Postischemic changes in cardiac sarcoplasmic reticulum Ca2+ channels. A possible mechanism of ischemic preconditioning

Preconditioning or Postconditioning with 8-Br-cAMP-AM Protects the Heart against Regional Ischemia and Reperfusion: A Role for Mitochondrial Permeability Transition

The cAMP analogue 8-Br-cAMP-AM (8-Br) confers marked protection against global ischaemia/reperfusion of isolated perfused heart. We tested the hypothesis that 8-Br is also protective under clinically relevant conditions (regional ischaemia) when applied either before ischemia or at the beginning of reperfusion, and this effect is associated with the mitochondrial permeability transition pore (MPTP). 8-Br (10 μM) was administered to Langendorff-perfused rat hearts for 5 min either before or at the end of 30 min regional ischaemia. Ca2+-induced mitochondria swelling (a measure of MPTP opening) and binding of hexokinase II (HKII) to mitochondria were assessed following the drug treatment at preischaemia. Haemodynamic function and ventricular arrhythmias were monitored during ischaemia and 2 h reperfusion. Infarct size was evaluated at the end of reperfusion. 8-Br administered before ischaemia attenuated ventricular arrhythmias, improved haemodynamic function, and reduced infarct size during ischaemia/reperfusion. Application of 8-Br at the end of ischaemia protected the heart during reperfusion. 8-Br promoted binding of HKII to the mitochondria and reduced Ca2+-induced mitochondria swelling. Thus, 8-Br protects the heart when administered before regional ischaemia or at the beginning of reperfusion. This effect is associated with inhibition of MPTP via binding of HKII to mitochondria, which may underlie the protective mechanism.

Cardioprotection by ranolazine in perfused rat heart

: We used the isolated working rat model to evaluate the effect of therapeutic concentrations (5-10 μM) of ranolazine on contractile performance, oxygen consumption, irreversible ischemic injury, and sarcoplasmic reticulum (SR) function. Ischemic injury was induced by 30 minutes of global ischemia followed by 120 minutes of Langendorff reperfusion and evaluated on the basis of triphenyltetrazolium chloride staining. SR function was determined on the basis of [H]-ryanodine binding, the kinetics of calcium-induced calcium release, measured by quick filtration technique, and oxalate-supported calcium uptake. In working hearts, ranolazine significantly reduced oxygen consumption (P = 0.031), in the absence of significant changes in contractile performance, and decreased irreversible ischemic injury (P = 0.011), if administered either before ischemia-reperfusion (25.4% ± 4.7% vs. 42.7% ± 6.0%) or only at the time of reperfusion (20.2% ± 5.2% vs. 43.7% ± 9.9%). In SR experiments, treatment with ranolazine determined a significant reduction in [H]-ryanodine binding (P = 0.029), because of decreased binding site density (369 ± 9 vs. 405 ± 12 fmol/mg), and in the kinetics of SR calcium release (P = 0.011), whose rate constant was decreased, whereas active calcium uptake was not affected. Ranolazine effectiveness at reperfusion and its ability to module SR calcium release suggest that this drug might be particularly useful to induce cardioprotection during coronary revascularization interventions, although the relevance of the effects on calcium homeostasis remains to be determined.

Cardioprotective effect of 3-iodothyronamine in perfused rat heart subjected to ischemia and reperfusion

3-iodothyronamine (T(1)AM) is an endogenous compound which shares structural and functional features with biogenic amines and is able to interact with a specific class of receptors, designed as trace amine associated receptors. T(1)AM has significant physiological effects in mammals and produces a reversible, dose-dependent negative inotropic and chronotropic effect in heart. The aim of the present study was to investigate if T(1)AM is able to reduce irreversible tissue injury in isolated rat hearts subjected to ischemia and reperfusion, as evaluated by triphenyltetrazolium chloride staining. We observed that T(1)AM reduced infarct size at concentrations (125 nM to 12.5 μM) which did not produce any significant hemodynamic action. The dose-response curve was bell-shaped and peaked at 1.25 μM. T(1)AM-induced cardioprotection was completely reversed by the administration of chelerythrine and glibenclamide, suggesting a protein kinase C and K (ATP) (+) -dependent pathway, while it was not additive to the protection induced by cyclosporine A, suggesting modulation of mitochondrial permeability transition. At cardioprotective concentration, T(1)AM reduced the time needed for cardiac attest during ischemia, but it did not affect sarcoplasmatic reticulum Ca(2+) handling, as demonstrated by unaltered ryanodine receptor binding properties. In conclusion, in isolated rat heart T(1)AM produces a cardioprotective effect which is mediated by a protein kinase C and K (ATP) (+) -dependent pathway and is probably linked to modulation of mitochondrial permeability transition and/or ischemic arrest time.

Effects of zofenopril on cardiac sarcoplasmic reticulum calcium handling

Isolated rat hearts were perfused for 120 minutes in the presence or in the absence of 10 microM zofenoprilat, the active metabolite of zofenopril. At the end of perfusion, cardiac tissue was used to assay sarcoplasmic reticulum (SR) (45)Ca uptake and SR calcium release, which was determined by automatized quick filtration technique after SR vesicle loading with (45)Ca. The expression of genes involved in the control of calcium homeostasis was evaluated by polymerase chain reaction after reverse transcription. In chronic experiments, SR (45)Ca uptake and gene expression were measured in hearts derived from rats treated with 15 mg*kg(-1)*day(-1) zofenopril for 15 days. Acute or chronic zofenopril administration did not produce any change in contractile performance. In acute experiments, SR (45)Ca uptake was significantly increased after exposure to zofenoprilat. The rate constant of calcium-induced calcium release was slightly although not significantly higher, and the calcium leak measured under conditions promoting SR channel closure was significantly increased. In the chronic model, significant increase in the rate of SR (45)Ca uptake was confirmed. Gene expression was not modified, except for decreased phospholamban expression, which is observed in the acute but not in the chronic model. In conclusion, zofenopril increases SR calcium cycling and stimulates active calcium uptake into the SR.

Myocardial Hsp70 phosphorylation and PKC-mediated cardioprotection following exercise

Both protein kinase C (PKC) activation and Hsp70 expression have been shown to be key components for exercise-mediated myocardial protection during ischemia-reperfusion injury. Given that Hsp70 has been shown to undergo inducible phosphorylation in striated muscle and liver, we hypothesized that PKC may regulate myocardial Hsp70 function and subsequent exercise-conferred cardioprotection through this phosphorylation. Hence, acute exercise of male Sprague-Dawley rats (30 m/min for 60 min at 2% grade) was employed to assess the role of PKC and its selected isoforms in phosphorylation of Hsp70 and protection of the myocardium during ischemia-reperfusion injury. It was observed that administration of the PKC inhibitor chelerythrine chloride (5 mg/kg) suppressed the activation of three exercise-induced PKC isoforms (PKCalpha, PKCdelta, and PKCepsilon) and attenuated the exercise-mediated reduction of myocardial infarct size during ischemia-reperfusion injury. While this study also demonstrated that exercise led to an alteration in the phosphorylation status of Hsp70, this posttranslational modification appeared to be dissociated from PKC activation, as exercise-induced phosphorylation of Hsp70 was unchanged following inhibition of PKC. Taken together, these results indicate that selected isoforms of PKC play an important role in exercise-mediated protection of the myocardium during ischemia-reperfusion injury. However, exercise-induced phosphorylation of Hsp70 does not appear to be a mechanism by which PKC induces this cardioprotective effect.

Modulation of cardiac ionic homeostasis by 3-iodothyronamine

3-iodothyronamine (T₁AM) is a novel endogenous relative of thyroid hormone, able to interact with trace amine-associated receptors, a class of plasma membrane G protein-coupled receptors, and to produce a negative inotropic and chronotropic effect. In the isolated rat heart 20–25 μM T₁AM decreased cardiac contractility, but oxygen consumption and glucose uptake were either unchanged or disproportionately high when compared to mechanical work. In adult rat cardiomyocytes acute exposure to 20 μM T₁AM decreased the amplitude and duration of the calcium transient. In patch clamped cardiomyocytes sarcolemmal calcium current density was unchanged while current facilitation by membrane depolarization was abolished consistent with reduced sarcoplasmic reticulum (SR) calcium release. In addition, T₁AM decreased transient outward current (I_to) and I_K1 background current. SR studies involving 20 μM T₁AM revealed a significant decrease in ryanodine binding due to reduced B_max, no significant change in the rate constant of calcium-induced calcium release, a significant increase in calcium leak measured under conditions promoting channel closure, and no effect on oxalate-supported calcium uptake. Based on these observations we conclude T₁AM affects calcium and potassium homeostasis and suggest its negative inotropic action is due to a diminished pool of SR calcium as a result of increased diastolic leak through the ryanodine receptor, while increased action potential duration is accounted for by inhibition of I_to and I_K1 currents.

Cardioprotection by ouabain and digoxin in perfused rat hearts

This work was aimed at determining the cardioprotective effect of digitalis glycosides in rat heart, and to relate it with Na, K-ATPase inhibition and ERK1/2 activation. Isolated working rat hearts were perfused in the presence of ouabain or digoxin, which were used at concentrations ranging from 10 to 10 M. The hearts were then subjected to 30 minutes of global normothermic ischemia followed by 120 minutes of retrograde reperfusion; irreversible tissue injury was determined on the basis of triphenyltetrazolium chloride staining. Significant cardioprotection was observed with 10 M and 10 M ouabain (ischemic injury averaged 7.0 +/- 3.5% and 8.3 +/- 0.6% versus 37.3 +/- 2.0% in controls, P < 0.01 in each case). Hearts treated with digoxin showed decreased ischemic injury at 10 M and 10 M (18.0 +/- 1.5% and 14.2 +/- 1.0%, P < 0.01 versus control in both cases). In parallel experiments, ERK2 phosphorylation was increased by 10 to 10 M ouabain, while ERK1 and ERK2 phosphorylation was increased by 10 to 10 M digoxin. The cardioprotective effect was not related to Na, K-ATPase inhibition, since Rbuptake was not significantly different between control and treated hearts.

Mechanisms of vitexin preconditioning effects on cultured neonatal rat cardiomyocytes with anoxia and reoxygenation

This study was aimed at investigating the protective effect and mechanism of vitexin preconditioning (VPC) on cultured neonatal rat cardiomyocytes after anoxia and reoxygenation (A/R). An A/R model was established by using cultured neonatal rat cardiomyocytes. Cellular injury was evaluated by measuring cell viability, the releases of creatine kinase (CK), and lactate dehydrogenase (LDH). The apoptosis rate of cardiomyocytes after Anoxia/reoxygenation and the activities of extracellular signal-regulated protein kinases (ERKs) were measured. The intracellular calcium indicated by the fluorescence in cardiomyocytes was measured by the laser confocal microscope. Vitexin preconditioning (10, 30 and 100 microM) significantly enhanced the cell viability, markedly inhibited A/R-induced increases of LDH and CK release, obviously decreased the number of apoptotic cardiomyocytes and markedly decreased the fluorescence intensity value of [Ca(2+)](i) in cardiomyocytes. Exposure to anoxia or vitexin preconditioning significantly increased the phospho-ERK level, and the increase was markedly inhibited by PD98059, an inhibitor of the upstream kinase of ERK. These results suggest that vitexin preconditioning has a protective effect on cardiomyocytes A/R injury through the improvement of cell viability, decrease of LDH and CK release, such that the protective mechanism may relate to its ability to inhibit the cardiomyocytes apoptosis, reduce the cardiomyocytes calcium overload and increase the abundance of phosphor-ERK1/2 of the cardiomyocytes after anoxia and reoxygenation.

Apelin decreases the SR Ca2+content but enhances the amplitude of [Ca2+]itransient and contractions during twitches in isolated rat cardiac myocytes

Apelin has been reported to have a positive inotropic action in the isolated rat heart. However, the effect of apelin on sarcoplasmic reticulum (SR) Ca2+content and its influence on intracellular Ca2+transient during excitation-contraction coupling remains poorly understood. In the present study, we determined the effect of apelin on Ca2+transient and contractions in isolated rat cardiomyocytes. When compared with control, treatment with apelin caused a 55.7 ± 13.9% increase in sarcomere fraction shortening and a 43.6 ± 4.56% increase in amplitude of electrical-stimulated intracellular Ca2+concentration (E[Ca2+]i) transients ( n = 14, P < 0.05). But SR Ca2+content measured by caffeine-induced [Ca2+]i(C[Ca2+]i) transient was decreased 8.41 ± 0.92% in response to apelin ( n = 14, P < 0.05). Na+/Ca2+exchanger (NCX) function was increased since half-decay time of C[Ca2+]iwas decreased 16.22 ± 1.36% in response to apelin. Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) activity was also increased by apelin. These responses can be partially or completely blocked by chelerythrine chloride, a PKC inhibitor. In addition, to confirm our data, we used indo-1 as another Ca2+indicator and rapid cooling as another way to measure SR Ca2+content, and we observed similar results. So we conclude that apelin has a positive inotropic effect on isolated myocytes, and increased amplitude of E[Ca2+]iis at least partially involved in the mechanism. NCX function and SERCA activity are increased by apelin, and the SR Ca2+content is decreased by apelin during twitches. PKC played an important role in these signaling mechanisms.

Potential role and mechanisms of subcellular remodeling in cardiac dysfunction due to ischemic heart disease

Several studies have revealed varying degrees of changes in sarcoplasmic reticular and myofibrillar activities, protein content, gene expression and intracellular Ca-handling during cardiac dysfunction due to ischemia-reperfusion (I/R); however, relatively little is known about the sarcolemmal and mitochondrial alterations, as well as their mechanisms in the I/R hearts. Because I/R is associated with oxidative stress and intracellular Ca-overload, it has been indicated that changes in subcellular activities, protein content and gene expression due to I/R are related to both oxidative stress and Ca-overload. Intracellular Ca-overload appears to induce changes in subcellular activities, protein contents and gene expression (subcellular remodeling) by activation of proteases and phospholipases, as well as by affecting the genetic apparatus, whereas oxidative stress is considered to cause oxidation of functional groups of different subcellular proteins in addition to modifying the genetic machinery. Ischemic preconditioning, which is known to depress the development of both intracellular Ca-overload and oxidative stress due to I/R, was observed to attenuate the I/R-induced subcellular remodeling and improve cardiac performance. It is suggested that a combination therapy with antioxidants and interventions, which reduce the development of intracellular Ca-overload, may improve cardiac function by preventing or attenuating the occurrence of subcellular remodeling due to ischemic heart disease. It is proposed that defects in the activities of subcellular organelles may serve as underlying mechanisms for I/R-induced cardiac dysfunction under acute conditions, whereas subcellular remodeling due to alterations in gene expression may explain the impaired cardiac performance under chronic conditions of I/R.

Protein kinase C and preconditioning: role of the sarcoplasmic reticulum

Activation of protein kinase C (PKC) is cardioprotective, but the mechanism(s) by which PKC mediates protection is not fully understood. Inasmuch as PKC has been well documented to modulate sarcoplasmic reticulum (SR) Ca2+and because altered SR Ca2+handling during ischemia is involved in cardioprotection, we examined the role of PKC-mediated alterations of SR Ca2+in cardioprotection. Using isolated adult rat ventricular myocytes, we found that addition of 1,2-dioctanoyl- sn-glycerol (DOG), to activate PKC under conditions that reduced myocyte death associated with simulated ischemia and reperfusion, also reduced SR Ca2+. Cell death was 57.9 ± 2.9% and 47.3 ± 1.8% in untreated and DOG-treated myocytes, respectively ( P < 0.05). Using fura 2 fluorescence to monitor Ca2+transients and caffeine-releasable SR Ca2+, we examined the effect of DOG on SR Ca2+. Caffeine-releasable SR Ca2+was significantly reduced (by ∼65%) after 10 min of DOG treatment compared with untreated myocytes ( P < 0.05). From our examination of the mechanism by which PKC alters SR Ca2+, we present the novel finding that DOG treatment reduced the phosphorylation of phospholamban (PLB) at Ser16. This effect is mediated by PKC-ε, because a PKC-ε-selective inhibitory peptide blocked the DOG-mediated decrease in phosphorylation of PLB and abolished the DOG-induced reduction in caffeine-releasable SR Ca2+. Using immunoprecipitation, we further demonstrated that DOG increased the association between protein phosphatase 1 and PLB. These data suggest that activated PKC-ε reduces SR Ca2+content through PLB dephosphorylation and that reduced SR Ca2+may be important in cardioprotection.

Dysfunction of myocardial sarcoplasmic reticulum in rats with myocardial calcification

We investigated the relationship between cardiac dysfunction and Ca2+ transport in the myocardial sarcoplasmic reticulum (SR) during the pathogenesis of cardiovascular calcification in rats. The possible mechanism of SR dysfunction was explored by detecting the alteration of the nitric oxide/nitric oxide synthase (NO/NOS) pathway in the SR. Using the vitamin D plus nicotine (VDN treatment for 2 week and 6 week) experimental model of cardiac calcification, cardiac function and sarcoplasmic reticulum function were measured. Inhibition of cardiac functions in vivo (peak rate of contraction and peak rate of relaxation, P < 0.05 or P < 0.01) were observed in all calcification groups, simultaneously, Ca2+ release and uptake in the SR as well as the Ca2+ release channel and Ca2+ pump activity were inhibited. Myocardial Ca2+ concentration and cardiac and SR dysfunction were inversely related (P < 0.05). The specific NO/NOS pathway (NO production, NOS activity and nNOS expression in the SR) was upregulated in the SR and associated with calcification (both 2- and 6 week VDN groups). These results indicate that cardiac dysfunction associated with myocardial calcification might be mediated by SR dysfunction, which may result from an impaired SR-specific NO/NOS pathway.

Increased tolerance to hypoxic metabolic inhibition and reoxygenation of cardiomyocytes from apolipoprotein E-deficient mice

Although hypercholesterolemia is a strong risk factor for cardiovascular disease, it has in some instances paradoxically been associated with reduced infarct size and preserved contractile function in isolated hearts after ischemia and reperfusion. To elucidate potential cellular protective mechanisms, myocytes of hypercholesterolemic apolipoprotein E-deficient (ApoE−/−) and wild-type mice were subjected to hypoxic metabolic inhibition (I) with subsequent reoxygenation (R). Intracellular Ca2+concentration ([Ca2+]i) and pH (pHi) were monitored as well as cell length and arrhythmic events. Force measurements in papillary muscles were also recorded, and myocardial expression of Na+/H+exchanger 1 (NHE1) and three Ca2+handling proteins [sarco(endo)plasmic reticulum Ca2+-ATPase, Na+/Ca2+exchanger, and plasma membrane Ca2+-ATPase] was quantified. After 30 min of I and 35 min of R, Ca2+overload was more pronounced in wild-type cells ( P < 0.05). In these myocytes, pHialso dropped faster and remained below those values determined in ApoE−/−cells ( P < 0.05). Furthermore, more wild-type myocytes remained in a contracted state ( P < 0.05). This group also showed a higher incidence of arrhythmic events during R ( P < 0.05). No group difference was found in the expression of the Ca2+handling proteins. However, NHE1 protein was downregulated in hearts of ApoE−/−mice ( P < 0.05). Histological results depict hyperplasia in ApoE−/−hearts without atherosclerosis of the coronaries. Contractile dysfunction was not observed in papillary muscles from ApoE−/−hearts. Our results suggest that downregulated myocardial NHE1 expression in hypercholesterolemic ApoE−/−mice could have contributed to increased tolerance to I/R. It remains to be elucidated whether NHE1 downregulation is a unique feature of these genetically altered animals.

Protective effects of dantrolene against myocardial injury induced by isoproterenol in rats: biochemical and histological findings

PURPOSE

We investigated whether dantrolene might protect the heart against myocardial injury (MI) induced by isoproterenol (ISO), using an experimental model in rats.

METHODS

Twenty-eight rats were randomized to treatment with saline only (control group, n=8), ISO only (ISO group, n=8), low-dose dantrolene (LDD)+ISO (LDD group, n=6) and high-dose dantrolene (HDD)+ISO (HDD group, n=6). ISO (150 mg/kg/day, s.c.), LDD (5 mg/kg/day, i.p.) and HDD (10 mg/kg/day, i.p.) were given once a day for two consecutive days. At the end of the second day, blood samples were taken from abdominal aorta shortly after the rats were anesthetised for cardiac troponins T (cTnT) and I (cTnI) assay, and the hearts were removed and observed microscopically.

RESULTS

cTnT and cTnI levels were increased in the ISO group when compared with the control group (p<0.001). LDD and HDD significantly reduced cTnT and cTnI levels when compared with the ISO group. Elevations of cTnT and cTnI appeared to relate to the severity of histological changes. The rate of animals that exhibited marked MI was higher in the ISO group than in the control group (p<0.001). The rats in both LDD and HDD groups showed less histological changes when compared to the ISO group (p<0.01). There was no significant difference between the control group and both LDD and HDD groups.

CONCLUSIONS

This study shows that dantrolene has a significant effect in the protection of the heart against MI induced by ISO. We believe that pretreatment with dantrolene may contribute to developing novel strategies in the cardiotoxicity animal models and in the prevention of the cardiotoxic effects of elevated levels of catecholamines.

Production of ouabain-like factor in normal and ischemic rat heart

Endogenous ouabain-like factor (OLF) has been detected in mammalian plasma, adrenal gland, and hypothalamus. We investigate whether cardiac tissue may also produce OLF. HPLC chromatographic separation of cardiac extracts showed that RIA-determined OLF activity coincided with the elution profile of exogenous ouabain and with the ability to inhibit 86Rb uptake in human erythrocytes. OLF activity was remarkably higher in excised hearts (3.94 +/- 0.84 pmol/g wet weight by RIA) than in rat blood (0.05 +/- 0.02 pmol/ml). Similar values were obtained in perfused working hearts, without significant changes over time from 5 to 30 minutes of aerobic perfusion. Significant OLF release in the perfusion buffer was also observed (0.54 +/- 0.05 pmoles over 30 minutes). In hearts subjected to 15 minutes of aerobic perfusion followed by 15 minutes of global myocardial ischemia OLF concentration was remarkably increased (8.59 +/- 1.13 versus 4.58 +/- 0.57 pmol/g wet weight by RIA, P < 0.01; an increase after ischemia was confirmed by the assay of 86Rb uptake). Our findings suggest that the rat heart is able to produce OLF, and that its concentration increases during ischemia. Myocardial OLF might modulate the Na/K-ATPase, producing relevant effects on ionic homeostasis and/or gene transcription.

Preconditioning: evolution of basic mechanisms to potential therapeutic strategies

Preconditioning describes the phenomenon by which a traumatic or stressful stimulus confers protection against subsequent injury. Originally recognized in dog heart subjected to ischemic challenges, preconditioning has been demonstrated in multiple species, can be induced by various stimuli, and is applicable in different organ systems. Tremendous progress has been made elucidating the signal transduction cascade of preconditioning. Preconditioning represents a potent tissue-protective condition, and mechanistic understanding may allow safe clinical application. This review recalls the history of preconditioning and how it relates to the history of the investigation of endogenous adaptation; summarizes the current mechanistic understanding of acute preconditioning; outlines the signal transduction cascade leading to the development of delayed preconditioning; discusses preconditioning in noncardiac tissue; and explores the potential of using preconditioning clinically.

S-nitrosothiol detection in isolated perfused rat heart

Nitrogen monoxide (NO) has important cardiovascular actions, and it has been suggested that they may be partly mediated by the reaction with protein sulfhydryl groups to produce S-nitrosothiols. In this work we describe and test a method that allows S-nitrosothiol detection in crude membrane preparations obtained from isolated perfused rat hearts. Isolated rat hearts were perfused under control conditions or in the presence of the NO donors SIN-1 and isosorbide dinitrate. Additional hearts were subjected to 10-20 min of ischemia followed or not by 10-20 min of reperfusion. At the end of perfusion a crude membrane fraction was prepared, and S-nitrosothiol concentration was assayed fluorometrically, on the basis of 2,3-naphthotriazole production from 2,3-diaminonaphthylene. The sensitivity of the method, as evaluated using S-nitrosoalbumin, was on the order of 1-2 pmol/mg of protein. S-nitrosothiols were undetectable under control conditions, as well as after ischemia or ischemia-reperfusion. On the other hand, significant S-nitrosothiol formation was observed after infusion of SIN-1 or isosorbide dinitrate (26.4 +/- 7.4 and 19.9 +/- 5.6 pmol per mg of protein, respectively). In conclusion, S-nitrosothiol production was observed in rat heart membranes after exposure to NO donors, while S-nitrosothiol concentration was below the sensitivity limits of the assay either under baseline conditions or after acute ischemia and reperfusion.

Resident cardiac mast cells and ischemia-reperfusion injury

BACKGROUND

The intense inflammatory reaction following reperfusion of ischemic myocardium has been implicated as a factor in the extension of myocardial injury. One of the therapeutic goals of modern cardiology is to design strategies to limit the infarct size following myocardial infarction. A sound understanding of the inflammatory cascade that involves the release of various proinflammatory mediators from cardiac cells is necessary before a specific intervention is pursued.

OBSERVATION

Summarized is the role of resident cardiac mast cells, which are noted to release inflammatory mediators, in ischemia-reperfusion-induced myocardial injury. Various pharmacologic interventions, such as disodium cromoglycate and ketotifen, that stabilize cardiac mast cells, or agents such as chlorpheniramine and cetirizine that prevent their degranulation during ischemia and reperfusion, may prove to be potential therapeutic agents to limit or salvage ischemia-reperfusion-induced injury.

CONCLUSION

On the basis of the effects of histamine H1 antagonists, adrenoceptor blockers, cellular calcium and nitric oxide modulators, as well as inhibitors of phosphodiesterase and mitogen-activated protein kinase on mast cells, cardiac resident mast cells may represent a novel target for the development of cardioprotective agents.

Ca2+ channel remodeling in perfused heart: Effects of mechanical work and interventions affecting Ca2+ cycling on sarcolemmal and sarcoplasmic reticulum Ca2+ channels

We investigated whether changes in cardiac work or in Ca2+ fluxes may affect the expression of sarcolemmal or sarcoplasmic reticulum Ca2+ channels (DHPRs and RyRs, respectively). Isolated rat hearts were perfused at low Ca2+ concentration (0.8 mM instead of 1.5 mM), at low preload (5 cm instead of 20 cm), in the presence of 100 nM nifedipine or with a cardioplegic solution. After 60 min, hypocalcemic perfusion produced significant reduction in [3H]-PN 200-110 and [3H]-ryanodine binding, due to approximately 30% reduction in Bmax (P<0.01), with unchanged Kd. Such modifications were reversible. Similar results were obtained in the nifedipine and cardioplegia groups. Low preload perfusion produced similar contractile effects as hypocalcemic perfusion, but it had no effect on radioligand binding. After hypocalcemic perfusion, DHPR and RyR gene expression, evaluated by RT-PCR, were not modified. Chelerythrine (protein kinase C inhibitor) and lavendustin C (Ca2+/calmodulin-dependent protein kinase II inhibitor), but not H-89 (protein kinase A inhibitor), abolished the effects of hypocalcemic perfusion on [3H]-PN 200-110 and [3H]-ryanodine binding. We conclude that reduced Ca2+ entry and/or intracellular Ca2+ cycling determines DHPR and RyR remodeling through posttranslational protein modifications. Both protein kinase C and Ca2+/calmodulin-dependent protein kinase II appear to play a role in this phenomenon.

Cardioprotective effects of 9-hydroxyellipticine on ischemia and reperfusion in isolated rat heart

We determined the effect of 9-hydroxyellipticine (9HE) on ryanodine receptor (RyR) and cardiac function after global ischemia in isolated rat hearts. The binding of [3H]-ryanodine in rabbit cardiac sarcoplasmic reticulum was displaced by 9HE in a biphasic manner corresponding to the two sites model with IC50 values of 6.1 microM and 55 mM. The increase of the intracellular Ca2+ concentration induced by caffeine in CHO cells expressing cardiac-type RyR was suppressed by 9HE in a concentration-dependent manner. Pretreatment of the heart with 9HE decreased the total duration of reperfusion-induced ventricular fibrillation (VF) and delayed the onset of VF. There was also a significant recovery of contractile force of ischemic hearts following 9HE. Unlike nifedipine, an L-type Ca2+-channel blocker, 9HE did not suppress the contraction of rat papillary muscles. Thus, 9HE exerts the cardioprotective effects against ischemia /reperfusion injury without changing hemodynamic indices.

Preconditioning prevents alterations in cardiac SR gene expression due to ischemia-reperfusion

We have previously shown that ischemic preconditioning (IP) improves cardiac performance and sarcoplasmic reticulum (SR) function in hearts subjected to ischemia-reperfusion (I/R). In this study, we examined the effect of IP on I/R-induced changes in gene expression for SR proteins such as the Ca(2+) release channel, Ca(2+) pump ATPase, phospholamban, and calsequestrin in the isolated rat heart. Normal isolated rat hearts exposed to three brief cycles of IP (5-min ischemia and 5-min reperfusion) exhibited a significant decrease in the transcript levels of SR genes. Nonpreconditioned I/R hearts when subjected to 30-min ischemia and 30-min reperfusion showed a marked decrease in mRNA levels for the SR proteins compared with normal hearts; this decrease was attenuated by preconditioning. Although hearts subjected to Ca(2+) paradox (CP) have been shown to exhibit intracellular Ca(2+) overload and SR dysfunction like those in I/R hearts, virtually nothing is known regarding the effect of CP on cardiac SR gene expression. Accordingly, CP (5-min Ca(2+)-free perfusion and 30-min reperfusion with normal medium) was observed to produce dramatic changes in SR gene expression, and the heart failed to contract; these alterations were attenuated by IP. Our results show that 1) both I/R and CP depress SR gene expression in the normal heart, 2) IP attenuates I/R- and CP-induced depression in cardiac function and SR gene expression, and 3) intracellular Ca(2+) overload may play a role in depressing SR gene expression in both I/R and CP hearts.

Ischemic and anesthetic preconditioning reduces cytosolic [Ca2+] and improves Ca(2+) responses in intact hearts

Ca(+) loading during reperfusion after myocardial ischemia is linked to reduced cardiac function. Like ischemic preconditioning (IPC), a volatile anesthetic given briefly before ischemia can reduce reperfusion injury. We determined whether IPC and sevoflurane preconditioning (SPC) before ischemia equivalently improve mechanical and metabolic function, reduce cytosolic Ca(2+) loading, and improve myocardial Ca(2+) responsiveness. Four groups of guinea pig isolated hearts were perfused: no ischemia, no treatment before 30-min global ischemia and 60-min reperfusion (control), IPC (two 2-min occlusions) before ischemia, and SPC (3.5 vol%, two 2-min exposures) before ischemia. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured at the left ventricular (LV) free wall with the fluorescent probe indo 1. Ca(2+) responsiveness was assessed by changing extracellular [Ca(2+)]. In control hearts, initial reperfusion increased diastolic [Ca(2+)] and diastolic LV pressure (LVP), and the maximal and minimal derivatives of LVP (dLVP/dt(max) and dLVP/dt(min), respectively), O(2) consumption, and cardiac efficiency (CE). Throughout reperfusion, IPC and SPC similarly reduced ischemic contracture, ventricular fibrillation, and enzyme release, attenuated rises in systolic and diastolic [Ca(2+)], improved contractile and relaxation indexes, O(2) consumption, and CE, and reduced infarct size. Diastolic [Ca(2+)] at 50% dLVP/dt(min) was right shifted by 32-53 +/- 8 nM after 30-min reperfusion for all groups. Phasic [Ca(2+)] at 50% dLVP/dt(max) was not altered in control but was left shifted by -235 +/- 40 nM [Ca(2+)] after IPC and by -135 +/- 20 nM [Ca(2+)] after SPC. Both SPC and IPC similarly reduce Ca(2+) loading, while augmenting contractile responsiveness to Ca(2+), improving postischemia cardiac function and attenuating permanent damage.

Effects of cardioplegic arrest and reperfusion on rabbit cardiac ryanodine receptors

Calcium overload is considered to be a primary contributor to ischemia-reperfusion injury. Cardiac sarcoplasmic reticulum (SR), the main regulator of intracellular Ca2+ concentration under normal conditions, is a target for ischemic myocardial injury. The ryanodine receptor (RyR) is the SR Ca2+ release channel. Previous reports have shown that a reduction in RyR activity during global myocardial ischemia correlates with concomitant myocardial dysfunction. Crystalloid cardioplegia, a technique for myocardial protection during heart operations, reduces Ca2+ accumulation during global ischemia. Hence, the effects of cardioplegia on RyR in isolated rabbit hearts was investigated. The study also compared [3H] ryanodine binding before ischemia (control group), after 30 min of ischemia (either global ischemia (GI group) or cardioplegic arrest (CA group)), and after 20 min of reperfusion. The GI group, but not the CA group, showed a significant reduction in the maximum number of binding sites (Bmax) for RyR compared with the control group (Control vs GI group: after ischemia, 1.33+/-0.27 vs 0.83+/-0.12 pmol/mg protein, p<0.05; after reperfusion, 1.33+/-0.27 vs 0.80+/-0.08 pmol/mg protein; p<0.05). CA group: after ischemia, 1.22+/-0.20 pmol/mg protein; after reperfusion, 1.15+/-0.28 pmol/mg protein). The affinity (Kd) values for [3H] ryanodine binding were not different among the 3 groups at any point. The preservation of RyR numbers during cardioplegia correlated with the concomitant preservation of cardiac functions. The results indicate that number of functional RyR was much better preserved during cardioplegia than during global ischemia. It is postulated that cardioplegia-induced protection of cardiac RyR may result in the protection of SR function during ischemia-reperfusion.

Myocardial ischemic preconditioning and mitochondrial F1F0-ATPase activity

A short period of ischemia followed by reperfusion (ischemic preconditioning) is known to trigger mechanisms that contribute to the prevention of ATP depletion. In ischemic conditions, most of the ATP hydrolysis can be attributed to mitochondrial F1F0-ATPase (ATP synthase). The purpose of the present study was to examine the effect of myocardial ischemic preconditioning on the kinetics of ATP hydrolysis by F1F0-ATPase. Preconditioning was accomplished by three 3-min periods of global ischemia separated by 3 min of reperfusion. Steady state ATP hydrolysis rates in both control and preconditioned mitochondria were not significantly different. This suggests that a large influence of the enzyme on the preconditioning mechanism may be excluded. However, the time required by the reaction to reach the steady state rate was increased in the preconditioned group before sustained ischemia, and it was even more enhanced in the first 5 min of reperfusion (101 +/- 3.0 sec in preconditioned vs. 83.4 +/- 4.4 sec in controls, p < 0.05). These results suggest that this transient increase in activation time may contribute to the cardioprotection by slowing the ATP depletion in the very critical early phase of post-ischemic reperfusion.

Effect of MEN 10755, a new disaccharide analogue of doxorubicin, on sarcoplasmic reticulum Ca(2+) handling and contractile function in rat heart

1. The use of anthraquinone antineoplastic agents is limited by their cardiac toxicity, which is largely due to activation of the sarcoplasmic reticulum (SR) Ca(2+) release channel (ryanodine receptor). MEN 10755 is a new disaccharide analogue of doxorubicin. We have evaluated its effects on SR function and its toxicity in isolated working rat hearts. 2. In rat SR vesicles, doxorubicin stimulated [(3)H]-ryanodine binding by increasing its Ca(2+)-sensitivity. At 1 microM Ca(2+), ryanodine binding increased by 15.3+/-2.5 fold, with EC(50)=20.6 microM. Epirubicin produced a similar effect, i.e. 9.7+/-0.6 fold stimulation with EC(50)=11.1 microM. MEN 10755 increased ryanodine binding by 1.9+/-0.3 fold (P:<0.01 vs doxorubicin and epirubicin), with EC(50)=38.9 microM. 3. Ca(2+)-induced Ca(2+) release experiments were performed by quick filtration technique, after SR loading with (45)Ca(2+). At 2 microM Ca(2+), doxorubicin (50 microM) increased the rate constant of Ca(2+) release to 82+/-5 s(-1) vs a control value of 22+/-2 s(-1) (P:<0.01), whereas 50 microM MEN 10755 did not produce any significant effect (24+/-3 s(-1)). 4. Ca(2+)-ATPase activity and (45)Ca(2+)-uptake were not significantly affected by doxorubicin, its 13-dihydro-derivative, epirubicin, MEN 10755 and the 13-dihydro-derivative of MEN 10755, at concentrations < or =100 microM. 5. In isolated heart experiments, administration of 30 microM doxorubicin or epirubicin caused serious contractile impairment, whereas 30 microM MEN 10755 produced only minor effects. 6. In conclusion, in acute experiments MEN 10755 was much less cardiotoxic than equimolar doxorubicin or epirubicin. This result might be accounted for by reduced activation of SR Ca(2+) release.

Intracellular free calcium and mitochondrial membrane potential in ischemia/reperfusion and preconditioning

Moderation of calcium perturbations has been implicated in ischemic preconditioning. As mitochondria possess an effective Ca(2+)transporting system driven by the mitochondrial membrane potential, experiments were performed to study time-averaged intracellular free calcium and the mitochondrial membrane potential during preconditioning and ischemia-reperfusion. Isolated rat hearts were subjected to 5 min of preconditioning, a 9-min intervening reperfusion and 21 min of ischemia with subsequent reperfusion. The hearts were preloaded with the Ca(2+)indicator Fura-2 or the mitochondrial membrane potential probe safranine. A method was devised for correction for NADH autofluorescence in time-averaged Ca(2+)probing with Fura-2. The pH dependence of the apparent dissociation constant of the Ca(2+)complex of Fura-2 was determined. Intracellular free Ca(2+)increased during the 5-min ischemia, and this was reversed upon reperfusion. During protracted ischemia a continual Ca(2+)rise was observed when the fluorescence data were corrected for changes in pH. An initial sharp Fura-2 fluorescence spike upon final reperfusion was caused by a pH-dependent change in the dissociation constant of the Ca(2+)complex of Fura-2. In preconditioned hearts the free Ca(2+)was somewhat lower during reperfusion, but a major effect of preconditioning was observed during the prolonged ischemia. The decrease in mitochondrial membrane potential during prolonged ischemia was faster in the preconditioned heart with no difference during the final reperfusion. The effect of preconditioning on cell survival was reflected in a decrease in the post-ischemic washout of creatine kinase. The moderation of the ischemic and post-ischemic intracellular Ca(2+)increase, and the acceleration of the ischemic mitochondrial membrane potential decrease by ischemic preconditioning is in accord with the notion of the involvement of mitochondrial ATP sensitive K(+)channels in preconditioning. In studies on ischemia it is absolutely necessary to correct for the pH-sensitivity of the apparent dissociation constant of the calcium complex of Fura-2 to obtain reliable data for intracellular free calcium.

Ischemic preconditioning prevents I/R-induced alterations in SR calcium-calmodulin protein kinase II

Although Ca(2+)/calmodulin-dependent protein kinase II (CaMK II) is known to modulate the function of cardiac sarcoplasmic reticulum (SR) under physiological conditions, the status of SR CaMK II in ischemic preconditioning (IP) of the heart is not known. IP was induced by subjecting the isolated perfused rat hearts to three cycles of brief ischemia-reperfusion (I/R; 5 min ischemia and 5 min reperfusion), whereas the control hearts were perfused for 30 min with oxygenated medium. Sustained I/R in control and IP groups was induced by 30 min of global ischemia followed by 30 min of reperfusion. The left ventricular developed pressure, rate of the left ventricular pressure, as well as SR Ca(2+)-uptake activity and SR Ca(2+)-pump ATPase activity were depressed in the control I/R hearts; these changes were prevented upon subjecting the hearts to IP. The beneficial effects of IP on the I/R-induced changes in contractile activity and SR Ca(2+) pump were lost upon treating the hearts with KN-93, a specific CaMK II inhibitor. IP also prevented the I/R-induced depression in Ca(2+)/calmodulin-dependent SR Ca(2+)-uptake activity and the I/R-induced decrease in the SR CaMK II activity; these effects of IP were blocked by KN-93. The results indicate that IP may prevent the I/R-induced alterations in SR Ca(2+) handling abilities by preserving the SR CaMK II activity, and it is suggested that CaMK II may play a role in mediating the beneficial effects of IP on heart function.

Mechanisms of ischemic preconditioning effects on Ca(2+) paradox-induced changes in heart

The effects of ischemic preconditioning (IP) on changes in cardiac performance and sarcoplasmic reticulum (SR) function due to Ca(2+) paradox were investigated. Isolated perfused hearts were subjected to IP (three cycles of 3-min ischemia and 3-min reperfusion) followed by Ca(2+)-free perfusion and reperfusion (Ca(2+) paradox). Perfusion of hearts with Ca(2+)-free medium for 5 min followed by reperfusion with Ca(2+)-containing medium for 30 min resulted in a dramatic decrease in the left ventricular (LV) developed pressure and a marked increase in LV end-diastolic pressure. Alterations in cardiac contractile activity due to Ca(2+) paradox were associated with depressed SR Ca(2+)-uptake, Ca(2+)-pump ATPase, and Ca(2+)-release activities as well as decreased SR protein contents for Ca(2+)-pump and Ca(2+) channels. All these changes due to Ca(2+) paradox were significantly prevented in hearts subjected to IP. The protective effects of IP on Ca(2+) paradox changes in cardiac contractile activity as well as SR Ca(2+)-pump and Ca(2+)-release activities were lost when the hearts were treated with 8-(p-sulfophenyl)-theophylline, an adenosine receptor antagonist; KN-93, a specific Ca(2+)/calmodulin-dependent protein kinase II (CaMK II) inhibitor; or chelerythrine chloride, a protein kinase C (PKC) inhibitor. These results indicate that IP rendered cardioprotection by preventing a depression in SR function in Ca(2+) paradox hearts. Furthermore, these beneficial effects of IP may partly be mediated by adenosine receptors, PKC, and CaMK II.

Is a functional sarcoplasmic reticulum necessary for preconditioning?

Several studies have shown that the protective effect of ischemic preconditioning (PC) is associated with decreased calcium release from the sarcoplasmic reticulum (SR). However, no study has yet demonstrated whether these changes are essential in the mechanism of PC. In order to investigate whether a functional SR was necessary for PC, we manipulated SR calcium handling using (i) 0.1microM ryanodine (RY), a concentration known to lock the SR calcium release channel in the open state and (ii) 50microM cyclopiazonic acid (CPA), a specific inhibitor of the SR calcium ATPase. Initial experiments confirmed that both RY and CPA eliminated the ability of the SR to accumulate calcium. Isolated rat hearts (n=6-7/group) were perfused normoxically for 30 min prior to either a further 40 min of perfusion [control (C)] or 4x[5 min ischemia (I) + 5 min reperfusion (R)] (PC). All hearts were then subjected to a further 40 min I + 40 min R. The C and PC protocols were then repeated in the presence of RY or CPA, introduced after 10 min of perfusion.(31)P-NMR was used to measure ATP, PCr, P(i)and intracellular pH. RY and CPA decreased developed pressure (DP) by 75% and 59%, respectively. Percentage recovery of LVDP was significantly higher in PC (72+/-8%), PC+RY (72+/-7%) and PC+CPA (49+/-7%) groups compared with their respective controls (43+/-7%, 47+/-7% and 10+/-4%) (P<0.05). Thus, PC remains protective in the presence of a SR unable to accumulate calcium, suggesting that the changes in SR calcium release are not essential in the mechanism of preconditioning.

Alterations in cardiac sarcoplasmic reticulum Ca2+ regulatory proteins in the atrial tissue of patients with chronic atrial fibrillation

OBJECTIVES

Our purpose was to determine whether atrial fibrillation (AF) patients have alterations in sarcoplasmic reticulum (SR) Ca2+ regulatory proteins in the atrial myocardium.

BACKGROUND

Clinically, AF is the most frequently encountered arrhythmia. Recent studies indicate that an inability to maintain intracellular Ca2+ homeostasis with a consequent increase in membrane-triggered activity could be the primary initiating factor in some circumstances, and that cytosolic Ca2+ abnormalities are an important mediator of sustained AF.

METHODS

We measured the maximum number of [3H]ryanodine binding sites (Bmax) and the expression levels of ryanodine receptor (RyR) mRNA and calcium-adenosine triphosphatase (Ca2+-ATPase) mRNA in atrial myocardial tissue from 13 patients with AF due to mitral valvular disease (MVD) and 9 patients with normal sinus rhythm (NSR).

RESULTS

In AF patients, 1) Bmax was significantly lower in each atrium (0.21+/-0.03 pmol/mg [right], 0.16+/-0.04 pmol/mg [left]) than in the right atrium (0.26+/-0.08 pmol/mg) of NSR patients; 2) Bmax was significantly lower in the left atrium than in the right atrium; 3) Bmax in the left atrium was significantly lower at higher levels of pulmonary capillary wedge pressure; 4) the expression level of RyR mRNA was significantly lower in both the left (1.24 x 10(-2)+/-1.28 x 10(-2)) and right (1.70 x 10(-2)+/-1.78 x 10(-2)) atrium than in the right atrium of NSR patients (6.11 x 10(-2)+/-2.79 x 10(-2)); and 5) the expression level of Ca2+-ATPase mRNA was significantly lower in both the left (5.67 x 10(-2)+/-4.01 x 10(-2)) and right (7.71 x 10(-2)+/-3.56 x 10(-2)) atrium than in the right atrium (12.60 x 10(-2)+/-3.92 x 10(-2)) of NSR patients.

CONCLUSIONS

These results provide the first direct evidence of abnormalities in the Ca2+ regulatory proteins of the atrial myocardium in chronic AF patients. Conceivably, such abnormalities may be involved in the initiation and/or perpetuation of AF.

Cellular mechanisms of infarct size reduction with ischemic preconditioning. Role of calcium?

Brief episodes of ischemia protect or "precondition" the heart and reduce infarct size caused by a subsequent sustained ischemic insult. Despite a decade of intensive investigation, the cellular mechanism(s) responsible for this paradoxical protection remain poorly understood. In this review, we focus on the emerging concept that alterations in intracellular calcium homeostasis may participate in either triggering and/or mediating infarct size reduction with preconditioning.

Growth hormone preserves cardiac sarcoplasmic reticulum Ca2+ release channels (ryanodine receptors) and enhances cardiac function in cardiomyopathic hamsters

OBJECTIVE

Growth hormone (GH) improves cardiac function in experimental models of heart failure and human dilated cardiomyopathy. However, the mechanism by which GH increases myocardial contractility is not entirely clear. Our aim was to examine the effects of GH on cardiac function and cardiac sarcoplasmic reticulum Ca2+ release channels (ryanodine receptors, RyR) in the hearts of UM-X7.1 cardiomyopathic hamsters during the development of heart failure.

METHODS

Experimental and healthy control hamsters were examined at the age of 20 weeks. Recombinant human GH (2 mg/kg/day, s.c.) or vehicle was then administered for 3 weeks. We examined (i) the in vivo left ventricular (LV) size and LV systolic function using transthoracic echocardiography, (ii) the density (Bmax) and affinity (Kd) of high-affinity [3H] ryanodine binding sites in crude homogenates from normal and cardiomyopathic hamster hearts.

RESULTS

Vehicle-treated UM-X7.1 hamsters exhibited significant increases in left ventricular end-diastolic diameter and end-systolic diameter (LVESd), and a significant decrease in LV fractional shortening (FS). GH-treatment attenuated the increase in LVESd and reduced the LV chamber size, and also significantly increased LVFS. Vehicle-treated UM-X7.1 hamsters exhibited a significantly lower Bmax than control hamsters (0.34 +/- 0.04 vs 0.44 +/- 0.06 pmol/mg, p < 0.05), and the treatment with GH in UM-X7.1 hamsters significantly attenuated the reduction of Bmax (0.42 +/- 0.03 pmol/mg vs vehicle-treated group (0.34 +/- 0.04 pmol/mg), p < 0.05). Kd did not differ significantly between the experimental groups. In normal control hamsters, GH treatment with this dose did not significantly enhance LV systolic function or the density of RyRs. There was no significant difference in terms of the connective-tissue volume-fraction, myocyte size and capillary density between the GH- and vehicle-treated groups of UM-X7.1 hamsters.

CONCLUSIONS

GH treatment may improve cardiac function by preserving the density of RyRs and enhancing cellular function in cardiomyopathic hamster hearts.

Modification of ischemia-reperfusion-induced changes in cardiac sarcoplasmic reticulum by preconditioning

To examine the effects of ischemic preconditioning on ischemia-reperfusion-induced changes in the sarcoplasmic reticulum (SR) function, isolated rat hearts were either perfused with a control medium for 30 min or preconditioned with three episodes of 5-min ischemia and 5-min reperfusion before sustained ischemia for 30 min followed by reperfusion for 30 min was induced. Preconditioning itself depressed cardiac function (left ventricular developed pressure, peak rate of contraction, and peak rate of relaxation) and SR Ca2+-release and -uptake activities as well as protein content and Ca2+/calmodulin-dependent protein kinase (CaMK) phosphorylation of Ca2+-release channels by 25-60%. Global ischemia for 30 min produced marked depressions in SR Ca2+-release and -uptake activities as well as SR Ca2+-pump protein content in control hearts; these changes were significantly attenuated by preconditioning. Compared with the control preparations, preconditioning improved the recovery of cardiac function and SR Ca2+-release and -uptake activities as well as Ca2+-release channel and Ca2+-pump protein contents in the ischemic-reperfused hearts. Unlike the protein kinase A-mediated phosphorylation in SR membranes, the CaMK-mediated phosphorylations at Ca2+-release channels, Ca2+ pump, and phospholamban were depressed in the ischemic hearts; these changes were prevented by preconditioning. These results indicate that ischemic preconditioning may exert beneficial effects on ischemia-reperfusion-induced alterations in SR function by preventing changes in Ca2+-release channel and Ca2+-pump protein contents in the SR membrane.

Ischemic preconditioning: exploring the paradox

Brief transient episodes of nonlethal myocardial ischemia protect or "precondition" the heart and render the myocardium resistant to a subsequent more sustained ischemic insult. The hallmark of this phenomenon--documented in virtually all species and experimental models evaluated to date in countless laboratories worldwide--is the profound reduction in infarct size seen in preconditioned groups versus time-matched controls. Efforts to identify the cellular mechanisms responsible for this paradoxical ischemia-induced cardioprotection, to expand the definition of ischemic preconditioning beyond infarct size reduction, and, perhaps most importantly, to evaluate the efficacy of preconditioning in disease models and in the clinical setting, are all topics of intensive ongoing investigation.

Tumor necrosis factor in the heart

The heart is a tumor necrosis factor (TNF)-producing organ. Both myocardial macrophages and cardiac myocytes themselves synthesize TNF. Accumulating evidence indicates that myocardial TNF is an autocrine contributor to myocardial dysfunction and cardiomyocyte death in ischemia-reperfusion injury, sepsis, chronic heart failure, viral myocarditis, and cardiac allograft rejection. Indeed, locally (vs. systemically) produced TNF contributes to postischemic myocardial dysfunction via direct depression of contractility and induction of myocyte apoptosis. Lipopolysaccharide or ischemia-reperfusion activates myocardial P38 mitogen-activated protein (MAP) kinase and nuclear factor kappa B, which lead to TNF production. TNF depresses myocardial function by nitric oxide (NO)-dependent and NO-independent (sphingosine dependent) mechanisms. TNF activation of TNF receptor 1 or Fas may induce cardiac myocyte apoptosis. MAP kinases and TNF transcription factors are feasible targets for anti-TNF (i.e., cardioprotective) strategies. Endogenous anti-inflammatory ligands, which trigger the gp130 signaling cascade, heat shock proteins, and TNF-binding proteins, also control TNF production and activity. Thus modulation of TNF in cardiovascular disease represents a realistic goal for clinical medicine.

Alterations in cardiac SR Ca(2+)-release channels during development of heart failure in cardiomyopathic hamsters

The cardiomyopathic Syrian hamster develops a progressive cardiomyopathy characterized by cellular necrosis, hypertrophy, cardiac dilatation, and congestive heart failure. This study aimed to identify alterations in cardiac mechanical function and in the cellular content of sarcoplasmic reticulum (SR) Ca(2+)-release channels (ryanodine receptors, RyR) in the heart of the UM-X7.1 cardiomyopathic hamster during the development of heart failure. Experimental and healthy control hamsters were examined at 8, 18, and 28 wk of age. The UM-X7.1 hamsters had developed left ventricular (LV) hypertrophy at 8 wk and a marked LV dilatation at 18-28 wk. During the latter stage, the UM-X7.1 hamster hearts showed global hypokinesis. Equilibrium binding assays of high-affinity sites for [3H]ryanodine were performed in ventricular homogenate preparations. There was no significant difference between the two groups in the maximum number of [3H]ryanodine binding sites (Bmax) at either 8 or 18 wk of age, although the cardiac pump function was impaired in UM-X7.1 hamsters at 18 wk of age. By 28 wk, Bmax was significantly lower in the UM-X7.1 hamsters. Quantitative immunoblot assay revealed that the content of RyR protein in cardiomyopathic hearts, which was increased at the early stage, declined to below normal as heart failure advanced. These results suggest that the number of RyR in the UM-X7.1 cardiomyopathic hamsters was preserved at both the hypertrophic and early stages of heart failure with a possibly compensatory increase in the level of protein expression, although the cardiac function already showed a tendency to be impaired.

Ischemic preconditioning: antiarrhythmic effects and electrophysiological mechanisms in isolated ventricle

The objective of this study was to identify cellular electrophysiological mechanisms by which ischemic preconditioning decreases arrhythmias in an isolated ventricular tissue model of ischemia and reperfusion. Electrical activity was recorded with microelectrodes from endocardium and epicardium of paced guinea pig right ventricular free walls. Control preparations were exposed for 15 min to Tyrode solution modified to simulate selected ischemic conditions and then were reperfused for 30 min with normal solution. Preconditioned tissues were exposed to a 2- or 5-min period of simulated ischemia before this same protocol. Neither preconditioning protocol affected incidence of ventricular tachycardia (VT) in ischemia; however, the 5-min protocol significantly decreased premature beats (PVB) and transmural conduction block. Preconditioning for 5 min, but not 2 min, significantly decreased reperfusion-induced VT and PVB. Ischemic preconditioning did not change effects of ischemia or reperfusion on action potential duration, effective refractory period, or endocardial conduction time. However, preconditioning markedly attenuated depression of transmural conduction by ischemia and early reperfusion and thereby prevented conduction delays necessary for transmural reentry.

Preconditioning the globally ischaemic, isolated rat heart: the impact of the preconditioning model on post-ischaemic systolic and diastolic function

In studies of preconditioning, a variety of models have been used. The aim of the present study was to find the optimal preconditioning model for preservation of cardiac function during reperfusion of globally ischaemic, Langendorff-perfused rat hearts. Cardiac function was assessed by the occurrence of severe reperfusion arrhythmias (ventricular fibrillation or asystolia), heart rate (HR), left ventricular systolic (LVSP), end diastolic (LVEDP), and developed pressures (LVDP = LVSP - LVEDP), as well as coronary flow (CF). Series 1 (n = 17) in each group: control perfusion for 20 min without preconditioning or 2 episodes of 2, 3, 4, or 5 min of ischaemia, each followed by 5 min reperfusion, before 25 min ischaemia and 60 min reperfusion. Preconditioning reduced the incidence of reperfusion arrhythmias, attenuated the reperfusion-induced increase of LVEDP, and increased CF, but did not influence LVSP, LVDP, or rate x pressure-product (RPP = LVSP x HR) during reperfusion. The greatest effect was found by 2 min ischaemia and 5 min reperfusion. In series 2 (n = 17 in each group) control perfusion for 7 or 28 min, or preconditioning with 1-4 episodes of 2 min ischaemia and 5 min reperfusion before 35 min ischaemia and 60 min reperfusion were compared. Reduction of severe reperfusion arrhythmias and LVEDP elevation, as well as improvement of CF, LVDP, and HR in preconditioned hearts were observed in series 2. Optimal cardioprotection was achieved by only one episode of preconditioning. In conclusion, preconditioning before global ischaemia improved cardiac function during reperfusion of isolated rat hearts. The most marked effects were reduction of severe reperfusion arrhythmias and attenuation of diastolic dysfunction. Although all preconditioning models employed were cardioprotective, 1 episode of 2 min ischaemia provided optimal protection.

Adaptive and maladaptive mechanisms of cellular priming

OBJECTIVE

The mechanisms of cellular priming resulting in both adaptive and maladaptive responses to subsequent injury and strategies for manipulating this priming to constructive therapeutic advantage are explored.

BACKGROUND DATA

A cell is prepared or educated by an initial insult (priming stimulus). Investigations in both laboratory animals and humans indicate that cells, organs, and perhaps even whole patients respond differently to a proximal second insult ("second hit") by virtue of this prior environmental history. The opportunity to achieve the primed state appears to be conserved across almost all cell types. The initial stimulus transmits a message to the cellular machinery that influences the cell's response to a subsequent challenge. This response may result in an exaggerated inflammatory response in the case of the neutrophil (an often maladaptive process) or an improved tolerance to injury by the myocyte (adaptive response). Our global hypothesis is that cellular priming is a conserved, receptor-dependent process that invokes common intracellular targets across multiple cell types. We further postulate that these targets create a language based on the transient phosphorylation and dephosphorylation of intracellular enzymes that is therapeutically accessible.

CONCLUSIONS

Priming is a conserved, receptor-dependent process transduced by means of intracellular targets across multiple cell types. The potential therapeutic strategies outlined involve the receptor-mediated manipulation of cellular events. These events are transmitted through an intracellular language that instructs the cell regarding its behavior in response to subsequent stimulation. Understanding these intracellular events represents a realistic goal of priming and preconditioning biology and will likely lead to clinical control of the primed state.

Mechanisms of cardiac preconditioning: ten years after the discovery of ischemic preconditioning

Cardiac preconditioning describes the phenomenon by which transient ischemia induces myocardial protection against subsequent ischemia and reperfusion injury. Ten years have passed since the original description of this potent cardiac protective strategy and within this period tremendous progress has been made elucidating the mechanisms of preconditioning. Mechanistic understanding may allow safe clinical application. This review (1) recalls the history of preconditioning and how it relates to the history of the investigation of endogenous adaptation; (2) summarizes the current mechanistic understanding of early preconditioning; (3) compares and contrasts the mechanisms of early versus delayed preconditioning; (4) suggests potential anti-inflammatory aspects of preconditioning; (5) examines limitations in laboratory models of preconditioning; and (6) explores the potential of using preconditioning clinically.

Compensatory up-regulation of cardiac SR Ca2+-pump by heat-shock counteracts SR Ca2+-channel activation by ischemia/reperfusion

We tested the hypothesis that heat-shock protected myocardial Ca2+-cycling by sarcoplasmic reticulum from ischemia and reperfusion (I/R) injury. Twenty-four hours after increasing body temperature to 42 degrees C for 15 min, rat hearts were isolated, Langendorff-perfused, and subjected to 30 min ischemia then 30 min reperfusion. Left ventricles were homogenized and their ionized Ca2+ concentration monitored with indo- during Ca2+-uptake in the presence and absence of the Ca2+-release channel (CRC) modulator ryanodine. Tissue content of heat-shock protein 72 (HSP 72) was analyzed. Exposure to I/R resulted in a 37% enhancement of CRC activity but no effect on Ca2+-pumping activity, resulting in 25% decreased net Ca2+-uptake activity. Pre-exposure to heat-shock resulted in a 10-fold increase in HSP 72, and a 25% enhancement of maximal Ca2+-pumping activity which counteracted the effect of I/R on CRC and net Ca2+-uptake activities. This protection of SR Ca2+-cycling was associated with partial protection of myocardial physiological performance. Net Ca2+-uptake activity was correlated with the left ventricular developed pressure and its rate of change. We conclude that one of the mechanisms by which heat-shock protects myocardium from I/R injury is to upregulate SR Ca2+-pumping activity to counteract the enhanced SR Ca2+-release produced by I/R.

Protein kinase C isoform diversity in preconditioning

Protein kinase C (PKC) appears to be a common intracellular effector and signal collector during cardiac preconditioning; however, it remains unknown whether agonists that activate different PKC isoforms are also linked to select aspects of myocardial protection. Using agonists that are known to activate unique combinations of PKC isoforms, we interrogated the relationship between isoform activation and the different aspects (pH, function, and viability) of endogenous myocardial protection. To study this, isolated rat hearts were subjected to ischemia-reperfusion (I/R) (20 min/40 min), without (control = Ctrl) or with receptor-dependent [phenylephrine (PE), 50 microM; adenosine (ADO), 125 microM] or -independent [phorbol myristate acetate (PMA), 100 nM] activation of PKC. Function, pH, and viability were assessed by rate pressure product (%RPP) and coronary flow (CF; ml/min), by 31P NMR, and by CF creatine kinase (CK; U/liter) leak, respectively. PMA, which activates PKC delta but not eta, resulted in intracellular pH (pHi) and viability protection, but did not protect against postischemic myocardial stunning. ADO, which activates PKC eta but not delta, protects against stunning, but not acidosis or necrosis. PE, which activates PKC delta and eta, provided global myocardial protection against necrosis, acidosis, and stunning. Different PKC isoforms may be linked to distinct aspects of myocardial protection. Targeted activation of PKC isoforms may allow precise mechanistic application of preconditioning-like myocardial protection.

Protection induced by a brief ischemic episode in the Langendorff perfused rat heart

RATIONALE AND OBJECTIVES

Ischemic episodes lasting approximately 1 min may be associated with coronary angioplasty. We explored whether such episodes could induce myocardial protection against prolonged ischemic episodes in an ex vivo model.

METHODS

Protection afforded by pretreatment with a 1-min ischemic episode (ischemic preconditioning) against prolonged ischemia was investigated in the isolated rat heart. Left ventricular developed pressure (LVDP; LV systolic pressure-LV end-diastolic pressure), heart rate (HR), and enzyme leakage (lactate dehydrogenase) were indexes of protection.

RESULTS

An increased recovery of LVDP x HR after 16 and 19 min of ischemia of 37% and 28%, respectively, paralleled by reduced enzyme leakage, was observed in preconditioned hearts after 10 min of reperfusion. However, the difference between preconditioned and control hearts was lost after 30 min of reperfusion.

CONCLUSION

Ischemic episodes lasting approximately 1 min are not sufficient to initiate stable protection even if initial functional and metabolic indexes suggest a protective effect.

Effect of gallopamil on excitation-contraction coupling

1. Investigations performed in skeletal muscle have suggested that phenylalkylamine calcium antagonists, particularly gallopamil, affect excitation-contraction coupling independently of their effect on the sarcolemmal calcium current. 2. Sarcoplasmic reticulum and single channel studies have provided evidence that phenylalkylamine calcium antagonists inhibit calcium release through the sarcoplasmic reticulum calcium channel/ryanodine receptor. This action has not been observed with dihydropyridine calcium antagonists. 3. Binding experiments have confirmed the existence of intracellular binding sites for phenylalkylamines, and have shown that gallopamil interferes with the binding of ryanodine to its low affinity sites. 4. The dose-response relationship for the effect of gallopamil on excitation-contraction coupling has not been definitely established. However, there is evidence that gallopamil may be effective at concentrations that are close to the therapeutic range.

Is potassium channel opening an effective form of preconditioning before cardioplegia?

BACKGROUND

Opening of adenosine triphosphate-sensitive potassium channels might be one of the mechanisms by which preconditioning preserves the myocardium against ischemic damage. The present study was therefore designed to compare the protective efficacy of ischemic preconditioning with that of pharmacologic preconditioning involving the use of a potassium channel opener in a surgically relevant model of cold cardioplegic arrest.

METHODS

Thirty isolated isovolumic rat hearts were subjected to 2 hours of potassium arrest at an average myocardial temperature of 23 degrees C, followed by 1 hour of reperfusion. Three groups (n = 10 per group) were studied: (1) control (no prearrest intervention); (2) ischemic preconditioning, achieved with 5 minutes of noflow ischemia followed by 5 minutes of reperfusion before arrest; and (3) pharmacologic preconditioning, achieved with a 5-minute infusion of the potassium channel opener nicorandil (10 mumol/L) followed by 5 minutes of drug-free perfusion before arrest. Standard functional indices were measured at multiple times during reperfusion, at the end of which pressure-volume curves were constructed and compared with those obtained at baseline.

RESULTS

Both ischemically and pharmacologically preconditioned hearts recovered systolic and diastolic function to a significantly greater extent than the controls. There was no difference in the recovery patterns between the forms of preconditioning. However, analysis of the postischemic pressure-volume curves demonstrated that nicorandil-preconditioned hearts incurred the smallest losses of compliance throughout the ischemia-reperfusion sequence.

CONCLUSIONS

The protective effects of a standard ischemic preconditioning challenge on functional recovery after an episode of moderately hypothermic cardioplegic arrest can be duplicated by pharmacologic opening of adenosine triphosphate-sensitive potassium channels. This finding may be of clinical relevance because of the availability of potassium channel openers, such as nicorandil, for human use.