Possibilities for the pharmacological exploitation of ischaemic preconditioning

Na/K-ATPase Signaling and Cardiac Pre/Postconditioning with Cardiotonic Steroids

The first reports of cardiac Na/K-ATPase signaling, published 20 years ago, have opened several major fields of investigations into the cardioprotective action of low/subinotropic concentrations of cardiotonic steroids (CTS). This review focuses on the protective cardiac Na/K-ATPase-mediated signaling triggered by low concentrations of ouabain and other CTS, in the context of the enduring debate over the use of CTS in the ischemic heart. Indeed, as basic and clinical research continues to support effectiveness and feasibility of conditioning interventions against ischemia/reperfusion injury in acute myocardial infarction (AMI), the mechanistic information available to date suggests that unique features of CTS-based conditioning could be highly suitable, alone /or as a combinatory approach.

Adenosine and chronic ischemia of the lower limbs

Adenosine is an endogenous nucleoside with multiple biological properties which plays a central role in the pathophysiology of tissue ischemia. Adenosine signals an imbalance between oxygen demand and supply, and it initiates responses to redress such a discrepancy. Besides its vasodilating properties, adenosine possesses anti-platelet and anti-neutrophil activities and provides cytoprotection. Adenosine is presumably the main mediator of the preconditioning phenomenon. During ischemia of the lower limbs, adenosine plays a physiological role by inducing vasodilatation and by preventing microcirculatory failure. Exercise training prolongs claudication distance possibly by inducing pulse increases of adenosine and consequently skeletal muscle preconditioning. Moreover, the adenosine increase which follows the administration of some drugs, such as buflomedil and propionylcarnitine, opens new perspectives in the management of leg ischemia. In fact, the concept arises of an ischemic (exercise-dependent) or pharmacologic preconditioning in the treatment of patients with claudication.

Ischemic and postischemic conditioning of the myocardium in clinical practice: challenges, expectations and obstacles

Conditioning refers to endogenous mechanisms rendering the myocardium more tolerant against reperfusion injury. Application of brief ischemia-reperfusion cycles prior to the index ischemia has a beneficial effect and limits the infarct size. This is called preconditioning and is mainly mediated by activation of adenosine, bradykinin, opioid and other receptors, with subsequent activation of intracellular mediators leading to mitochondrial protection. A clinical equivalent of preconditioning is preinfarction angina. Benefits for the ischemic and reperfused myocardium are also provided by repetitive short-lived cycles of ischemia-reperfusion applied after the index ischemia. This is termed postconditioning, shares a common pathway with preconditioning, and is more useful and relevant in clinical practice. Finally, benefits are also derived from remote conditioning, i.e. ischemia applied in a remote vascular territory parallel with or immediately after the index myocardial ischemia. Several pharmacological interventions may interfere with these mechanisms leading to enhanced protection of the myocardium and limitation of the infarct size. Despite the huge interest and the great body of evidence that verify the effectiveness of conditioning, clinical application has remained limited due to controversies over the appropriate intervention protocol, but also interference of medication, comorbidities and other factors that may enhance or blur the protective effect.

Tolerance of the developing cyanotic heart to ischemia-reperfusion injury in the rat

OBJECTIVE

Whether chronic hypoxia attenuates myocardial ischemia-reperfusion injury remains controversial because conflicting data have been reported probably due to the existence of many factors influencing the functional recovery of hearts. These factors include the differences of species, the time at which hypoxia begins, the degree of hypoxia, and so on. Regarding chronic hypoxia from birth, so far the only available data are based on findings in rabbit hearts. The purpose of this study was to describe the effect of chronic hypoxia from birth on myocardial reperfusion injury in the rat heart.

METHODS

Normoxic hearts were obtained from rats housed in ambient air for 6 weeks (normoxic group); hypoxic hearts were obtained from rats housed in a hypoxic chamber (13%-14% oxygen) from birth for 6 weeks (hypoxic group). Isolated, crystalloid perfused working hearts were subjected to 30 min of global normothermic ischemia followed by 15 min of reperfusion; functional recovery was then measured in the two groups. The excretion of cyclic guanosine monophosphate (cGMP) in the coronary drainage was measured at the end of the preischemia and reperfusion periods.

RESULTS

The percent recovery of the left ventricular developed pressure and the first derivative of left ventricular pressure were significantly better in the hypoxic group than in the normoxic group. cGMP excretion in the coronary drainage was significantly increased during both the preischemia and reperfusion periods.

CONCLUSION

Chronic hypoxia from birth increased myocardial tolerance to ischemia-reperfusion injury with increased cGMP synthesis in the isolated heart model in rats.

Hypoxia tolerance and preconditioning are not additive in the trout(Oncorhynchus mykiss) heart

Research has shown that the trout heart is normally hypoxia-sensitive, and that it can be preconditioned. However, we have identified a group of rainbow trout Oncorhynchus mykiss that shows a surprising degree of myocardial hypoxia tolerance. In this study, we used in situ hearts from these fish as a comparative model to examine whether the cardioprotective effects afforded by hypoxic adaptation and preconditioning are additive. In situ trout hearts were exposed to severe hypoxia (perfusate PO2 5–10 mmHg) in the absence and presence of a transient hypoxic pre-exposure (preconditioning). The four groups studied were: (1) control (no hypoxia); (2) 5 min of severe hypoxia;(3) 30 min of severe hypoxia; and (4) 5 min of severe hypoxia (hypoxic preconditioning) followed 20 min later by 30 min of severe hypoxia. 30 min of severe hypoxia significantly decreased maximum cardiac output and stroke volume by 15–30%. However, hypoxic preconditioning failed to confer any protection against post-hypoxic myocardial dysfunction. This work shows that the protection afforded by inherent myocardial hypoxia tolerance and preconditioning are not additive in this population of trout, and strongly suggests that the relationship between hypoxic adaptation and preconditioning in fishes resembles that of the neonatal/immature, not adult, mammalian heart. Further, our results (1) indicate that stretch (volume loading) and chronic exposure to low levels of adrenaline (15 nmol l-1) do not confer any protection against hypoxia-related myocardial dysfunction in this population, and (2) validate the use of the in situ trout heart as a comparative model for studying aspects of myocardial hypoxia tolerance and preconditioning in vertebrates.

Expression and localization of the multidrug resistance protein 5 (MRP5/ABCC5), a cellular export pump for cyclic nucleotides, in human heart

The multidrug resistance protein 5 (MRP5/ABCC5) has been recently identified as cellular export pump for cyclic nucleotides with 3',5'-cyclic GMP (cGMP) as a high-affinity substrate. In view of the important role of cGMP for cardiovascular function, expression of this transport protein in human heart is of relevance. We analyzed the expression and localization of MRP5 in human heart [21 auricular (AS) and 15 left ventricular samples (LV) including 5 samples of dilated and ischemic cardiomyopathy]. Quantitative real-time polymerase chain reaction normalized to beta-actin revealed expression of the MRP5 gene in all samples (LV, 38.5 +/- 12.9; AS, 12.7 +/- 5.6; P < 0.001). An MRP5-specific polyclonal antibody detected a glycoprotein of approximately 190 kd in crude cell membrane fractions from these samples. Immunohistochemistry with the affinity-purified antibody revealed localization of MRP5 in cardiomyocytes as well as in cardiovascular endothelial and smooth muscle cells. Furthermore, we could detect MRP5 and ATP-dependent transport of [(3)H]cGMP in sarcolemma vesicles of human heart. Quantitative analysis of the immunoblots indicated an interindividual variability with a higher expression of MRP5 in the ischemic (104 +/- 38% of recombinant MRP5 standard) compared to normal ventricular samples (53 +/- 36%, P < 0.05). In addition, we screened genomic DNA from our samples for 20 single-nucleotide polymorphisms in the MRP5 gene. These results indicate that MRP5 is localized in cardiac and cardiovascular myocytes as well as endothelial cells with increased expression in ischemic cardiomyopathy. Therefore, MRP5-mediated cellular export may represent a novel, disease-dependent pathway for cGMP removal from cardiac cells.

ATP-sensitive K(+) channel activation by nitric oxide and protein kinase G in rabbit ventricular myocytes

The present investigation tested the hypothesis that nitric oxide (NO) potentiates ATP-sensitive K(+) (K(ATP)) channels by protein kinase G (PKG)-dependent phosphorylation in rabbit ventricular myocytes with the use of patch-clamp techniques. Sodium nitroprusside (SNP; 1 mM) potentiated K(ATP) channel activity in cell-attached patches but failed to enhance the channel activity in either inside-out or outside-out patches. The 8-(4-chlorophenylthio)-cGMP Rp isomer (Rp-CPT-cGMP, 100 microM) suppressed the potentiating effect of SNP. 8-(4-Chlorophenylthio)-cGMP (8-pCPT-cGMP, 100 microM) increased K(ATP) channel activity in cell-attached patches. PKG (5 U/microl) added together with ATP and cGMP (100 microM each) directly to the intracellular surface increased the channel activity. Activation of K(ATP) channels was abolished by the replacement of ATP with ATPgammaS. Rp-pCPT-cGMP (100 microM) inhibited the effect of PKG. The heat-inactivated PKG had little effect on the K(ATP) channels. Protein phosphatase 2A (PP2A, 1 U/ml) reversed the PKG-mediated K(ATP) channel activation. With the use of 5 nM okadaic acid (a PP2A inhibitor), PP2A had no effect on the channel activity. These results suggest that the NO-cGMP-PKG pathway contributes to phosphorylation of K(ATP) channels in rabbit ventricular myocytes.

Hyperlipidemia induced by high cholesterol diet inhibits heat shock response in rat hearts

We examined whether heat shock response is affected by experimental hyperlipidemia in rat hearts. Therefore, isolated hearts of male Wistar rats fed a 2% cholesterol-enriched diet or standard diet for 12 weeks were subjected to either 20 min heat stress at 42 degrees C or global normothermic ischemia followed by 120 min normothermic, normoxic perfusion. Both heat stress and ischemia resulted in a significant increase in cardiac mRNA and protein levels of the inducible member of the 70-kDa heat shock protein family (HSP70) when compared to time-matched controls as assessed by reverse transcriptase polymerase chain reaction and Western blotting in hearts of normal rats. However, in hyperlipidemic groups, increase in cardiac hsp70 mRNA and HSP70 protein in response to heat stress and ischemia was markedly attenuated. We further observed that the basal level of hsp70 mRNA was significantly higher in the hyperlipidemic group when compared to normal controls; however, the HSP70 protein level was not different. This is the first demonstration that hyperlipidemia inhibits cardiac heat shock response. We further conclude that basal HSP70 expression might be downregulated at a posttranscriptional level in hyperlipidemia.

Clinical relevance of ischemic preconditioning

The mechanisms of ischemic cell death and reperfusion injury in the myocardium and the ways to limit these have been under extensive research for decades. The discovery of the phenomenon of ischemic preconditioning, i.e. endogenous protection against ischemia-reperfusion injury obtained by one or more brief preceding episodes of ischemia, really boosted this research 15 years ago. Even though extensive research in experimental animals has provided data on the cellular mechanisms of ischemic preconditioning, such as adenosine receptor activation, opening of mitochondrial adenosine triphosphate (ATP)-sensitive potassium channels and production of endogenous protective stress proteins, direct clinical applications are still missing. The purpose of this study is to summarize the latest progress in solving the cellular and molecular mechanisms of the phenomenon, as well as the evidence for the existence of this phenomenon in humans and its clinical relevance.

Protective effects of nitroglycerin-induced preconditioning mediated by calcitonin gene-related peptide in rat small intestine

Previous studies of myocardium have shown that ischemic preconditioning could be mimicked by nitroglycerin through stimulating the release of calcitonin gene-related peptide (CGRP). The present study examined whether nitroglycerin could also provide a preconditioning stimulus in the peripheral vascular bed (the anse intestinalis of rat), and whether endogenous CGRP is involved in this process. The model of in situ perfusion was prepared with rat small intestine. One hour of ischemia and 15 min of reperfusion caused a significant impairment of intestinal morphology and an increase in the release of both lactate dehydrogenase and malondialdehyde. Pretreatment with nitroglycerin, 10(-7), 3 x 10(-7), 10(-6) M for 5 min produced a significant improvement of intestinal tissue morphology and a decrease in the release of both lactate dehydrogenase and malondialdehyde. However, the protection afforded by nitroglycerin was abolished by CGRP-(8-37), a selective CGRP acceptor antagonist. Pretreatment with capsaicin, which specifically depletes the transmitter content of sensory nerves, also abolished the protection by nitroglycerin. In addition, the content of CGRP-like immunoreactivity in the effluent was increased during nitroglycerin perfusion. On the other hand, the results from the in vivo experiment showed that nitroglycerin (i.v. 0.13 mg/kg) injected 5 min before prolonged ischemia could provide significant protection against the injury caused by 30-min ischemia and 1-h reperfusion in the rat small intestine, but would also cause a significant increase in the levels of CGRP in the plasma. All these findings suggest that nitroglycerin-induced preconditioning is related to stimulation of CGRP release in the rat small intestine.

Modulation of ATP-sensitive potassium channels by cGMP-dependent protein kinase in rabbit ventricular myocytes

This investigation used a patch clamp technique to test the hypothesis that protein kinase G (PKG) contributes to the phosphorylation and activation of ATP-sensitive K(+) (K(ATP)) channels in rabbit ventricular myocytes. Nitric oxide donors and PKG activators facilitated pinacidil-induced K(ATP) channel activities in a concentration-dependent manner, and a selective PKG inhibitor abrogated these effects. In contrast, neither a selective protein kinase A (PKA) activator nor inhibitor had any effect on K(ATP) channels at concentrations up to 100 and 10 microm, respectively. Exogenous PKG, in the presence of both cGMP and ATP, increased channel activity, while the catalytic subunit of PKA had no effect. PKG activity was prevented by heat inactivation, replacing ATP with adenosine 5'-O-(thiotriphosphate) (a nonhydrolyzable analog of ATP), removing Mg(2+) from the internal solution, applying a PKG inhibitor, or by adding exogenous protein phosphatase 2A. The effects of cGMP analogs and PKG were observed under conditions in which PKA was repressed by a selective PKA inhibitor. The results suggest that K(ATP) channels are regulated by a PKG-signaling pathway that acts via PKG-dependent phosphorylation. This mechanism may, at least in part, contribute to a signaling pathway that induces ischemic preconditioning in rabbit ventricular myocytes.

Does nitric oxide generation contribute to the mechanism of remote ischemic preconditioning?

The protective effect of local or remote ischemic preconditioning (IPC) on subsequent 40-min ischemic and 120-min reperfusion myocardial damage was investigated. Preconditioned rats underwent one cycle of myocardial ischemia/reperfusion consisting of 5-min ischemia produced as a left coronary artery (LCA) occlusion and 5 min of reperfusion. Remote IPC was produced as 15 min of small intestinal ischemia with 15 min of reperfusion as well as 30 min of limb ischemia with 15 min of reperfusion. A marked protective action was afforded by both IPC protocols with a more significant effect of local (classic) ischemic preconditioning. Since the protective effect of remote IPC was not abolished by nitric oxide (NO) synthase inhibition with Nomega-nitro-L-arginine (L-NNA) it is concluded that NO generation may not be involved in the mechanism of remote IPC.

Nitric oxide-induced cardioprotection in cultured rat ventricular myocytes

The aim of this study was to investigate the role of nitric oxide (NO) in a cellular model of early preconditioning (PC) in cultured neonatal rat ventricular myocytes. Cardiomyocytes "preconditioned" with 90 min of stimulated ischemia (SI) followed by 30 min reoxygenation in normal culture conditions were protected against subsequent 6 h of SI. PC was blocked by N(G)-monomethyl-L-arginine monoacetate but not by dexamethasone pretreatment. Inducible nitric oxide synthase (NOS) protein expression was not detected during PC ischemia. Pretreatment (90 min) with the NO donor S-nitroso-N-acetyl-L,L-penicillamine (SNAP) mimicked PC, resulting in significant protection. SNAP-triggered protection was completely abolished by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) but was unaffected by chelerythrine or the presence of glibenclamide and 5-hydroxydecanoate. With the use of RIA, SNAP treatment increased cGMP levels, which were blocked by ODQ. Hence, NO is implicated as a trigger in this model of early PC via activation of a constitutive NOS isoform. After exposure to SNAP, the mechanism of cardioprotection is cGMP dependent but independent of protein kinase C or ATP-sensitive K(+) channels. This differs from the proposed mechanism of NO-induced cardioprotection in late PC.

Glycolipid RC-552 induces delayed preconditioning-like effect via iNOS-dependent pathway in mice

We recently demonstrated that monophosphoryl lipid A (MLA)-induced delayed cardioprotection is mediated by inducible nitric oxide synthase (iNOS) in mice. In the present study, we determined whether RC-552, a novel synthetic glycolipid related in chemical structure to MLA, could afford similar protection. Adult mice were pretreated with vehicle or RC-552 (350 microg/kg ip, n = 7 mice/group) 24 h before global ischemia and reperfusion in a Langendorff isolated, perfused heart model. A group of RC-552-treated mice received S-methylisothiourea (SMT), a selective inhibitor of iNOS (3 mg/kg ip), 30 min before heart perfusion. Myocardial infarct size was significantly reduced from 19.2 +/- 2.0% in vehicle to 8.2 +/- 2.9% in RC-552 group (P < 0.05). Treatment with SMT abolished RC-552-induced reduction in infarct size (20.0 +/- 3.9%). In addition, RC-552 failed to reduce infarct size in isolated hearts from iNOS knockout mice (27.1 +/- 2.8%) compared with that in hearts from control knockout mice without drug treatment (22.9 +/- 5.4%). Acute buffer perfusion with RC-552 (0.1, 1.0, or 2.5 microg/ml) for 8 min immediately before ischemia-reperfusion did not reduce infarct size significantly. We concluded that RC-552 induces delayed cardioprotection via an iNOS-dependent pathway.

Chronic dipyridamole therapy produces sustained protection against cardiac ischemia-reperfusion injury

Sustained protection against ischemia-reperfusion injury is not available for patients at risk for myocardial infarction who may require emergent reperfusion therapy. Whereas ischemic preconditioning and adenosinergic agents reduce myocardial injury, they are only effective when given immediately before ischemia or reperfusion. We recently found chronic ethanol exposure, an adenosine uptake inhibitor, produced sustained cardioprotection against ischemia-reperfusion injury. We now ask whether chronic dipyridamole therapy, a clinically usable nucleoside transport inhibitor, induces similar cardioprotection. Perfused hearts from guinea pigs, given dipyridamole (4 mg. kg(-1). day(-1)) in their water for 2-6 wk (n = 10 for each group), underwent ischemia-reperfusion. Injury was assessed by recovery of left ventricular developed (LVDP) and end-diastolic (LVEDP) pressures and creatine kinase release. During reperfusion, hearts from dipyridamole-treated animals (6 wk) had 74% higher LVDP, 28% lower LVEDP, and 61% lower creatine kinase release versus controls. Adenosine A(1)-receptor antagonism (8-cyclopentyl-1, 3-dipropylxanthine; 200 nM) abolished the protection of dipyridamole but A(2) antagonism (3,7-dimethyl-1-propargylxanthine; 10 mM) did not. Dipyridamole therapy produces sustained protection against ischemia-reperfusion injury in guinea pigs. This cardioprotection requires adenosine A(1) receptor signaling at the time of ischemia.

Adaptation to high altitude hypoxia protects the rat heart against ischemia-induced arrhythmias. Involvement of mitochondrial K(ATP) channel

The aim was to determine whether adaptation to chronic hypoxia protects the heart against ischemic arrhythmias and whether ATP-dependent potassium channels (K(ATP)) play a role in the antiarrhythmic mechanism. Adult male rats were adapted to intermittent high altitude hypoxia (5000 m, 4 h/day) and susceptibility to ischemia-induced ventricular arrhythmias was evaluated in the Langendorff-perfused hearts subjected to either an occlusion of the coronary artery for 30 min or pre-conditioning by brief occlusion of the same artery prior to 30-min reocclusion. In separate groups, either a K(ATP) blocker, glibenclamide (10 micromol/l), or a mitochondrial K(ATP) opener, diazoxide (50 micromol/l), were added to a perfusion medium 20 min before the occlusion. Adaptation to hypoxia reduced the total number of ventricular arrhythmias by 64% as compared with normoxic controls. Preconditioning by a single 3-min coronary artery occlusion was antiarrhythmic only in the normoxic group, while two occlusion periods of 5 min each were needed to pre-condition the hypoxic hearts. Glibenclamide increased the number of arrhythmias in the normoxic hearts from 1316+/-215 to 2091+/-187 (by 59%) and in the hypoxic group from 636+/-103 to 1777+/-186 (by 179%). In contrast, diazoxide decreased the number of arrhythmias only in the normoxic group from 1374+/-96 to 582+/-149 (by 58%), while its effect in the hypoxic group was not significant. It is concluded that long-term adaptation of rats to high altitude hypoxia decreases the susceptibility of their hearts to ischemic arrhythmias and increases an antiarrhythmic threshold of pre-conditioning. The mitochondrial K(ATP) channel, rather than the sarcolemmal K(ATP) channel, appears to be involved in the protective mechanism afforded by adaptation.

Development of cardiac sensitivity to oxygen deficiency: comparative and ontogenetic aspects

Hypoxic states of the cardiovascular system are undoubtedly associated with the most frequent diseases of modern times. They originate as a result of disproportion between the amount of oxygen supplied to the cardiac cell and the amount actually required by the cell. The degree of hypoxic injury depends not only on the intensity and duration of the hypoxic stimulus, but also on the level of cardiac tolerance to oxygen deprivation. This variable changes significantly during phylogenetic and ontogenetic development. The heart of an adult poikilotherm is significantly more resistant as compared with that of the homeotherms. Similarly, the immature homeothermic heart is more resistant than the adult, possibly as a consequence of its greater capability for anaerobic glycolysis. Tolerance of the adult myocardium to oxygen deprivation may be increased by pharmacological intervention, adaptation to chronic hypoxia, or preconditioning. Because the immature heart is significantly more dependent on transsarcolemmal calcium entry to support contraction, the pharmacological protection achieved with drugs that interfere with calcium handling is markedly altered. Developing hearts demonstrated a greater sensitivity to calcium channel antagonists; a dose that induces only a small negative inotropic effect in adult rats stops the neonatal heart completely. Adaptation to chronic hypoxia results in similarly enhanced cardiac resistance in animals exposed to hypoxia either immediately after birth or in adulthood. Moreover, decreasing tolerance to ischemia during early postnatal life is counteracted by the development of endogenous protection; preconditioning failed to improve ischemic tolerance just after birth, but it developed during the early postnatal period. Basic knowledge of the possible improvements of immature heart tolerance to oxygen deprivation may contribute to the design of therapeutic strategies for both pediatric cardiology and cardiac surgery.

Antiischemic effect of monophosphoryl lipid A in conscious rabbits with hypercholesterolemia and atherosclerosis

We studied whether monophosphoryl lipid A (MLA), an endotoxin derivative, protected the heart from planned ischemia in hypercholesterolemic conscious rabbits. Normal and hypercholesterolemic (8-week exposure to 1.5% cholesterol-enriched diet) conscious rabbits with right ventricular electrode and left ventricular polyethylene catheters were subjected to ventricular overdrive pacing (VOP: 500 beats/min over 10 min = control VOP). The resulting intracavitary ST-segment elevation, increase in left ventricular end-diastolic pressure (LVEDP), and a reduction of ventricular effective refractory period (VERP) were measured. Three days later the animals were given a single intravenous bolus of 10 or 30 microg/kg MLA or its solvent or both, and a second VOP (test VOP) was applied 24 h later. MLA decreased ST elevation and LVEDP increase from 2.1 +/- 0.16 to 1.27 +/- 0.25 and 0.97 +/- 0.13 mV and 14.6 +/- 1.2 to 11.1 +/- 1.0 and 12.4 +/- 1.2 mm Hg in normal animals and from 2.55 +/- 0.14 to 1.31 +/- 0.12 and 0.96 +/- 0.30 mV and from 21.0 +/- 1.6 to 11.7 +/- 1.3 and 12.4 +/- 1.3 mm Hg in atherosclerotic animals after 10- and 30-microg/kg doses, respectively (p < 0.001 for each). VOP-induced VERP reduction was also significantly alleviated by both MLA doses; nevertheless, 30-microg/kg MLA significantly prolonged resting VERP with a slight VERP reduction in response to pacing in both normal and atherosclerotic animals. We conclude that MLA produces a delayed antiischemic effect in both normal and hypercholesterolemic/atherosclerotic conscious rabbits.

Alcohol consumption reduces ischemia-reperfusion injury by species-specific signaling in guinea pigs and rats

We recently discovered that regular alcohol consumption reduces ischemia-reperfusion injury to the same degree as ischemic preconditioning in guinea pig hearts. Ischemic preconditioning, like this cardioprotective effect of alcohol, is mediated by adenosine signaling in guinea pigs. In rats, ischemic preconditioning may be mediated predominantly by alpha1-adrenergic signaling. To be certain that this protective effect of alcohol is a general biological response, we searched for alcohol's cardioprotection in rat and identified a potential signaling mechanism. Hearts isolated from alcohol-fed guinea pigs and rats were subjected to ischemia-reperfusion. Hearts from alcohol-fed animals showed greater recovery of left ventricular developed pressure than controls (guinea pigs, 46 vs. 29%; rats, 50 vs. 31%) and decreased myocyte necrosis assessed by creatine kinase release (guinea pigs, 204 +/- 42 vs. 440 +/- 70 U . ml-1 . g dry wt-1; rats 158 +/- 13 vs. 328 +/- 31 U . ml-1 . g dry wt-1). Adenosine receptor blockade [8-(p-sulfophenyl)theophylline] abolished alcohol's protection in guinea pig but not rat hearts. By contrast, alpha1-adrenergic blockade (prazosin) abolished alcohol's protection in rat but not guinea pig hearts. We conclude that regular alcohol consumption reduces ischemia-reperfusion injury and is mediated by species-specific signaling mechanisms. A major goal of cardiovascular research is to find a pharmacologically induced chronic state of preconditioning. Understanding the mechanisms of alcohol's cardioprotection against ischemia-reperfusion injury may aid in reaching this goal.

Monophosphoryl lipid A attenuates myocardial stunning in dogs: role of ATP-sensitive potassium channels

Results of previous studies indicate that monophosphoryl lipid A (MLA) reduces myocardial infarct size when administered 24 but not 1 h before a prolonged period of regional ischemia in dogs and rabbits. This cardioprotective effect of MLA could be reversed by the administration of the adenosine triphosphate (ATP)-sensitive potassium channel (K(ATP)) blockers, glibenclamide, or 5-hydroxydecanoate. MLA also was shown to attenuate myocardial stunning in dogs; however, its mechanism in this model remains unknown. Therefore the major aim of our study was to determine the dose-related effect of MLA to enhance contractile function in stunned myocardium and to determine the role of the K(ATP) channel in mediating its cardioprotective effect. To produce myocardial stunning, barbital-anesthetized dogs were subjected to five cycles of 5 min of left anterior descending (LAD) coronary artery occlusion interspersed with 10 min of reperfusion and finally followed by 2 h of reperfusion. Regional segment shortening (%SS) was determined by sonomicrometers implanted in the subendocardium of the ischemic region. Single intravenous doses of MLA in the range of 10-35 microg/kg given 24 h before ischemia resulted in an improvement in %SS over a 2-h reperfusion period. Similar to results obtained in the canine and rabbit infarct models, cardioprotection against stunning with MLA appears to require activation of K(ATP) channels during ischemia, because glibenclamide (50 microg/kg, 15 min before ischemia) completely blocked the effect of MLA to improve regional %SS during reperfusion. Cardioprotective doses of MLA were without effect on systemic hemodynamics, blood gases, and pH throughout the experiment. No treatment-related effects on regional myocardial blood flow were observed during ischemia or reperfusion. These results suggest that MLA improves %SS at doses of 10-35 microg/kg by an ATP-sensitive potassium channel-dependent process, and that MLA may mimic the antistunning effects observed during the second window of ischemic preconditioning.

Adaptation to myocardial stress in disease states: is preconditioning a healthy heart phenomenon?

Effective therapeutic strategies for protecting the ischaemic myocardium are much sought after. Ischaemic heart disease in humans is a complex disorder, often associated with other systemic diseases such as dyslipidaemia, hypertension and diabetes that exert multiple biochemical effects on the heart, independently of ischaemia. Ischaemic preconditioning of myocardium is a well-described adaptive response in which brief exposure to ischaemia markedly enhances the ability of the heart to withstand a subsequent ischaemic insult. The underlying molecular mechanisms of this phenomenon have been extensively investigated in the hope of identifying new rational approaches to therapeutic protection of the ischaemic myocardium. However, most studies have been undertaken in animal models in which ischaemia is imposed in the absence of other disease processes. In this article, Peter Ferdinandy, Zoltan Szilvassy and Gary Baxter review the ways in which systemic diseases might modify the preconditioning response and they emphasize the importance of further preclinical studies that specifically examine preconditioning in relation to complicating disease states.

Cardiac mast cell stabilization and cardioprotective effect of ischemic preconditioning in isolated rat heart

This study was designed to investigate the effect of disodium cromoglycate (DSCG), a mast cell stabilizer, on cardioprotective effect of ischemic preconditioning. Isolated rat heart was subjected to 30 min of global ischemia followed by 30 min of reperfusion. Ischemic preconditioning was provided by four episodes of 5-min global ischemia followed by 5 min of reperfusion before sustained ischemia. Ischemic preconditioning and DSCG (10 and 100 microM) treatment markedly decreased the release of lactate dehydrogenase (LDH) and creatine kinase (CK) in coronary effluent and percentage incidence of ventricular premature beats (VPBs) and ventricular tachycardia/fibrillation (VT/VF) during reperfusion. Ischemic preconditioning and DSCG treatment also significantly reduced ischemia/reperfusion-induced mast cell peroxidase (MPO) release, a marker of mast cell degranulation. A significant increase in MPO release was observed immediately after ischemic preconditioning, and the release was found to be inhibited in hearts perfused with DSCG (10 and 100 microM) during ischemic preconditioning. DSCG administered during ischemic preconditioning (DSCG in ischemic preconditioning) attenuated the cardioprotective and antiarrhythmic effects of ischemic preconditioning. DSCG in ischemic preconditioning produced no marked effect on ischemia/reperfusion-induced MPO release. These findings tentatively suggest that DSCG administration during ischemic preconditioning abolishes its cardioprotective effect, perhaps by stabilizing resident cardiac mast cells.

Metabolic and antioxidant effects of R(+/-)-N6-(2-phenylisopropyl)-adenosine following regional ischemia and reperfusion in canine myocardium

Recent studies have indicated that activation of A1/A2-receptors may mediate metabolic adaptation of the heart to ischemia/reperfusion stress. This study tests whether pretreatment with A1-selective agonist R(-)-N6-(2-phenylisopropyl) adenosine (R-PIA) might mimic effects of a brief period of coronary occlusion (ischemic preconditioning, IP) on energy metabolism and hydroxyl radical (OH.) formation in canine myocardium following subsequent prolonged ischemia and reperfusion. Anaesthetized dogs were randomized to a control group subjected to 40-min occlusion of a diagonal branch of left anterior descending coronary artery (LAD) followed by 1-h reperfusion, or a preconditioned group (PC) in which the same period of sustained ischemia and reperfusion was preceded by a single cycle of IP (5-min occlusion of the same LAD branch and 10-min reperfusion), or to PIA group in which R-PIA infusion into the same branch of LAD (0.4 microg/kg per min during 5 min) was followed by 10 min of perfusion prior to sustained ischemia-reperfusion. Pretreatment with R-PIA similarly to IP reduced lactate (Lac), creatine (Cr) and inorganic phosphate (Pi) release from myocytes into the interstitial fluid during sustained ischemia compared to these indices in control. By the end of reperfusion, both IP and R-PIA infusion enhanced recovery of myocardial ATP and phosphocreatine (PCr) and attenuated the total creatine (sigmaCr = PCr + Cr) loss, an index of cell membrane damage. A1-receptor activation by R-PIA, as IP, led to a significant reduction in OH. radical generation following reperfusion assessed by a spin trap 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) using cardiac microdialysis. R-PIA pretreatment did not affect systemic and cardiac hemodynamic parameters. We conclude that (1) adaptive mechanisms of IP involve A1-receptor activation that contributes to the overall metabolic response and (2) R-PIA acts as a useful preconditioning-mimetic and anti-ischemic agent in dogs.

Resident cardiac mast cells and the cardioprotective effect of ischemic preconditioning in isolated rat heart

Our study was designed to investigate the role of resident cardiac mast cells in the cardioprotective effect of ischemic preconditioning. Ischemic/compound 48/80 preconditioning and treatment with compound 48/80, a mast cell degranulator (1 microg/ml), produced cardioprotective and antiarrhythmic effects in isolated perfused rat heart subjected to 30-min global ischemia followed by 30-min reperfusion. Four episodes of ischemic/compound 48/80 preconditioning and compound 48/80 treatment markedly reduced the release of lactate dehydrogenase (LDH) and creatine kinase (CK) in coronary perfusate and the incidence of ventricular premature beats (VPBs) and ventricular tachycardia or fibrillation (VT/VF) during the reperfusion phase. The release of mast cell peroxidase (MPO), a marker of mast cell degranulation in coronary perfusate, increased immediately after ischemic and compound 48/80 preconditioning. The cardioprotective and antiarrhythmic effect of ischemic/compound 48/80 preconditioning was lost within 60 min. It is proposed that the cardioprotective effect of ischemic preconditioning, which lasts for 60 min in isolated rat heart, may be ascribed to degranulation of resident cardiac mast cells.

The effect of continuous versus intermittent treatment with transdermal nitroglycerin on pacing-induced preconditioning in conscious rabbits

1. Tolerance to the hypotensive effect of nitroglycerin (NG) blocks preconditioning induced by rapid ventricular pacing (RVP) in rabbits. In the present work the effect of continuous versus intermittent treatment with transdermal nitroglycerin on the pacing-induced preconditioning phenomenon was studied in conscious rabbits. 2. RVP (500 beats min-1 over 5 min) increased left ventricular end-diastolic pressure (LVEDP) from baseline 4.1 +/- 0.9 to postpacing 13.8 +/- 2.9 mmHg (P < 0.001) with a right intraventricular ST-segment elevation of 1.25 +/- 0.13 mV, two indicators of myocardial ischaemia. These changes were significantly attenuated when the RVP period was preceded by a preconditioning pacing of the same rate and duration with an interpacing interval of 5 min. 3. Protection by preconditioning was abolished when the animals had been made tolerant to the vasodilator effect of 30 micrograms kg-1 NG by the application of transdermal NG (approx. 0.07 mg kg-1 h-1) over 7 days. Furthermore, transdermal NG per se attenuated both RVP-induced ST-segment elevation and LVEDP-increase over the 7 day period. 4. With intermittent transdermal NG treatment (12 h 'patch on' vs 'patch off'), neither development of vascular tolerance nor attenuation of the NG- or preconditioning-induced anti-ischaemic effects were observed. However, the severity of pacing-induced myocardial ischaemia was significantly increased during the 'patch off' periods. 5. In a second set of experiments, postpacing changes in cardiac cyclic GMP and cyclic AMP levels were determined by means of radioimmunoassay in chronically instrumented anaesthetized open-chest rabbits with the same NG-treatment protocols. Preconditioning reduced postpacing increase in cyclic AMP with an increase in cyclic GMP concentrations in hearts of the untreated animals and in those given patches intermittently during both 'patch on' and 'patch off' periods. However, the preconditioning effect on either cyclic nucleotide was blocked in the tolerant animals. 6. Transdermal NG increased resting levels of both cardiac cyclic nucleotides in the non-tolerant but not in the tolerant state. The postpacing increase in cyclic AMP content was inhibited by transdermal NG, independent of vascular tolerance development, whereas an cyclic GMP content was exclusively seen in the non-tolerant animals. 7. We conclude that the anti-ischaemic effect of NG is independent of the cyclic GMP mechanism in the tolerant state. While intermittent NG therapy prevents development of vascular tolerance and preserves preconditioning, the nitrate-free periods yield an increased susceptibility of the heart to ischaemic challenges.

Increased tolerance of the chronically hypoxic immature heart to ischemia. Contribution of the KATP channel

BACKGROUND

Hypoxia from birth in immature rabbits increases the tolerance of isolated hearts to ischemia compared with age-matched normoxic rabbits. We determined whether this increased tolerance to ischemia was due to an alteration in the ATP-sensitive potassium (KATP) channel and whether increased KATP channel activation was associated with increases in intracellular lactate.

METHODS AND RESULTS

Isolated immature rabbit hearts (7 to 10 days old) were perfused with bicarbonate buffer at 39 degrees C in the Langendorff mode at a constant pressure. Saline-filled latex balloons were placed in the left and right ventricles for measurement of developed pressure. A KATP channel agonist (bimakalim) or a KATP channel antagonist (glibenclamide) was added 15 minutes before a global ischemic period of 18 minutes, followed by 35 minutes of reperfusion. Rabbits raised from birth in hypoxic conditions (FIO2 = 0.12) displayed significantly enhanced recovery of developed pressure. The right ventricle was more tolerant of ischemia than the left ventricle in normoxic and hypoxic hearts. Bimakalim (1 mumol/L) increased the recovery of left ventricular developed pressure in normoxic hearts to values not different from those of hypoxic controls (43 +/- 3% to 67 +/- 5%) and slightly increased developed pressure in hypoxic hearts (67 +/- 5% to 72 +/- 5%). Glibenclamide (3 mumol/L) abolished the cardioprotective effect of hypoxia (67 +/- 5% to 43 +/- 5%). Constant-flow studies indicated that the effects of bimakalim and glibenclamide were independent of their actions on coronary flow. Ventricular lactate and lactate dehydrogenase concentrations were elevated in hypoxic hearts compared with normoxic control hearts.

CONCLUSIONS

Increased tolerance to ischemia exhibited by chronically hypoxic rabbit hearts is associated with increased activation of the KATP channel. This increased KATP activity may be the result of increased intracellular concentrations of lactate.

Activation of cardiac muscarinic receptor and ischemic preconditioning effects in in situ rat heart

Activation of cardiac muscarinic receptors by vagal stimulation decreases cardiac work, which may have a protective effect against ischemic injury. To determine whether cardiac muscarinic receptors contribute to the mechanisms of preconditioning effects, we examined the effect of carbachol on ischemia/reperfusion damage and the effect of vagotomy on cardioprotection induced by ischemic preconditioning. Rats were subjected to 30 min of left coronary artery occlusion followed by 30-min reperfusion in situ. Pre-conditioning was induced by three cycles of 2-min coronary artery occlusion and, subsequently by 5 min of reperfusion. The incidence of ischemic arrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF), and the development of myocardial infarction were markedly reduced by the preconditioning. Carbachol infusion (4 micrograms/kg per min) delayed the occurrence of VT and VF during ischemia and reduced the infarct size. Compared with non-ischemic left ventricle, the cyclic guanosine monophosphate (GMP) content in the ischemic region of the left ventricle was decreased by ischemia/reperfusion, whereas the cyclic adenosine monophosphate (AMP) content of this region was increased. These changes were reversed by preconditioning. Similar changes in cyclic GMP and AMP content in the ischemic region were seen in rats undergoing carbachol treatment. These results suggest the possible contribution of muscarinic receptor stimulation to preconditioning. Vagotomy prior to preconditioning diminished the antiarrhythmic effects, whereas it did not block the anti-infarct effect afforded by pre-conditioning. Vagotomy abolished the preconditioning effect on the tissue cyclic GMP, but it did not attenuate the decrease in tissue cyclic AMP. The results suggest that muscarinic stimulation exerts preconditioning-mimetic protective effects in ischemic/reperfused hearts, but that a contribution of reflective vagal activity to the mechanism for preconditioning is unlikely.

Ischemic preconditioning triggers the activation of MAP kinases and MAPKAP kinase 2 in rat hearts

While much is known about the beneficial effects of myocardial stress adaptation, relatively less information is available about the adaptive mechanisms. To explore the signaling pathways of stress adaptation, isolated working rat hearts were divided into three groups. Group I was adapted to stress by conventional technique of repeated ischemia and reperfusion consisting of 5 min of ischemia followed by 10 min of reperfusion, repeated four times. Group II was treated with 100 microM of genistein, a tyrosine kinase inhibitor, followed by preconditioning as described for group I. The third group, perfused with buffer only for 60 min, served as control. All hearts were subjected to 30 min of ischemia followed by 30 min of reperfusion. The results of our study demonstrated better postischemic myocardial functions in the preconditioned hearts as evidenced by increased aortic flow, coronary flow, developed pressure and lesser amount of tissue injury as evidenced by the decreased creatine kinase release. The preconditioning effects were associated with enhancement of phospholipase D activity in the heart. The preconditioning effect was almost abolished by the genistein treatment which also prevented the enhancement of phospholipase D activities. Additionally, preconditioning of the rat hearts stimulated protein kinase C, MAP kinase, and MAPKAP kinase 2 activities which were inhibited by genistein. The results identifies for the first time tyrosine kinase-phospholipase D as potential signaling pathway for ischemic preconditioning, and implicates the involvement of multiple protein kinases in myocardial adaptation to ischemia.

Stimulation of phosphatidylinositol hydrolysis, protein kinase C translocation, and mitogen-activated protein kinase activity by bradykinin in rat ventricular myocytes: dissociation from the hypertrophic response

In ventricular myocytes cultured from neonatal rat hearts, bradykinin (BK), kallidin or BK(1-8) [(Des-Arg9)BK] stimulated PtdinsP2 hydrolysis by 3-4-fold. EC50 values were 6 nM (BK), 2 nM (kallidin), and 14 microM [BK(1-8)]. BK or kallidin stimulated the rapid (less than 30 s) translocation of more than 80% of the novel protein kinase C (PKC) isoforms nPKC-delta and nPKC-epsilon from the soluble to the particulate fraction. EC50 values for nPKC-delta translocation by BK or kallidin were 10 and 2 nM respectively. EC50 values for nPKC-epsilon translocation by BK or kallidin were 2 and 0.6 nM respectively. EC50 values for the translocation of nPKC-delta and nPKC-epsilon by BK(1-8) were more than 5 microM. The classical PKC, cPKC-alpha, and the atypical PKC, nPKC-zeta, did not translocate. BK caused activation and phosphorylation of p42-mitogen-activated protein kinase (MAPK) (maximal at 3-5 min, 30-35% of p42-MAPK phosphorylated). p44-MAPK was similarly activated. EC50 values for p42/p44-MAPK activation by BK were less than 1 nM whereas values for BK(1-8) were more than 10 microM. The order of potency [BK approximately equal to kallidin >> BK (1-8)] for the stimulation of PtdInsP2 hydrolysis, nPKC-delta and nPKC-epsilon translocation, and p42/p44-MAPK activities suggests involvement of the B2 BK receptor subtype. In addition, stimulation of all three processes by BK was inhibited by the B2BK receptor-selective antagonist HOE140 but not by the B1-selective antagonist Leu8BK(1-8). Exposure of cells to phorbol 12-myristate 13-acetate for 24 h inhibited subsequent activation of p42/p44-MAPK by BK suggesting participation of nPKC (and possibly cPKC) isoforms in the activation process. Thus, like hypertrophic agents such as endothelin-1 (ET-1) and phenylephrine (PE), BK activates PtdInsP2 hydrolysis, translocates nPKC-delta, and nPKC-epsilon, and activates p42/p44-MAPK. However, in comparison with ET-1 and PE, BK was only weakly hypertrophic as assessed by cell morphology and patterns of gene expression. This difference could not be attributed to dissimilarities between the duration of activation of p42/p44-MAPK by BK or ET-1. Thus activation of these signalling pathways alone may be insufficient to induce a powerful hypertrophic response.

Protection from preconditioning can be reinstated at various reperfusion intervals

The aim of this study was to evaluate the way in which short-term protection declines and is eventually lost in preconditioning and to determine the efficacy of a second preconditioning at various reperfusion intervals. Male rabbits were divided into six groups. Forty-five minutes (sustained) ischemia followed by 120 minutes reperfusion was applied 60, 65, 70, 75, and 80 minutes after a 5 minute preconditioning (groups A, B, C, D, and E) and in a control group (F) after no preconditioning. The infarct to risk ratio (I/R) was 38.3 +/- 3.5% in group A, 46.0 +/- 7.8% in B, 61.6 +/- 9.7% in C, 68.1 +/- 4.2% in D, 64.5 +/- 7.8% in E, and 61.0 +/- 7.7% in F. Group A had a smaller I/R compared with groups C, D, E, and F (p < 0.05). In another series, groups G, H, and I were exposed to two 5-minute preconditioning stimuli, separated, respectively, by 45, 60, and 75 minutes of reperfusion; 10 minutes after the last preconditioning, the animals were exposed to 45-minutes ischemia and 120 minutes reperfusion. Groups A and D (with the smaller and higher I/R ratio) were also incorporated into this protocol in order to compare the effect of the additional preconditioning with the single one. The I/R ratio was 25.4 +/- 8.5% in group G, 22.8 +/- 7.0% in group H, and 14.7 +/- 4.0% in group I (p = NS). Group D showed a higher I/R compared with groups G, A, and H (p < 0.01), and group I had a smaller I/R compared with groups A (p < 0.01) and D (p < 0.001). Cardioprotection after a first preconditioning declines gradually and is eventually lost. An additional preconditioning is always effective, and the longer the interval from the first preconditioning, the more potent is the effect.

Alterations in circulating cyclic guanosine monophosphate (c-GMP) during short and long ischemia in preconditioning

The aim of this study was to investigate if levels of circulating cyclic guanosine monophosphate (c-GMP) alter in preconditioning. Twenty-eight rabbits were divided into four groups. In vivo hearts were preconditioned, either with 5 min (group A, n = 8) or with 1 min (group B, n = 8) ischemia, followed by 10 min reperfusion, while groups C (n = 7) and D (n = 5) had no interventions. Protection was determined by subjecting groups A, B and C (but not D) to 30 min regional ischemia which was followed (including group D) by 2 h reperfusion. Seven blood samples were collected for the assessment of circulating c-GMP at different points of time. All results were expressed in pmol/ml using radio-immunoassay and the infarcted to risk area in percent using fluorescent particles and tetrazolium chloride (TTC). Circulating c-GMP increased during long ischemia only in group A (baseline value 47 +/- 4, long ischemic values 60.5 +/- 4 and 60.4 +/- 4, p < 0.05). Circulating c-GMP in group A was significantly higher in the middle of the long ischemia in comparison to the groups B, C and D (60.5 +/- 4 vs 43.9 +/- 4, 45.8 +/- 5 and 43.6 +/- 4, p < 0.05). Infarcted to risk ratio was lower in group A than in groups B and C (12.2 +/- 4 vs 29.6 +/- 6 and 34.2 +/- 6 respectively, p < 0.05). Circulating c-GMP is increased in classically preconditioned in comparison to ineffectively preconditioned hearts or to control groups. This elevation may be related to the protective effect of this phenomenon.