, and organ injury during this disease process. Further elucidation of the molecular mechanisms should provide the rationale to identify much-needed novel therapeutic options to prevent or ameliorate organ damage due to ischemia and reperfusion in clinics.

Nitric oxide promotes distant organ protection: evidence for an endocrine role of nitric oxide

Endothelial NOS (eNOS)-derived NO has long been considered a paracrine signaling molecule only capable of affecting nearby cells because of its short half-life in blood and relatively limited diffusion distance in tissues. To date, no studies have demonstrated that endogenously generated NO possesses a clearly defined endocrine function. Therefore, we evaluated whether enzymatic generation of NO in the heart is capable of modulating remote physiological actions and cell signaling. Mice with cardiac-specific overexpression of the human eNOS gene (CS-eNOS-Tg) were used to address this hypothesis. Cardiac-specific eNOS overexpression resulted in significant increases in nitrite, nitrate, and nitrosothiols in the heart, plasma, and liver. To examine whether the increase in hepatic NO metabolites could modulate cytoprotection, we subjected CS-eNOS-Tg mice to hepatic ischemia-reperfusion (I/R) injury. CS-eNOS-Tg mice displayed a significant reduction in hepatic I/R injury (4.2-fold reduction in the aminotransferase and a 3.5-fold reduction in aspartate aminotransferase) compared with WT littermates. These findings demonstrate that endogenously derived NO is transported in the blood, metabolized in remote organs, and mediates cytoprotection in the setting of I/R injury. This study presents clear evidence for an endocrine role of NO generated endogenously from eNOS and provides additional evidence for the profound cytoprotective actions of NO in the setting of I/R injury.

Nitrite reductase activity of myoglobin regulates respiration and cellular viability in myocardial ischemia-reperfusion injury

The nitrite anion is reduced to nitric oxide (NO*) as oxygen tension decreases. Whereas this pathway modulates hypoxic NO* signaling and mitochondrial respiration and limits myocardial infarction in mammalian species, the pathways to nitrite bioactivation remain uncertain. Studies suggest that hemoglobin and myoglobin may subserve a fundamental physiological function as hypoxia dependent nitrite reductases. Using myoglobin wild-type ((+/+)) and knockout ((-/-)) mice, we here test the central role of myoglobin as a functional nitrite reductase that regulates hypoxic NO* generation, controls cellular respiration, and therefore confirms a cytoprotective response to cardiac ischemia-reperfusion (I/R) injury. We find that myoglobin is responsible for nitrite-dependent NO* generation and cardiomyocyte protein iron-nitrosylation. Nitrite reduction to NO* by myoglobin dynamically inhibits cellular respiration and limits reactive oxygen species generation and mitochondrial enzyme oxidative inactivation after I/R injury. In isolated myoglobin(+/+) but not in myoglobin(-/-) hearts, nitrite treatment resulted in an improved recovery of postischemic left ventricular developed pressure of 29%. In vivo administration of nitrite reduced myocardial infarction by 61% in myoglobin(+/+) mice, whereas in myoglobin(-/-) mice nitrite had no protective effects. These data support an emerging paradigm that myoglobin and the heme globin family subserve a critical function as an intrinsic nitrite reductase that regulates responses to cellular hypoxia and reoxygenation [corrected]

S-nitrosation and thiol switching in the mitochondrion: a new paradigm for cardioprotection in ischaemic preconditioning

Understanding the molecular mechanisms through which the heart could be protected from ischaemic injury is of major interest and offers a potential route for the development of new therapies. Recently, several studies have uncovered intriguing relationships between nitric oxide-induced protein thiol modifications and the cardioprotected phenotype. In a highly cited, seminal article published in the Biochemical Journal in 2006, Burwell and colleagues addressed this issue and provided direct evidence for S-nitrosation of complex I of the mitochondrial electron transport chain. These authors were the first to show increased S-nitrosation of mitochondrial proteins from hearts subjected to the cardioprotective process known as ischaemic preconditioning. This study has paved the way for further investigations that collectively reveal a potential link between the mitochondrial S-nitrosoproteome and ischaemic preconditioning.

The reno-vascular A2B adenosine receptor protects the kidney from ischemia

BACKGROUND

Acute renal failure from ischemia significantly contributes to morbidity and mortality in clinical settings, and strategies to improve renal resistance to ischemia are urgently needed. Here, we identified a novel pathway of renal protection from ischemia using ischemic preconditioning (IP).

METHODS AND FINDINGS

For this purpose, we utilized a recently developed model of renal ischemia and IP via a hanging weight system that allows repeated and atraumatic occlusion of the renal artery in mice, followed by measurements of specific parameters or renal functions. Studies in gene-targeted mice for each individual adenosine receptor (AR) confirmed renal protection by IP in A1(-/-), A2A(-/-), or A3AR(-/-) mice. In contrast, protection from ischemia was abolished in A2BAR(-/-) mice. This protection was associated with corresponding changes in tissue inflammation and nitric oxide production. In accordance, the A2BAR-antagonist PSB1115 blocked renal protection by IP, while treatment with the selective A2BAR-agonist BAY 60-6583 dramatically improved renal function and histology following ischemia alone. Using an A2BAR-reporter model, we found exclusive expression of A2BARs within the reno-vasculature. Studies using A2BAR bone-marrow chimera conferred kidney protection selectively to renal A2BARs.

CONCLUSIONS

These results identify the A2BAR as a novel therapeutic target for providing potent protection from renal ischemia.

Nitrite anion provides potent cytoprotective and antiapoptotic effects as adjunctive therapy to reperfusion for acute myocardial infarction

BACKGROUND

Accumulating evidence suggests that the ubiquitous anion nitrite (NO2-) is a physiological signaling molecule, with roles in intravascular endocrine nitric oxide transport, hypoxic vasodilation, signaling, and cytoprotection. Thus, nitrite could enhance the efficacy of reperfusion therapy for acute myocardial infarction. The specific aims of this study were (1) to assess the efficacy of nitrite in reducing necrosis and apoptosis in canine myocardial infarction and (2) to determine the relative role of nitrite versus chemical intermediates, such as S-nitrosothiols.

METHODS AND RESULTS

We evaluated infarct size, microvascular perfusion, and left ventricular function by histopathology, microspheres, and magnetic resonance imaging in 27 canines subjected to 120 minutes of coronary artery occlusion. This was a blinded, prospective study comparing a saline control group (n=9) with intravenous nitrite during the last 60 minutes of ischemia (n=9) and during the last 5 minutes of ischemia (n=9). In saline-treated control animals, 70+/-10% of the area at risk was infarcted compared with 23+/-5% in animals treated with a 60-minute nitrite infusion. Remarkably, a nitrite infusion in the last 5 minutes of ischemia also limited the extent of infarction (36+/-8% of area at risk). Nitrite improved microvascular perfusion, reduced apoptosis, and improved contractile function. S-Nitrosothiol and iron-nitrosyl-protein adducts did not accumulate in the 5-minute nitrite infusion, suggesting that nitrite is the bioactive intravascular nitric oxide species accounting for cardioprotection.

CONCLUSIONS

Nitrite has significant potential as adjunctive therapy to enhance the efficacy of reperfusion therapy for acute myocardial infarction.

Serial measurements of whole blood nitrite in an intensive care setting

Nitrite plays an eminent role in cardiovascular physiology and pathology, mediating hypoxic vasodilation, reducing ischemia-reperfusion injury, and regulating cardiac energetics and function. The role of circulating nitrite in critically ill patients has not been examined so far. To investigate whether whole blood nitrite can be determined reproducibly in an intensive care setting, 30 patients from a cardiology intensive care unit were enrolled in this study, no matter what the underlying disease. Blood was drawn from an arterial catheter and whole blood nitrite was determined, using a tri-iodide/ozone-based chemiluminescence assay after incubation with a ferricyanide-containing stabilization solution. Whole blood nitrite levels ranged from 35 to 1193 nmol/L (mean+/-SEM: 220+/-20 nmol/L). Myocardial infarction was associated with lower whole blood nitrite levels (200+/-53 nmol/L for elevated serum CK MB levels vs 432+/-95 nmol/L in the normal CK MB range, p=0.039). Neither impaired kidney function nor an inflammatory state was associated with higher or lower whole blood nitrite levels. In conclusion, whole blood nitrite can be measured easily and reproducibly in critically ill patients, regardless of renal function and inflammation. The origin of decreased nitrite levels in myocardial infarction is currently unclear and needs to be further elucidated.

Therapeutic uses of inorganic nitrite and nitrate: from the past to the future

Potential carcinogenic effects, blue baby syndrome, and occasional intoxications caused by nitrite, as well as the suspected health risks related to fertilizer overuse, contributed to the negative image that inorganic nitrite and nitrate have had for decades. Recent experimental studies related to the molecular interaction between nitrite and heme proteins in blood and tissues, the potential role of nitrite in hypoxic vasodilatation, and an unexpected protective action of nitrite against ischemia/reperfusion injury, however, paint a different picture and have led to a renewed interest in the physiological and pharmacological properties of nitrite and nitrate. The range of effects reported suggests that these simple oxyanions of nitrogen have a much richer profile of biological actions than hitherto assumed, and several efforts are currently underway to investigate possible beneficial effects in the clinical arena. We provide here a brief historical account of the medical uses of nitrite and nitrate over the centuries that may serve as a basis for a careful reassessment of the health implications of their exposure and intake and may inform investigations into their therapeutic potential in the future.

Attenuation of apoptosis and the eye of the beholder

OBJECTIVES

We consider the conundrum suggested by myocardial hibernation and late restoration of function despite the absence of a substantial lateral peri-infarction border zone with respect to oxygenation, and suggest a pivotal role for apoptosis and its attenuation in salvaging jeopardized myocardium.

METHODS

Selective pertinent literature is reviewed, and some recent observations indicating difficulties in identifying and quantifying apoptosis microscopically are summarized.

RESULTS

Apoptosis seems to occur primarily after reperfusion following ischemia rather than persistent ischemia leading to necrosis. Refinements of markers of its presence are needed in vitro for use ultimately in vivo and should be pivotal in defining the extent to which tissue-protective interventions can salvage myocardium in the context of a fixed magnitude and duration of ischemia.

CONCLUSION

Apoptosis is strongly implicated in the overall demise of jeopardized myocardium. Its attenuation seems likely to be potentially beneficial. Validation of this hypothesis will require progress in identification, delineation, and assessment of the extent of apoptosis in the threatened heart.

Endogenous neuroprotection: mitochondria as gateways to cerebral preconditioning?

From single to multicellular organisms, protective mechanisms have evolved against endogenous and exogenous noxious stimuli. Preconditioning paradigms, in which stimulation below the threshold of injury results in subsequent protection of the brain, have played an important role in elucidating such endogenous protective mechanisms. Consequently, over the past decades numerous signaling pathways have been discovered by which the brain senses and reacts to such insults as neurotoxins, substrate deprivation, or inflammation. Research on preconditioning is aimed at understanding endogenous neuroprotection to boost it, or to supplement its effectors therapeutically once damage to the brain has occurred, such as after stroke or brain trauma. Another goal of establishing preconditioning protocols is to induce endogenous neuroprotection in anticipation of incipient brain damage. Currently several endogenous neuroprotectants are being investigated in controlled clinical trials. In the present review we will give a short overview on the signals, sensors, transducers, and effectors of endogenous neuroprotection. We will first focus on common mechanisms, on which pathways of endogenous neuroprotection converge, and in particular on mitochondria, which may be considered master integrators of endogenous neuroprotection. We will then discuss various applications of preconditioning, including pharmacological and anesthetic preconditioning, as well as postconditioning, and explore the prospects of endogenous neuroprotective therapeutic approaches.

Mitochondria as a target for the cardioprotective effects of nitric oxide in ischemia-reperfusion injury

During cardiac ischemia-reperfusion (IR) injury, excessive generation of reactive oxygen species (ROS) and overload of Ca(2+) at the mitochondrial level both lead to opening of the mitochondrial permeability transition (PT) pore on reperfusion. This can result in the depletion of ATP, irreversible oxidation of proteins, lipids, and DNA within the cardiomyocyte, and can trigger cell-death pathways. In contrast, mitochondria are also implicated in the cardioprotective signaling processes of ischemic preconditioning (IPC), to prevent IR-related pathology. Nitric oxide (NO*) has emerged as a potent effector molecule for a variety of cardioprotective strategies, including IPC. Whereas NO* is most noted for its activation of the "classic" soluble guanylate cyclase (sGC) signaling pathway, emerging evidence indicates that NO can directly act on mitochondria, independent of the sGC pathway, affording acute cardioprotection against IR injury. These direct effects of NO* on mitochondria are the focus of this review.

Targeting the mitochondria to augment myocardial protection

The dynamic regulation of the structure, function and turnover of mitochondria is recognized as an immutable control node maintaining cellular integrity and homeostasis. The term 'mitohormesis' has recently been coined to describe the adaptive reprogramming of mitochondrial biology in response to low levels of metabolic substrate deprivation to augment subsequent mitochondrial and cellular tolerance to biological stress. Disruption of these regulatory programs gives rise to cardiovascular and neurodegenerative diseases, and augmentation or fine-tuning of these programs may ameliorate mitochondrial and global cellular stress tolerance. This is in part via the regulation of reactive oxygen species, calcium homeostasis, and in response to caloric restriction, the capacity to augment DNA repair. The objective of this manuscript is to briefly review these regulatory programs and to postulate novel therapeutic approaches with the primary goal of modulating mitochondria to enhance tolerance to cardiac ischemic stress.

The nitrate-nitrite-nitric oxide pathway in physiology and therapeutics

The inorganic anions nitrate (NO3-) and nitrite (NO2-) were previously thought to be inert end products of endogenous nitric oxide (NO) metabolism. However, recent studies show that these supposedly inert anions can be recycled in vivo to form NO, representing an important alternative source of NO to the classical L-arginine-NO-synthase pathway, in particular in hypoxic states. This Review discusses the emerging important biological functions of the nitrate-nitrite-NO pathway, and highlights studies that implicate the therapeutic potential of nitrate and nitrite in conditions such as myocardial infarction, stroke, systemic and pulmonary hypertension, and gastric ulceration.

Reductive gas-phase chemiluminescence and flow injection analysis for measurement of the nitric oxide pool in biological matrices

There is growing evidence for nitric oxide (NO.) being involved in cell signaling and pathology. Much effort has been made to elucidate and characterize the different biochemical reaction pathways of NO.in vivo. However, a major obstacle in assessing the significance of nitrosated species and oxidized metabolites often remains: a reliable analytical technique for the detection of NO. in complex biological matrices. This chapter presents refined methodologies, such as chemiluminescence detection and flow injection analysis, compared with adequate sample processing procedures to reliably quantify and assess the circulating and resident NO(.) pool, consisting of nitrite, nitrate, nitroso, and nitrosylated species.