The cardioprotection achieved by preconditioning can be differentiated into two phases: an early phase (EPC) and a late phase (LPC) of preconditioning. There is overwhelming evidence for an involvement of opioid receptors in the signal transduction cascade of early ischemic preconditioning (IP). Schultz et al. (1) demonstrated that naloxone (NAL), a nonspecific opioid receptor antagonist, completely abolished the cardioprotection of early IP. The early cardioprotection is promoted by activation of δ-opioid receptors (2) and adenosine triphosphate-sensitive potassium channels (3). Fryer et al. (4) provided evidence that opioid receptors are not only involved in EPC but also in LPC. In that study, a specific δ1-opioid receptor agonist, TAN-67, triggered the LPC. The de novo synthesis of mediator proteins is an essential step in the signaling cascade of LPC (5) and the nuclear transcription factor kappa B (NF-κB) is one important factor involved in LPC (6,7). Morphine (MO) is a commonly used analgesic that, besides having μ-opioid receptor-mediated analgesic effects, can also stimulate the δ1-opioid receptor. MO was shown to induce EPC (3) but little is known about potential late cardioprotective properties of MO and the potential underlying mechanisms. We hypothesized that MO induces LPC and that opioid receptors and NF-κB are involved in this MO-induced cardioprotection.
The study was performed in accordance with the regulations of the German Animal Protection Law and was approved by the District Government of Düsseldorf.
To investigate the late cardioprotective properties of MO and lipopolysaccharide of Escherichia coli (LPS) and the underlying mechanism, we performed three sub-studies.
Experiments for Infarct Size Determination
Eighty-eight male Wistar rats were enrolled in the first part of the study and were randomized into 11 groups. All animals underwent 25 min of regional ischemia followed by 2 h of reperfusion (I/R). Excepting the animals of the MO-12h and the LPS-12h groups, the animals were pretreated 24 h before I/R with MO (MO-24h: 3 mg/kg in 5 mL saline) LPS (LPS-24h: 1 mg/kg in 5 mL saline), or saline (NaCl 0.9% 5 mL) by intraperitoneal injection. The involvement of opioid receptors in this experimental setting was investigated using NAL (NAL 1 mg/kg). To differentiate between the trigger and the mediator phases of LPC, NAL was given 10 min before MO, saline, or LPS administration (trigger) or 10 min before I/R (mediator). A schematic illustration of the experimental protocol is given in Figure 1A. Five animals did not complete the experimental protocol: three animals of the LPS-groups were found dead in their cages after LPS administration and 2 animals died from surgical bleeding (NaCl- and Nal+MO-group). Eighty-three animals completed the experimental protocol.
Experiments for Western Blot Analysis
Twenty-four rats were randomly assigned to the six groups of the second part of the study. One of the essential steps in signal transduction of LPC is the activation of NF-κB by phosphorylation of the inhibitory protein IκB. We determined phosphorylation of IκB by Western blot analysis. Rats were treated with saline, MO, or LPS with or without pretreatment with NAL. After 2 h, the hearts were excised and prepared for Western blot analysis. A schematic illustration of the experimental protocol is given in Figure 1B.
Experiments for Elesctrophoretic Mobility Shift Assay (EMSA)
Forty-eight rats were randomly assigned to the six groups of the third part of the study. To confirm the indirect results from Western blots and to directly demonstrate that phosphorylation of IκB led to an activation of NF-κB DNA binding activity, we performed EMSA of NF-κB using the same protocol as for Western blot analysis (Fig. 1B).
Chemicals were purchased from Sigma (Taufkirchen, Germany) or Merck-Eurolab (Munich, Germany) unless otherwise stated in the manuscript.
General Surgical Preparation
After completing the preconditioning protocol, the rats (mean ± sd body weight, 444 ± 39 g) were anesthetized by intraperitoneal injection of S(+)-ketamine (250 mg/kg). After intubation of the trachea, the lungs were ventilated (Rhema-Labortechnik Beatmungsgerät, Typ 10 mL, Cass, Germany) with a tidal volume of 5 mL at 60 breaths/min to maintain Pco2 within physiological limits. Surface electrocardiogram (Siemens Elema AB EKG-Gerät, Germany) was recorded continuously. After cannulation of a femoral vein, the rats received a continuous infusion of α-chloralose (25 mg/kg) and saline 0.9% (5 mL/h) for maintenance of anesthesia and compensation of fluid loss, respectively. For measurement of aortic pressure (AOP), a polyethylene catheter was inserted into the descending aorta via a femoral artery and connected to a pressure transducer (Statham PD23; Gould, Cleveland, OH).
After left lateral thoracotomy in the fourth intercostal space and pericardiotomy were performed, a ligature snare was passed around a major left coronary artery for later occlusion. A temperature probe was placed sub-diaphragmatically (GTH 1160; Digital Thermometer, Geisinger Electronic, Germany) and body temperature was maintained at 37.9°C ± 0.7°C with a heating pad and a warming lamp. Arterial blood gas tensions were analyzed at baseline and during I/R and kept within physiological ranges (data not shown).
Infarct Size Assessment
After completing the experimental protocol, hearts were excised and perfused on a modified Langendorff apparatus with saline at 80 mm Hg perfusion pressure to wash out any remaining blood. The major left coronary artery was then re-occluded with the snare and 5-10 mL of 0.2% Evans Blue dye in 1% dextran was infused via the aortic root into the coronary system. This procedure identifies the area at risk as unstained. The heart was frozen and cut into 8-12 transverse slices of 1-mm thickness. The slices were incubated at 37°C for 15 min in buffered 1% triphenyltetrazolium chloride adjusted to pH 7.4 and then incubated for 24 h in 4% formaldehyde. Viable myocardium was then identified as stained red by triphenyltetrazolium chloride, whereas necrotic myocardium appears pale gray. The area at risk and the infarcted area were determined by a blinded investigator by planimetry using Sigma Scan Pro 5 computer software (SPSS Science Software, Chicago, IL) and corrected for dry weight of each slice.
Hearts were excised and shock-frozen in liquid nitrogen. The investigators involved in Western Blot analysis were blinded for the experimental protocol. The frozen tissue was pulverized and dissolved in 2.5 mL of lysis buffer containing: Tris base, EGTA, NaF, and Na3VO4 (as protease inhibitors), a freshly added proteinase inhibitor mix (aprotinin, leupeptin, and pepstatin), 100 μM/mL okadaic-acid and dithiothreitol (DTT). The solution was homogenized on ice (Homogenisator; IKA) and centrifuged at 1000g for 10 min at 4°C. After protein determination of the supernatant by the Bradford method (8), equal amounts of protein were mixed 1:1 with loading buffer containing Tris-HCl, glycerol, and bromphenol blue. Samples were vortexed and boiled for 5 min at 95°C before being subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (10%, 100 V, 80 min) After electrophoresis proteins were transferred to a polyvinylidene diflouride membrane by tank blotting, 100 V for 1 h. Nonspecific binding of the antibody was blocked by incubation with 5% dry milk powder solution in Tris-buffered saline containing Tween (TBS-T) for 2 h. The membrane was incubated overnight at 4°C with the respective first antibody (phospho IκB-α and non-phospho-IκB-α rabbit polyclonal antibody, Cell Signaling, Frankfurt/Main, Germany) at a dilution of 1:1000 in. 5% bovine serum albumin. After washing in fresh, cold TBS-T, the blot was subjected to the anti-rabbit secondary antibody conjugated to horseradish peroxidase (Cell Signaling, Frankfurt/Main, Germany) for 2 h at room temperature. The immunoreactive bands were visualized by chemoluminescence (Phototope®-HRP Western Detection Kit, Cell Signaling, Frankfurt/Main, Germany) detected on radiograph film (Hyperfilm ECL, Amersham). The blots were quantified by Sigma Scan software (Sigma Scan Pro 5®).
Nuclear Protein Isolation and EMSA
The investigators involved in EMSA were blinded to the experimental protocol. Frozen tissue was pulverized and dissolved in 3 mL of hypotonic buffer A (10 mM HEPES pH 7.9; 10 mM KCl; 0.1 mM EDTA; 0.1 mM EGTA; 1 mM DTT; 0.5 mM phenylmethylsulfonylflouride). The tissue suspension was divided into 3 portions and allowed to swell on ice for 10 min. After centrifugation at 1000g for 10 min at 4°C, 500 μL buffer A was added to each remaining pellet. Nonidet P-40 (10%, 30 μL), a detergent that renders the nuclear membranes permeable to obtain the nuclear protein containing NF-κB, was added followed by 10 s of vigorous vortexing and incubation on ice for 10 min. After centrifugation at 12,000g for 30 s, each supernatant was removed and the nuclear pellet was extracted with 66 μL of hypertonic buffer B (20 mM HEPES pH 7.9; 0.4 M NaCl; 1 mM EDTA; 1 mM EGTA; 1 mM DTT; 1 mM phenylmethylsulfonylflouride) by shaking at 4°C for 30 min. The extract was centrifuged at 12,000g and the supernatant was frozen at -85°C. Protein concentrations were determined by the method of Lowry et al. (9) Nuclear extracts and EMSA experiments were performed as described previously (10,11). A 22-mer double-stranded oligonucleotide probe containing a consensus binding-sequence for NF-κB (5′-AGT TGA GGG GAC TTT CCC AGG C-3′, Promega, Mannheim, Germany) was 5′ end-labeled with [γ32P]-ATP (10 μCi) using T4 polynucleotide kinase. Equal amounts of nuclear protein (10 – 20 μg) were incubated for 20 min at room temperature in a 15-μL reaction volume containing 10 mM Tris-HCl (pH 7.5), 5 × 104 cpm radiolabeled oligonucleotide probe, 2 μg poly(dIdC), 4% glycerol, 1 mM MgCl2, 0.5 mM EDTA, 50 mM NaCl, and 0.5 mM DTT. Specificity of the DNA-protein complex was confirmed by competition with a 100-fold excess of unlabeled NF-κB (cold NF-κB excess) and activating protein-1 (AP-1) (5′-CGC TTG ATG AGT CAG CCG GAA-3′) (cold AP-1 excess) binding sequences, respectively. Nucleoprotein-oligonucleotide complexes were resolved by electrophoresis (4.5% non-denaturing polyacrylamide gel, 100 V), and bands were visualized by detection on radiograph film (Hyperfilm). Shifts were quantified by Sigma Scan Pro 5®.
Data are presented as mean ± sd. Heart rate (HR, in bpm) and mean AOP (AOPmean, in mm Hg) were measured at regular intervals during baseline conditions, coronary artery occlusion, and reperfusion. The hemodynamic data were analyzed for time and group effects with analysis of variance followed by Dunnett’s post hoc test (SPSS 11.5.1®). Infarct size data were analyzed for group effects by analysis of variance followed by Bonferroni’s multiple comparison test (GraphPad Prism© 4.0; GraphPad, San Diego, CA). Western blot data are presented as phosphorylated IkB in percentage of total IκB and were analyzed by analysis of variance following Dunnett’s multiple comparison test (GraphPad Prism© 4.0). EMSA data are presented as x-fold increase of average light intensity and analyzed by analysis of variance following Dunnett’s multiple comparison test (GraphPad Prism© 4.0).
Infarct Size Determination
Infarct size was 59% ± 9% of the area at risk in saline-treated controls and was not affected by NAL administration during the trigger or mediator phases (Fig. 2). Both, morphine and LPS reduced infarct size after 24 h to 20% ± 6% and 23% ± 8%, respectively (P < 0.001 versus NaCl). The infarct size reduction by MO was abolished by NAL given either during the trigger phase or during the mediator phase. In contrast, blocking the opioid receptor during the trigger phase had no effect on LPS-induced cardioprotection. Blockade of the opioid receptors during the mediator phase also abolished the infarct size reduction after LPS treatment. The administration of MO or LPS 12 h before I/R did not reduce infarct size, demonstrating the absence of a cardioprotective effect between EPC and LPC. The dry heart weight and the area at risk in percentage of the heart for each group are given in Table 1, with no differences observed between the groups.
Table 2 shows the time course of HR and AoPmean during the experiments. No significant differences in HR and AoPmean were observed between the experimental groups during baseline conditions. At the beginning of ischemia, only in the NAL-LPS group, we did observe a statistically lower AOPmean. At the end of ischemia this difference was absent. At the end of the experiments, HR was lower in MO-NAL and LPS+NAL treated rats. The LPS-pretreated animals showed a slightly reduced AOPmean during baseline and a significant lower AOPmean at the end of the reperfusion period. Hemodynamic measurements are important to exclude any influence of the global hemodynamic on infarct size, especially during the ischemia.
After phosphorylation of IκB, NF-κB is released and can translocate to the nucleus, where it promotes gene transcription by binding to the DNA. MO and LPS led to a threefold increase of phosphorylated IkB (Fig. 3, NaCl, 8 ± 5; MO, 24 ± 16; P < 0.01 versus NaCl; LPS, 23 ± 12; P < 0.05 versus NaCl), providing indirect evidence for an activation of NF-kB. Pretreatment with NAL abolished this effect. NAL itself did not influence phosphorylation of IkB (Fig. 3).
To directly investigate the DNA binding activity of NF-κB, we performed electrophoretic mobility shift assays of NF-κB that was isolated from the nuclear cell extract. Figure 4 illustrates that both MO (8.0 ± 5.6) and LPS (8.1 ± 5.1) significantly increased DNA binding activity of NF-kB (P < 0.05 versus NaCl: 3.6 ± 2.4), and these effects were again blocked by pretreatment with NAL (NAL-NaCl, 4.7 ± 4.5; NAL–MO, 5.0 ± 2.6; Nal-LPS, 5.0 ± 1.7; P > 0.05 versus NaCl, MO, and LPS, respectively).
The present study extends our knowledge about MO-induced cardioprotection from the known EPC effect to the phase of LPC. In addition, we demonstrated for the first time that opioid receptors are not only involved in EPC but also involved in the trigger phase of MO-induced LPC and in the mediator phase of both MO- and LPS-induced LPC. Moreover, we could show that activation of the DNA binding activity of nuclear transcription factor NF-kB is involved in MO-induced LPC.
IP limits cellular damage after prolonged I/R in a time-dependent manner. Immediately after the preconditioning stimulus, cardioprotection can be observed for 3-4 h (EPC), followed by a protection-free interval. After 24 h, the protective effect reappears and lasts for 2-3 days (LPC) (12,13). Other than ischemia, several pharmacological stimuli induce both EPC and LPC (14,15). There is growing evidence that one of the three known opioid receptors, e.g., the δ1-opioid receptor, mediates EPC (2,4). Naltrindole, a specific δ1-opioid receptor antagonist, given before IP, abolished early cardioprotection (2). The analgesic effect of MO is mediated by μ-opioid receptors, but MO also interacts with δ-opioid receptors (16). Schultz et al. (3) demonstrated that MO mimics EPC, and there is strong evidence that MO-induced EPC is mediated by activation of the δ1-opioid receptor (2). The MO-induced cardioprotection is mediated by free oxygen radicals and opening of ATP-sensitive potassium channels in isolated rat hearts (17) and isolated myocytes (18). Peart and Gross (19) demonstrated in rat hearts that MO preconditioning requires both, activation of δ1-opioid receptor and adenosine A1 receptor. Recently, Gross et al. (20) investigated the interaction of the analgesic drugs MO, aspirin, and ibuprofen against reperfusion injury. Although aspirin had no protective effect, ibuprofen and MO given 5 minutes before reperfusion reduced infarct size. Aspirin in combination with MO abolished the cardioprotection, which was mediated by 12-lipoxygenase. The same group investigated the signal transduction of MO given before ischemia (preconditioning). They found that phosphatidylinositol-3 kinase, a target of rapamycin and, in combination with ischemia, a reduced glycogen synthase kinase β-activity are involved in MO-induced EPC (21). The combination of the volatile anesthetic isoflurane and MO enhanced the cardioprotection compared with the infarct size reduction achieved by sole administration of these anesthetics. This additive effect is mediated by opening of KATP-channels (22).
Fryer et al. (4) found that the specific δ1-opioid receptor agonist TAN-67 induced LPC. Recently, Jiang et al. (23) demonstrated that MO induced LPC in mice and that the inducible nitric oxide synthetase (iNOS) is a mediator of this cardioprotection. Our data now confirm these findings regarding MO-induced LPC in rats. In addition, the present data demonstrate that during the LPC, both MO- and LPS-induced cardioprotection is mediated by opioid receptors (Fig. 2B). The distinct subtype of the opioid receptor inducing LPC remains speculative and needs further investigation. The data of Fryer et al. may suggest that at least the δ1-opioid receptor could be involved (4).
The mechanism whereby the cardioprotection is finally mediated in ischemic, MO- or LPS-induced cardioprotection is unknown. It is most likely that endogenous opioid receptor ligands are involved: a direct MO effect 24 hours after administration can be eliminated because, first, the plasma half-life time of MO is 30 minutes in rats (24) and, second, 12 h after MO and LPS administration, no cardioprotection was observed (Fig. 2A). Furthermore, Fryer et al. demonstrated that δ1-opioid induced LPC requires δ1-opioid receptor activation during both phases (4). Blockade of extracellular signal regulated kinase (ERK) or the p38 stress activated mitogen activated protein kinase (p38 MAPK) by specific blockers given before administration of δ1-opioid receptor agonist abolished late cardioprotection. These findings indicate that ERK and p38 MAPK are involved at least in the trigger phase of opioid-induced IP (25).
LPS-induced late cardioprotection was abolished by NAL in the mediator phase, while, in the trigger phase, LPS-induced cardioprotection did not require opioid receptors. This surprising result is in line with the findings from Börner et al. (26), who demonstrated that the μ receptor gene in a human neuroblastoma cell line is regulated by the AP-1. AP-1 itself is upregulated by LPS (27), and it could be possible that LPS administration led to an increase in opioid receptor gene transcription.
A de novo synthesis of proteins is required for ischemic LPC (5), and iNOS, cyclooxygenase-2, and 12-lipoxygenase are mediators of δ1-opioid receptor agonist triggered LPC (28,29). The key step before a de novo synthesis of proteins is gene transcription. Activation of NF-κB is essential in ischemic (6) and LPS-induced (7) LPC. There is conflicting evidence regarding the influence of MO on NF-κB activity. Loop et al. (30) did not observe an activation of NF-κB after stimulation with MO in human lymphocytes in vitro. In contrast, Wang et al. (31) observed an increase in NF-κB activity via μ-opioid receptors in a human neuronal cell line (NT-2N). Our data now show that in the rat heart, MO and LPS increase NF-κB DNA binding activity via phosphorylation of the inhibitory IκB-protein. NF-κB is present in the cytosol in an inactivated state, complexed with the inhibitory IκB protein. Activation occurs via phosphorylation of IκB at serine 32 and 36, resulting in the release and nuclear translocation of NFκB. Therefore, phosphorylation of IκB is an indirect marker of cytosolic activation of NF-κB. After MO and LPS administration, an increase in phosphorylation of IκB, which was blocked by NAL administration, was observed (Fig. 3). To confirm this finding and to directly demonstrate the transcriptional activity of NF-κB, we performed EMSA. With this technique, we substantiated that MO and LPS administration led to an activation of NF-κB, which was mediated by opioid receptors (Fig. 4). The evidence from this study for the involvement of NF-κB in MO-induced LPC is indirect, but there is ample evidence that activation of NF-κB is an essential step in the signal transduction of LPC (6).
At first, the dosage of MO used in our experiments appears quite high compared with the dosage used in humans. However, rodents generally need larger dosages of analgesics compared with humans, and for analgesic properties in rats 10 mg/kg MO is recommended (32). Whether these observed species differences could be explained by differences in metabolism, receptor interactions, or signal transduction remains unclear, as no comprehensive studies have been published. Therefore, our dosage is in the lower limit for rodents. However, whether this cardioprotective effect could be observed in humans with human dosages remains unclear. No significant changes in behavior were observed after MO administration except for an initial lethargic period lasting 15 to 30 minutes. LPS treatment led to impaired well-being of the animals showing lethargic behavior and scrubby pelt, but all animals were still interested in food. No significant differences in hemodynamics were observed between LPS-treated and untreated animals during baseline conditions. Hemodynamic measurements are important to exclude any influence of the global hemodynamic on infarct size, especially during the ischemia. As several statistical comparisons were made (each group is compared with MO-24 and within groups each timepoint is compared with baseline values) the observed statistically significant differences could occur alone by chance, as our significant level was set before the study at P < 0.05.
Almost all known anesthetics have a negative or positive influence on IP. Volatile anesthetics induce cardioprotection similar to IP (33). Racemic ketamine and R(-)-ketamine abolished the cardioprotection induced by IP, whereas S(+)-ketamine did not influence the cardioprotection (34–36). We therefore decided to use S(+)-ketamine as the induction drug and α-chloralose, as a classical anesthetic for physiological experiments for maintenance. We can not exclude an influence of baseline anesthesia on our results, but as we used the same anesthesia for all experiments, this is unlikely.
To summarize, we demonstrate for the first time that MO is able to induce a late cardioprotection 24 h after its administration in rat hearts. If one assumes that LPC could be an important endogenous cardioprotective mechanism for patients with coronary artery disease (37,38), such a positive side effect of morphine, if confirmed in humans, could be of relevance when choosing the analgesic drug for patients with acute coronary syndromes. In addition, we provided indirect evidence that endogenous opioid receptor ligands must be involved in mediating cardioprotection of LPC.
The authors thank Jessica Wolter, cand. med., for her excellent assistance.
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