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Anesthesiology:
doi: 10.1097/ALN.0b013e3181cded1f
Perioperative Medicine

Hyperglycemia Adversely Modulates Endothelial Nitric Oxide Synthase during Anesthetic Preconditioning through Tetrahydrobiopterin– and Heat Shock Protein 90–mediated Mechanisms

Amour, Julien M.D., Ph.D.*; Brzezinska, Anna K. Ph.D.†; Jager, Zachary M.D.‡; Sullivan, Corbin M.D.‡; Weihrauch, Dorothee D.V.M., Ph.D.§; Du, Jianhai Ph.D.∥; Vladic, Nikolina M.D.*; Shi, Yang Ph.D.#; Warltier, David C. M.D., Ph.D.**; Pratt, Phillip F. Jr. Ph.D.††; Kersten, Judy R. M.D., F.A.C.C.‡‡

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Abstract

Background: Endothelial nitric oxide synthase activity is regulated by (6R-)5,6,7,8-tetrahydrobiopterin (BH4) and heat shock protein 90. The authors tested the hypothesis that hyperglycemia abolishes anesthetic preconditioning (APC) through BH4- and heat shock protein 90-dependent pathways.
Methods: Myocardial infarct size was measured in rabbits in the absence or presence of APC (30 min of isoflurane), with or without hyperglycemia, and in the presence or absence of the BH4 precursor sepiapterin. Isoflurane-dependent nitric oxide production was measured (ozone chemiluminescence) in human coronary artery endothelial cells cultured in normal (5.5 mm) or high (20 mm) glucose conditions, with or without sepiapterin (10 or 100 μm).
Results: APC decreased myocardial infarct size compared with control experiments (26 ± 6% vs. 46 ± 3%, respectively; P < 0.05), and this action was blocked by hyperglycemia (43 ± 4%). Sepiapterin alone had no effect on infarct size (46 ± 3%) but restored APC during hyperglycemia (21 ± 3%). The beneficial actions of sepiapterin to restore APC were blocked by the nitric oxide synthase inhibitior N (G)-nitro-l-arginine methyl ester (47 ± 2%) and the BH4 synthesis inhibitor N-acetylserotonin (46 ± 3%). Isoflurane increased nitric oxide production to 177 ± 13% of baseline, and this action was attenuated by high glucose concentrations (125 ± 6%). Isoflurane increased, whereas high glucose attenuated intracellular BH4/7,8-dihydrobiopterin (BH2) (high performance liquid chromatography), heat shock protein 90-endothelial nitric oxide synthase colocalization (confocal microscopy) and endothelial nitric oxide synthase activation (immunoblotting). Sepiapterin increased BH4/BH2 and dose-dependently restored nitric oxide production during hyperglycemic conditions (149 ± 12% and 175 ± 9%; 10 and 100 μm, respectively).
Conclusion: The results indicate that tetrahydrobiopterin and heat shock protein 90-regulated endothelial nitric oxide synthase activity play a central role in cardioprotection that is favorably modulated by volatile anesthetics and dysregulated by hyperglycemia. Enhancing the production of BH4 may represent a potential therapeutic strategy.
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What We Already Know about This Topic

❖ Myocardial protection through anesthetic preconditioning relies on nitric oxide production and is disrupted by hyperglycemia
❖ Tetrahydrobiopterin (BH4) and heat shock protein (Hsp) 90 also regulate nitric oxide production and may interact with anesthetic preconditioning
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What This Article Tells Us That Is New

❖ In rabbits, hyperglycemia decreased, whereas cardiac anesthetic preconditioning increased BH4 and Hsp 90 association with nitric oxide synthase. A BH4 precursor rescued nitric oxide synthase activity and nitric oxide production during hyperglycemia
❖ Enhancing BH4 production may enhance anesthetic preconditioning during hyperglycemia
NITRIC oxide derived from endothelial nitric oxide synthase (eNOS) is a critical mediator of anesthetic preconditioning (APC) against myocardial infarction,1–3 and this molecule protects against ischemia and reperfusion injury through activation of intracellular signaling pathways and direct effects on mitochondria.4 The volatile anesthetic isoflurane activates eNOS, as indicated by phosphorylation of Serine 1177,1 and nitric oxide production is directly related to the extent of eNOS phosphorylation.5 Conversely, hyperglycemia decreases the availability of nitric oxide6 and is an independent predictor of increased cardiovascular morbidity and mortality.7 APC is abolished by diabetes and acute hyperglycemia,2,8 but interestingly, cardioprotection is restored by treatment with an 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitor through a nitric oxide-mediated mechanism.2
There are several important regulators of eNOS function that are potentially modifiable by hyperglycemia and volatile anesthetics. For example, heat shock protein (Hsp) 90 is a physiologic binding partner of eNOS that regulates eNOS phosphorylation and modulates subsequent nitric oxide production.9–11 We recently demonstrated that Hsp90 impacts APC through protein–protein interactions, thereby, enhancing nitric oxide production in endothelial cells and decreasing myocardial ischemia and reperfusion injury.1 In contrast, diabetes and hyperglycemia have been shown to impair Hsp90–eNOS interactions.12 It is unknown whether attenuated associations between Hsp90 and eNOS might account for the deleterious effects of hyperglycemia on APC signal transduction.
eNOS is also regulated by (6R-)5,6,7,8-tetrahydrobiopterin (BH4), a reduced unconjugated protein and essential cofactor for the normal function of this enzyme. Decreases in BH4 contribute to endothelial dysfunction in diabetes,13 whereas BH4 supplementation decreases reactive oxygen species production and restores endothelial function induced by acute hyperglycemia,14 diabetes,15–17 and hypercholesterolemia.18,19 The actions of volatile anesthetics to alter BH4 concentrations have not been investigated previously.
Thus, the aim of this study was to test the hypothesis that hyperglycemia abolishes APC through Hsp90- and BH4-dependent mechanisms. Experiments in vivo were conducted to determine whether the metabolic precursor of BH4, sepiapterin, restores APC during hyperglycemia. In vitro experiments were conducted to define the mechanisms whereby hyperglycemia disrupts isoflurane-enhanced eNOS chaperone (Hsp90) and cofactor (BH4) function using ozone chemiluminescence to measure nitric oxide, high performance liquid chromatography to measure BH4, and immunoblotting and confocal microscopy to measure Hsp90–eNOS interactions.
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Materials and Methods

All the experimental procedures and protocols used in this investigation were reviewed and approved by the Institutional Animal Care and Use Committee of the Medical College of Wisconsin (Milwaukee, WI). Furthermore, all conformed to the Guiding Principles in the Care and Use of Animals of the American Physiologic Society and were in accordance with the Guide for the Care and Use of Laboratory Animals.
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In Vivo Myocardial Infarction Model
Male, New Zealand white rabbits were anesthetized with intravenous sodium pentobarbital (30 mg/kg) and instrumented as previously described.1,20 Briefly, a tracheotomy was performed, and rabbits were ventilated with positive pressure using an air–oxygen mixture (30% fractional inspired oxygen concentration). Arterial blood gas tensions and acid–base status were maintained within a normal physiologic range by adjusting the respiratory rate or tidal volume throughout the experiment. Heparin-filled catheters were positioned in the right carotid artery and the left jugular vein for continuous measurement of arterial blood pressure and fluid and drug administration (0.9% saline; 15 ml · kg−1 · h−1), respectively. After thoracotomy, a silk ligature was placed around the left anterior descending coronary artery approximately halfway between the base and the apex for the production of coronary artery occlusion and reperfusion. Coronary artery occlusion was verified by the presence of epicardial cyanosis and regional dyskinesia in the ischemic zone, and reperfusion was confirmed by observing an epicardial hyperemic response.
Fig. 1
Fig. 1
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The experimental protocol is illustrated in figure 1. All rabbits underwent a 30-min coronary artery occlusion followed by 3 h of reperfusion. Rabbits were randomly assigned to preconditioning with 30 min of isoflurane (2.1%, 1 minimum alveolar concentration; APC) followed by a 15-min washout. In separate experimental groups, rabbits were randomly assigned to receive 0.9% saline or 15% dextrose in water to increase blood glucose concentrations (glucometer) to approximately 270 mg/dl in the presence or absence of APC,2 with and without pretreatment with intravenous sepiapterin (2 mg/kg),21 which is converted to BH4 intracellularly, the NOS inhibitor N (G)-nitro-l-arginine methyl ester (10 mg/kg), or the sepiapterin reductase antagonist N-acetylserotonin (15 mg/kg).22
Myocardial infarct size was measured as previously described.23 Briefly, the left ventricular area at risk for infarction was separated from the normal area, and the two regions were incubated at 37°C for 20 to 30 min in 1% 2,3,5-triphenyltetrazolium chloride in 0.1 m phosphate buffer adjusted to pH 7.4. After overnight storage in 10% formaldehyde, infarcted and noninfarcted myocardial areas within the area at risk were carefully separated and weighed. Myocardial infarct size was expressed as a percentage of the area at risk. Rabbits that developed intractable ventricular fibrillation and those with an area at risk less than 15% of total left ventricular mass were excluded from subsequent analysis.
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Human Coronary Artery Endothelial Cells
Human coronary artery endothelial cells isolated from healthy coronary arteries (Cells Applications, San Diego, CA) were cultured without cryopreservation, propagated to fifth passage in growth medium (Cells Applications) and used for experiments between the fourth and the fifth passage. Cells were used for experiments when approximately 70 to 80% confluent. Human coronary artery endothelial cells were seeded on cell culture dishes (100 mm) and maintained at 37°C in growth medium (Cell Applications). Twenty-four hours before experimentation, the growth medium was removed and replaced with normal (d-glucose 5.5 mm, mannitol 14.5 mm, NaCl 81 mm, KCl 4.0 mm, CaCl2 1.6 mm, pH 7.4) or high (d-glucose 20.0 mm, NaCl 81 mm, KCl 4.0 mm, CaCl2 1.6 mm, pH 7.4) glucose media having the same osmolarity (290 mosmol/L). Additional cells were pretreated with sepiapterin (10 or 100 μm) for 1 h before exposure to isoflurane or air (control). Isoflurane was administered for 60 min, and the anesthetic concentrations (0.42 mm; the equivalent of 1 minimum alveolar concentration) were continuously monitored by a gas analyzer (POET IQ; Critcare System, Waukesha, Wisconsin), at continuous air flow (0.7 l/min) in a specific incubator chamber (Billups-Rothenberg, Del-Mar, CA) maintained at 37°C.1 Because gas flow can induce shear-stress-dependent nitric oxide release,24 the control group was exposed to air alone at the same flow rate.
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Ozone Chemiluminescence
Nitrite concentration corresponding to the stable breakdown product of nitric oxide in aqueous solution was quantified by ozone chemiluminescence. Previous evidence suggests that nitric oxide release in response to stimuli in coronary endothelial cells peaks at 60 min.1,10 Therefore, nitric oxide measurements were performed 60 min after isoflurane exposure. Samples (20 μl) were refluxed in glacial acetic acid containing potassium iodide and nitrite quantified in a nitric oxide chemiluminescence analyzer (Sievers Instruments, Boulder, CO) as described previously.25 Nitrite concentrations were calculated after subtraction of background levels and normalized to protein content (Bradford method).
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High Performance Liquid Chromatography
BH4 and 7,8-dihydrobiopterin (BH2) were quantified by high performance liquid chromatography with an electrochemical detector (ESA Biosciences CoulArray® system Model 542, Chelmsford, MA) as described previously.26 Endothelial cell pellets were immediately lysed in 300 μl of 50 mm phosphate buffer (pH 2.6) containing 0.2 mm diethylenetriaminepentaacetic acid and 1 mm dithioerythritol (freshly added) by shearing cells with a 28-gauge tuberculin syringe. Samples were centrifuged (12000g, 10 min, 4°C), and supernatants were filtered through a 10-kD cutoff column (Millipore, Billerica, MA). One hundred eighty microliters of the flow through was analyzed by using a Synergi Polar-RP column (Phenomex, Torrance, CA) eluted with argon-saturated 50 mm phosphate buffer (pH 2.6). Multichannel colorimetric detection was set between 0 and 600 mV. One channel was set at −250 mV to verify the reversibility of BH4 oxidative peak detection. Calibration curves were constructed by summation of peak areas collected at 0 and 150 mV for BH4 and 280 and 365 mV for BH2. Intracellular concentrations were calculated using authentic BH4 and BH2 as standards. Cellular BH4 and BH2 levels were then normalized to cell protein concentration and expressed as the ratio of BH4 to BH2.27
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Immunoblotting
Human coronary artery endothelial cells were lysed in 500 μl of lysis buffer [20.0 mm 3-(N-morpholino)propanesulfonic acid, 2.0 mm EGTA, 5.0 mm EDTA, protease inhibitor cocktail (1:100; Sigma-Aldrich, St. Louis, MO), phosphatase inhibitors cocktail (1:100; Calbiochem, San Diego, CA), 0.5% detergent (Nonidet P-40 detergent pH 7.4, Sigma-Aldrich)] 20 min after the beginning of isoflurane exposure, because this time period corresponds to the peak of eNOS phosphorylation in coronary endothelial cells in response to various stimuli.1,10,11 Fifteen to 25 μg of protein was loaded onto precast 7.5% tris-HCl gels (Criterion, BioRad, Hercules, CA) and transferred to polyvinylidene fluoride membranes. After blocking the membranes in 5% milk in tris-buffered saline, immunoblots were performed with rabbit monoclonal antiphospho-eNOS (Ser 1177;1:1,000; Cell Signaling Technology, Danvers, MA) and rabbit polyclonal anti-eNOS (1:5,000; Santa Cruz Biotechnologies, Delaware, CA) and were incubated overnight at 4°C. Membranes were washed and incubated with secondary antibodies horseradish peroxidase-conjugated donkey antirabbit IgG for eNOS (1:10,000; Santa Cruz Biotechnologies) and goat antirabbit for phospho-eNOS (1:10,000; Bio-Rad Laboratories, Hercules, CA). Membranes were developed using the ECLplus Western blot chemiluminescence detection reagent (Bio-Rad Laboratories), and densitometric analysis was carried out by using image acquisition and analysis software (Scion Image, Frederick, Maryland).
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Confocal Microscopy
Human coronary artery endothelial cells were cultured on gelatin-coated slides as described previously 1 to visualize colocalization of Hsp90 and eNOS. After 60 min of isoflurane exposure, cells were fixed in 1% paraformaldehyde, permeabilized in 0.5% TritonX-100 (Sigma-Aldrich), and incubated for 30 min at 37°C with primary monoclonal antibody anti-eNOS (1:100; Biomol International, Plymouth, PA) in phosphate-buffered saline. Incubations with corresponding biotinylated secondary antibodies Alexa 488 conjugated (1:1,000; Invitrogen, Eugene, OR) were conducted for 30 min at 37°C. After washing with phosphate-buffered saline, cells were incubated for 30 min at 37°C with monoclonal antibody anti-Hsp90 (1:50; Santa Cruz Biotechnologies). Incubations with corresponding biotinylated secondary antibodies Alexa 546 conjugated (1:1,000; Invitrogen) were conducted for 30 min at 37°C followed by 1:1000 TO-PRO-3 (nuclear stain; Molecular Probes, Eugene, OR) for 5 min at room temperature and washed again with phosphate-buffered saline. Images were visualized using confocal microscopy (Nikon Eclipse TE 200-U microscope with EZ C1 laser scanning software, Melville, NY) at excitation wavelengths of 488/546/633 nm and emission wavelengths of greater than 520/578/661 nm for eNOS, Hsp90, and TO-PRO-3 respectively. The number of double-stained cells indicating colocalization of Hsp90 and eNOS were counted and expressed as a percentage of the total cell count.
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Statistics
Data were expressed as mean ± SD. Comparison of two means was performed using the Student's-t test. Comparison of several means was performed using one-way (one factor tested) or two-way (systemic hemodynamics: two factors tested) analysis of variance, when appropriate, and the post hoc test Newman-Keuls test. Hemodynamic data were analyzed with repeated measures analysis of variance. All P values were two-tailed, and a P value less than 0.05 was considered significant. Statistical analysis was performed using NCSS 2007 software (Statistical Solutions Ltd., Cork, Ireland).
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Results

Influence of Hyperglycemia on APC: Modulation of BH4 and eNOS In Vivo
Table 1
Table 1
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Table 2
Table 2
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Fig. 2
Fig. 2
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Eighty rabbits were instrumented to obtain 76 successful experiments in which infarct size was measured. Four rabbits were excluded because intractable ventricular fibrillation occurred during coronary artery occlusion (two in the control group, one in the hyperglycemia alone group, and one in the hyperglycemia with APC group). Arterial blood gas tensions were maintained within the physiologic range in each group (data not shown). Systemic hemodynamics were similar at baseline among groups (table 1). Intravenous dextrose similarly increased (P < 0.05) blood glucose concentrations during coronary artery occlusion compared with baseline values in the presence (257 ± 33 mg/dl vs. 124 ± 11 mg/dl) or absence of APC (281 ± 46 mg/dl vs. 110 ± 33 mg/dl), during APC with sepiapterin (260 ± 62 mg/dl vs. 123 ± 9 mg/dl), and with N (G)-nitro-l-arginine methyl ester (295 ± 42 mg/dl) or N-acetylserotonin (288 ± 17 mg/dl). Left ventricular mass, area at risk mass, and the ratio of area at risk to left ventricular mass were similar between groups (table 2). APC decreased myocardial infarct size compared with control experiments (26 ± 6% vs. 46 ± 3% of the left ventricular area at risk, respectively; P < 0.05: fig. 2). Hyperglycemia alone had no effect on infarct size but abolished the protective effects of APC (44 ± 2% vs. 43 ± 4% of the left ventricular area at risk, respectively). Sepiapterin did not influence infarct size compared with control experiments (46 ± 3% vs. 46 ± 3%), but it restored the cardioprotective effect of APC during hyperglycemia (46 ± 3% vs. 21 ± 3%, respectively; P < 0.05). The beneficial actions of sepiapterin to restore APC during hyperglycemia were blocked by the NOS inhibitior N (G)-nitro-l-arginine methyl ester (47 ± 2%) and the BH4 synthesis inhibitor N-acetylserotonin (46 ± 3%). Sepiapterin had no effect on infarct size during hyperglycemia (n = 4; 39 ± 3%). N (G)-nitro-l-arginine methyl ester3 or N-acetylserotonin alone (n = 4; 46 ± 2%) did not alter the extent of myocardial necrosis.
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Effects of Glucose on Isoflurane-dependent Nitric Oxide Production and BH4 in Human Coronary Artery Endothelial Cells
Fig. 3
Fig. 3
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Fig. 4
Fig. 4
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Isoflurane significantly increased nitric oxide production in human coronary artery endothelial cells compared with control experiments (612 ± 63 nmol/mg vs. 344 ± 11 nmol/mg of protein, respectively; P < 0.05: fig. 3A), and this action was significantly attenuated by high (20 mm) glucose conditions (379 ± 32 nmol/mg of protein; fig. 3B). Sepiapterin alone (100 μm) slightly increased (418 ± 37 nmol/mg of protein) nitric oxide production compared with control experiments and enhanced isoflurane-dependent nitric oxide production (757 ± 99 nmol/mg of protein) in the absence of high glucose. Hyperglycemic conditions had a deleterious effect on nitric oxide production by isoflurane (125 ± 6% vs. 177 ± 13% of control values, respectively; fig. 4), whereas sepiapterin dose-dependently restored (149 ± 12 and 175 ± 9%, 10 and 100 um, respectively) isoflurane-enhanced nitric oxide concentrations in the presence of elevated glucose.
Fig. 5
Fig. 5
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BH4/BH2 was increased by isoflurane when compared with control experiments (0.11 ± 0.03 vs. 0.06 ± 0.01, respectively; P < 0.05: fig. 5A). Increases in glucose concentration had no effect on BH4/BH2 (0.07 ± 0.01) but abolished increases in the ratio of reduced to oxidized biopterin produced by isoflurane (0.05 ± 0.01). Sepiapterin, in both low and high concentrations, profoundly increased BH4/BH2 (fig. 5B) in the presence of increased glucose. During hyperglycemic conditions and low dose sepiapterin, isoflurane did not further augment BH4/BH2 levels. However, this ratio was further increased when isoflurane was combined with high dose sepiapterin.
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eNOS Activation and Hsp90–eNOS Interactions during APC and Increased Glucose
Fig. 6
Fig. 6
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Fig. 7
Fig. 7
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Isoflurane increased the ratio of phospho-eNOS to total eNOS during normoglycemic but not hyperglycemic conditions (135 ± 17% vs. 85 ± 19% of baseline values, respectively; P < 0.05: fig. 6). Sepiapterin significantly enhanced (132 ± 26% of control values) isoflurane-induced eNOS activation (phospho-eNOS) during hyperglycemia (fig. 6). Similarly, isoflurane increased colocalization of Hsp90 with eNOS (fig. 7) in cells cultured in normal but not high glucose media.
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Discussion

The loss of nitric oxide bioavailability because of reduced synthesis or scavenging by oxidative species is the sine qua non of endothelial dysfunction and an independent predictor of adverse cardiovascular events.28 Dysregulation of eNOS plays a critical role in the pathogenesis of cardiovascular disease during hyperglycemia and diabetes mellitus,29 hypercholesterolemia,18 hypertension,30 aging,31 and chronic smoking.32 The current results extend previous findings demonstrating that diabetes and hyperglycemia may increase cardiovascular risk by impairing ischemic and pharmacologic preconditioning2,33,34 and further demonstrate that this action is mediated by glucose-induced modulation of eNOS chaperone and cofactor function during APC.
Hsp90 is a highly abundant molecular chaperone protein involved in protein folding and maturation, and it is a physiologic binding partner and regulator of eNOS.9 Impairment of eNOS–Hsp90 interaction disrupts nitric oxide-dependent signaling and increases the production of superoxide anion.1,9,10 We have previously demonstrated that APC increases the association between Hsp90 and eNOS.1 Hsp90 antagonists, geldanamycin and radicicol, abolished infarct size reduction afforded by either anesthetic or ischemic preconditioning, and inhibition of Hsp90 prevented isoflurane-induced production of nitric oxide.1 The current results similarly indicate that hyperglycemia attenuates APC, in part, by disrupting Hsp90 association with eNOS and decreasing nitric oxide production. This finding confirms previous work showing that hyperglycemia and diabetes induce translocation of Hsp90 to the outside of endothelial cells, which decreases Hsp90-eNOS association and subsequent nitric oxide production.12
BH4 is an essential cofactor that also regulates nitric oxide synthesis.18,35 eNOS enzyme consists of a reductase domain that transfers electrons from reduced nicotinamide adenine dinucleotide phosphate to the flavoproteins, flavin adenine dinucleotide, and flavin mononucleotide; and a heme-containing oxygenase domain that binds BH4, molecular oxygen, and substrate l-arginine.36 Electrons transferred from the reductase domain to the oxygenase domain enable ferric heme to bind oxygen, forming a ferrous-dioxy complex. A second electron may be preferentially transferred from BH4 to activate oxygen and catalyze l-arginine hydroxylation. Thus, in the presence of adequate substrate and BH4, heme and oxygen reduction are coupled to the synthesis of nitric oxide. However, in the presence of low concentrations of intracellular BH4, electron transfer within the active site of eNOS becomes uncoupled from l-arginine oxidation, and molecular oxygen is reduced to superoxide anion.37,38
The intracellular regulation of BH4 is dependent on the cellular redox state.36 Therefore, it is likely that hyperglycemia and accompanying increases in oxidative stress adversely modify eNOS regulation and cardioprotection during APC by modulating biopterin concentrations. For example, the oxidant species peroxynitrite that is increased by hyperglycemia has been shown to oxidize BH4 to the catalytically incompetent pterin species BH2. This action leads to uncoupling of eNOS and enhanced production of superoxide anion by the uncoupled enzyme.39,40 Hyperglycemia may overwhelm the natural antioxidant defense mechanisms that maintain BH4 in its reduced form.27 Experimental findings demonstrate that high concentrations of glucose decrease intracellular BH4/BH2 levels in parallel with decreases in nitric oxide production and overproduction of superoxide anion.15,27,41 In fact, superoxide anion is the sole product of recombinant eNOS either in the absence of BH4 or in the presence of excess BH2.27 Conversely, increases in BH4/BH2 favor an increased production of nitric oxide production, a decrease in superoxide anion,18,35 and maintenance of eNOS in its active phosphorylated state.42
BH4 is synthesized in the cell cytoplasm by either de novo or salvage pathways.43 The enzyme guanosine 5′-triphosphate cyclohydrolase I is the first and rate-limiting step in the biosynthesis of BH4 through the de novo pathway. BH4 is also synthesized from BH2 through the activity of two salvage pathway reduced nicotinamide adenine dinucleotide phosphate-dependent enzymes, sepiapterin reductase, and dihydrofolate reductase. Although sepiapterin is not an endogenous precursor of BH4, it can serve as an effective substrate for BH4 synthesis through the salvage pathway.
The current results demonstrate that isoflurane increases BH4/BH2 levels concomitantly with enhanced eNOS activation (increased phospho-eNOS to total eNOS) and nitric oxide production in human coronary artery endothelial cells. In contrast, isoflurane failed to increase BH4/BH2, phosphorylated eNOS and nitric oxide production during hyperglycemia, confirming that hyperglycemia adversely impacts isoflurane-induced nitric oxide signaling. In contrast, sepiapterin dose-dependently restored eNOS function during hyperglycemia. Although low concentrations (10 μm) of sepiapterin profoundly increased BH4/BH2 levels, 10-fold higher concentrations (100 μm) of sepiapterin were required to restore APC-induced nitric oxide production during hyperglycemia. This result could indicate that excess concentrations of BH4 might be required during hyperglycemia to compensate for sustained eNOS dysfunction due to impaired Hsp90–eNOS association.
Hyperglycemia blocked the cardioprotective effects of APC; however, isoflurane-induced reductions of myocardial infarct size were restored by sepiapterin during increases in blood glucose concentration. Thus, our findings indicate that BH4 is an important cofactor that affects outcome after ischemia and reperfusion injury in the setting of hyperglycemia. This contention is supported by other evidence that myocardial ischemia and reperfusion decreased intracellular BH4 concentrations in parallel with overproduction of superoxide anion.44 Conversely, BH4 supplementation enhanced left ventricular developed pressure and decreased left ventricular end-diastolic pressure after ischemia and reperfusion injury in the isolated perfused rat heart.45 Supplementation with BH4 enhanced nitric oxide production and decreased superoxide anion release during hypercholesterolemia,18 chronic smoking,32 diabetes,16,46 and acute hyperglycemia.14,27,47 Interestingly, 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors have been shown to stimulate the synthesis of BH4 by increasing the expression of guanosine 5′-triphosphate cyclohydrolase I.47 We previously demonstrated that simvastatin restored ischemic preconditioning during hyperglycemia through a nitric oxide-dependent mechanism.2 Taken together, these results suggest that pharmacologic strategies that target BH4 either directly (sepiapterin) or indirectly (statins) may enhance eNOS signaling and cardioprotection during diabetes and hyperglycemia.
Hyperglycemia also impairs APC by increasing concentrations of deleterious reactive oxygen species8 and by attenuating activation of ATP-regulated potassium channels.34 ATP-regulated potassium channels are known downstream targets of nitric oxide, and superoxide anion has previously been shown to decrease nitric oxide concentrations by reacting with and inactivating this cardioprotective molecule. Thus, hyperglycemia produces a wide range of adverse effects on multiple components of the APC signaling pathway, including alterations in the regulation of eNOS, and on downstream effectors such as ATP-regulated potassium channels.
The current results should be interpreted within the constraints of several potential limitations. The role of BH4, Hsp90, and eNOS during APC was investigated only with isoflurane. Although a recent study performed in humans confirmed a protective effect of other halogenated anesthetics such as sevoflurane on endothelial function,48 we cannot substantiate that the current results indicate a class effect of volatile anesthetic agents on eNOS regulation. The actions of isoflurane on Hsp90 interactions with eNOS, BH4, and nitric oxide were measured in human coronary artery endothelial cells in vitro. Results may be different in cardiomyocytes or myocardium in vivo. The role of paracrine interactions between endothelial cells that produce nitric oxide and effects on neighboring cardiomyocytes during cardioprotection in vivo are unknown and is an important focus for future investigations. There is remarkable consistency in the identified signaling pathways demonstrated by experiments conducted in multiple animal and human models. Although the current results obtained in human coronary artery endothelial cells confirmed those obtained in rabbits in vivo, altered anesthetic signaling mechanisms that are dependent on species or cell lineage cannot be totally excluded from the analysis. Different severity of hyperglycemia was used during in vivo (moderate; approximately 14 mm), when compared with in vitro (severe; 20 mm), experiments. Moderate hyperglycemia was used during myocardial infarction experiments because severe hyperglycemia alone increases infarct size. In contrast, endothelial cells in culture are relatively resistant to moderate hyperglycemia.
In conclusion, the current results demonstrate that hyperglycemia adversely modulates APC through Hsp90 chaperone and BH4 cofactor function. Although isoflurane enhances eNOS function by increasing Hsp90–eNOS interactions and the ratio of reduced to oxidized biopterin, these actions are attenuated by hyperglycemia. Conversely, sepiapterin, a precursor of BH4, restores APC-induced cardioprotection in the presence of hyperglycemia.
The authors thank David Schwabe, B.S., Research Technician, for technical assistance and Shelly Logsdon, Administrative Assistant, for manuscript preparation (both Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisconsin).
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