Opposing effects on infarction of delta and kappa opioid receptor activation in the isolated rat heart: implications for ischemic preconditioning

The Role of Endogenous Opioids in Cardioprotection

The opioid system involves opioid receptors (OPRs) and endogenous opioid peptides.This chapter will focus on the distribution of OPRs in the cardiovascular system, the expression pattern in the heart, the activation by opioid peptides, and the effects of OPRs activation with potential relevance in cardiovascular performance. In the heart, OPRs are co-expressed with beta adrenergic receptors (β-ARs) in the G-protein-coupled receptor (GPCR) superfamily, functionally cross-talk with β-Ars and modify catecholamine-induced effects. They are involved in cardiac contractility, energy metabolism, myocyte survival or death, vascular resistance. The effects of the opioid system in the regulation of systemic circulation at both the central and peripheral level are presented. The pathways are discussed under physiological (i.e., aging) and pathological conditions (atherosclerosis, heart failure, essential hypertension, ischemic stress). Stimulation of OPRs not only inhibits cardiac excitation-contraction coupling, but also protects the heart against hypoxic and ischemic injury. An enhanced sensitivity to opioids of endocrine organs and neuronal systems is operative in hypertensive patients. The opioid system can be pharmacologically engaged to selectively mimic these responses via cardiac and nervous signaling. The clinical opportunities for the use of cardioprotective effects of opioids require future investigations to provide more specific details of the impact on cardiac performance and electrophysiological properties.

Impacts of opium addiction on patterns of angiographic findings in patients with acute coronary syndrome

Opium is one of the most abused substances in the Middle East. The effects of opium use on coronary artery disease (CAD) are a matter of debate. This study aimed to assess the association between opium use and angiographic findings as well as the complexity of CAD in patients with acute coronary syndrome (ACS) diagnosis. In this case–control study, all patients admitted for coronary angiography from 2019 to 2020 were evaluated. After applying the eligibility criteria, they were categorized into two groups opium and non-opium based on their history of opium use. Both groups were matched regarding the demographic features. The prevalence, location, and severity of obstruction of the vessels were compared between the non-opium and opium groups. The SYNTAX score was also calculated and compared between the two groups. The scores ≤ 22 are considered low risk and the higher scores are a non-low risk. P value < 0.05 is considered significant. A total of 170 patients with a mean age of 61.59 ± 9.07 years were finally enrolled in our study. Regarding the severity of vascular involvement, there was a significant difference between the non-opium and opium groups in LAD (P = 0.025), and PLV (P = 0.018) vessels. From the location points of view of obstructive coronary artery involved segments, only in the PDA (P = 0.006), and LCX (P = 0.004) vessels, a significant difference was observed. Moreover, 47.1% of opium and 30.6% of non-opium use group were in the non-low risk SYNTAX score classification which is a statistically significant difference between these two groups (P value = 0.048). Opium, as an independent risk factor for cardiovascular diseases, can have specific effects on angiographic findings in patients with acute coronary syndrome. Likewise, the complexity of CAD in opium users who undergo percutaneous coronary intervention is significantly higher.

Diagnosis of coronary artery rethrombosis after effective systemic thrombolytic therapy in patients with ST-segment elevation myocardial infarction

The aim of the study was to evaluate the diagnostic significance of ST-segment re-elevation episodes registered with telemetric ECG monitoring in patients with ST-segment elevation myocardial infarction (STEMI) treated with thrombolytic therapy (TLT). The study included 117 patients with STEMI following effective TLT. The elective coronary angiography followed by percutaneous coronary interventions was performed in the interval from 3 to 24 hours after a successful systemic TLT. Before and after cardiac catheterization, the telemetric ECG monitoring was performed using AstroCard Telemetry system (Meditec, Russia). During the study, two groups of patients were formed. Group 1 included 85 patients (72.6%) without new ST-segment deviations on telemetry. 77 patients (90.6%) had no recurrent coronary artery thrombosis at angiography. Eight patients (9.4%) from group 1 were diagnosed with thrombosis of the infarct-related coronary artery. Group 2 included 32 patients (27.4%) who underwent TLT and then had ST-segment re-elevation episodes of 1 mV or more in the infarct-related leads, lasting for at least 1 minute. In group 2, in 27 of 32 patients (84.4%), thrombosis of the infarct-related coronary artery was confirmed (p<0.01 compared with group 1). In 71.9% cases, the recurrent ischemic episodes were asymptomatic ('painless myocardial ischemia') (p<0.01). Thus, in patients with STEMI and successful TLT, re-elevation of ST-segment during remote ECG monitoring is strongly related to angiographically documented coronary artery thrombotic reocclusion. The absence of chest pain during recurrent myocardial ischemia requires continuous ECG telemetry to select patients for the rescue percutaneous coronary interventions at an earlier stage.

Surveying the Effect of Opioid Abuse on the Extent of Coronary Artery Diseases in Diabetic Patients

Background

Diabetes mellitus is recognized as one of the most common, serious, and costly chronic diseases. Opium addiction is also a common health problem in Iran. Given the high prevalence of opium use in South Khorasan Province and the increasing prevalence of opioid abuse in the community, this study was performed to investigate the effect of opioid abuse on the extent of disease in diabetic patients undergoing coronary angiography in the cardiology department of Vali-e-Asr Hospital in Birjand city, South Khorasan Province, Iran.

Methods

This study recruited a total of 1051 diabetic patients who underwent coronary angiography in the cardiology department of Vali-e-Asr Hospital of Birjand city from 2011 to 2015. The collected data were analyzed using SPSS version 22.0 with the chi-square test and univariate regression analysis. P value <0.05 was considered as statistically significant.

Results

Among opiate-addicted diabetics, the risk of coronary artery disease was 0.44 times higher than among nonaddicted diabetics (range 0.24–0.77, P=0.004). The extent of coronary vessel involvement, when present, was not different between the two groups.

Conclusion

Opiate-addicted diabetics appear to be more susceptible to CAD than their nonaddicted counterparts. The determinants and correlates of this interaction must be the subject of further study.

The effects of opium on the cardiovascular system: a review of side effects, uses, and potential mechanisms

BACKGROUND

In Iran, as in many other Asian and Middle Eastern countries, some believe that opium has beneficial effects on cardiovascular system. Dependent patients suppose that opium has positive effects on cardiovascular function and can prevent or improve cardiovascular diseases; however, only few comprehensive studies evaluating such effects have been performed.

OBJECTIVES

In this study, we sought to clarify the effect of opium on cardiovascular problems by incorporating the previous findings and the current information on the issue and to explain the possible mechanisms of this effect.

METHODS

The available human studies published up to October 30, 2019, were searched in different databases. Case-control, cohort, and cross-sectional studies were retrieved. Papers published in English or those with an English abstract were included. The risk of bias for each included study was assessed based on the Newcastle-Ottawa Scale (NOS). We then categorized the effects of opium on cardiovascular problems along with its probable underlying mechanisms of action.

RESULTS

In this study, most of the published articles suggested the adverse effects of opium on the cardiovascular system, including atherosclerosis, myocardial infarction, arrhythmia, low ejection fraction, and cardiovascular mortality; however, some articles reported the beneficial or impartial effects of opium on the cardiovascular system. In this article, we have categorized all the effects of opium on cardiovascular system; also, the proposed mechanisms of action of opium in each of the above-mentioned disorders are summarized.

CONCLUSION

Although the available evidences were incoherent, it was mostly suggested that opium use does not protect against or improve cardiovascular problems.

Opium use and cardiovascular diseases: a systematic review and meta-analysis

The effects of opium on cardiovascular diseases (CVDs) have been extensively studied. However, there are few studies that summarize this research comprehensively; thus, this systematic review and meta-analysis is a collection of the newest information combined with previous findings to furthermore illuminate the effects of opium on CVDs. In this systematic review, all observational studies were systematically searched using the main international databases such as PubMed/Medline, Web of Sciences, and Scopus until October 2018. After the quality assessment of the articles, the fixed or random model meta-analysis was used to pool the results. I-square test was used to assess the heterogeneity of the studies. Overall, 41 studies were identified. Based on the random model, the pooled odds ratio (OR) (95% confidence interval (CI)) of opium use and coronary artery diseases (CAD) was estimated at 2.75 (95% CI = 2.04-3.75; I²=47%). The pooled OR of opium use and CVD in-hospital mortality was not statistically significant (OR: 1.44, 95% CI = 0.88-2.36, I²  = 51%). In the stratified analysis, in the patients who had undergone heart surgery, the average of ejection fraction (EF) in the opium users was significantly lower than those not using opium (mean differences: -3.06, CI 95% = -4.40 to -1.71, I² = 60%) but in the patients with acute myocardial infarction undergoing angiography, the average EF was not significantly different in the opium users compared to non-users (mean difference: 0.30, CI: -0. 55 to 1.15). The results of this meta-analysis revealed that opium might be a risk factor for CAD and EF but not in-hospital mortality.

[D-Ala2, D-Leu5] Enkephalin Improves Liver Preservation During Normothermic Ex Vivo Perfusion

BACKGROUND

Meeting the metabolic demands of donor livers using normothermic ex vivo liver perfusion (NEVLP) preservation technology is challenging. The delta opioid agonist [D-Ala2, D-Leu5] enkephalin (DADLE) has been reported to decrease the metabolic demand in models of ischemia and cold preservation. We evaluated the therapeutic potential of DADLE by investigating its ability to protect against oxidative stress and hepatic injury during normothermic perfusion.

MATERIALS AND METHODS

Primary rat hepatocytes were used in an in vitro model of oxidative stress to determine the minimum dose of DADLE needed to induce protection and the mechanisms associated with protection. NEVLP was then used to induce injury in rat livers and determine the effectiveness of DADLE in preventing liver injury.

RESULTS

In hepatocytes, DADLE was protective against oxidative stress and led to a decrease in phosphorylation of JNK and p38. Naltrindole, a δ-opioid receptor antagonist, blocked this effect. DADLE also activated the PI3K/Akt signaling pathway, and PI3K/Akt inhibition decreased the protective effects of DADLE treatment. In addition, DADLE treatment during NEVLP resulted in lower perfusate alanine aminotransferase and tissue malondialdehyde and better tissue adenosine triphosphate and glutathione. Furthermore, perfusion with DADLE compared with perfusate alone preserved tissue architecture.

CONCLUSIONS

DADLE confers protection against oxidative stress in hepatocytes and during NEVLP. These data suggest that the mechanism of protection involved the prevention of mitochondrial dysfunction by opioid receptor signaling and subsequent increased expression of prosurvival/antiapoptotic signaling pathways. Altogether, data suggest that opioid receptor agonism may serve as therapeutic target for improved liver protection during NEVLP.

DADLE improves hepatic ischemia/reperfusion injury in mice via activation of the Nrf2/HO‑1 pathway

Hepatic ischemia/reperfusion (I/R) injury is a common pathophysiological process that occurs following liver surgery, which is associated with oxidative stress, and can cause acute liver injury and lead to liver failure. Recently, the development of drugs for the prevention of hepatic I/R injury has garnered interest in the field of liver protection research. Previous studies have demonstrated that [D‑Ala2, D‑Leu5]‑Enkephalin (DADLE) exerts protective effects against hepatic I/R injury. To further clarify the specific mechanism underlying the effects of DADLE on hepatic I/R injury, the present study aimed to observe the effects of various doses of DADLE on hepatic I/R injury in mice. The results indicated that DADLE, at a concentration of 5 mg/kg, significantly reduced the levels of alanine aminotransferase and aspartate aminotransferase in the serum, and the levels of malondialdehyde in the liver homogenate. Conversely, the levels of glutathione, catalase and superoxide dismutase in the liver homogenate were increased. In addition, DADLE was able to promote nuclear factor, erythroid 2 like 2 (Nrf2) nuclear translocation and upregulate the expression of heme oxygenase (HO)‑1, which is a factor downstream of Nrf2, thus improving hepatic I/R injury in mice. In conclusion, the present study demonstrated that DADLE was able to significantly improve hepatic I/R injury in mice, and the specific mechanism may be associated with the Nrf2/HO‑1 signaling pathway.

Prospects for Creation of Cardioprotective and Antiarrhythmic Drugs Based on Opioid Receptor Agonists

It has now been demonstrated that the μ, δ1 , δ2 , and κ1 opioid receptor (OR) agonists represent the most promising group of opioids for the creation of drugs enhancing cardiac tolerance to the detrimental effects of ischemia/reperfusion (I/R). Opioids are able to prevent necrosis and apoptosis of cardiomyocytes during I/R and improve cardiac contractility in the reperfusion period. The OR agonists exert an infarct-reducing effect with prophylactic administration and prevent reperfusion-induced cardiomyocyte death when ischemic injury of heart has already occurred; that is, opioids can mimic preconditioning and postconditioning phenomena. Furthermore, opioids are also effective in preventing ischemia-induced arrhythmias.

Remifentanil preconditioning protects the small intestine against ischemia/reperfusion injury via intestinal δ- and μ-opioid receptors

BACKGROUND

Intestinal ischemia/reperfusion (I/R) injury can cause a high rate of mortality in the perioperative period. Remifentanil has been reported to provide protection for organs against I/R injury. We hypothesized that remifentanil preconditioning would attenuate the small intestinal injury induced by intestinal I/R.

METHODS

We used both an in vivo rat model of intestinal I/R injury and a cell culture model using IEC-6 cells (the rat intestinal epithelial cell line) subjected to oxygen and glucose deprivation (OGD). Remifentanil was administered before ischemia or OGD, and 3 specific opioid receptors antagonists, naltrindole (a δ-OR selective antagonist), nor-binaltorphimine (nor-BNI, a κ-OR selective antagonist), and CTOP (a μ-OR selective antagonist), were administered before preconditioning to determine the role of opioid receptors in the intestinal protection mediated by remifentanil.

RESULTS

In the in vivo rat model, intestinal I/R induced obvious intestinal injury as evidenced by increases in the Chiu score, serum diamine oxidase activity, the apoptosis index, and the level of cleaved caspase-3 protein expression, whereas remifentanil preconditioning significantly improved these changes in vivo. In the in vitro cell culture exposed to OGD, cell viability (MTT, ie, (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay and flow cytometric analysis showed that remifentanil preconditioning enhanced IEC-6 cell viability and decreased apoptosis. In both in vitro and in vivo models, the aforementioned protective effects of remifentanil preconditioning were abolished completely by previous administration of the δ- or μ-opioid markedly attentuated but not the κ-opioid receptor antagonist.

CONCLUSION

Remifentanil preconditioning appears to act via δ- and μ-opioid receptors to protect the small intestine from intestinal I/R injury by attenuating apoptosis of the intestinal mucosal epithelial cells.

Extracellular signalling molecules in the ischaemic/reperfused heart - druggable and translatable for cardioprotection?

In patients with acute myocardial infarction, timely reperfusion is essential to limit infarct size. However, reperfusion also adds to myocardial injury. Brief episodes of ischaemia/reperfusion in the myocardium or on organ remote from the heart, before or shortly after sustained myocardial ischaemia effectively reduce infarct size, provided there is eventual reperfusion. Such conditioning phenomena have been established in many experimental studies and also translated to humans. The underlying signal transduction, that is the molecular identity of triggers, mediators and effectors, is not clear yet in detail, but several extracellular signalling molecules, such as adenosine, bradykinin and opioids, have been identified to contribute to cardioprotection by conditioning manoeuvres. Several trials have attempted the translation of cardioprotection by such autacoids into a clinical scenario of myocardial ischaemia and reperfusion. Adenosine and its selective agonists reduced infarct size in a few studies, but this benefit was not translated into improved clinical outcome. All studies with bradykinin or drugs which increase bradykinin's bioavailability reported reduced infarct size and some of them also improved clinical outcome. Synthetic opioid agonists did not result in a robust infarct size reduction, but this failure of translation may relate to the cardioprotective properties of the underlying anaesthesia per se or of the comparator drugs. The translation of findings in healthy, young animals with acute coronary occlusion/reperfusion to patients of older age, with a variety of co-morbidities and co-medications, suffering from different scenarios of myocardial ischaemia/reperfusion remains a challenge.

Delta opioid receptors: the link between exercise and cardioprotection

This study investigated the role of opioid receptor (OR) subtypes as a mechanism by which endurance exercise promotes cardioprotection against myocardial ischemia-reperfusion (IR) injury. Wistar rats were randomly divided into one of seven experimental groups: 1) control; 2) exercise-trained; 3) exercise-trained plus a non-selective OR antagonist; 4) control sham; 5) exercise-trained plus a kappa OR antagonist; 6) exercise-trained plus a delta OR antagonist; and 7) exercise-trained plus a mu OR antagonist. The exercised animals underwent 4 consecutive days of treadmill training (60 min/day at ∼70% of maximal oxygen consumption). All groups except the sham group were exposed to an in vivo myocardial IR insult, and the myocardial infarct size (IS) was determined histologically. Myocardial capillary density, OR subtype expression, heat shock protein 72 (HSP72) expression, and antioxidant enzyme activity were measured in the hearts of both the exercised and control groups. Exercise training significantly reduced the myocardial IS by approximately 34%. Pharmacological blockade of the kappa or mu OR subtypes did not blunt exercise-induced cardioprotection against IR-mediated infarction, whereas treatment of animals with a non-selective OR antagonist or a delta OR antagonist abolished exercise-induced cardioprotection. Exercise training enhanced the activities of myocardial superoxide dismutase (SOD) and catalase but did not increase the left ventricular capillary density or the mRNA levels of HSP72, SOD, and catalase. In addition, exercise significantly reduced the protein expression of kappa and delta ORs in the heart by 44% and 37%, respectively. Together, these results indicate that ORs contribute to the cardioprotection conferred by endurance exercise, with the delta OR subtype playing a key role in this response.

Detection and distribution of opioid peptide receptors in porcine myocardial tissue

There is growing evidence that opioid peptide receptors (OPRs) play an important role in cardiovascular function. Many studies have been conducted in swine, in view of their anatomic and physiologic similarities to humans. Until now, the presence and particularly distribution of OPRs has been unclear. Porcine myocardial tissue was obtained from both the left and right atria and ventricles. Expression of mRNA for μ-, δ- and κ-OPR was determined by reverse transcription PCR. OPR proteins were detected by Western blot, distribution and cellular location were identified using immunohistochemistry. Homogenous expression of mRNA and protein for δ- and κ-OPRs were demonstrated in all porcine myocardial tissue tested, whereas expression of μ-OPR mRNA was not demonstrated in any of the tissues tested. This study demonstrates the expression of δ- and κ-OPRs in porcine myocardial tissue. No differences in distribution of δ- and κ-OPRs were found between the four heart cavities. Modulation of cardiac function by δ- and κ-OPR agonists or antagonists is therefore possible, while μ-OPR-mediated direct cardiac effects appear unlikely, due to nonexpression of the receptor. This study demonstrates that porcine studies can further elucidate the role of OPRs in cardiac (patho-)physiology.

Alpinetin activates the δ receptor instead of the κ and μ receptor pathways to protect against rat myocardial cell apoptosis

Alpinetin is a natural flavonoid that protects cells against fatal injury in ischemia-reperfusion. δ receptor activation protects myocardial cells from trauma; however, the mechanism is unknown. The aim of this study was to explore the function of alpinetin in δ receptor-mediated myocardial apoptosis. The myocardial cells of newly born rats were cultivated and myocardial apoptosis was induced by serum deprivation. The MTT method was used to evaluate cell viability and Annexin V-fluorescein isothiocyanate (FITC)/propidium iodide (PI) staining was used to analyze apoptosis. The expression levels of opioid receptor mRNA and protein were tested using reverse transcription-polymerase reaction (RT-PCR) and western blot assays. In addition, an opioid receptor antagonist, as well as protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) inhibitors, were used to determine the inferred signaling pathway. The results showed that that alpinetin reduced the myocardial apoptosis induced by serum deprivation in a concentration-dependent manner. However, the protection conferred to the myocardial cells by alpinetin was blocked by the δ opioid receptor antagonist naltrindole, as well as by PKC and ERK inhibitors (GF109203X and U0126, respectively). In addition, it was shown that alpinetin was able to maintain the stability of the mitochondrial membrane potential, lower the level of intracytoplasmic cytochrome c and reduce Bax displacement from the cytoplasm to the mitochondria. It was concluded that alpinetin was able to activate δ receptors to induce the endogenous protection of myocardial cells via the PKC/ERK signaling pathway.

Role of endogenous opioid peptides in the infarct size-limiting effect of adaptation to chronic continuous hypoxia

AIMS

The objective of this study was to examine the involvement of endogenous opioid peptides and opioid receptor (OR) subtypes in the cardioprotective effect of adaptation to chronic hypoxia in rats.

MAIN METHODS

Rats were exposed to continuous normobaric hypoxia (CNH; 12% oxygen) for 3 weeks. Myocardial ischemia was induced by 20-min coronary artery occlusion followed by 3-h reperfusion in anesthetized open-chest animals. Various OR antagonists were administered to rats prior to ischemia. The size of myocardial infarction and the incidence of ischemic ventricular arrhythmias were assessed. Myocardial and plasma concentrations of opioid peptides (met-enkephalin, β-endorphin, and endomorphins) were determined.

KEY FINDINGS

Adaptation to CNH significantly increased myocardial and plasma concentrations of opioids, potentiated their further elevation by ischemia/reperfusion, and reduced myocardial infarct size, but it did not affect the incidence of ischemic arrhythmias. The infarct size-limiting effect of CNH was abolished by OR antagonists naltrexone (non-selective), naloxone methiodide (non-selective peripherally acting), TIPP[ψ] (δ-OR), naltriben (δ2-OR), or CTAP (μ-OR), while BNTX (δ1-OR) and nor-binaltorphimine (κ-OR) had no effect.

SIGNIFICANCE

The results suggest that the infarct size-limiting effect afforded by adaptation to CNH is mediated by activation of peripheral δ2- and μ-ORs by elevated levels of endogenous opioid peptides.

Role of Intracellular Calcium and Cyclic Nucleotides in Realization of Cardioprotective Effects of δ(1)- and κ(1)-Opioid Receptor Agonists

The role of cyclic nucleotides (cAMP, cGMP) and Ca(2+)-ATPase of the sarcoplasmic reticulum in the mechanism of cardioprotective effects of selective δ(1)- and κ(1)-opioid receptor agonists DPDPE and U-50488 was studied under conditions of global ischemia and reperfusion of isolated and perfused rat heart. Activation of both types of opioid receptors 2-fold reduced the reperfusion release of creatine phosphokinase. The cardioprotective effect of U-50488 was paralleled by a 2-fold decrease in cAMP content in the myocardium, while DPDPE did not modify the content of cAMP throughout the experiment. None of these substances changed the content of cGMP in the myocardium. The cardioprotective effect of DPDPE was not observed after inhibition of sarcoplasmic reticulum Ca(2+)-ATPase with cyclopiazonic acid. The cardioprotective effect of U-50488 was associated with reduction of cAMP level in the myocardium, while the cytoprotective effect of DPDPE was mediated by opioidergic modulation of Ca(2+) transport at the level of the sarcoplasmic reticulum.

Comparative analysis of the cardioprotective properties of opioid receptor agonists in a rat model of myocardial infarction

OBJECTIVES

This study was conducted to test the hypothesis that opioid receptor (OR)-mediated cardioprotection is agonist specific when administered prior to coronary artery occlusion and reperfusion in a rat model.

METHODS

Anesthetized open-chest male Wistar rats were subjected to 45 minutes of left coronary artery occlusion and 2 hours of reperfusion. Opioid agonists were infused 15 minutes prior to coronary artery occlusion. Two control groups and 15 opioid-treated groups were studied. Controls were infused with either saline alone (n = 16) or dimethyl sulfoxide plus hydroxypropyl-β-cyclodextrin in saline (n = 19). The μ-selective agonist DAMGO was infused at either 150 nmol/kg (n = 15) or 1500 nmol/kg (n = 14), and dermorphin-H was infused at 150 nmol/kg (n = 14). The δ₁ -selective agonist d-Pen²(,)⁵ enkephalin (DPDPE) was infused at 150 nmol/kg (n = 16) or 1500 nmol/kg (n = 14). The δ₂ -selective agonists deltorphin II (n = 16), deltorphin-D(variant) (n = 15), and deltorphin-E (n = 14) were infused at 150 nmol/kg. The selective κ₁ opioid agonist U-50488 was infused at 240 nmol/kg (n = 14), 1500 nmol/kg (n = 14), and 2,400 nmol/kg (n = 14). The selective κ₂ opioid agonist GR-89696 was infused at 150 nmol/kg (n = 14) and 1500 nmol/kg (n = 15). Orphinan FQ (nociceptin), also referred to as OR-like 1 (ORL1), was infused at 220 nmol/kg (n = 15) and 1500 nmol/kg (n = 15). The infarct size/area at risk (IS/AAR) ratio was determined after reperfusion by negative staining with patent blue violet dye. Hemodynamic parameters including heart rate, mean arterial blood pressure (MAP), and rate pressure product (RPP) were determined.

RESULTS

Pretreatment with the δ₂ OR agonist deltorphin II (150 nmol/kg) significantly reduced the IS/AAR ratio, while deltorphin-D(variant) and deltorphin-E did not exhibit an infarct-sparing effect at that treatment dose. Activation of δ₁ OR by DPDPE, κ₁ OR by U-50488, κ₂ OR by GR-89696, μ OR by DAMGO, dermorphin-H, and nociceptin had no effect on the IS/AAR ratio. U-50488 at 2,400 nmol/L induced a bradycardic effect. All other opioids had no effect on hemodynamic parameters at the doses tested.

CONCLUSIONS

Peripheral δ₂ OR activation by deltorphin II induces infarct size reduction in this animal model. Agonists of μ, δ₁, κ₁, κ₂, and nociceptin receptors at the doses tested did not induce cardiac tolerance to ischemia/reperfusion injury in vivo.

Kappa and delta opioid receptor signaling is augmented in the failing heart

The opioidergic system, an endogenous stress pathway, modulates cardiac function. Furthermore, opioid peptide and receptor expression is altered in a number of cardiac pathologies. However, whether the response of myocardial opioid receptor signaling is altered in heart failure progression is currently unknown. Elucidating possible alterations in and effects of opioidergic signaling in the failing myocardium is of critical importance as opioids are commonly used for pain management, including in patients at risk for cardiovascular disease. A hamster model of cardiomyopathy and heart failure (Bio14.6) was used to investigate cardiac opioidergic signaling in heart failure development. This study found an augmented negative inotropic and lusitropic response to administration of agonists selective for the kappa opioid receptor and delta opioid receptor in the failing heart that was mediated by a pertussis toxin-sensitive G-protein. The augmented decrease in cardiac function was manifested by increased inhibition of cAMP accumulation and the amplitude of the systolic Ca(2+) transient. Furthermore, increased depression of cardiac function and of two important second messengers, cAMP and intracellular Ca(2+), were independent of changes in cardiac opioid peptide or receptor expression. Thus, the cardiomyopathy-induced failing heart experiences increased cardiac depressant effects following opioid receptor stimulation which could exacerbate diminished cardiac function in end-stage heart failure. As cardiac function is already depressed in heart failure patients, administration of opioids could exacerbate the degree of cardiac dysfunction and worsen disease progression.

Signaling pathways of cardioprotective ischemic preconditioning

BACKGROUND

Ischemia/reperfusion (I/R) injury is a major contributory factor to cardiac dysfunction and infarct size that determines patient prognosis after acute myocardial infarction. During the last 20 years, since the appearance of the first publication on ischemic preconditioning (IP), our knowledge of this phenomenon has increased exponentially.

RESULTS AND CONCLUSION

Basic scientific experiments and preliminary clinical trials in humans suggest that IP confers resistance to subsequent sustained ischemic insults not only in the regional tissue but also in distant organs (remote ischemic preconditioning), which may provide a simple, cost-effective means of reducing the risk of perioperative myocardial ischemia. The mechanism may be humoral, neural, or a combination of both, and involves adenosine, bradykinin, protein kinases and K(ATP) channels, although the precise end-effector remains unclear. This review describes different signaling pathways involved in acute ischemic preconditioning in detail.

Cardioprotective effect of kappa1-opioid receptor activation and role of cAMP in its realization

The cardioprotective and antiarrhythmic effects of a selective kappa1-opioid receptor agonist U-50,488 were studied during experimental 45-min total ischemia and 30-min reperfusion of isolated rat heart. The opioid had no effect on the incidence and type of reperfusion arrhythmias. U-50,488 in a concentration of 0.1 microM inhibited reperfusion-induced release of creatine phosphokinase and decreased cAMP concentration in the myocardium by 2 times. These parameters remained unchanged after treatment with U-50,488 in a concentration of 1 microM. The cardioprotective effect of U-50,488 was probably associated with a decrease in cAMP concentration in heart cells. U-50,488 in a concentration of 1 microM produced no cardioprotective effect, which can be explained by its interaction with an unknown non-opioid receptor in cardiomyocytes.

Activations of nPKCε and ERK1/2 Were Involved in Oxygen-Glucose Deprivation-induced Neuroprotection via NMDA Receptors in Hippocampal Slices of Mice

Accumulated reports have suggested that activation of protein kinase C (PKC) isoforms may involve the activation of extracellular signal-regulated kinases (ERKs) in the neuronal response to ischemic/hypoxic stimuli. We have previously demonstrated that the membrane translocation of novel PKC (nPKC) epsilon increased in the early phase of cerebral ischemic/hypoxic preconditioning of mice. In this study, we used Western blot analysis and propidium iodide stain to determine whether the activations of nPKCepsilon and ERKs were involved in oxygen-glucose deprivation (OGD)-induced neuroprotection via N-methyl-D-aspartate (NMDA) receptors. The hippocampal slices of mice were exposed to OGD for 10 (OGD10) or 45 minutes (OGD45) to mimic mild (causing ischemic/hypoxic preconditioning) and severe (causing severe OGD) ischemia/hypoxia, respectively. We found that OGD10-induced nPKCepslilon membrane translocation was mediated by NMDA receptors, and both OGD10 and NMDA (1 microM, 30 min) pretreatment could protect Cornu Ammonis region 1 neurons against the subsequent severe OGD45. In addition, nPKCepsilon translocation inhibitor, epsilonV1-2 (1 microM, 30 min), and ERKs upstream mitogen-activated protein/extracellular signal regulated kinase kinase inhibitor, PD-98059 (20 microM, 30 min), could significantly inhibit OGD10 and NMDA-induced neuroprotection. These results suggest that OGD10-induced neuroprotection against severe OGD45 in the Cornu Ammonis region 1 region of the hippocampal slices was mediated by the activations of NMDA receptors, nPKCepsilon, and the downstream ERKs.

??-Opioid Receptor Antagonism Improves Recovery from Myocardial Stunning in Chronically Instrumented Dogs

We tested the hypothesis that the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) improves recovery from myocardial stunning. Ten dogs were chronically instrumented for measurement of heart rate, left atrial, aortic and left ventricular pressure (LVP), and the maximum rate of LVP increase (LV dP/dt(max)) and decrease (LV dP/dt(max)), coronary blood flow velocity and myocardial wall-thickening fraction. Regional myocardial blood flow was determined with fluorescent microspheres. Catecholamine plasma levels were measured by high-performance liquid chromatography, and beta-endorphin and dynorphin plasma levels by radioimmunoassay. An occluder around the left anterior descending artery (LAD) allowed induction of a reversible LAD-ischemia. Animals underwent two experiments in a randomized crossover fashion on separate days: (a) 10 min LAD-occlusion (control experiment), (b) second ischemic episode 24 h after nor-BNI (2.5 mg/kg IV) (intervention). Dogs receiving nor-BNI showed an increase in wall-thickening fraction, LV dP/dt(max) and LV dP/dt(min) before ischemia and during the whole reperfusion (P < 0.05 versus control experiment). After nor-BNI pretreatment, dynorphin levels increased after induction of ischemia to a peak level of 15.1 +/- 3.6 pg/mL (P < 0.05 versus control experiment). The increase in plasma beta-endorphin during ischemia and early reperfusion was attenuated after nor-BNI. Compared with the control experiment, nor-BNI left global hemodynamics, regional myocardial blood flow, and catecholamine levels unchanged. In conclusion, nor-BNI improves recovery from myocardial stunning after regional myocardial ischemia in chronically instrumented dogs.

Bremazocine: a kappa-opioid agonist with potent analgesic and other pharmacologic properties

Bremazocine is a kappa-opioid receptor agonist with potent analgesic and diuretic activities. As an analgesic it is three- to four-times more potent than morphine, as determined in both hot plate and tail flick tests. Bremazocine and other benzomorphan analogs were synthesized in an effort to produce opiates with greater kappa-opioid receptor selectivity and with minimal morphine-like side effects. Unlike morphine bremazocine is devoid of physical and psychological dependence liability in animal models and produces little or no respiratory depression. While bremazocine does not produce the characteristic euphoria associated with morphine and its abuse, it has been shown to induce dysphoria, a property that limits its clinical usefulness. Similarly to morphine, repeated administration of bremazocine leads to tolerance to its analgesic effect. It has been demonstrated that the marked diuretic effect of bremazocine is mediated primarily by the central nervous system. Because of its psychotomimetic side effects (disturbance in the perception of space and time, abnormal visual experience, disturbance in body image perception, de-personalization, de-realization and loss of self control) bremazocine has limited potential as a clinical analgesic. However, its possible utility for the therapy of alcohol and drug addiction warrants further consideration because of its ability to decrease ethanol and cocaine self-administration in non-human primates. In addition, the ability of bremazocine-like drugs to lower intraocular pressure and to minimize ischemic damage in animal models suggests their possible use in the therapy of glaucoma and cardiovascular disease.

Myocardial preconditioning and remote renal preconditioning--identifying a protective factor using proteomic methods?

It is still unknown whether remote ischemic preconditioning is mediated by a humoral or a neurogenic mechanism from the preconditioning to the preconditioned tissue. The purpose of the following study was to identify a possible humoral trigger of ischemic myocardial preconditioning and remote renal preconditioning. Open chest rats were subjected to a coronary artery occlusion period of 45 min followed by 2 h of reperfusion (Control animals; n = 6). The coronary preconditioned group (IPC, n = 6) was subjected to a preceding preconditioning period of 5 min coronary artery occlusion followed by 5 min of reperfusion, repeated three times. The renal preconditioned group (IPR, n = 6) was subjected to a preceding renal artery occlusion period of 10 min followed by 20 min of reperfusion. Area at risk (AAR) and infarcted area (IA) were determined at the end of each protocol. Blood samples were taken at the end of the preconditioning protocols from parallel experiments for proteomic analysis using two-dimensional gel electrophoresis (2-DE), matrix assisted laser desorption and ionization-time of flight-mass spectrometry (MALDI-TOF-MS), and liquid chromatography-electrospray ionization-tandem mass spectrometry (nanoLC-ESI-MS/MS). IA/AAR was 87.8 +/- 10.7% in the control group. IPC and IPR significantly reduced IA/AAR (58.2 +/- 9.3% and 56.9 +/- 9.0%, p < 0.001). Proteomic analyses detected four protein spots which were either up- (n = 3) or down-regulated in the preconditioned groups vs. the control group. The three up-regulated protein spots were identified as albumin fragments, whereas the down-regulated spot was identified as liver regeneration-related protein (LRRG03). Interestingly, albumin modification by brief ischemia has been recently shown and evaluated for the clinical diagnosis of sublethal myocardial ischemia. However, no differentially abundant proteins which possess a known signaling function could be found. Hence, though there is a differential protein expression in blood following IPC and IPR, our data are not in favor of a humoral mediator of remote preconditioning with a molecular weight of more than 8 kDa. Our results rather suggest either a neurogenic pathway or a mediator smaller than 8 kDa.

Opioid-induced preconditioning: recent advances and future perspectives

Opioids, named by Acheson for compounds with morphine-like actions despite chemically distinct structures, have received much research interest, particularly for their central nervous system (CNS) actions involved in pain management, resulting in thousands of scientific papers focusing on their effects on the CNS and other organ systems. A more recent area which may have great clinical importance concerns the role of opioids, either endogenous or exogenous compounds, in limiting the pathogenesis of ischemia-reperfusion injury in heart and brain. The role of endogenous opioids in hibernation provides tantalizing evidence for the protective potential of opioids against ischemia or hypoxia. Mammalian hibernation, a distinct energy-conserving state, is associated with depletion of energy stores, intracellular acidosis and hypoxia, similar to those which occur during ischemia. However, despite the potentially detrimental cellular state induced with hibernation, the myocardium remains resilient for many months. What accounts for the hypoxia-tolerant state is of great interest. During hibernation, circulating levels of opioid peptides are increased dramatically, and indeed, are considered a "trigger" of hibernation. Furthermore, administration of opioid antagonists can effectively reverse hibernation in mammals. Therefore, it is not surprising that activation of opioid receptors has been demonstrated to preserve cellular status following a hypoxic insult, such as ischemia-reperfusion in many model systems including the intestine [Zhang, Y., Wu, Y.X., Hao, Y.B., Dun, Y. Yang, S.P., 2001. Role of endogenous opioid peptides in protection of ischemic preconditioning in rat small intestine. Life Sci. 68, 1013-1019], skeletal muscle [Addison, P.D., Neligan, P.C., Ashrafpour, H., Khan, A., Zhong, A., Moses, M., Forrest, C.R., Pang, C.Y., 2003. Noninvasive remote ischemic preconditioning for global protection of skeletal muscle against infarction. Am. J. Physiol. Heart Circ. Physiol. 285, H1435-H1443], the CNS [Borlongan, C.V., Wang, Y., Su, T.P., 2005. Delta opioid peptide (d-ala 2, d-leu 5) enkephalin: linking hiberation and neuroprotection. Front Biosci. 9, 3392-3398] and the myocardium [Romano, M.A., Seymour, E.M., Berry, J.A., McNish, R.A., Bolling, S.F., 2004. Relative contribution of endogenous opioids to myocardial ischemic tolerance. J Surg Res. 118, 32-37; Peart, J.N., Gross, G.J., 2004a. Exogenous activation of delta- and kappa-opioid receptors affords cardioprotection in isolated murine heart. Basic Res Cardiol. 99(1), 29-37]. For the purpose of this review, we will focus primarily on the protective effects of opioids against post-reperfusion myocardial stunning and infarction.

Roles of KATP channels in delayed cardioprotection and intracellular Ca(2+) in the rat heart as revealed by kappa-opioid receptor stimulation with U50488H

The effect of preconditioning with U50488 H (UP), a selective kappa-opioid receptor (kappa-OR) agonist, on infarct size and intracellular Ca2+ ([Ca2+]i) in the heart subjected to ischaemic insults were studied and evaluated. U50488 H administered intravenously reduced the infarct size 18-48 h after administration in isolated hearts subjected to regional ischaemia/reperfusion (I/R). The effect was dose dependent. A peak effect was reached at 10 mg x kg-1 U50488 H and at 24 h after administration. The effect of 10 mg x kg-1 U50488 H at 24 h after administration was abolished by nor-binaltorphimine (nor-BNI), a selective kappa-OR antagonist, indicating the effect was kappa-OR mediated. The infarct reducing effect of U50488 H was attenuated when a selective blocker of mitochondrial (5-hydroxydecanoic acid, 5-HD) or sarcolemmal (HRM-1098) ATP-sensitive potassium channel (KATP) was coadministered with U50488 H 24 h before ischaemia or when 5-HD was administered just before ischaemia. U50488 H also attenuated the elevation in [Ca2+]i and reduction in electrically induced [Ca2+]i transient in cardiomyocytes subjected to ischaemic insults. The effects were reversed by blockade of KATP channel, which abolished the protective effect of preconditioning with U50488 H. The results indicated that mitochondrial KATP channel serves as both a trigger and a mediator, while sarcolemmal KATP channel as a trigger only, of delayed cardioprotection of kappa-OR stimulation. The effects of these channels may result from prevention/attenuation of [Ca2+]i overload induced by ischaemic insults.

Remifentanil limits infarct size but attenuates preconditioning-induced infarct limitation

OBJECTIVES

We investigated the influence of the narcotic anesthetic remifentanil on irreversible myocardial ischemic injury.

METHODS

New Zealand White rabbits were anesthetized with propofol (0.7-1.8 mg.kg.min) and then subjected to 30 min regional myocardial ischemia and 3 h reperfusion (CON). Some animals also underwent ischemic preconditioning, elicited by either one (IP1) or two (IP2) cycles of 5 min ischemia and 5 min reperfusion, and/or remifentanil, administered either as a transient infusion mimicking the preconditioning protocol (RP2, 10 microg x kg x min) or as a continuous infusion (R, 3-10 microg x kg x min). Rabbits were randomly assigned to experimental groups. Infarct size was assessed with tetrazolium. Results are reported as mean+/-SD.

RESULTS

Non-preconditioned infarct size was approximately 50% of the area-at-risk (49.6+/-20.1% CON). Both one and two cycles of ischemic preconditioning markedly reduced infarct size (49.6+/-20.1% CON versus 18.6+/-8.6% IP and versus 7.5+/-7.6% IP2; both p<0.001). Preconditioning with remifentanil modestly reduced infarct size (49.6+/-20.1% CON versus 29.3+/-8.5% RP2; p<0.01). However, sustained administration of remifentanil did not provide protection (49.6+/-20.1% CON versus 43.9+/-16.2% R), and it attenuated the protection offered by preconditioning (49.6+/-20.1% CON versus 35.6+/-20.7% R+IP1, p=NS; and versus 14.5+/-14.5% R+IP2; p<0.05).

CONCLUSION

Transient pre-ischemic administration of remifentanil modestly reduces infarct size in propofol-anesthetized rabbits, but continuous administration of remifentanil increases the threshold for ischemic preconditioning-induced infarct limitation.

Effect of exogenous kappa-opioid receptor activation in rat model of myocardial infarction

The involvement of opioid receptor activation during ischemia-reperfusion is somewhat controversial. While it is generally accepted that activation of the delta-opioid receptor (DOR) is cardioprotective, and may indeed be an important mediator of ischemic preconditioning, the role of the kappa-opioid receptor (KOR) is less well understood. To this end, we examined three different KOR agonists and their effects upon infarct size and arrhythmia development. Male Sprague-Dawley rats were subjected to 30 minutes of occlusion followed by 90 minutes of reperfusion. Opioid receptor agonists were administered 10 minutes before the onset of ischemia, while the opioid antagonists were given 20 minutes before occlusion. Untreated rats exhibited an infarct size (IS/AAR%) of 52.4 +/- 2.7%. Pretreatment with the DOR agonist, BW373U86, limited infarct development to 37.2 +/- 1.8%, which was reversed by the selective DOR antagonist, BNTX. All three KOR agonists studied, U50,488, ICI 204,448, and BRL 52537 significantly reduced infarct size to levels comparable to that of BW373U86. The infarct-sparing effects of U50,488 and ICI 204,448 were abolished by the selective KOR antagonist, nor-BNI. Nor-BNI failed to inhibit the cardioprotective effects of BRL 52537. Furthermore, U50,488 and BRL 52537, but not ICI 204,448, significantly reduced the incidence of arrhythmias. These effects were not blocked by nor-BNI. These data demonstrate that KOR activation provides a similar degree of infarct size reduction as DOR activation. KOR agonists also reduced arrhythmogenesis; however, these responses appear to be independent of KOR activation.

[D-Ala2, D-Leu5] enkephalin (DADLE) protects liver against ischemia-reperfusion injury in the rat

BACKGROUND

[D-Ala(2), D-Leu(5)] enkephalin (DADLE) is a synthetic delta class of opioid and is reported to induce hibernation as well as hibernation induction trigger (HIT) in the serum of hibernating mammals. DADLE and HIT have been demonstrated to protect the heart, lung, and jejunum against ischemia-reperfusion (I-R) injury. In the present study, we examined the effect of DADLE on I-R injury of the liver in rats.

METHODS

After administration of DADLE (DADLE group) or normal saline as a vehicle (Control group), partial hepatic ischemia was induced by occluding the vessels supplying 92% of the liver for 45 min, followed by declamping the vessels and resection of the non-ischemic lobe. After 120 min of reperfusion, serum glutamic-pyruvic transaminase (GPT), hyaluronic acid (HA) levels, and concentrations of malondialdehyde (MDA) of the liver tissue were measured. Additionally, bile output from the ischemic lobes was measured after reperfusion.

RESULTS

GPT levels were significantly lower in the DADLE group as compared to those of the Control group (P < 0.05), but the serum levels of HA were not different between the two groups. The concentrations of MDA of the liver tissue were significantly lower in the DADLE group than in the Control group (P < 0.01). The bile output after reperfusion was not significantly different between the two groups.

CONCLUSIONS

DADLE protects against I-R injury in hepatocytes, but not in the sinusoidal endothelial cells of the liver in rats. An anti-oxidative effect is suggested to be responsible for this effect.

Anaesthetics and cardiac preconditioning. Part II. Clinical implications

There is compelling evidence that preconditioning occurs in humans. Experimental studies with potential clinical implications as well as clinical studies evaluating ischaemic, pharmacological and anaesthetic cardiac preconditioning in the perioperative setting are reviewed. These studies reveal promising results. However, there are conflicting reports on the efficacy of preconditioning in the diseased and aged myocardium. In addition, many anaesthetics and a significant number of perioperatively administered drugs affect the activity of cardiac sarcolemmal and mitochondrial K(ATP) channels, the end-effectors of cardiac preconditioning, and thereby markedly modulate preconditioning effects in myocardial tissue. Although these modulatory effects on K(ATP) channels have been investigated almost exclusively in laboratory investigations, they may have potential implications in clinical medicine. Important questions regarding the clinical utility and applicability of perioperative cardiac preconditioning remain unresolved and need more experimental work and randomized controlled clinical trials.

Cardioprotection through a PKC-dependent decrease in myofilament ATPase

Activation of myocardial kappa-opioid receptor-protein kinase C (PKC) pathways may improve postischemic contractile function through a myofilament reduction in ATP utilization. To test this, we first examined the effects of PKC inhibitors on kappa-opioid receptor-dependent cardioprotection. The kappa-opioid receptor agonist U50,488H (U50) increased postischemic left ventricular developed pressure and reduced postischemic end-diastolic pressure compared with controls. PKC inhibitors abolished the cardioprotective effects of U50. To determine whether kappa-opioid-PKC-dependent decreases in Ca2+-dependent actomyosin Mg2+-ATPase could account for cardioprotection, we subjected hearts to three separate actomyosin ATPase-lowering protocols. We observed that moderate decreases in myofibrillar ATPase were equally cardioprotective as kappa-opioid receptor stimulation. Immunoblot analysis and confocal microscopy revealed a kappa-opioid-induced increase in myofilament-associated PKC-epsilon, and myofibrillar Ca2+-independent PKC activity was increased after kappa-opioid stimulation. This PKC-myofilament association led to an increase in troponin I and C-protein phosphorylation. Thus we propose PKC-epsilon activation and translocation to the myofilaments causes a decrease in actomyosin ATPase, which contributes to the kappa-opioid receptor-dependent cardioprotective mechanism.

Delta-opioid receptor activation mimics ischemic preconditioning in the canine heart

The role of delta-opioid receptors in mediating ischemic preconditioning (IPC) in rats, rabbits, and pigs has been well-established; however, no studies have been performed in dogs. Therefore, the purpose of the present study was to determine if activation of delta-opioid receptors can mimic the cardioprotective effects of IPC in the canine heart and to determine if a nonselective opioid receptor antagonist could block IPC. All dogs were subjected to 60 minutes of left anterior descending (LAD) coronary artery occlusion and 3 hours of reperfusion. Ischemic preconditioning was produced by one 5-minute period of ischemia 10 minutes before LAD coronary artery occlusion. Infarct size (IS) expressed as a percent of the area at risk (AAR; IS/AAR) was determined by triphenyltetrazolium staining. Two selective delta-opioid receptor (DOR) agonists, TAN-67 and BW373U86, were administered by intracoronary infusion for 30 minutes before LAD occlusion and the opioid receptor antagonist naloxone was administered 30 minutes before IPC. Both TAN-67 and BW373U86 produced significant reductions in IS/AAR similar to that of IPC (control: 28+/-2.1; TAN: 12.3+/-2.2; IPC: 9.3+/-3.0: BW: 11.7+/-2.6). Naloxone attenuated the effect of IPC (control: 28+/-2.1; naloxone: 18.2+/-4.5). These results suggest that opioid receptors are important in IPC in dogs, and stimulation of delta-opioid receptors with selective agonists can mimic the cardioprotective effects of IPC and may have therapeutic potential.

Role of kappa-opioid receptor activation in pharmacological preconditioning of swine

Pharmacological preconditioning with kappa-opioid receptor agonists is proarrhythmic and exerts antipreconditioning effects in rats. In swine, it is unknown whether kappa-opioid receptor stimulation plays a role in pharmacological preconditioning. Swine were preconditioned with 1) saline (controls), 2) [d-Ala(2),d-Leu(5)]enkephalin (DADLE), 3) morphine, 4) pentazocine, 5) norbinaltorphimine (nor-BNI), 6) DADLE + nor-BNI, 7) morphine + nor-BNI, or 8) pentazocine + nor-BNI before occlusion (45 min) and reperfusion (180 min) of the left anterior descending coronary artery. Infarct size to area at risk (IS), regional (systolic shortening) and global (pressures and flows) myocardial function, and arrhythmia occurrence were assessed. Only DADLE + nor-BNI preconditioning significantly decreased infarct size compared with controls (47 +/- 13 vs. 65 +/- 5%, P < 0.05); morphine preconditioning was not cardioprotective with or without kappa-opioid receptor blockade (nor-BNI). DADLE preconditioning significantly increased ischemia-induced arrhythmias relative to controls, whereas pentazocine-preconditioned animals (n = 2) experienced intractable ventricular fibrillation during ischemia. kappa-Opioid receptor blockade with DADLE or pentazocine preconditioning alleviated proarrhythmic effects. These results suggest that kappa-opioid receptor activation during pharmacological preconditioning is proarrhythmic in swine.