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(Circulation Research. 2002;91:240.)
© 2002 American Heart Association, Inc.

Integrative Physiology

Aldose Reductase Is an Obligatory Mediator of the Late Phase of Ischemic Preconditioning

Ken Shinmura, Roberto Bolli, Si-Qi Liu, Xian-Liang Tang, Eitaro Kodani, Yu-ting Xuan, Sanjay Srivastava, Aruni Bhatnagar

From the Experimental Research Laboratory, Division of Cardiology, University of Louisville and Jewish Hospital Heart and Lung Institute, Louisville, Ky. Present address for K.S. is the Division of Geriatric Medicine, Department of Internal Medicine, Keio University School of Medicine, Japan.

Correspondence to Aruni Bhatnagar, PhD, Division of Cardiology, Jewish Cardiovascular Research Center, 500 South Floyd St, Louisville, KY 40202. E-mail aruni{at}louisville.edu

	Abstract

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Aldose reductase (AR), a member of the aldo-keto reductase superfamily, has been shown to metabolize toxic aldehydes generated by lipid peroxidation, suggesting that it may serve as an antioxidant defense. To investigate its role in the late phase of ischemic preconditioning (PC), conscious rabbits underwent 6 cycles of 4-minute coronary occlusion/4-minute reperfusion. Twenty-four hours later, there was a marked increase in AR protein and activity and in the myocardial content of sorbitol, a unique product of AR catalysis. Pretreatment with N-nitro-L-arginine, a nitric oxide synthase inhibitor, or chelerythrine, a protein kinase C inhibitor (both given at doses that block late PC in this model), prevented the increase in AR protein 24 hours later, demonstrating that ischemic PC upregulates AR via nitric oxide- and protein kinase C-dependent signaling pathways. The AR-selective inhibitors tolrestat and sorbinil prevented AR-mediated accumulation of sorbitol and abrogated the infarct-sparing effect of late PC, demonstrating that enhanced AR activity is necessary for this cardioprotective phenomenon to occur. Inhibition of AR did not affect infarct size or the myocardial accumulation of the lipid peroxidation product 4-hydroxy trans-2-nonenal (HNE) in nonpreconditioned rabbits. The accumulation of HNE was inhibited by late PC, and this effect was abrogated by sorbinil. Taken together, these results establish AR as an essential mediator of late PC. Furthermore, the data suggest that myocardial ischemia/reperfusion injury is due in part to the generation of lipid peroxidation products and that late PC diminishes this source of injury by upregulating AR.

Key Words: myocardial ischemia/reperfusion injury • aldose reductase • myocardial infarction • 4-hydroxy-trans-2-nonenal • ischemic preconditioning

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion References

 
Aldose reductase (AR) is a member of the aldo-keto reductase superfamily.¹ Although this enzyme is expressed in most eukaryotic cells and is known to catalyze the reduction of a several aldehydes including aldo-sugars, its physiological role remains unclear.² Recent investigations have shown that AR exhibits high affinity for hydrophobic aldehydes, such as those generated during lipid peroxidation.³ The most abundant among the lipid-derived aldehydes, 4-hydroxy trans-2-nonenal (HNE) and its glutathione conjugate, are excellent substrates of AR.³ Because lipid-derived aldehydes are cytotoxic,⁴ the ability of AR to metabolize them suggests that this enzyme may be involved in protection against oxidative injury. This function of AR could be important in myocardial ischemia/reperfusion, which is associated with increased generation of reactive oxygen species.⁵ Indeed, mounting evidence indicates that HNE is a major product of lipid peroxidation during myocardial ischemia/reperfusion⁶ and that the formation of HNE and the accumulation of HNE-modified proteins are related to the extent of tissue damage.^7,8

The late phase of ischemic preconditioning (PC) is a long-lasting adaptive response of the myocardium to a mild ischemic stress that confers relative resistance against both mild, reversible ischemia/reperfusion injury (myocardial stunning) and severe, irreversible injury (myocardial infarction).^9–16 The protection afforded by late PC appears 12 to 24 hours after the initial ischemic challenge, lasts 3 to 4 days, and is mediated by the synthesis of cardioprotective proteins.¹⁷ Among these, recent pharmacological and/or genetic studies have identified inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2) as essential mediators of the infarct-sparing effects of late PC.^12,15,16,18 However, in view of the evidence that late PC is a complex polygenic adaptation, ¹⁷ it seems plausible to postulate that other protein(s) may also be involved. Although AR is known to be a stress-responsive enzyme^19,20 and could be potentially cardioprotective by virtue of its antioxidant activity, virtually nothing is known regarding its involvement in late PC and its role in myocardial protection remains obscure.

In the present study, we tested the hypothesis that AR plays an obligatory role in mediating the protective effects of the late phase of ischemic PC. Using a well-characterized rabbit model, we first sought to determine whether AR is upregulated 24 hours after a PC protocol known to elicit delayed protection against myocardial infarction.^12,14,16 We next examined whether upregulation of AR is mediated by NO and/or protein kinase C (PKC), two early elements of the signaling pathways of late PC.^{11,13–15,17,21,22} Finally, we determined whether blocking AR activity with AR selective inhibitors interferes with the delayed protective effects elicited by ischemic PC and prevents the detoxification of HNE generated during ischemia/reperfusion. These studies were conducted in conscious animals to obviate the potentially confounding influence of conditions associated with open-chest animal preparations,^11,12,16 particularly in view of the fact that ROS generation is markedly exaggerated in open-chest models.²³ The results demonstrate that ischemic PC upregulates AR in the heart and that AR activity is essential for the infarct-sparing actions of the late phase of PC, thereby identifying, for the first time, AR as a cardioprotective protein.

	Materials and Methods

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The conscious rabbit model of myocardial ischemia has been described in detail previously.^11–14,16 Briefly, New Zealand White male rabbits (obtained from Myrtle’s Rabbitry, Thompson Station, Tenn; weight, 2.3±0.1 kg) were instrumented under sterile conditions with a balloon occluder around a major branch of the left coronary artery, a 10-MHz pulsed Doppler ultrasonic crystal in the center of the region to be rendered ischemic, and bipolar ECG leads on the chest wall. Rabbits were allowed to recover for a minimum of 14 days after surgery. The present study was performed in accordance with the guidelines of the Animal Care and Use Committee of the University of Louisville (Ky) School of Medicine and with the Guide for the Care and Use of Laboratory Animals (Department of Health and Human Services, publication No. [NIH] 86-23).

Phases of Study
The study consisted of 3 consecutive phases (Phases I through III).

Phase I: Studies of AR Expression and AR and Sorbitol Dehydrogenase (SDH) Activity
Rabbits were assigned to 5 groups (online Figure 1 found in the online data supplement available at http://www.circresaha.org). Rabbits in group I (control) did not receive any treatment and did not undergo coronary occlusion. They were euthanized and samples of the anterior and posterior left ventricular (LV) wall were rapidly removed and stored at -140°C until use. Rabbits in groups II (PC) underwent a sequence of six 4-minute coronary occlusion/4-minute reperfusion cycles and were euthanized 24 hours later. Myocardial samples were removed from the ischemic/reperfused and the nonischemic region, and stored at -140°C until use. Rabbits in groups III-V underwent the same sequence of occlusion/reperfusion cycles. Group III (L-NA+PC) received N-nitro-L-arginine (L-NA, IV, 13 mg/kg) for 10 minutes, starting 20 minutes before and ending 10 minutes before coronary occlusion. Group IV (CHE+PC) received chelerythrine (IV, 5 mg/kg) 5 minutes before the first coronary occlusion. These doses of L-NA and chelerythrine block the cardioprotective effects of late PC in this model.^11–14 L-NA (Sigma) was dissolved in normal saline. Chelerythrine chloride (Research Biomedicals International) was dissolved in dimethyl sulfoxide (DMSO) and then diluted with normal saline to a final concentration of 50% DMSO in saline (v/v). Rabbits in group V (PC+tolrestat) underwent a sequence of 6 occlusion/reperfusion cycles without any treatment on day 1. Twenty-four hours later, they received tolrestat (10 mg/kg; IV) and were euthanized 30 minutes after the injection. Tolrestat (Wyeth-Ayerst) was dissolved in 25 mmol/L bicarbonate buffer, pH 8.5.

View larger version (56K): [in this window] [in a new window]   Figure 1. Effect of ischemic PC on the expression of AR protein in rabbit myocardium. Tissue samples were obtained from the anterior and posterior LV wall (AW and PW, respectively) of control rabbits (group I) and from the ischemic/reperfused (IZ) and nonischemic (NZ) zones of the heart of rabbits that underwent ischemic PC with six 4-minute coronary occlusion/4-minute reperfusion cycles and were euthanized 24 hours later. AR immunoreactivity (Western) in the membranous fractions increased markedly in both the ischemic/reperfused and nonischemic region 24 hours after ischemic PC (A), whereas no change was observed in AR protein in the cytosolic fractions (B). Pretreatment before the occlusion/reperfusion cycles with L-NA (C) or chelerythrine (D) prevented the increase in AR protein expression in the membranous fractions 24 hours later.

The frozen samples were powdered in liquid nitrogen and suspended in 50 mmol/L potassium phosphate buffer, pH 7.4, containing 5 mmol/L dithiothreitol (DTT) and 1 mmol/L PMSF, 25 µg/mL leupeptin, 25 mmol/L NaF, and 1 mmol/L Na₃VO₄. The extract was centrifuged at 13 000g for 30 minutes at 4°C, and the supernatant was used for the measurements of AR and sorbitol dehydrogenase (SDH) activity. Western analysis was performed using standard procedures¹⁶ using anti-SDH or anti-AR antibodies²⁴ (see the expanded Materials and Methods section, which can be found in the online data supplement available at http://www.circresaha.org). The myocardial content of sorbitol, a unique product of AR catalysis, was determined as described previously.^25,26

Phase II: Studies of Myocardial Infarction
All rabbits were subjected to a 30-minute coronary artery occlusion followed by 3 days of reperfusion. Rabbits were assigned to eight groups (online Figure 2). Rabbits in group VI (control) underwent the 30-minute occlusion without PC or drug pretreatment. Rabbits in group VII (PC) were preconditioned with a sequence of six 4-minute coronary occlusion/4-minute reperfusion cycles 24 hours before the 30-minute coronary occlusion. Rabbits in groups IX, X, XII, and XIII were also preconditioned with the sequence of 6 occlusion/reperfusion cycles on day 1. Twenty-four hours later, they received either an IV injection of tolrestat (Wyeth-Ayerst; 10 mg/kg [group IX: PC+tolrestat]) or an IP injection of sorbinil (Pfizer; 40 mg/kg [group XII: PC+sorbinil]), 30 minutes before the coronary occlusion. Sorbinil was dissolved in DMSO and then diluted with normal saline to a final concentration of 50% DMSO in saline (vol/vol). Rabbits in group X (PC+vehicle I) and XIII (PC+vehicle II) received only the vehicle used for dissolving tolrestat (intravenous) or sorbinil (intraperitoneal). Rabbits in groups VIII (tolrestat) and XI (sorbinil) received the same doses of tolrestat or sorbinil without a PC protocol 24 hours earlier.

View larger version (29K): [in this window] [in a new window]   Figure 2. Densitometric analysis of AR signals. A, AR protein in the membranous fractions of the ischemic and nonischemic zone increased markedly (+179% and +185%, respectively) 24 hours after ischemic PC (group II), compared with control hearts (group I). In contrast, AR protein in the cytosolic fraction did not change (B). In all samples, the densitometric measurements of AR immunoreactivity were expressed as a percentage of the average value measured in the anterior LV wall of control rabbits. C, Densitometric analysis of AR signals in membranous fractions of myocardial samples obtained from rabbits treated with chelerythrine or L-NA. Pretreatment before ischemic PC with chelerythrine (group III) or L-NA (group IV) completely abrogated the increase in AR protein in the membranous fractions. Blots were normalized by Ponceau staining as described previously13–14 (online Figure 5).

Postmortem Tissue Analysis
At the conclusion of the study, the occluded/reperfused vascular bed and the infarct were identified by postmortem perfusion of the heart with Phthalo blue dye and triphenyltetrazolium.^14,16 Infarct size was calculated by using computerized video planimetry.^14,16

Phase III: Measurements of HNE
To measure ischemia-induced changes in HNE, rabbits in groups XIV through XVII underwent 30 minutes of coronary occlusion and were euthanized after 60 minutes of reperfusion (online Figure 3). Group XIV (I/R) was subjected to a 30-minute occlusion followed by 1 hour of reperfusion. Group XV received sorbinil (40 mg/kg, IP) 30 minutes before occlusion. Groups XVI (PC+I/R) and XVII (PC+I/R+sorbinil) were preconditioned with 6 occlusion/reperfusion cycles 24 hours before the 30-minute occlusion; additionally, rabbits in group XVII received sorbinil (40 mg/kg, IP) 30 minutes before occlusion on day 2. The concentration of free HNE was measured according to the method of Luo et al²⁷ (see online data supplement).

View larger version (21K): [in this window] [in a new window]   Figure 3. Ischemia-induced changes in AR activity (A) and sorbitol formation (B). AR activity was measured in extracts of the ischemic/reperfused section 24 hours after PC (group II) or in control hearts (group I). On average, a 165% increase in the enzyme activity was observed. In parallel experiments, the myocardial content of sorbitol was measured in control (group I) hearts and in hearts 24 after PC without (group II) or with (group V) tolrestat treatment.

Statistical Analysis
Data are reported as mean±SEM. Data were analyzed with either a 1-way or a 2-way repeated-measures ANOVA, as appropriate, followed by paired or unpaired Student’s t test with the Bonferroni correction.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
A total of 121 rabbits were instrumented for this study.

Phase I: Studies of AR Expression and Activity
Exclusions
Of the 26 rabbits instrumented for phase I, 6 were assigned to group I (control), 6 to group II (PC), 5 to group III (L-NA+PC), 5 to group IV (CHE+PC), and 4 to group V (PC+tolrestat). All rabbits completed the protocol successfully.

Expression of AR Protein
A representative Western blot of AR in membranous fractions prepared from groups I through III is illustrated in Figure 1. In control rabbits (group I), the AR protein was detected in both the cytosolic and the membranous fractions, although more than 85% of the protein was associated with the cytosolic fraction (estimated from the relative band intensities on Western blots). Isolated rabbit myocytes displayed immunoreactivity to anti-AR antibodies, indicating that AR is expressed in cardiac myocytes (online Figure 4). Our previous studies show that myocytes, isolated from rabbit²⁸ or guinea pig²⁹ hearts, metabolize aldehydes via AR. When rabbits were preconditioned with six 4-minute occlusion/4-minute reperfusion cycles 24 hours earlier (group II), the expression of AR in the membranous fractions increased markedly in both the ischemic/reperfused and the nonischemic regions (2.8-fold and 2.9-fold increase, respectively, versus the anterior LV wall of control rabbits; Figures 1 and 2A and online Figure 5). No increase in AR protein was detectable in the cytosolic fractions (Figures 1 and 2B). Treatment with L-NA or chelerythrine before ischemic PC prevented the increase in AR (groups III and IV;Figures 1 and 2C), indicating that ischemic PC upregulates AR via NO- and PKC-dependent signaling pathways.

View larger version (20K): [in this window] [in a new window]   Figure 4. Effect of AR inhibitors on infarct size. Myocardial infarct size in groups VI (control, n=8), VII (PC, n=8), VIII (tolrestat, n=7), IX (PC+tolrestat, n=9), X (PC+tolrestat, n=9), XI (sorbinil, n=8), XII (PC+sorbinil, n=8), and XIII (PC+vehicle, n=7). Infarct size is expressed as a percentage of the region at risk of infarction. Open circles represent individual rabbits, whereas filled circles represent mean±SEM.

View larger version (27K): [in this window] [in a new window]   Figure 5. Myocardial level of free HNE. Concentration of HNE was measured using gas chromatography-mass spectrometry in the ischemic/reperfused (anterior wall) and nonischemic (posterior wall) regions of the hearts of rabbits that underwent 30 minutes of occlusion and 1 hour of reperfusion without (group XIV) or with sorbinil (group XV) administered 30 minutes before occlusion. Group XVI underwent ischemic PC 24 hours before occlusion. Rabbits in group XVII were preconditioned 24 hours before occlusion, but were treated with sorbinil 30 minutes before occlusion.

Myocardial AR Activity and Sorbitol Levels
The increase in AR protein was accompanied by an increase in sorbinil-sensitive AR activity (Figure 3A) and an increase in the myocardial levels of sorbitol (Figure 3B). Administration of the selective AR inhibitor, tolrestat, completely abrogated the increase in myocardial sorbitol levels induced by ischemic PC (Figure 3B). Thus, the dose of tolrestat used in the present study was effective in blocking the increase in AR activity associated with ischemic PC in vivo. In contrast to AR, we found no detectable expression of SDH in the rabbit heart, and SDH was not induced by ischemic PC (online Figure 6).

Phase II: Studies of Myocardial Infarction
Exclusions
Of the 74 rabbits instrumented for the studies of myocardial infarction, 10, 10, 9, 9, 9, 9, 9, and 9 were assigned to groups VI, VII, VIII, IX, X, XI, XII, and XIII, respectively. Seven rabbits died of ventricular fibrillation during the 30-minute coronary occlusion (2 in group VI, 2 in group VII, 1 in group XII, and 2 in group XIII). Two rabbits were excluded because of a small region at risk (<10% of the left ventricle) (one each in group VIII and XI). One rabbit in group VIII was excluded because of postoperative problems (pericarditis).

Hemodynamic Parameters
There were no appreciable differences in heart rate throughout the experimental protocol among groups V through XII (data from groups IX, XII, and XIII are shown in online Table 1, which can be found in the online data supplement available at http://www.circresaha.org). In addition, there were no differences in thickening fraction among the eight groups at baseline (before administration of vehicle, tolrestat, or sorbinil) and just before coronary occlusion (data not shown). In four rabbits in each of groups IX, XII, and XIII, arterial blood pressure was measured using a 22-gauge angiocatheter inserted into ear dorsal artery as previously described.¹¹ There was no difference in mean arterial pressure at baseline or before coronary occlusion (online Table 1).

Region at Risk and Infarct Size
The size of the LV and of the region at risk did not differ among the 8 groups (online Table 2). The average infarct size was 47% smaller in group VII (PC) compared with group VI (control), indicating late PC against myocardial infarction (Figure 4). In contrast, in preconditioned rabbits treated with either tolrestat (group IX) or sorbinil (group XII), infarct size was similar to that measured in controls (Figure 4), indicating that both drugs abrogated the protective effect of late PC. Administration of vehicle (groups X and XIII) did not interfere with late PC. In groups VIII and XI, infarct size did not differ from that observed in controls, indicating that tolrestat and sorbinil did not affect the extent of cell death in nonpreconditioned myocardium. For any given size of the region at risk, the resulting infarction was greater in preconditioned rabbits treated with either tolrestat or sorbinil than in untreated preconditioned rabbits (online Figure 7).

Phase III: Measurements of HNE
Exclusions
Of the 21 rabbits instrumented for phase III, 6, 5, 5, and 5 were assigned to groups XIV, XV, XVI, and XVII, respectively. One rabbit was excluded in group XIV due to ventricular fibrillation. All other rabbits completed the protocol successfully.

As shown in Figure 5, 30 minutes of occlusion followed by 60 minutes of reperfusion (group XIV) led to a 5-fold increase in the content of free HNE in the ischemic/reperfused zone as compared with the nonischemic zone. Treatment with sorbinil 30 minutes before coronary occlusion did not affect HNE accumulation in nonpreconditioned hearts (group XV), indicating that AR activity is not a significant mechanism for HNE detoxification in the absence of PC. However, the increase in HNE was attenuated in rabbits that were preconditioned 24 hours earlier (group XVI). When preconditioned rabbits were given sorbinil (group XVII), a striking increase in free HNE content was observed (Figure 5): in the ischemic/reperfused region, the free HNE content was 4- to 5-fold greater than in untreated preconditioned rabbits (group XVI) and 2- to 2.5-fold greater than in nonpreconditioned rabbits (group XIV) (Figure 5), indicating that AR activity plays an important role in HNE metabolism during ischemia/reperfusion of preconditioned hearts. In the nonischemic region, the content of free HNE was also markedly increased compared with the other groups (Figure 5).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
Although AR and its homologs are widely distributed aldo-keto reductases, the physiological role of these enzymes remains unclear. In vitro, AR catalyzes the reduction of a large array of aldehydes including aldo-sugars.² Our recent observations showing that the enzyme displays 10³- to 10⁴-fold lower K_m for lipid-derived aldehydes as compared with glucose^3,30 suggest that AR may be involved in the metabolism of toxic aldehydes such as those generated during lipid peroxidation. To test whether AR provides antioxidant protection by diminishing tissue injury caused by lipid peroxidation, we examined its regulation by ischemia and its involvement in ischemia/reperfusion injury, which is accompanied by enhanced generation of oxygen-derived free radicals.

Our results show that brief episodes of myocardial ischemia (ischemic PC) upregulate AR. The elevated levels of AR were associated with a parallel increase in the myocardial content of sorbitol and in the sorbinil-sensitive myocardial AR activity, indicating that the newly-synthesized protein is catalytically active. Inhibition of AR with two unrelated compounds completely abrogated the infarct-sparing effects of late PC (Figure 4), demonstrating that AR plays an essential role in the cardioprotection afforded by the late phase of ischemic PC. Inhibition of AR also abrogated the protective effects of late PC against the accumulation of the cytotoxic lipid aldehyde HNE (Figure 5), demonstrating that AR activity is important for the antioxidant actions of late PC. Taken together, these results expand our understanding of late PC by identifying a new molecular effector of this cardioprotective phenomenon and by demonstrating that AR serves an important antioxidant function.

The increase in the expression of AR after ischemia is consistent with its role as a stress-responsive protein. The expression of AR is known to be upregulated by oxidative, osmotic, or chemical stress.^2,19,20,24 Moreover, AR was reported to be one of the few genes whose expression was consistently enhanced in failing human hearts,³¹ suggesting that upregulation of AR may be a general protective response of the stressed heart. However, the mechanism whereby ischemia upregulates AR is unknown. We tested the role of NO and PKC because previous studies have demonstrated that ischemic PC activates PKC via formation of NO¹⁴ and that the generation of NO and the activation of PKC are essential for the development of late PC.^11,13,14,22 Our results demonstrate that the induction of AR was prevented by the same doses of L-NA and chelerythrine that block the activation of NOS and PKC, respectively, in this model,^13,14 demonstrating that the generation of NO and the subsequent activation of PKC is essential for the induction of AR by ischemic PC. These results suggest that upregulation of AR is an important component of the molecular changes that underlie the development of late PC, thereby prompting us to examine the function of AR in this phenomenon.

To test the role of AR, we used two structurally unrelated inhibitors, so as to minimize the possibility that the results could be due to nonspecific actions of either drug. We found that the infarct-sparing effects of late PC were completely abrogated by both tolrestat and sorbinil. Our measurements of myocardial sorbitol and HNE levels provide direct evidence that tolrestat and sorbinil effectively blocked the increase in AR activity associated with PC (Figures 3B and 5). However, neither drug decreased infarct size in nonpreconditioned rabbits, indicating that AR does not modulate ischemia/reperfusion injury in the absence of ischemic PC. The reason for this selective role of AR in protecting the preconditioned heart is presently unclear, but may relate to the low abundance of AR in the rabbit heart. Although AR protein, activity, and sorbitol contents were detectable in the rabbit heart, the AR activity was only one-tenth of that detected in the rat heart (unpublished observations, 2002). Another possibility is that the ischemia-induced upregulation of AR in the membranous fraction (Figures 1 and 2) may be essential for this enzyme to reduce lipid-derived aldehydes that are generated and retained in the membrane due to their high hydrophobicity. A third possibility, supported by the greater accumulation of HNE in sorbinil-treated PC hearts (Figure 5), is that late PC induces a prooxidant state, which is counteracted by the upregulation of AR but is unmasked by AR inhibitors. This would be in agreement with previous reports showing that the late preconditioned phenotype is established by an increase in the levels of oxidant-generating enzymes such as COX-2 and iNOS.^{11,12,14–17} According to this hypothesis, the role of AR may be more important in preconditioned hearts because they generate higher levels of lipid peroxidation products compared with nonpreconditioned hearts. That inhibition of AR did not increase HNE levels in the nonpreconditioned myocardium (Figure 5, group XV) rules out the possibility that HNE generation is less in preconditioned vis-à-vis nonpreconditioned hearts and that AR is unable to effectively remove all the HNE that is produced in the nonpreconditioned hearts. Rather, our results are consistent with the view that the nonpreconditioned rabbit heart does not contain enough AR in the appropriate compartment (membrane) to prevent HNE accumulation.

In apparent contradiction to our findings, two recent studies^25,26 have concluded that pharmacological inhibition of AR reduces ischemia/reperfusion injury in isolated perfused hearts. It was speculated that AR inhibitors exert their protective effects by limiting the increase in the NADH/NAD⁺ ratio as a result of inhibition of the polyol pathway, thereby maintaining glycolysis and limiting ATP depletion during ischemia.²⁵ However, these studies^25,26 examined only one AR inhibitor and it is unclear whether the effects were specific to this drug. One investigation²⁵ was conducted in diabetic rats, and therefore comparison with our results is not possible. In the study that examined nondiabetic rabbit hearts,²⁶ the infarct size measured after treatment with the AR inhibitor zopolrestat ranged widely (from 8% to 70% of the risk region), making conclusions difficult. Importantly, we found no detectable expression or activity of SDH in the heart either in control conditions or after ischemia (online Figure 6), implying that in the rabbit heart sorbitol is not further metabolized to fructose. Consequently, in this species inhibition of AR cannot protect the heart by increasing the NADH/NAD⁺ ratio.

The finding that infarct size limitation by ischemic PC is mediated by AR, which is primarily an aldehyde-metabolizing enzyme, suggests that ischemia/reperfusion causes lipid peroxidation and that the toxicity of lipid-derived products is a significant component of tissue injury. Although the occurrence of oxidative stress during ischemia/reperfusion is well documented, conflicting results have been reported regarding the ability of antioxidants to limit infarct size.⁶ To date, almost all antioxidant interventions have been targeted at oxygen-free radicals such as superoxide anion or hydroxyl radical. However, these species are highly reactive and short-lived. Consequently, they would be expected to cause damage at or near the site of their formation. In contrast, the toxic aldehydes generated by lipid peroxidation have a considerably longer half-life, and thus can mediate or amplify the cellular effects of their radical precursors.⁴ Because in vitro HNE causes metabolic inhibition and death of cardiac myocytes,³² it is likely that accumulation of HNE is toxic to the ischemic heart. Unsaturated aldehydes have been shown to be generated during myocardial ischemia/reperfusion.^6,7 Recently, Eaton et al⁸ have reported that proteins modified by HNE increase in ischemic rat hearts with increasing durations of ischemia.⁸ Nevertheless, the functional role of unsaturated aldehydes in ischemia/reperfusion injury remains virtually unknown.

Our observations provide important new insights into this issue. Consistent with previous reports,^6–8 we found that ischemia/reperfusion leads to a marked accumulation of free HNE in the heart (Figure 5), indicating increased lipid peroxidation. The accumulation of HNE was significantly reduced by ischemic PC (Figure 5). To our knowledge, this is the first demonstration that late PC is associated with mitigation of oxidant stress. Sorbinil, given at doses that abrogated protection against infarction, completely blocked the antioxidant actions of late PC, resulting in a marked increase in free HNE formation (Figure 5). Taken together, these data provide the first evidence to suggest that formation of lipid-derived electrophiles is a significant cause of ischemia/reperfusion injury and that the mechanism whereby late PC confers cytoprotection involves, at least in part, AR-dependent attenuation of aldehyde accumulation. Nevertheless, our data suggest that HNE accumulation, at the levels noted in our model, is not sufficient to precipitate cell death in the absence of ischemia/reperfusion injury because the HNE content of the nonischemic region of preconditioned hearts treated with sorbinil (group XVII) was comparable to that of the ischemic/reperfused zone of nonpreconditioned hearts (group XIV), yet no infarction was present in the nonischemic region of sorbinil-treated preconditioned hearts (group XII). The observation that the HNE content increased in the nonischemic zone of preconditioned hearts treated with sorbinil (group XVII) is somewhat surprising, and suggests that PC is associated with a global increase in oxidative stress. We propose that this increased prooxidant state does not result in increased HNE levels as long as AR activity is upregulated; however, when AR is inhibited, HNE levels increase (Figure 5). The mechanism for the increased prooxidant state in the nonischemic preconditioned myocardium remains speculative at present. Even in nonpreconditioned hearts, regional ischemia leads to a global increase in lipid peroxidation products.^33,34 Our data indicate that the accumulation of these products per se does not lead to tissue injury, and that high levels of these oxidants can be tolerated by nonischemic myocardium, whereas ischemic/reperfused myocardium is more sensitive to their toxic actions.

In summary, the present study identifies AR as an essential mediator responsible for the beneficial effects of the late phase of ischemic PC. Our results support the concept that cytotoxic aldehydes are a significant cause of myocardial ischemia/reperfusion injury and that AR is an antioxidant enzyme that detoxifies aldehydes generated by lipid peroxidation. This study also suggests that late PC exerts its cytoprotective effects by a hitherto unrecognized mechanism, namely, decreased accumulation of lipid peroxidation products. Although further investigation will be required to elucidate the relationship between aldehyde detoxification and the other known cardioprotective mechanisms recruited by ischemic PC, the findings reported herein bring a new focus on AR and lipid peroxidation and should stimulate further research into their role in myocardial ischemia/reperfusion. Moreover, the finding that AR mediates the protective effects of ischemic preconditioning has implications for the clinical use of AR inhibitors for treating diabetic complications.

	Acknowledgments

 
This study was supported by NIH grants HL55477 and HL59378 (to A.B.), HL43151, HL55757, and HL68088 (to R.B.), and by the Scientist Development Award of the American Heart Association (S.-Q.L.). We are grateful to Dr S.S. Chung for providing anti-SDH antibodies.

	Footnotes

 
This manuscript was sent to Eugene Braunwald, Consulting Editor, for review by expert referees, editorial decision, and final disposition.

Received December 26, 2001; revision received July 9, 2002; accepted July 9, 2002.

	References

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