Challenges to be faced in the reconstruction of metabolic networks from public databases

In the post-genomic era, the biochemical information for individual compounds, enzymes, reactions to be found within named organisms has become readily available. The well-known KEGG and BioCyc databases provide a comprehensive catalogue for this information and have thereby substantially aided the scientific community. Using these databases, the complement of enzymes present in a given organism can be determined and, in principle, used to reconstruct the metabolic network. However, such reconstructed networks contain numerous properties contradicting biological expectation. The metabolic networks for a number of organisms are reconstructed from KEGG and BioCyc databases, and features of these networks are related to properties of their originating database.

Expression and activity of cAMP phosphodiesterase isoforms in pulmonary artery smooth muscle cells from patients with pulmonary hypertension: role for PDE1

Pulmonary hypertension (PHT) is associated with increased vascular resistance due to sustained contraction and enhanced proliferation of pulmonary arterial smooth muscle cells (PASMC); the abnormal tone and remodeling in the pulmonary vasculature may relate, at least in part, to decreased cyclic nucleotide levels. Cyclic nucleotide phosphodiesterases (PDEs), of which 11 families have been identified, catalyze the hydrolysis of cAMP and cGMP. We tested the hypothesis that PASMC isolated from patients with PHT, either idiopathic pulmonary arterial hypertension (IPAH) or secondary pulmonary hypertension (SPH), have increased expression and activity of PDE isoforms that reduce the responsiveness of agents that raise cellular cAMP. Real-time PCR and immunoblotting demonstrated that the expression of PDE1A, PDE1C, PDE3B, and PDE5A was enhanced in PASMC from both IPAH and SPH patients compared with control PASMC. Consistent with this enhanced expression of PDEs, agonist-stimulated cAMP levels were significantly reduced in IPAH and SPH PASMC unless a PDE inhibitor was present. The use of specific PDE inhibitors revealed that an increase in PDE1 and PDE3 activity largely accounted for reduced agonist-induced cAMP levels and increased proliferation in IPAH and SPH PASMC. Treatment with PDE1C-targeted small interference RNA enhanced cAMP accumulation and inhibited cellular proliferation to a greater extent in PHT PASMC than controls. The results imply that an increase in PDE isoforms, in particular PDE1C, contributes to decreased cAMP and increased proliferation of PASMC in patients with PHT. PDE1 isoforms may provide novel targets for the treatment of both primary and secondary forms of the disease.

Role of 12-lipoxygenase in volatile anesthetic-induced delayed preconditioning in mice

Delayed cardiac protection mediated by 12-lipoxygenase (12-LO) expression and activity has been linked to opioid receptor stimulation. The role of 12-LO in volatile anesthetic-induced delayed cardiac protection has not been determined. We tested the hypothesis that expression and activity of 12-LO mediate delayed cardiac protection induced by isoflurane in the mouse heart in vivo. Mice were pretreated with 1.4% isoflurane for 30 min and allowed to recover for 1, 12, or 24 h. Immunoblot analysis showed isoflurane significantly enhanced 12-LO protein expression at 12 and 24 h after isoflurane exposure, and this induction of 12-LO was confirmed by immunohistochemistry of whole heart sections at 24 h. The induced protein expression appeared to be localized to intercalated disc regions adjoining adjacent cardiac myocytes. Additionally, mice ± isoflurane (24 h previously) were subjected to 30 min coronary artery occlusion followed by 2 h of reperfusion in the presence and absence of a 12-LO inhibitor. Isoflurane reduced infarct size (27.1 ± 2.2% of the area at risk; n = 8) compared with the control group (44.6 ± 3.6%, n = 8). Baicalein (3 mg/kg), a selective 12-LO inhibitor, blocked the delayed protective effects of isoflurane pretreatment on infarct size (40.6 ± 3.6%, n = 8). These data suggest that 12-LO is an important mediator of isoflurane-induced delayed preconditioning.

Cardiac-directed expression of adenylyl cyclase VI facilitates atrioventricular nodal conduction

OBJECTIVES

The purpose of this study was to test the hypothesis that cardiac-directed expression of adenylyl cyclase VI (AC(VI)) facilitates atrioventricular (AV) nodal conduction.

BACKGROUND

Cardiac-directed expression of AC(VI), unlike other strategies to increase cyclic adenosine monophosphate generation, reduces mortality in murine cardiomyopathy. Recent reports suggest that AC(VI) expression may also protect against lethal bradycardia.

METHODS

We performed immunofluorescence staining for AC(VI) in the AV node of transgenic mice. We then performed electrophysiologic studies (EPSs) using a 1.7-F octapolar catheter at the AV junction in 11 transgenic AC(VI) mice and 14 control mice.

RESULTS

Immunofluorescence staining revealed increased AC(VI) expression in the AV node of transgenic mice versus controls. During EPS, AV intervals approximated PR intervals (R2 = 0.99) and related linearly to atrial-to-His intervals (R2 = 0.98; both p < 0.0001). Thus, we studied AV intervals to avoid electrocardiogram pacing artifacts and inconsistent inscription of His bundle electrograms. At baseline, AC(VI) mice had shorter AV intervals (47 +/- 9 ms) than controls (57 +/- 11 ms; p = 0.02), despite similar sinus rates. In pacing, AV intervals were shorter in AC(VI) mice than controls for a wide cycle-length range (p < 0.01). The AC(VI) mice also had shorter AV Wenckebach cycle lengths (AC(VI): 114 +/- 12 ms; control: 131 +/- 28 ms; p = 0.05) and ventriculo-atrial effective refractory periods (AC(VI): 97 +/- 21 ms; control: 127 +/- 15 ms; p = 0.05). We observed no differences between groups in sinus node function, and ventricular arrhythmias were not inducible.

CONCLUSIONS

Cardiac-directed expression of AC(VI) facilitates AV nodal conduction without altering sinus node function. These results suggest the need to define a role for AC(VI) gene transfer in treating diseases of AV conduction.

Microtubules and actin microfilaments regulate lipid raft/caveolae localization of adenylyl cyclase signaling components

Microtubules and actin filaments regulate plasma membrane topography, but their role in compartmentation of caveolae-resident signaling components, in particular G protein-coupled receptors (GPCR) and their stimulation of cAMP production, has not been defined. We hypothesized that the microtubular and actin cytoskeletons influence the expression and function of lipid rafts/caveolae, thereby regulating the distribution of GPCR signaling components that promote cAMP formation. Depolymerization of microtubules with colchicine (Colch) or actin microfilaments with cytochalasin D (CD) dramatically reduced the amount of caveolin-3 in buoyant (sucrose density) fractions of adult rat cardiac myocytes. Colch or CD treatment led to the exclusion of caveolin-1, caveolin-2, beta1-adrenergic receptors (beta1-AR), beta2-AR, Galpha(s), and adenylyl cyclase (AC)5/6 from buoyant fractions, decreasing AC5/6 and tyrosine-phosphorylated caveolin-1 in caveolin-1 immunoprecipitates but in parallel increased isoproterenol (beta-AR agonist)-stimulated cAMP production. Incubation with Colch decreased co-localization (by immunofluorescence microscopy) of caveolin-3 and alpha-tubulin; both Colch and CD decreased co-localization of caveolin-3 and filamin (an F-actin cross-linking protein), decreased phosphorylation of caveolin-1, Src, and p38 MAPK, and reduced the number of caveolae/mum of sarcolemma (determined by electron microscopy). Treatment of S49 T-lymphoma cells (which possess lipid rafts but lack caveolae) with CD or Colch redistributed a lipid raft marker (linker for activation of T cells (LAT)) and Galpha(s) from lipid raft domains. We conclude that microtubules and actin filaments restrict cAMP formation by regulating the localization and interaction of GPCR-G(s)-AC in lipid rafts/caveolae.

Focal adhesions in (myo)fibroblasts scaffold adenylyl cyclase with phosphorylated caveolin

Fibroblast-myofibroblast transformation, a critical event for enhanced extracellular matrix deposition, involves formation of an actin stress fiber contractile apparatus that radiates from focal adhesions (FA) in the plasma membrane. Activation of adenylyl cyclase (AC, i.e. increases in cAMP) negatively regulates such transformation. Caveolae and their resident protein caveolins scaffold signaling molecules, including AC isoforms, whereas phosphorylated caveolin-1 (phospho-cav-1) may localize at FA. Here, we used adult rat cardiac fibroblasts to examine distribution and expression of AC, phospho-cav-1, and FA proteins to define mechanisms that link increases in cAMP to caveolin-1 phosphorylation, actin/FA assembly, and fibroblast-myofibroblast transformation. Sucrose density gradient centrifugation, immunoblot, and immunohistochemical analysis revealed that, unlike cav-1, phospho-cav-1 enriches in membrane fractions that express FA proteins and localize at the ends of actin stress fibers. We detected AC in both cav-1 and phospho-cav-1 immunoprecipitates, but FA kinase (FAK), phospho-FAK (FAK Tyr-397), paxillin, and vinculin were detected only in phospho-cav-1 immunoprecipitates. Treatment with the AC activator forskolin or a cAMP analog increased cav-1 phosphorylation but decreased FAK Tyr-397 phosphorylation in a cAMP-dependent protein kinase-dependent manner. These events preceded actin cytoskeletal disruption, an effect that was blocked by small interfering RNA knock-down of cav-1. Inhibition of protein tyrosine phosphatase 1B abrogated cAMP-mediated disruption of actin cytoskeleton, cav-1 phosphorylation, and FAK Tyr-397 dephosphorylation. The data thus define a novel organization of signaling molecules that regulate fibroblasts: scaffolding of AC by phospho-cav-1 at FA sites in a caveolae-free microdomain along with components that mediate inhibition of actin/FA assembly and fibroblast-myofibroblast transformation via increases in cAMP.

Isoflurane produces sustained cardiac protection after ischemia-reperfusion injury in mice

BACKGROUND

Isoflurane reduces myocardial ischemia-reperfusion injury within hours to days of reperfusion. Whether isoflurane produces sustained cardiac protection has never been examined. The authors studied isoflurane-induced cardiac protection in the intact mouse after 2 h and 2 weeks of reperfusion and determined the dependence of this protection on adenosine triphosphate-dependent potassium channels and the relevance of this protection to myocardial function and apoptosis.

METHODS

Mice were randomly assigned to receive oxygen or isoflurane for 30 min with 15 min of washout. Some mice received mitochondrial (5-hydroxydecanoic acid) or sarcolemmal (HMR-1098) adenosine triphosphate-dependent potassium channel blockers with or without isoflurane. Mice were then subjected to a 30-min coronary artery occlusion followed by 2 h or 2 weeks of reperfusion. Infarct size was determined at 2 h and 2 weeks of reperfusion. Cardiac function and apoptosis were determined 2 weeks after reperfusion.

RESULTS

Isoflurane did not change hemodynamics. Isoflurane reduced infarct size after reperfusion when compared with the control groups (27.7 +/- 6.3 vs. 41.7 +/- 6.4% at 2 h and 19.6 +/- 5.9 vs. 28.8 +/- 9.0% at 2 weeks). Previous administration of 5-hydroxydecanoic acid, but not HMR-1098, abolished isoflurane-induced cardiac protection. At 2 weeks, left ventricular end-diastolic diameter was decreased significantly and end-systolic pressure and maximum and minimum dP/dt were improved by isoflurane. Isoflurane-treated mice subjected to ischemia and 2 weeks of reperfusion showed less expression of proapoptotic genes, significantly decreased expression of cleaved caspase-3, and significantly decreased deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling-positive nuclei compared with the control group.

CONCLUSIONS

Cardiac protection induced by isoflurane against necrotic and apoptotic cell death is associated with an acute memory period that is sustained and functionally relevant 2 weeks after ischemia-reperfusion injury in mice in vivo.

P2Y2 receptor polymorphisms and haplotypes in cystic fibrosis and their impact on Ca2+ influx

OBJECTIVE

Activation of P2Y2 receptors in airway epithelia by ATP and UTP stimulates a Ca2+-regulated Cl- channel, which regulates Cl- secretion in cystic fibrosis (CF). We hypothesized that genetic alterations in the P2Y2 receptor may act as disease modifiers in CF and thus analyzed the coding region of this gene for polymorphisms in 146 CF patients and 64 healthy controls. We also assessed the impact of the genetic variants on Ca2+-influx of P2Y2-null cells transfected with several P2Y2 receptor haplotypes.

RESULTS

We identified three frequent nonsynonymous P2Y2 receptor polymorphisms: Leu46Pro; Arg312Ser and Arg334Cys, of which only Arg312Ser was significantly more common in CF: Arg = 0.80, Ser = 0.20 (CF) vs. Arg = 0.72, Ser = 0.28 (controls), P < 0.05; for Leu46Pro, Leu = 0.92, Pro = 0.08 (CF) vs. Leu = 0.96, Pro = 0.04 (controls), P = 0.65 and for Arg334Cys, Arg = 0.79, Cys = 0.21 (CF) vs. Arg = 0.84, Cys = 0.16 (controls), P = 0.79. The most frequent haplotype was Leu46Leu/Arg312Arg/Arg334Arg (28% in CF, 31% in controls) but 6% of CF patients and none of the controls had Leu46Leu/Ser312Ser/Arg334Cys or Leu46Leu/Arg312Arg/Cys334Cys. To assess function of the receptor haplotypes, we stably transfected 1321N1 (P2Y-null) cells to similar levels of mRNA expression with Leu46Leu/Arg312Arg/Arg334Arg (wild-type), Leu46Leu/Ser312Ser/Arg334Arg and Leu46Leu/Arg312Arg/Cys334Cys and measured ATP-stimulated transient Ca2+-influx. Cells expressing the homozygous Cys334 variant had significantly increased Ca2+-influx compared to wild-type (P<0.01). The increase in Ca2+-influx was more pronounced in cells carrying the homozygous Ser312 variant than in cells with the other two genotypes (P<0.01).

CONCLUSIONS

These data indicate that P2Y2 receptor gene haplotypes influence intracellular Ca2+-release. Such genetic variants might therefore represent modifiers of Cl- secretion or of response to P2Y2 agonist therapy in CF.

Protection of adult rat cardiac myocytes from ischemic cell death: role of caveolar microdomains and delta-opioid receptors

The role of caveolae, membrane microenvironments enriched in signaling molecules, in myocardial ischemia is poorly defined. In the current study, we used cardiac myocytes prepared from adult rats to test the hypothesis that opioid receptors (OR), which are capable of producing cardiac protection in vivo, promote cardiac protection in cardiac myocytes in a caveolae-dependent manner. We determined protein expression and localization of delta-OR (DOR) using coimmunohistochemistry, caveolar fractionation, and immunoprecipitations. DOR colocalized in fractions with caveolin-3 (Cav-3), a structural component of caveolae in muscle cells, and could be immunoprecipitated by a Cav-3 antibody. Immunohistochemistry confirmed plasma membrane colocalization of DOR with Cav-3. Cardiac myocytes were subjected to simulated ischemia (2 h) or an ischemic preconditioning (IPC) protocol (10 min ischemia, 30 min recovery, 2 h ischemia) in the presence and absence of methyl-beta-cyclodextrin (MbetaCD, 2 mM), which binds cholesterol and disrupts caveolae. We also assessed the cardiac protective effects of SNC-121 (SNC), a selective DOR agonist, on cardiac myocytes with or without MbetaCD and MbetaCD preloaded with cholesterol. Ischemia, simulated by mineral oil layering to inhibit gas exchange, promoted cardiac myocyte cell death (trypan blue staining), a response blunted by SNC (37 +/- 3 vs. 59 +/- 3% dead cells in the presence and absence of 1 muM SNC, respectively, P < 0.01) or by use of the IPC protocol (35 +/- 4 vs. 62 +/- 3% dead cells, P < 0.01). MbetaCD treatment, which disrupted caveolae (as detected by electron microscopy), fully attenuated the protective effects of IPC or SNC, resulting in cell death comparable to that of the ischemic group. By contrast, SNC-induced protection was not abrogated in cells incubated with cholesterol-saturated MbetaCD, which maintained caveolae structure and function. These findings suggest a key role for caveolae, perhaps through enrichment of signaling molecules, in contributing to protection of cardiac myocytes from ischemic damage.

Caveolae and lipid rafts: G protein-coupled receptor signaling microdomains in cardiac myocytes

A growing body of data indicates that multiple signal transduction events in the heart occur via plasma membrane receptors located in signaling microdomains. Lipid rafts, enriched in cholesterol and sphingolipids, form one such microdomain along with a subset of lipid rafts, caveolae, enriched in the protein caveolin. In the heart, a key caveolin is caveolin-3, whose scaffolding domain is thought to serve as an anchor for other proteins. In spite of the original morphologic definition of caveolae ("little caves"), most work related to their role in compartmenting signal transduction molecules has involved subcellular fractionation or immunoprecipitation with anti-caveolin antibodies. Use of such approaches has documented that several G protein-coupled receptors (GPCR), and their cognate heterotrimeric G proteins and effectors, localize to lipid rafts/caveolae in neonatal cardiac myocytes. Our recent findings support the view that adult cardiac myocytes appear to have different patterns of localization of such components compared to neonatal myocytes and cardiac fibroblasts. Such results imply the existence of multiple subcellular microdomains for GPCR-mediated signal transduction in cardiac myocytes, in particular adult myocytes, and raise a major unanswered question: what are the precise mechanism(s) that determine co-localization of GPCR and post-receptor components with lipid rafts/caveolae in cardiac myocytes and other cell types?

Compartmentation of G-protein-coupled receptors and their signalling components in lipid rafts and caveolae

G-protein-coupled receptors (GPCRs) and post-GPCR signalling components are expressed at low overall abundance in plasma membranes, yet they evoke rapid, high-fidelity responses. Considerable evidence suggests that GPCR signalling components are organized together in membrane microdomains, in particular lipid rafts, enriched in cholesterol and sphingolipids, and caveolae, a subset of lipid rafts that also possess the protein caveolin, whose scaffolding domain may serve as an anchor for signalling components. Caveolae were originally identified based on their morphological appearance but their role in compartmentation of GPCR signalling has been primarily studied by biochemical techniques, such as subcellular fractionation and immunoprecipitation. Our recent studies obtained using both microscopic and biochemical methods with adult cardiac myocytes show expression of caveolin not only in surface sarcolemmal domains but also at, or close to, internal regions located at transverse tubules/sarcoplasmic reticulum. Other results show co-localization in lipid rafts/caveolae of AC (adenylyl cyclase), in particular AC6, certain GPCRs, G-proteins and eNOS (endothelial nitric oxide synthase; NOS3), which generates NO, a modulator of AC6. Existence of multiple caveolin-rich microdomains and their expression of multiple modulators of signalling strengthen the evidence that caveolins and lipid rafts/caveolae organize and regulate GPCR signal transduction in eukaryotic cells.

G-protein-coupled Receptor Signaling Components Localize in Both Sarcolemmal and Intracellular Caveolin-3-associated Microdomains in Adult Cardiac Myocytes

This study tests the hypothesis that G-protein-coupled receptor (GPCR) signaling components involved in the regulation of adenylyl cyclase (AC) localize with caveolin (Cav), a protein marker for caveolae, in both cell-surface and intracellular membrane regions. Using sucrose density fractionation of adult cardiac myocytes, we detected Cav-3 in both buoyant membrane fractions (BF) and heavy/non-buoyant fractions (HF); beta2-adrenergic receptors (AR) in BF; and AC5/6, beta1-AR, M4-muscarinic acetylcholine receptors (mAChR), mu-opioid receptors, and Galpha(s) in both BF and HF. In contrast, M2-mAChR, Galpha(i3), and Galpha(i2) were found only in HF. Immunofluorescence microscopy showed co-localization of Cav-3 with AC5/6, Galpha(s), beta2-AR, and mu-opioid receptors in both sarcolemmal and intracellular membranes, whereas M2-mAChR were detected only intracellularly. Immunofluorescence of adult heart revealed a distribution of Cav-3 identical to that in isolated adult cardiac myocytes. Upon immunoelectron microscopy, Cav-3 co-localized with AC5/6 and Galpha(s) in sarcolemmal and intracellular vesicles, the latter closely allied with T-tubules. Cav-3 immunoprecipitates possessed components that were necessary and sufficient for GPCR agonist-promoted stimulation and inhibition of cAMP formation. The distribution of GPCR, G-proteins, and AC with Cav-3 in both sarcolemmal and intracellular T-tubule-associated regions indicates the existence of multiple Cav-3-localized cellular microdomains for signaling by hormones and drugs in the heart.

Sarcolemmal KATP channel triggers delayed ischemic preconditioning in rats

Previous work from our laboratory has shown that the sarcolemmal K(ATP) channel (sK(ATP)) is required as a trigger for delayed cardioprotection upon exogenous opioid administration. We also established that the mitochondrial K(ATP) (mK(ATP)) channel is not required for triggering delayed delta-opioid-induced infarct size reduction. Because mechanistic differences have been found among delta-opioids and that due to ischemic preconditioning (IPC), we determined whether the triggering mechanism of delayed IPC-induced infarct size reduction involves either the sK(ATP) or mK(ATP). Male Sprague-Dawley rats received either sham surgery or IPC (3- to 5-min cycles of ischemia and reperfusion) 24 h before being subjected to 30 min of ischemia and 2 h of reperfusion. Infarct size was determined and expressed as a percentage of the area at risk, with significance compared with sham reported at P </= 0.001. A subset of both sham and IPC-treated rats received either the selective sK(ATP) channel antagonist, HMR-1098 (6 mg/kg), or the selective mK(ATP) channel antagonist, 5-hydroxydeconoic acid (5-HD; 10 mg/kg), given 5 min before IPC. Rats subjected to IPC demonstrated a significant reduction in infarct size compared with sham (29.2 +/- 4.7 vs. 59.3 +/- 2.5%, respectively; P </= 0.001). Prior administration of HMR-1098, but not 5-HD, abolished IPC-induced infarct size reduction (48.8 +/- 2.9 and 28.8 +/- 4.0%, respectively; P </= 0.001). Furthermore, administration of HMR 24 h after IPC, before index ischemia, did not abrogate IPC-induced infarct size reduction (33.0 +/- 5.0 vs. 29.2 +/- 4.7%, respectively; P </= 0.001). These data suggest that the sK(ATP) channel is required as a trigger but not a mediator for delayed IPC-induced infarct size reduction in rat hearts.

COX-2 and iNOS in opioid-induced delayed cardioprotection in the intact rat

Cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) have been previously implicated in the late phase of cardioprotection associated with opioid-induced and ischemic preconditioning (IPC) in conscious rabbits and COX-2 in isolated rat hearts pretreated with an exogenous delta opioid agonist. However, it is not know if both iNOS and COX-2 mediate the late phase of cardioprotection induced by opioids in the intact blood-perfused rat. Therefore, we investigated the role of COX-2 and iNOS in the delayed phase of protection mediated by delta opioid receptor activation. Rats were pretreated 24 hours prior to an occlusion/reperfusion protocol with the selective non-peptide delta opioid agonists, BW373U86 (BW) and SNC-121 (SNC). NS-398, a selective COX-2 inhibitor was administered after the 24-hour recovery period just prior to index ischemia. The selective iNOS inhibitors, S-methylthiourea (SMT) and aminoguanidine (AG), were administered in conjunction with opioid pretreatment or were also given 24 hours after opioid administration just prior to index ischemia. COX-2 inhibition by NS-398 given 24 hours after opioid administration attenuated the protective effects of both BW and SNC (46 +/- 6 vs. 13 +/- 3 and 51 +/- 5 vs. 29 +/- 2, p < 0.001, respectively). Similarly, inhibition of iNOS following 24 hours of treatment with opioids also attenuated the protective effects of BW and SNC. However, the delayed protective effects of the opioids were not attenuated by pretreatment with the iNOS inhibitors 24 hours prior to the infarct protocol. These results suggest that both COX-2 and iNOS are mediators of delayed protection induced by non-peptide delta opioid agonists. It appears that the trigger effect is not dependent on the activity of iNOS or COX-2 but the late phase of cardioprotection is dependent on the upregulation of these enzymes.

Delayed cardioprotection is mediated via a non-peptide ? opioid agonist, SNC-121, independent of opioid receptor stimulation

Acute cardioprotection is mediated primarily through delta opioid receptor stimulation independent of micro or kappa opioid receptor stimulation. Delayed cardioprotection is mediated by delta opioid receptor agonists but ambiguity remains about direct receptor involvement. Therefore, we investigated the potential of SNC-121, a non-peptide delta opioid agonist, to produce delayed cardioprotection and characterized the role of opioid receptors in this delayed response. All rats underwent 30 minutes of ischemia followed by 2 hours of reperfusion. SNC-121 induced a significant delayed cardioprotective effect. To determine the nature of this SNC-121-induced delayed cardioprotection, rats were treated with specific opioids receptor antagonists and underwent pertussis toxin (PT) treatment prior to opioid agonist stimulation. Control rats were injected with saline and allowed to recover for 24 hours. Pretreatment and early treatment with opioid receptor antagonists failed to inhibit the delayed protective effects of SNC-121, as did pretreatment with PT. Treatment with a free radical scavenger, 2-mercaptopropionyl glycine, at the time of opioid stimulation attenuated the delayed cardioprotective effects of SNC-121. These data suggest that delayed cardioprotection is stimulated via non-peptide delta opioid agonists by a mechanism unrelated to opioid receptor activation. The mechanism appears to be a non-opioid receptor mediated production of reactive oxygen species that triggers the signaling cascade leading to delayed cardioprotection.

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.

12-lipoxygenase in opioid-induced delayed cardioprotection: gene array, mass spectrometric, and pharmacological analyses

12-lipoxygenase (12-LO) has been shown to be a factor in acute ischemic preconditioning (IPC) in the isolated rat heart; however, no studies have been reported in delayed PC. We characterized the role of 12-LO in an intact rat model of delayed PC induced by a delta-opioid agonist SNC-121 (SNC). Rats were pretreated with SNC and allowed to recover for 24 hours. They were then treated with either baicalein or phenidone, 2 selective 12-LO inhibitors. In addition, SNC-pretreated rats had plasma samples isolated at different times after ischemia-reperfusion for liquid chromatographic-mass spectrometric analysis of the major metabolic product of 12-LO, 12-HETE. Similar studies were conducted with inhibitors. Gene array data showed a significant induction of 12-LO message (P<0.05) after opioid pretreatment. This induction in 12-LO mRNA was confirmed by real-time polymerase chain reaction, and 12-LO protein expression was enhanced by SNC pretreatment at 24 hours relative to vehicle treatment. Both baicalein and phenidone attenuated the protective effects of SNC pretreatment on infarct size (50+/-4% and 42+/-3% versus 29+/-2%, P<0.05, respectively). No significant differences were observed in 12-HETE concentrations between baseline control and SNC-treated rats. However, 12-HETE concentrations were increased significantly at both 15 minutes during ischemia and at 1 hour of reperfusion in the SNC-treated rats compared with controls. Baicalein and phenidone attenuated the increase in 12-HETE at 1 hour of reperfusion. These data suggest that SNC-121 appears to enhance message and subsequently the activity and expression of 12-LO protein during times of stress, resulting in delayed cardioprotection.

Morphine enhances pharmacological preconditioning by isoflurane: role of mitochondrial K(ATP) channels and opioid receptors

BACKGROUND

Adenosine triphosphate-regulated potassium channels mediate protection against myocardial infarction produced by volatile anesthetics and opioids. We tested the hypothesis that morphine enhances the protective effect of isoflurane by activating mitochondrial adenosine triphosphate-regulated potassium channels and opioid receptors.

METHODS

Barbiturate-anesthetized rats (n = 131) were instrumented for measurement of hemodynamics and subjected to a 30 min coronary artery occlusion followed by 2 h of reperfusion. Myocardial infarct size was determined using triphenyltetrazolium staining. Rats were randomly assigned to receive 0.9% saline, isoflurane (0.5 and 1.0 minimum alveolar concentration [MAC]), morphine (0.1 and 0.3 mg/kg), or morphine (0.3 mg/kg) plus isoflurane (1.0 MAC). Isoflurane was administered for 30 min and discontinued 15 min before coronary occlusion. In eight additional groups of experiments, rats received 5-hydroxydecanoic acid (5-HD; 10 mg/kg) or naloxone (6 mg/kg) in the presence or absence of isoflurane, morphine, and morphine plus isoflurane.

RESULTS

Isoflurane (1.0 MAC) and morphine (0.3 mg/kg) reduced infarct size (41 +/- 3%; n = 13 and 38 +/- 2% of the area at risk; n = 10, respectively) as compared to control experiments (59 +/- 2%; n = 10). Morphine plus isoflurane further decreased infarct size to 26 +/- 3% (n = 11). 5-HD and naloxone alone did not affect infarct size, but abolished cardioprotection produced by isoflurane, morphine, and morphine plus isoflurane.

CONCLUSIONS

Combined administration of isoflurane and morphine enhances the protection against myocardial infarction to a greater extent than either drug alone. This beneficial effect is mediated by mitochondrial adenosine triphosphate-regulated potassium channels and opioid receptors in vivo.

Cardioprotection at a distance: mesenteric artery occlusion protects the myocardium via an opioid sensitive mechanism

Preconditioning in remote organs protects the myocardium; however, mediators of the protection remain unknown. Protection of the heart is linked to opioids; therefore, we hypothesized that mesenteric preconditioning (MPC) releases endogenous opioids that protect the myocardium from ischemic injury. In an intact rat model of myocardial infarction, all rats underwent 30 min of coronary artery occlusion followed by 2 h of reperfusion. Prior to coronary artery occlusion, control rats were subjected to sham surgery in which the mesenteric artery was isolated but not occluded both with and without naloxone (10mg/kg) pretreatment. Experimental groups underwent isolation and occlusion of the mesenteric artery for 15 min followed by a 10 min reperfusion period with and without naloxone pretreatment. At the end of 2 h of coronary reperfusion, myocardial infarct size (IS) was determined by tetrazolium staining and expressed as a percent of the area at risk (AAR). Control rats had an IS/AAR of 57.3+/-2. MPC resulted in a significant reduction in infarct size compared to controls (32.2+/-3, P<0.001). Pretreatment with naloxone significantly attenuated the protective effects of MPC (53.8+/-4, P<0.0002). Therefore, it appears that MPC releases endogenous opioids that protect the myocardium from ischemic injury.

Delta opioid agonists and volatile anesthetics facilitate cardioprotection via potentiation of K(ATP) channel opening

Opioids and volatile anesthetics produce marked cardioprotective effects against myocardial infarction via the activation of ATP-sensitive potassium (K(ATP)) channels, however, the effect of combined treatment with both drugs is unknown. We examined the hypothesis that opioids and volatile anesthetics potentiate cardiac K(ATP) channel opening, thereby enhancing cardioprotection. Rats were treated with the delta opioid agonists, TAN-67 or BW373U86, or isoflurane, together or alone with and without diazoxide, a mitochondrial K(ATP) channel opener. Glibenclamide, a non-selective K(ATP) channel blocker, was used to further characterize the signaling mechanism involved. Myocardial infarct size (IS) was determined by tetrazolium staining and was expressed as a percent of the area at risk (AAR). High doses of TAN-67 (10 mg/kg), diazoxide (10 mg/kg), and isoflurane (1 MAC) produced a significant reduction in IS compared with the control group (30+/-3%, 36+/-5%, and 42+/-2 vs. 58+/-2%, respectively), whereas lower doses of the drugs had no effect except for the low dose of isoflurane (0.5 MAC). The combination of TAN-67 and diazoxide or isoflurane and diazoxide resulted in a marked reduction in IS compared with controls in the presence of high (9+/-3% and 14+/-3%) and low (17+/-7% and 31+/-7%) dose combinations, respectively. The combination of TAN-67 or BW373U86 and isoflurane also caused a striking reduction in IS/AAR (16+/-7% and 7+/-2%, respectively). To date, this is the first demonstration that opioids and volatile anesthetics work in conjunction to confer protection against myocardial infarction through potentiation of cardiac K(ATP) channel opening.

Sarcolemmal K(ATP) channel triggers opioid-induced delayed cardioprotection in the rat

Recently, the involvement of sarcolemmal K(ATP) (sarcK(ATP)) channels in ischemic and pharmacological preconditioning (IPC and PPC) has been minimized by numerous studies suggesting a primary role for mitochondrial K(ATP) (mitoK(ATP)) channels in early and delayed cardioprotection. Although the mitoK(ATP) channel has clearly been shown to be a distal effector of delayed IPC and PPC, studies implicating it as a trigger of protection in delayed IPC are lacking. Accordingly, we characterized the role of cardiac K(ATP) channels as triggers or distal effectors of delayed cardioprotection induced by opioids in rats, and the data suggest that the sarcK(ATP) channel triggers and that the mitoK(ATP) channel is a distal effector of opioid-induced delayed cardioprotection.

Attenuation of heat shock-induced cardioprotection by treatment with the opiate receptor antagonist naloxone

Whole body hyperthermia induces heat shock proteins (HSPs), which confer cardioprotection. Several opioid receptor subtypes are expressed in the heart and are linked to cardioprotection; however, no one has attempted to link the protection elicited by heat stress (HS) to opioids. Therefore, we investigated the effect of an opiate receptor antagonist, naloxone, on HS-induced cardioprotection. Anesthetized Sprague-Dawley rats were subjected to HS (42 degrees C for 20 min) with and without naloxone pretreatment and were allowed to recover for 48 h. They then underwent 30 min of ischemia followed by 2 h of reperfusion. An acute HS group was given an intravenous bolus of naloxone (3 mg/kg) 10 min before index ischemia. Infarct size (IS), expressed as a percentage of the area at risk (IS/AAR), was determined. The right heart was excised for analysis of HSP content by Western blot. Heat-shocked rats showed significant reductions in IS/AAR versus control (16 +/- 3 vs. 58 +/- 4%, P < 0.001). Pretreatment with naloxone before HS attenuated the protective effects in a dose-dependent fashion, with significant attenuation of protection occurring at 15 mg/kg naloxone versus heat shock (42 +/- 6 vs. 16 +/- 3%, P < 0.001). Acute treatment with naloxone (3 mg/kg) 48 h after recovery from HS also significantly attenuated the delayed protective effect (47 +/- 4 vs. 16 +/- 3%, P < 0.001). No difference was seen in the level of HSP70 induced in the different groups. We conclude that heat shock-induced cardioprotection can be attenuated by naloxone, an opiate receptor antagonist, without reducing the levels of certain HSPs. These results suggest there may be a link between the endogenous release of opioids and HS that mediates cardioprotection.