Ischemic preconditioning modulates mitochondrial respiration, irrespective of the employed signal transduction pathway

The Ambivalence of Connexin43 Gap Peptides in Cardioprotection of the Isolated Heart against Ischemic Injury

The present study investigates infarct-reducing effects of blocking ischemia-induced opening of connexin43 hemichannels using peptides Gap19, Gap26 or Gap27. Cardioprotection by ischemic preconditioning (IPC) and Gap peptides was compared, and combined treatment was tested in isolated, perfused male rat hearts using function and infarct size after global ischemia, high-resolution respirometry of isolated mitochondrial and peptide binding kinetics as endpoints. The Gap peptides reduced infarct size significantly when given prior to ischemia plus at reperfusion (Gap19 76.2 ± 2.7, Gap26 72.9 ± 5.8 and Gap27 71.9 ± 5.8% of untreated control infarcts, mean ± SEM). Cardioprotection was lost when Gap26, but not Gap27 or Gap19, was combined with triggering IPC (IPC 73.4 ± 5.5, Gap19-IPC 60.9 ± 5.1, Gap26-IPC 109.6 ± 7.8, Gap27-IPC 56.3 ± 8.0% of untreated control infarct). Binding stability of peptide Gap26 to its specific extracellular loop sequence (EL2) of connexin43 was stronger than Gap27 to its corresponding loop EL1 (dissociation rate constant K_d 0.061 ± 0.004 vs. 0.0043 ± 0.0001 s^-1, mean ± SD). Mitochondria from IPC hearts showed slightly but significantly reduced respiratory control ratio (RCR). In vitro addition of Gap peptides did not significantly alter respiration. If transient hemichannel activity is part of the IPC triggering event, inhibition of IPC triggering stimuli might limit the use of cardioprotective Gap peptides.

Cardioprotection induced by remote ischemic preconditioning preserves the mitochondrial respiratory function in acute diabetic myocardium

A 2×2 factorial design was used to evaluate possible preservation of mitochondrial functions in two cardioprotective experimental models, remote ischemic preconditioning and streptozotocin-induced diabetes mellitus, and their interaction during ischemia/reperfusion injury (I/R) of the heart. Male Wistar rats were randomly allocated into four groups: control (C), streptozotocin-induced diabetic (DM), preconditioned (RPC) and preconditioned streptozotocin-induced diabetic (DM+RPC). RPC was conducted by 3 cycles of 5-min hind-limb ischemia and 5-min reperfusion. DM was induced by a single dose of 65 mg/kg streptozotocin. Isolated hearts were exposed to ischemia/reperfusion test according to Langendorff. Thereafter mitochondria were isolated and the mitochondrial respiration was measured. Additionally, the ATP synthase activity measurements on the same preparations were done. Animals of all groups subjected to I/R exhibited a decreased state 3 respiration with the least change noted in DM+RPC group associated with no significant changes in state 2 respiration. In RPC, DM and DM+RPC group, no significant changes in the activity of ATP synthase were observed after I/R injury. These results suggest that the endogenous protective mechanisms of RPC and DM do preserve the mitochondrial function in heart when they act in combination.

Pre-ischaemic mitochondrial substrate constraint by inhibition of malate-aspartate shuttle preserves mitochondrial function after ischaemia-reperfusion

KEY POINTS

Pre-ischaemic administration of aminooxiacetate (AOA), an inhibitor of the malate-aspartate shuttle (MAS), provides cardioprotection against ischaemia-reperfusion injury. The underlying mechanism remains unknown. We examined whether transient inhibition of the MAS during ischaemia and early reperfusion by AOA treatment could prevent mitochondrial damage at later reperfusion. The AOA treatment preserved mitochondrial respiratory capacity with reduced mitochondrial oxidative stress during late reperfusion to the same extent as ischaemic preconditioning (IPC). However, AOA treatment, but not IPC, reduced the myocardial interstitial concentration of tricarboxylic acid cycle intermediates at the onset of reperfusion. The results obtained in the present study demonstrate that metabolic regulation by inhibition of the MAS at the onset of reperfusion may be beneficial for the preservation of mitochondrial function during late reperfusion in an IR-injured heart.

ABSTRACT

Mitochondrial dysfunction plays a central role in ischaemia-reperfusion (IR) injury. Pre-ischaemic administration of aminooxyacetate (AOA), an inhibitor of the malate-aspartate shuttle (MAS), provides cardioprotection against IR injury, although the underlying mechanism remains unknown. We hypothesized that a transient inhibition of the MAS during ischaemia and early reperfusion could preserve mitochondrial function at later phase of reperfusion in the IR-injured heart to the same extent as ischaemic preconditioning (IPC), which is a well-validated cardioprotective strategy against IR injury. In the present study, we show that pre-ischaemic administration of AOA preserved mitochondrial complex I-linked state 3 respiration and fatty acid oxidation during late reperfusion in IR-injured isolated rat hearts. AOA treatment also attenuated the excessive emission of mitochondrial reactive oxygen species during state 3 with complex I-linked substrates during late reperfusion, which was consistent with reduced oxidative damage in the IR-injured heart. As a result, AOA treatment reduced infarct size after reperfusion. These protective effects of MAS inhibition on the mitochondria were similar to those of IPC. Intriguingly, the protection of mitochondrial function by AOA treatment appears to be different from that of IPC because AOA treatment, but not IPC, downregulated myocardial tricarboxilic acid (TCA)-cycle intermediates at the onset of reperfusion. MAS inhibition thus preserved mitochondrial respiratory capacity and decreased mitochondrial oxidative stress during late reperfusion in the IR-injured heart, at least in part, via metabolic regulation of TCA cycle intermediates in the mitochondria at the onset of reperfusion.

Autophagy: an affair of the heart

Whether an element of routine housekeeping or in the setting of imminent disaster, it is a good idea to get one’s affairs in order. Autophagy, the process of recycling organelles and protein aggregates, is a basal homeostatic process and an evolutionarily conserved response to starvation and other forms of metabolic stress. Our understanding of the role of autophagy in the heart is changing rapidly as new information becomes available. This review examines the role of autophagy in the heart in the setting of cardioprotection, hypertrophy, and heart failure. Contradictory findings are reconciled in light of recent developments. The preponderance of evidence favors a beneficial role for autophagy in the heart under most conditions.

Ischemic preconditioning protects cardiomyocyte mitochondria through mechanisms independent of cytosol

Mitochondria play a central role in the protection conferred by ischemic preconditioning (IP) by not fully elucidated mechanisms. We investigated whether IP protects mitochondria against ischemia-reperfusion (IR) injury through mechanisms independent of cytosolic signaling. In isolated rat hearts, sublethal IR increased superoxide production and reduced complex-I- and II-mediated respiration in subsarcolemmal (SS), but not interfibrillar (IF) mitochondria. This effect of IR on mitochondrial respiration was significantly attenuated by IP. Similar results were obtained in isolated cardiac mitochondria subjected to in vitro IR. The reduction in SS mitochondrial respiration in the heart and in vitro model was paralleled by an increase in oxidized cysteine residues, which was also prevented by IP. IP was also protective in mitochondria submitted to lethal IR. The protective effect of IP against respiratory failure was unaffected by inhibition of mitochondrial KATP channels or mitochondrial permeability transition. However, IP protection was lost in mitochondria from genetically-modified animals in which connexin-43, a protein present in SS but not IF mitochondria, was replaced by connexin-32. Our results demonstrate the existence of a protective mitochondrial mechanism or "mitochondrial preconditioning" independent of cytosol that confers protection against IR-induced respiratory failure and oxidative damage, and requires connexin-43.

Altered expression of mitochondrial electron transport chain proteins and improved myocardial energetic state during late ischemic preconditioning

Altered expression of mitochondrial electron transport proteins has been shown in early preconditioned myocardial tissue. We wished to determine whether these alterations persist in the Second Window of Protection (SWOP) and if so, whether a favorable energetic state is facilitated during subsequent ischemia. Fourteen pigs underwent a SWOP protocol with ten 2-minute balloon inflations in the LAD artery, each separated by 2 minutes reperfusion. Twenty-four hours later, mitochondria were isolated from SWOP and SHAM pig hearts and analyzed for uncoupling protein (UCP)-2 content by western blot analysis, proteomic changes by iTRAQ(®) and respiration by an oxygen electrode. In parallel in vivo studies, high-energy nucleotides were obtained by transmural biopsy from anesthetized SWOP and SHAM pigs at baseline and during sustained low-flow ischemia. Compared with SHAM mitochondria, ex vivo SWOP heart tissue demonstrated increased expression of UCP-2, Complex IV (cytochrome c oxidase) and Complex V (ATPase) proteins. In comparison with SHAM pigs during in vivo conditions, transmural energetics in SWOP hearts, as estimated by the free energy of ATP hydrolysis (ΔG(0)), were similar at baseline but had decreased by the end of low-flow ischemia (-57.0 ± 2.1 versus -51.1 ± 1.4 kJ/mol; P < 0.05). In conclusion, within isolated mitochondria from preconditioned SWOP hearts, UCP-2 is increased and in concert with enhanced Complex IV and V proteins, imparts a favorable energetic state during low-flow ischemia. These data support the notion that mitochondrial adaptations that may reduce oxidant damage do not reduce the overall efficiency of energetics during sustained oxygen deprivation.

Preconditioning approach in stem cell therapy for the treatment of infarcted heart

Nearly two decades of research in regenerative medicine have been focused on the development of stem cells as a therapeutic option for treatment of the ischemic heart. Given the ability of stem cells to regenerate the damaged tissue, stem-cell-based therapy is an ideal approach for cardiovascular disorders. Preclinical studies in experimental animal models and clinical trials to determine the safety and efficacy of stem cell therapy have produced encouraging results that promise angiomyogenic repair of the ischemically damaged heart. Despite these promising results, stem cell therapy is still confronted with issues ranging from uncertainty about the as-yet-undetermined "ideal" donor cell type to the nonoptimized cell delivery strategies to harness optimal clinical benefits. Moreover, these lacunae have significantly hampered the progress of the heart cell therapy approach from bench to bedside for routine clinical applications. Massive death of donor cells in the infarcted myocardium during acute phase postengraftment is one of the areas of prime concern, which immensely lowers the efficacy of the procedure. An overview of the published data relevant to stem cell therapy is provided here and the various strategies that have been adopted to develop and optimize the protocols to enhance donor stem cell survival posttransplantation are discussed, with special focus on the preconditioning approach.

Sunitinib-Induced Systemic Vasoconstriction in Swine Is Endothelin Mediated and Does Not Involve Nitric Oxide or Oxidative Stress

Angiogenesis inhibition with agents targeting tyrosine kinases of vascular endothelial growth factor receptors is an established anticancer treatment, but is, unfortunately, frequently accompanied by systemic hypertension and cardiac toxicity. Whether vascular endothelial growth factor receptor antagonism also has adverse effects on the pulmonary and coronary circulations is presently unknown. In chronically instrumented awake swine, the effects of the vascular endothelial growth factor receptor antagonist sunitinib on the systemic, pulmonary, and coronary circulation were studied. One week after sunitinib (50 mg PO daily), mean aortic blood pressure (MABP) had increased from 83±5 mm Hg at baseline to 97±6 mm Hg ( P <0.05) because of a 57±20% increase in systemic vascular resistance as cardiac output decreased. In contrast, sunitinib had no discernible effects on pulmonary and coronary hemodynamics or cardiac function. We subsequently investigated the mechanisms underlying the sunitinib-induced systemic hypertension. Intravenous administration of NO synthase inhibitor N G -nitro- l -arginine increased MABP by 24±1 mm Hg under baseline conditions, whereas it increased MABP even further after sunitinib administration (32±3 mm Hg; P <0.05). Reactive oxygen species scavenging with a mixture of antioxidants lowered MABP by 13±2 mm Hg before but only by 5±2 mm Hg ( P <0.05) after sunitinib administration. However, intravenous administration of the dual endothelin A/endothelin B receptor blocker tezosentan, which did not lower MABP at baseline, completely reversed MABP to presunitinib values. These findings indicate that sunitinib produces vasoconstriction selectively in the systemic vascular bed, without affecting pulmonary or coronary circulations. The sunitinib-mediated systemic hypertension is principally attributed to an increased vasoconstrictor influence of endothelin, with no apparent contributions of a loss of NO bioavailability or increased oxidative stress.

Myocardial perfusion and contraction in acute ischemia and chronic ischemic heart disease

A large body of evidence has demonstrated that there is a close coupling between regional myocardial perfusion and contractile function. When ischemia is mild, this can result in the development of a new balance between supply and energy utilization that allows the heart to adapt for a period of hours over which myocardial viability can be maintained, a phenomenon known as "short-term hibernation". Upon reperfusion after reversible ischemia, regional myocardial function remains depressed. The "stunned myocardium" recovers spontaneously over a period of hours to days. The situation in myocardium subjected to chronic repetitive ischemia is more complex. Chronic dysfunction can initially reflect repetitive stunning with insufficient time for the heart to recover between episodes of spontaneous ischemia. As the frequency and/or severity of ischemia increases, the heart undergoes a series of adaptations which downregulate metabolism to maintain myocyte viability at the expense of contractile function. The resulting "hibernating myocardium" develops regional myocyte cellular hypertrophy as a compensatory response to ischemia-induced apoptosis along with a series of molecular adaptations that while regional, are similar to global changes found in advanced heart failure. As a result, flow-function relations become independently affected by tissue remodeling and interventions that stimulate myocyte regeneration. Similarly, chronic vascular remodeling may alter flow regulation in a fashion that increases myocardial vulnerability to ischemia. Here we review our current understanding of myocardial flow-function relations during acute ischemia in normal myocardium and highlight newly identified complexities in their interpretation in viable chronically dysfunctional myocardium with myocyte cellular and molecular remodeling. This article is part of a Special Issue entitled "Coronary Blood Flow".

Involvement of reperfusion injury salvage kinases in preconditioning depends critically on the preconditioning stimulus

Different preconditioning stimuli can activate divergent signaling pathways. In rats, adenosine-independent pathways (triple 3-min coronary artery occlusion [3CAO3]) and adenosine-dependent pathways (one 15-min coronary artery occlusion [ICAO15]) exist, both ultimately converging at the level of the mitochondrial respiratory chain. Furthermore, while 3CAO3, 1CAO15 and exogenous adenosine (ADO) are equally cardioprotective, only 1CAO15 increases interstitial myocardial adenosine levels. Reperfusion Injury Salvage Kinase (RISK) pathway kinases have been implicated in ischemic preconditioning, but not all preconditioning stimuli activate this pathway. Consequently, we evaluated in anesthetized rats the effects of three distinctly different preconditioning stimuli (3CAO3, 1CAO15 or ADO) on infarct size (IS), signaling pathways with a special emphasis on kinases belonging to the RISK pathway (phosphatidylinositol 3-kinase-Akt-nitric oxide synthase and extracellular signal-related kinase [ERK]) and mitochondrial respiration. All three stimuli increased state-2 respiration (using succinate as complex-II substrate), thereby decreasing the respiratory control index, which was accompanied by a limitation of IS produced by a 60-min coronary artery occlusion (CAO). Nitric oxide synthase inhibition abolished the mitochondrial effects and the cardioprotection by 3CAO3, 1CAO15 or ADO. In contrast, the PI3 kinase inhibitor, wortmannin, blocked protection by 1CAO15, but did not affect protection by 3CAO3 or ADO. Western blotting confirmed that phosphorylation of Akt and ERK were increased by 1CAO15 (which was inhibited by wortmannin), but not by 3CAO3 or ADO. In conclusion, while the three cardioprotective stimuli 3CAO3, 1CAO15 and ADO afford cardioprotection via nitric oxide-mediated modulation of mitochondrial respiration, only the 1CAO15 exerts its protection via activation of kinases belonging to the RISK pathway.

Ischemic preconditioning decreases mitochondrial proton leak and reactive oxygen species production in the postischemic heart

BACKGROUND

Proton leak (H(+) leak) dissipates mitochondrial membrane potential (mΔΨ) through the re-entry of protons into the mitochondrial matrix independent of ATP synthase. Changes in H(+) leak may affect reactive oxygen species (ROS) production. We measured H(+) leak and ROS production during ischemia-reperfusion and ischemic preconditioning (IPC) and examined how changing mitochondrial respiration affected mΔΨ and ROS production.

MATERIALS AND METHODS

Isolated rat hearts (n = 6/group) were subjected to either control-IR or IPC. Rate pressure product (RPP) was measured. Mitochondria were isolated at end reperfusion. Respiration was measured by polarography and titrated with increasing concentrations of malonate (0.5-2 mM). mΔΨ was measured using a tetraphenylphosphonium electrode. H(+) leak is the respiratory rate required to maintain membrane potential at -150 mV in the presence of oligomycin-A. Mitochondrial complex III ROS production was measured by fluorometry using Amplex-red.

RESULTS

IPC improved recovery of RPP at end reperfusion (63% ± 4% versus 21% ± 2% in control-IR, P < 0.05). Ischemia-reperfusion caused increased H(+) leak (94 ± 12 versus 31 ± 1 nmol O/mg protein/min in non-ischemic control, P < 0.05). IPC attenuates these increases (55 ± 9 nmol O/mg protein/min, P < 0.05 versus control-IR). IPC reduced mitochondrial ROS production compared with control-IR (31 ± 2 versus 40 ± 3 nmol/mg protein/min, P < 0.05). As mitochondrial respiration decreased, mΔΨ and mitochondrial ROS production also decreased. ROS production remained lower in IPC than in control-IR for all mΔΨ and respiration rates.

CONCLUSIONS

Increasing H(+) leak is not associated with decreased ROS production. IPC decreases both the magnitude of H(+) leak and ROS production after ischemia-reperfusion.

Ischaemic and morphine-induced post-conditioning: impact of mK(Ca) channels

BACKGROUND

Mitochondrial calcium-sensitive potassium (mK(Ca)) channels are involved in cardiac preconditioning. In the present study, we investigated whether also ischaemic-, morphine-induced post-conditioning, or both is mediated by the activation of mK(Ca) channels in the rat heart in vitro.

METHODS

Animals were treated in compliance with institutional and national guidelines. Male Wistar rats were randomly assigned to one of seven groups (each n = 7). Control animals were not further treated. Post-conditioning was induced either by 3 × 30 s of ischaemia/reperfusion (I-PostC) or by administration of morphine (M-PostC, 1 µM) for 15 min at the onset of reperfusion. The mK(Ca)-channel inhibitor paxilline (1 µM) was given with and without post-conditioning interventions (M-PostC+Pax, I-PostC+Pax, and Pax). As a positive control, we determined whether direct activation of mK(Ca) channels with NS1619 (10 µM) induced cardiac post-conditioning (NS1619). Isolated hearts underwent 35 min ischaemia followed by 120 min reperfusion. At the end of reperfusion, infarct sizes were measured by triphenyltetrazolium chloride staining.

RESULTS

In the control group, infarct size was 53 (5)% of the area at risk. Morphine- and ischaemic post-conditioning reduced infarct size in the same range [M-PostC: 37 (4)%, I-PostC: 35 (5)%; each P<0.05 vs control]. The mK(Ca)-channel inhibitor paxilline completely blocked post-conditioning [M-PostC+Pax: 47 (7)%, I-PostC+Pax: 51 (3)%; each P<0.05 vs M-PostC and I-PostC, respectively]. Paxilline itself had no effect on infarct size (NS vs control). NS1619 reduced infarct size to 33 (4)% (P < 0.05 vs control).

CONCLUSIONS

Ischaemic- and morphine-induced post-conditioning is mediated by the activation of mK(Ca) channels.

Morphine induces preconditioning via activation of mitochondrial K(Ca) channels

Purpose

Mitochondrial calcium sensitive potassium (mK_Ca) channels are involved in cardioprotection induced by ischemic preconditioning. In the present study we investigated whether morphine-induced preconditioning also involves activation of mK_Ca channels.

Methods

Isolated rat hearts (six groups; each n = 8) underwent global ischemia for 30 min followed by a 60-min reperfusion. Control animals were not further treated. Morphine preconditioning (MPC) was initiated by two five-minute cycles of morphine 1 μM infusion with one five-minute washout and one final ten-minute washout period before ischemia. The mK_Ca blocker, paxilline 1 μM, was administered, with and without morphine administration (MPC + Pax and Pax). As a positive control, we added an ischemic preconditioning group (IPC) alone and combined with paxilline (IPC + Pax). At the end of reperfusion, infarct sizes were determined by triphenyltetrazoliumchloride staining.

Results

Infarct size was (mean ± SD) 45 ± 9% of the area at risk in the Control group. The infarct size was less in the morphine or ischemic preconditioning groups (MPC: 23 ± 8%, IPC: 20 ± 5%; each P < 0.05 vs Control). Infarct size reduction was abolished by paxilline (MPC + Pax: 37 ± 7%, P < 0.05 vs MPC and IPC + Pax: 36 ± 6%, P < 0.05 vs IPC), whereas paxilline alone had no effect (Pax: 46 ± 7%, not significantly different from Control).

Conclusion

Cardioprotection by morphine-induced preconditioning is mediated by activation of mK_Ca channels.

Parstatin(1-26): the putative signal peptide of protease-activated receptor 1 confers potent protection from myocardial ischemia-reperfusion injury

Parstatin, the N-terminal 41-amino-acid peptide cleaved by thrombin from the protease-activated receptor 1, protects against rat myocardial ischemia and reperfusion injury. In this study, we determined that the parstatin fragment 1-26, the putative signal peptide of protease-activated receptor 1, contains the functional domain of parstatin. We assessed a synthesized parstatin(1-26) peptide in an in vivo rat model of myocardial regional ischemia-reperfusion injury (n = 6/group). Infarct size in control rat hearts was 58 +/- 1% area at risk. Parstatin(1-26) was able to reduce infarct size to 13 +/- 1% (P < 0.001) and 22 +/- 1% area at risk (P < 0.01) when given before or after reperfusion. The infarct-sparing effects of parstatin(1-26) were abolished by inhibition of G(i) proteins (pertussis toxin), phosphoinositide 3-kinase/Akt (wortmannin), nitric-oxide synthase (NOS; N(G)-monomethyl-l-arginine), soluble guanylyl cyclase [1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ)], and sarcolemmal and mitochondrial K(ATP) channels [glibenclamide, 5-hydroxydecanoic acid, and sodium (5-(2-(5-chloro-2-methoxybenzamido)ethyl)-2-methoxyphenylsulfonyl) (methylcarbamothioyl)amide (HMR 1098)]. Parstatin(1-26) cardioprotection was also abolished by atractyloside, a mitochondrial permeability transition pore (mPTP) opener. The inhibitors and opener alone had no effect on infarct size. Furthermore, preischemic treatment with parstatin(1-26) increased Akt and endothelial NOS phosphorylation at the time of reperfusion. After a 120-min reperfusion, parstatin(1-26) increased nitric oxide levels (12 +/- 0.4 to 17 +/- 0.9 mmol/g tissue) and cyclic GMP levels (87 +/- 21 to 395 +/- 36 pmol/g tissue). Parstatin(1-26) treatment either before or after ischemia results in an extremely efficacious protection against ischemia-reperfusion injury that depends on a G(i) protein-mediated pathway involving mPTP, the end effector of the preconditioning pathway. This suggests that parstatin(1-26) has a potential therapeutic role in the treatment of ischemia and reperfusion injury.

Both ischemic preconditioning and ghrelin administration protect hippocampus from ischemia/reperfusion and upregulate uncoupling protein-2

BACKGROUND

A major endogenous protective mechanism in many organs against ischemia/reperfusion (I/R) injury is ischemic preconditioning (IPC). By moderately uncoupling the mitochondrial respiratory chain and decreasing production of reactive oxygen species (ROS), IPC reduces apoptosis induced by I/R by reducing cytochrome c release from the mitochondria. One element believed to contribute to reduce ROS production is the uncoupling protein UCP2 (and UCP3 in the heart). Although its implication in IPC in the brain has been shown in vitro, no in vivo study of protein has shown its upregulation. Our first goal was to determine in rat hippocampus whether UCP2 protein upregulation was associated with IPC-induced protection and increased ROS production. The second goal was to determine whether the peptide ghrelin, which possesses anti-oxidant and protective properties, alters UCP2 mRNA levels in the same way as IPC during protection.

RESULTS

After global forebrain ischemia (15 min) with 72 h reperfusion (I/R group), we found important neuronal lesion in the rat hippocampal CA1 region, which was reduced by a preceding 3-min preconditioning ischemia (IPC+I/R group), whereas the preconditioning stimulus alone (IPC group) had no effect. Compared to control, UCP2 protein labelling increased moderately in the I/R (+39%, NS) and IPC+I/R (+28%, NS) groups, and substantially in the IPC group (+339%, P < 0.05). Treatment with superoxide dismutase (10000 U/kg ip) at the time of a preconditioning ischemia greatly attenuated (-73%, P < 0.001) the increase in UCP2 staining at 72 h, implying a role of oxygen radicals in UCP2 induction.Hippocampal UCP2 mRNA showed a moderate increase in I/R (+33%, P < 0.05) and IPC+I/R (+40%, P < 0.05) groups versus control, and a large increase in the IPC group (+333%, P < 0.001). In ghrelin experiments, the I/R+ghrelin group (3 daily administrations) showed considerable protection of CA1 neurons versus I/R animals, and increased hippocampal UCP2 mRNA (+151%, P < 0.001).

CONCLUSION

We confirm that IPC causes increased expression of UCP2 protein in vivo, at a moment appropriate for protection against I/R in the hippocampus. The two dissimilar protective strategies, IPC and ghrelin administration, were both associated with upregulated UCP2, suggesting that UCP2 may often represent a final common pathway in protection from I/R.

Interaction between pre- and postconditioning in the in vivo rat heart

Patients with an impending myocardial infarction may be preconditioned by pre-infarct angina. Hence, it is important to establish whether ischemic postconditioning is still effective in preconditioned hearts. We therefore studied in anesthetized rats the effect of postconditioning after coronary artery occlusions (CAO) of 60 min in control hearts, hearts preconditioned by a single 15-min CAO (1IPC15) or a triple 3-min CAO (3IPC3). Furthermore, we studied the effect of postconditioning in hearts that had been pharmacologically preconditioned with intravenous adenosine and in hearts that had become tolerant to 1IPC15. Postconditioning limited infarct size in control hearts, but did not afford additional protection in preconditioned hearts, irrespective of the IPC stimulus. NO synthase inhibition abolished the cardioprotection by postconditioning, both IPC stimuli, and the combination of postconditioning and either IPC stimulus. Postconditioning also failed to afford cardioprotection in hearts protected by adenosine, and in hearts that had become tolerant to cardioprotection by 1IPC15. In accordance with previous observations, postconditioning paradoxically increased infarct size following a 30-min CAO. This detrimental effect was prevented by either IPC stimulus, in a NO synthase-dependent manner. In conclusion, postconditioning does not afford additional protection in preconditioned hearts, irrespective of the preconditioning stimulus and the presence of tolerance to preconditioning. Lack of additional protection may be related to the observation that postconditioning and preconditioning are both mediated via NO synthase. In contrast, the increase in infarct size by postconditioning following a 30-min CAO is abolished by either IPC stimulus. These findings indicate that the interaction between preconditioning and postconditioning is highly dependent on the duration of index ischemia, but independent of the preconditioning stimulus.

Adenosine receptors and reperfusion injury of the heart

Adenosine, a catabolite of ATP, exerts numerous effects in the heart, including modulation of the cardiac response to stress, such as that which occurs during myocardial ischemia and reperfusion. Over the past 20 years, substantial evidence has accumulated that adenosine, administered either prior to ischemia or during reperfusion, reduces both reversible and irreversible myocardial injury. The latter effect results in a reduction of both necrosis or myocardial infarction (MI) and apoptosis. These effects appear to be mediated via the activation of one or more G-protein-coupled receptors (GPCRs), referred to as A(1), A(2A), A(2B) and A(3) adenosine receptor (AR) subtypes. Experimental studies in different species and models suggest that activation of the A(1) or A(3)ARs prior to ischemia is cardioprotective. Further experimental studies reveal that the administration of A(2A)AR agonists during reperfusion can also reduce MI, and recent reports suggest that A(2B)ARs may also play an important role in modulating myocardial reperfusion injury. Despite convincing experimental evidence for AR-mediated cardioprotection, there have been only a limited number of clinical trials examining the beneficial effects of adenosine or adenosine-based therapeutics in humans, and the results of these studies have been equivocal. This review summarizes our current knowledge of AR-mediated cardioprotection, and the roles of the four known ARs in experimental models of ischemia-reperfusion. The chapter concludes with an examination of the clinical trials to date assessing the safety and efficacy of adenosine as a cardioprotective agent during coronary thrombolysis in humans.