Enhanced gene expression of calcium regulatory proteins in stunned porcine myocardium

Genetics and genomics of ischemic tolerance: focus on cardiac and cerebral ischemic preconditioning

A subthreshold ischemic insult applied to an organ such as the heart and/or brain may help to reduce damage caused by subsequent ischemic episodes. This phenomenon is known as ischemic tolerance mediated by ischemic preconditioning (IPC) and represents the most powerful endogenous mechanism against ischemic injury. Various molecular pathways have been implicated in IPC, and several compounds have been proposed as activators or mediators of IPC. Recently, it has been established that the protective phenotype in response to ischemia depends on a coordinated response at the genomic, molecular, cellular and tissue levels by introducing the concept of 'genomic reprogramming' following IPC. In this article, we sought to review the genetic expression profiles found in cardiac and cerebral IPC studies, describe the differences between young and aged organs in IPC-mediated protection, and discuss the potential therapeutic application of IPC and pharmacological preconditioning based on the genomic response.

Role of cytokines in myocardial ischemia and reperfusion

Mediators of myocardial inflammation, predominantly cytokines, have for many years been implicated in the healing processes after infarction. In recent years, however, more attention has been paid to the possibility that the inflammation may result in deleterious complications for myocardial infarction. The proinflammatory cytokines may mediate myocardial dysfunction associated with myocardial infarction, severe congestive heart failure, and sepsis. A growing body of literature suggests that inflammatory mediators could play a crucial role in ischemia–reperfusion injury. Furthermore, ischemia–reperfusion not only results in the local transcriptional and translational upregulation of cytokines but also leads to tissue infiltration by inflammatory cells. These inflammatory cells are a ready source of a variety of cytokines which could be lethal for the cardiomyocytes. At the cellular level it has been shown that hypoxia causes a series of well documented changes in cardiomyocytes that includes loss of contractility, changes in lipid metabolism and subsequent irreversible cell membrane damage leading to cell death. For instance, hypoxic cardiomyocytes produce interleukin-6 (IL-6) which could contribute to the myocardial dysfunction observed in ischemia reperfusion injury. Ischemia followed by reperfusion induces a number of other multi-potent cytokines, such as IL-1, IL-8, tumor necrosis factor-α (TNF-α), transforming growth factor-β1 (TGF-β1) as well as an angiogenic cytokine/ growth factor, vascular endothelial growth factor (VEGF), in the heart. Intrestingly, these multipotent cytokines (e.g. TNF-α) may induce an adaptive cytoprotective response in the reperfused myocardium. In this review, we have included a number of cytokines that may contribute to ventricular dysfunction and/or to the cytoprotective and adaptive changes in the reperfused heart.

Myocardial stunning is associated with impaired calcium uptake by sarcoplasmic reticulum

Myocardial stunning (temporary post-ischaemic contractile dysfunction) may be caused by oxidative stress and/or impaired myocyte calcium homeostasis. Regional myocardial stunning was induced in open-chest pigs (segment shortening reduced to 68.3+/-4.7% of baseline) by repetitive brief circumflex coronary occlusion (I/R). Reduced glutathione was depleted in stunned myocardium (1.34+/-0.06 vs. 1.77+/-0.11 nmol/mg, p=0.02 vs. remote myocardium) indicating regional oxidant stress, but no regional differences were observed in protein-bound 3-nitrotyrosine or S-nitrosothiol content. Repetitive I/R did not affect myocardial quantities of the sarcolemmal sodium-calcium exchanger, L-type channel, SR calcium ATPase and phospholamban, or the kinetics of ligand binding to L-type channels and SR calcium release channels. However, initial rates of oxalate-supported (45)Ca uptake by SR were impaired in stunned myocardium (41.3+/-13.5 vs. 73.0+/-15.6 nmol/min/mg protein, p=0.03). The ability of SR calcium ATPase to sequester cytosolic calcium is impaired in stunned myocardium. This is a potential mechanism underlying contractile dysfunction.

Early reperfusion phenomena

Cycles of ischemia-reperfusion are ubiquitous in clinical cardiology. Depending on the duration and intensity of the ischemic episode as well as its repetition mode, several pathophysiologic syndromes have been identified, such as myocardial stunning, hibernation, and preconditioning. It remains a difficult exercise to distinguish ischemic from reperfusion damage. Production of oxygen free radicals and alteration in calcium homeostasis are major players during early reperfusion, responsible for the pathologic and functional alterations. At the molecular level, upregulation and downregulation of a number of genes have been observed in stunned myocardium, pointing toward some inborn survival adaptive mechanism. The no-reflow phenomenon, a most paradoxic event after reperfusion, usually occurs after more prolonged episodes of ischemia. The underlying mechanism involves additional lesions to the microvasculature interacting with myocytes lesions. Further insight into molecular and genomic adaptation to ischemia and reperfusion will undoubtedly help to improve our ability to fight reperfusion injury.

Gene Expression of the Na+–Ca2+ Exchanger, SERCA2a and Calsequestrin after Myocardial Ischemia in the Neonatal Rabbit Heart

BACKGROUND

Neonatal hearts are less susceptible to developing myocardial dysfunction after hypoxia and/or ischemia than adult hearts. Differences in intracellular calcium homeostasis may be responsible for reduced calcium overload of the immature myocardium leading to the observed protection against ischemia.

OBJECTIVE

To assess differences in baseline and post-ischemic gene expression of calcium handling proteins after ischemia in neonatal and adult rabbit hearts.

METHODS

We used isolated antegrade perfused rabbit hearts (age 2 days, 28 days, n = 32), which were exposed to ischemia and hypothermia simulating myocardial stunning comparable to neonatal asphyxia. Gene and protein expression of the sodium-calcium exchanger (NCX), the sarco-endoplasmatic reticulum Ca2+-ATPase 2a (SERCA) and calsequestrin (CSQ) were measured using quantitative real-time PCR and Western blotting.

RESULTS

After ischemia and reperfusion in neonatal and adult hearts, a significant decrease in myocardial performance was recorded. At the mRNA level, significant differences in the baseline expression of NCX, SERCA and CSQ between neonatal and adult hearts were observed. In neonatal post-ischemic hearts, NCX and CSQ expression were significantly higher at the mRNA level than in controls. In contrast, SERCA expression remained unchanged in neonatal hearts and decreased in adult hearts compared to the non-ischemic controls.

CONCLUSION

These findings suggest that changes in gene expression of calcium handling proteins may be involved in the different susceptibility of neonatal compared to adult hearts to developing myocardial dysfunction after ischemia.

Mechanisms of Cell Survival in Myocardial Hibernation

Myocardial hibernation represents a condition of regional ventricular dysfunction in patients with chronic coronary artery disease, which reverses gradually after revascularization. The precise mechanism mediating the regional dysfunction is still debated. One hypothesis suggests that chronic hypoperfusion results in a self-protecting downregulation in myocardial function and metabolism to match the decreased oxygen supply. An alternative hypothesis suggests that the myocardium is subject to repetitive episodes of ischemic dysfunction resulting from an imbalance between myocardial metabolic demand and supply that eventually creates a sustained depression of contractility. It is generally agreed that hibernating myocardium is submitted repeatedly to ischemic stress, and therefore one question persists: how do myocytes survive in the setting of chronic ischemia? The hallmark of hibernating myocardium is a maintained viability of the dysfunctional myocardium which relies on an increased uptake of glucose. We propose that, in addition to this metabolic adjustment, there must be molecular switches that confer resistance to ischemia in hibernating myocardium. Such mechanisms include the activation of a genomic program of cell survival as well as autophagy. These protective mechanisms are induced by ischemia and remain activated chronically as long as either sustained or intermittent ischemia persists.

Gene expression profiling--a new approach in the study of myocardial ischemia

Current technologies make it possible to study thousands of genes simultaneously in the same biological sample - an approach termed gene expression profiling. Several techniques, including (i) differential display, (ii) serial analysis of gene expression (SAGE), (iii) subtractive hybridization and (iv) gene microarrays (Gene Chips), have been developed. Recently, gene profiling was applied in studying the mechanisms of ischemic injury and ischemic preconditioning. In the case of reversible ischemia caused by one or several brief transient episodes of complete coronary occlusion (as with ischemic preconditioning), or with a more prolonged but partial coronary ligation, many up-regulated genes were related to the "cell survival program". Protective genes included mitogen-activated protein kinase-activated protein kinase 3 (MAPKAPK 3), heat shock proteins 70, 27, 22, B-crystalline, vascular endothelial growth factor, inducible nitric oxide synthase and plasminogen activator inhibitors 1 and 2. With permanent coronary occlusion lasting from 24 h to several weeks, and resulting in a true myocardial infarction (MI), the list of up-regulated genes included those related to remodeling (e.g., collagens I and III, fibronectin, laminin) and apoptosis (Bax), while many down-regulated genes were related to major energy-generating pathways in the heart, namely, fatty acid metabolism. Gene expression profiling experiments have resulted in the discovery of two different genetic programs in the heart, namely, a protective program activated upon brief episodes of transient ischemia and an injury-related one activated in response to irreversible ischemic injury. Searching for factors turning on protective genes, and turning down injury-related ones, is a justifiable approach in developing new therapeutic strategies aimed to fight ischemic heart disease.

Novel mechanisms mediating stunned myocardium

Myocardial stunning is defined as the prolonged contractile dysfunction following an ischemic episode that does not result in necrosis, which also occurs in patients with coronary artery disease. There is also evidence to consider myocardial stunning as a fundamental component of hibernating myocardium. Various experimental approaches (from a brief episode to prolonged partial ischemia) and animal models (from rodents to large mammals) have been developed to investigate the pathogenesis of myocardial stunning. Three hypotheses to explain the mechanism, i.e. oxygen radical, Troponin I degradation, and Ca(2+), have been proposed. The first was tested primarily using large mammalian models, whereas the others were tested primarily using rodent models. Recently, the Ca(2+) handling hyothesis has been tested in a large mammalian swine model of myocardial stunning, in which both Ca(2+) and transients and L-type Ca(2+) current density were decreased. Relaxation function and phospholamban phosphorylation are also radically different in large mammalian and rodent models. In addition, troponin I degradation, which was identified as the mechanism of stunning in rodent models, was not found in stunned swine myocardium. Interestingly, the large mammalian model demonstrates that stunning elicits broad changes in gene and protein regulation, some of which have not been observed in the heart previously. The overall genomic adaptation upregulates the expression of survival genes that prevent irreversible damage. Pursuing these new concepts derived from large mammalian models of ischemia/reperfusion will provide more comprehensive mechanistic information underlying myocardial stunning and will serve to devise new therapeutic modalities for patients.

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.

Calcium sensitizer EMD 57033, but not the beta1-adrenoreceptor agonist dobutamine, increases mechanical efficiency in stunned myocardium

External work, efficiency of energy transfer (EET), and mechanical efficiency (defined as the ratio of external work over myocardial oxygen consumption (MVO2) are reduced in stunned myocardium. We therefore evaluated how inotropic stimulation by dobutamine and the calcium sensitizer EMD 57033 affected the regional stress-strain relationship as, from which contractility (E(es)), external work at the working point (EWwp), maximal external work (EWmax), EETwp (%), and EETmax are determined. Thirty minutes after regional stunning in 11 open chest pigs by two 10-min coronary occlusions separated by 10 min of reperfusion, dobutamine (0.5, 1, and 2 microg x kg(-1) x min(-1)) was infused, after an ample washout period followed by infusion of EMD 57033 (0.05, 0.1, 0.2 mg x kg(-1) x min(-1)). Stunning decreased E(es) (30%), EWwp (56%), EWmax (63%), EETwp (34%), EETmax (33%) and mechanical efficiency (55%), but MVO2 was unaffected. EWwp, EWmax, EETwp, and EETmax increased similarly with the two drugs, whereas MVO2 increased only after dobutamine. Consequently, mechanical efficiency increased linearly with contractility during EMD 57033 infusion but remained constant during infusion of dobutamine.

Myocyte apoptosis and reduced SR gene expression precede the transition from chronically stunned to hibernating myocardium

A systematic transition from chronic stunning to hibernation occurs as coronary flow reserve decreases to a critical level. Hibernating myocardium exhibits apoptosis-induced myocyte loss and a reduction in the expression of the sarcoplasmic reticulum (SR) Ca2+ ATPase but whether similar cellular changes occur in chronic stunning is unknown. Pigs with a chronic left anterior descending coronary artery (LAD) stenosis were studied one (n=9) or two (n=10) months after instrumentation. Anterior hypokinesis with normal levels of resting perfusion developed at each time-point, consistent with chronic stunning. After 1 month, sub-endocardial flow reserve was moderately reduced (adenosine/rest, LAD: 3.60+/-0.91 v Remote: 6.00+/-0.54, P<0.01) with no regional differences in SR protein expression, no increase in apoptosis (32+/-6 v 21+/-5 nuclei/10(6) myocyte nuclei, p-ns) and no regional myocyte loss (1976+/-44 v 1955+/-30 nuclei/mm2, p-ns). After 2 months, sub-endocardial flow reserve in chronically stunned myocardium was severely impaired (LAD: 1.41+/-0.21 v Remote: 5.59+/-0.96, P<0.01). There were small but significant reductions in LAD mRNA and protein levels for the SRCa2+ ATPase and phospholamban whereas calsequestrin was unchanged. In addition, regional myocyte apoptosis increased (127+/-24 v 55+/-9 nuclei/10(6) myocyte nuclei, P<0.01), resulting in the onset of myocyte loss (1293+/-50 v 1394+/-32 nuclei/mm2, P<0.01). Apoptosis-induced myocyte loss and reductions in SR protein expression are not invariably present in viable chronically dysfunctional myocardium. They are induced as the propensity of a region to develop reversible ischemia increases (as reflected by coronary flow reserve). The temporal progression indicates that alterations in SR protein expression and myocyte apoptosis precede the transition from chronically stunned to hibernating myocardium.

Modulation of cardiac sarcoplasmic reticulum gene expression by lack of oxygen and glucose

Although ischemia reperfusion has been shown to depress gene expression of the sarcoplasmic reticulum (SR) proteins, such as the ryanodine receptor, Ca2+-pump ATPase, phospholamban, and calsequestrin in the heart, the mechanisms of these changes are not understood. Given the occurrence of hypoxia and the lack of glucose during the ischemic phase, we investigated the effects of these factors on the cardiac SR gene expression. Isolated rat hearts perfused in the absence of oxygen and/or glucose for 30 min showed an increase in the expression of SR genes. However, perfusion of hearts for 60 min with normal oxygenated medium after 30 min of lack of both oxygen and glucose depressed the transcript levels for the SR proteins; these changes did not occur when hearts were deprived of either oxygen or glucose. The effect of intracellular Ca2+-overload, which occurs during reperfusion, was studied by using hearts perfused for 5 min with Ca2+-free medium and then reperfused for 30 min. Ca2+-depletion/repletion induced a dramatic decrease in the transcript levels of the SR genes. These results suggest that the lack of both oxygen and glucose during ischemia are necessary for reperfusion-induced depression in SR gene expression, possibly due to the occurrence of intracellular Ca2+-overload.

Gene program for cardiac cell survival induced by transient ischemia in conscious pigs

Therapy for ischemic heart disease has been directed traditionally at limiting cell necrosis. We determined by genome profiling whether ischemic myocardium can trigger a genetic program promoting cardiac cell survival, which would be a novel and potentially equally important mechanism of salvage. Although cardiac genomics is usually performed in rodents, we used a swine model of ischemia/reperfusion followed by ventricular dysfunction (stunning), which more closely resembles clinical conditions. Gene expression profiles were compared by subtractive hybridization between ischemic and normal tissue of the same hearts. About one-third (23/74) of the nuclear-encoded genes that were up-regulated in ischemic myocardium participate in survival mechanisms (inhibition of apoptosis, cytoprotection, cell growth, and stimulation of translation). The specificity of this response was confirmed by Northern blot and quantitative PCR. Unexpectedly, this program also included genes not previously described in cardiomyocytes. Up-regulation of survival genes was more profound in subendocardium over subepicardium, reflecting that this response in stunned myocardium was proportional to the severity of the ischemic insult. Thus, in a swine model that recapitulates human heart disease, nonlethal ischemia activates a genomic program of cell survival that relates to the time course of myocardial stunning and differs transmurally in relation to ischemic stress, which induced the stunning. Understanding the genes up-regulated during myocardial stunning, including those not previously described in the heart, and developing strategies that activate this program may open new avenues for therapy in ischemic heart disease.

Adenosine and cardioprotection during ischaemia and reperfusion--an overview

Adenosine is a local hormone, with numerous tissue-specific biological functions. In the myocardium, adenosine is released in small amounts at constant basal rate during normoxia. During ischaemia the production of adenosine increases several fold due to breakdown of adenosine triphosphate (ATP). Increased production of adenosine causes coronary vasodilatation. Thus, adenosine couples myocardial metabolism and flow during ischaemia and is called a homeostatic or "retaliatory metabolite". Furthermore, adenosine has electrophysiological effects in supraventricular tissue, causing a decrease in heart rate. In 1985 it was discovered that adenosine also exerts cardioprotective effects directly on cardiomyocytes. The aim of this review is to give an overview of the role of adenosine as a directly cytoprotective agent during myocardial ischaemia and reperfusion. We will focus on its effects on the myocytes, elicited by stimulation of adenosine receptors in sarcolemma, which triggers intracellular signalling systems. We will also address the new aspect that adenosine can influence regulation of gene expression. There is evidence that the myocardium is capable of endogenous adaptation in response to ischaemia, namely "hibernation" and early and late phases of "preconditioning". Endogenous substances produced during ischaemia probably trigger these responses. We will discuss the role of adenosine in these different settings. Adenosine can be given exogenously through intravasal routes; however, this review will also focus on the effects of endogenously produced adenosine. We will discuss pharmacological ways to increase endogenous levels of adenosine, and the effects of such interventions during ischaemia and reperfusion. Finally, we will review results from studies in humans together with relevant experimental studies, and indicate potential therapeutic implications of adenosine.

Biochemical mechanism(s) of stunning in conscious dogs

The mechanism(s) underlying contractile dysfunction in cardiac stunning is not completely understood. The expression and/or the phosphorylation state of cardiac Ca(2+) homoeostasis-regulating proteins might be altered in stunning. We tested this hypothesis in a well-characterized model of stunning. Conscious dogs were chronically instrumented, and the left anterior descending artery (LAD) was occluded for 10 min. Thereafter, reperfusion of the LAD was initiated. Tissues from reperfused LAD (stunned) and Ramus circumflexus (control) areas were obtained when left ventricular regional wall thickening fraction had recovered by 50%. Northern and Western blotting revealed no differences in the expression of the following genes: phospholamban, calsequestrin, sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a, and the inhibitory subunit of troponin I (TnI). However, the phosphorylation state of TnI and phospholamban were reduced in the LAD area. Fittingly, cAMP levels were reduced by 28% (P < 0.05). It is concluded that the contractile dysfunction in cardiac stunning might be mediated in part by decreased levels of cAMP and subsequently a reduced phosphorylation state of phospholamban and TnI.

Identification of regulated genes in rat heart after myocardial infarction by means of differential mRNA display

In order to testify the hypothesis that unknown mechanisms are involved in the process of cardiac remodeling after myocardial infarction (MI), we employed differential display reverse transcription-polymerase chain reaction (DDRT-PCR) as our primary inspection tool. An animal model of MI was established by ligation of the left anterior descending coronary artery (LAD) in rat. Fifty upregulated candidate cDNA fragments were obtained in the right ventricle (RV) of the heart six weeks after MI. Eight cDNA fragments isolated from DD denaturing gel were extracted and reamplified, cloned into pCR II vector and sequenced. A Genbank search of these clones showed that three of them have a high homology with known genes not previously associated with cardiac remodeling, i.e., mouse interleukin-4 receptor gene, rat ferritin mRNA, and T-cell receptor beta chain V beta 5. The remaining clones have no similarity to known sequences. These data suggest that certain genes which were not previously being associated with cardiac hypertrophy are turned on during the process of cardiac remodeling after MI.

Regional alterations in SR Ca(2+)-ATPase, phospholamban, and HSP-70 expression in chronic hibernating myocardium

We sought to identify mechanisms for chronic dysfunction in hibernating myocardium. Pigs were instrumented with a left anterior descending artery stenosis for 3 mo. Angiography demonstrated high-grade stenoses and hibernating myocardium with 1) severe anterior hypokinesis (P < 0.001 vs. shams), 2) reduced subendocardial perfusion [0.73 +/- 0.05 (SE) vs. 1.01 +/- 0.06 ml. min(-1). g(-1) in normal, P < 0.001], and 3) critically reduced adenosine flow (1.0 +/- 0.17 vs. 3.84 +/- 0.26 ml. min(-1). g(-1) in normal, P < 0.001). Histology did not reveal necrosis. Northern blot analysis of hibernating myocardium demonstrated regional downregulation in mRNAs for sarcoplasmic reticulum (SR) proteins phospholamban (0.76 +/- 0.08 vs. 1.07 +/- 0.06, P < 0.02) and SR Ca(2+)-ATPase (0.83 +/- 0.06 vs. 1.02 +/- 0.06, P < 0.05) with no change in calsequestrin (1.08 +/- 0.06 vs. 0.96 +/- 0.05, P = not significant). Heat shock protein (HSP)-70 mRNA was regionally induced in hibernating myocardium (2.4 +/- 0.3 vs. 1.0 +/- 0.11, P < 0.01). Directionally similar changes were confirmed by Western blot analysis of respective proteins. Our results indicate that hibernating myocardium exhibits a molecular phenotype that on a regional basis is similar to end-stage ischemic cardiomyopathy. This supports the hypothesis that SR dysfunction from reversible ischemia may be an early defect in the progression of left ventricular dysfunction.

Elevated levels of endogenous adenosine alter metabolism and enhance reduction in contractile function during low-flow ischemia: associated changes in expression of Ca(2+)-ATPase and phospholamban

Adenosine has several potentially cardioprotective effects including vasodilatation, reduction in heart rate and alterations in metabolism. Adenosine inhibits catecholamine-induced increase in contractile function mainly through inhibition of phosphorylation of phospholamban (PLB), the main regulatory protein of Ca(2+)-ATPase in sarcoplasmic reticulum (SR), and during ischemia it reduces calcium (Ca2+) overload. In this study we examined the effects of endogenous adenosine on contractile function and metabolism during low-flow ischemia (LFI) and investigated whether endogenous adenosine can alter expression of the Ca(2+)-ATPase/PLB-system and other Ca(2+)-regulatory proteins. Isolated blood-perfused piglet hearts underwent 120 min 10% flow. Hearts were treated with either saline, the adenosine receptor blocker (8)-sulfophenyl theophylline (8SPT, 300 micromol/l) or the nucleoside transport inhibitor draflazine (1 micromol/l). During LFI, 8SPT did not substantially influence metabolic or functional responses. However, draflazine enhanced the reduction in heart rate, contractile force and MVO(2), with less release of H+ and CO2. Before LFI there were no significant differences between groups for any of the proteins (Ca(2+)-ATPase, ryanodine-receptor, Na+/K(+)-ATPase) or mRNAs (Ca(2+)-ATPase, PLB, calsequestrin, Na+/Ca(2+)-exchanger) measured. At end of LFI mRNA-level of PLB was higher in draflazine-treated hearts compared to both other groups (P<0.01 vs both). Also, at end of LFI protein-level of Ca(2+)-ATPase was lower in draflazine-treated hearts (P<0.05 vs both), and a parallel trend towards a lower mRNA-level was seen (P=0.11 vs saline and P=0.43 vs 8SPT). During LFI tissue Ca2+ tended to rise in saline- and 8SPT-treated hearts but not in draflazine-treated hearts (at end of LFI, P=0.01 vs 8SPT). We conclude that the amount of adenosine normally produced during LFI does not substantially influence function and metabolism. However, increased endogenous levels by draflazine enhance downregulation of function and reduce signs of anaerobic metabolism. At end of LFI associated changes in expression of PLB and Ca(2+)-ATPase were seen. The functional significance was not determined in the present study. However, altered protein-levels might influence Ca(2+)-handling in sarcoplasmic reticulum and thus affect contractile force and tolerance to ischemia.

Absence of troponin I degradation or altered sarcoplasmic reticulum uptake protein expression after reversible ischemia in swine

The findings of troponin I (TnI) proteolysis (in isolated rat hearts) and induction of selected sarcoplasmic reticulum (SR) calcium-regulatory genes (after repetitive total coronary occlusions in swine) have given rise to the hypothesis that the time course of functional recovery of stunned myocardium reflects the resynthesis of reversibly damaged proteins. Although stunning occurs after brief total occlusions and prolonged partial occlusions (ie, short-term hibernation), the time course of functional recovery varies from a few hours to several days, suggesting that the severity of protein damage or mechanisms responsible for the dysfunction may differ. To study this, we examined SR gene expression and TnI degradation in stunned myocardium produced by 10-minute total left anterior descending coronary artery (LAD) occlusions (n=4) or 1-hour partial LAD occlusions, in which flow was reduced to approximately 50% of control values for 60 minutes (n=6) in swine. One hour after reperfusion, LAD wall thickening was severely depressed in both models despite normal perfusion and no triphenyltetrazolium chloride evidence of necrosis. Normal myocardium exhibited TnI immunoreactivity at 31 kDa and a weak secondary band at 27 kDa. Irreversible injury or calpain activation in vitro produced a marked increase in the intensity of the 27-kDa band, consistent with TnI degradation. Stunned myocardium demonstrated no change in the 31- or the 27-kDa band, and the percentage of the 27- to 31-kDa band remained constant after 10-minute total occlusions (LAD, 5.9+/-0.9%; normal, 4.9+/-1.6%) and 1-hour partial occlusions (LAD, 8.5+/-1.9%; normal, 7.3+/-1.4%) and in sham controls (LAD, 10.9+/-1.5%; normal, 9.8+/-1.4%). Northern analysis showed no alterations in TnI or SR gene expression, but the stress protein HSP-70 was variably induced. Thus, stunned myocardium occurs without TnI degradation or altered SR gene expression, indicating that additional mechanisms are responsible for the reversible dysfunction after single episodes of regional ischemia in swine.

Alterations in sarcoplasmic reticulum function and gene expression in ischemic-reperfused rat heart

In view of the critical role of sarcoplasmic reticular (SR) Ca(2+) release and the Ca(2+) pump in cardiac contraction-relaxation, this study was undertaken to assess the status of SR function, protein content, and gene expression in isolated rat hearts subjected to global ischemia for 30 min followed by 60 min of reperfusion (I/R). Attenuated recovery of contractile function in the I/R hearts was associated with reduced SR Ca(2+) uptake, Ca(2+) release, and ryanodine-binding activities. mRNA levels and protein contents for SR Ca(2+) pump ATPase and Ca(2+) release channels were markedly depressed in the I/R hearts. Perfusion of hearts with superoxide dismutase plus catalase, well-known scavengers of oxyradicals, prevented the I/R-induced alterations in cardiac function and partially prevented SR Ca(2+) transport activities and mRNA abundance. In hearts perfused with xanthine plus xanthine oxidase or H(2)O(2), changes similar to those in the I/R hearts were observed. These results indicate that oxyradicals may participate in depressing the SR Ca(2+) handling and gene expression in the I/R heart. It is suggested that treatment of hearts with antioxidants may improve the recovery of cardiac function by preserving the SR function and partially protecting the SR gene expression.

Oxygen free radical signaling in ischemic preconditioning

This review will focus on the free radical signaling mechanism of preconditioning. The results from our laboratory as well as studies from other laboratories suggest that reactive oxygen species function as second messenger during myocardial adaptation to ischemia. This review provides evidence for the first time that tyrosine kinase and MAP kinases are the targets for reactive oxygen species generated in the preconditioned myocardium. The finding that p38 MAP kinase might be upstream of NF kappa B further supports our previous reports that MAPKAP kinase 2 could be the most likely link between the preconditioning and adaptation mediated by gene expression. p38 activation appears to be an important step in the translocation and activation of the nuclear transcription factor NF kappa B, which in turn may be involved in the induction of the expression of a variety of stress-inducible genes.

Acute brain death alters left ventricular myocardial gene expression

OBJECTIVES

The depressed myocardial function observed in brain dead organ donors has been attributed to massive sympathetic discharge and catecholamine cardiotoxicity. Because elevated catecholamines are associated with altered myocardial gene expression, we investigated whether acute brain death from increased intracranial pressure alters the expression of myocardial gene products important in contractility.

METHODS

A balloon expansion model was used to increase intracranial pressure in rabbits (n = 22). At timed intervals after brain death, mean arterial pressure, heart rate, electrocardiograms, histologic myocardial injury, and systemic catecholamines were assessed. Messenger RNA levels encoding myofilaments, adrenergic receptors, sarcoplasmic reticulum proteins, transcription factors, and stress-induced programs were measured with blot hybridization of total left ventricular RNA.

RESULTS

Increased intracranial pressure induced an immediate pressor response that temporally coincided with diffuse electrocardiographic ST segment changes. Systemic epinephrine and norepinephrine levels concurrently increased (5- to 8-fold within 1 minute), then fell below baseline within 2 hours, and remained depressed at 4 hours. By 1 hour, histologic injury was evident. Four hours after the induction of increased intracranial pressure, levels of messenger RNA-encoding skeletal and cardiac alpha-actins, egr-1, and heat shock protein 70 were significantly increased. Sham-operated animals did not exhibit these changes.

CONCLUSIONS

Select changes in myocardial gene expression occur in response to increased intracranial pressure and implicate ventricular remodeling in the myocardial dysfunction associated with acute brain death.

Expression of calcium regulatory proteins in short-term hibernation and stunning in the in situ porcine heart

BACKGROUND

Myocardial hibernation and stunning are characterised by a reversible contractile dysfunction during and after ischaemia, respectively. Calcium homeostasis might be disturbed in hibernation and stunning due to altered expression of cardiac proteins involved in calcium handling.

METHODS

In enflurane-anaesthetised swine the coronary blood flow through the left anterior descending coronary artery was decreased to reduce regional contractile function (microsonometry) by approximately 50%. In transmural biopsies obtained during ischaemia and reperfusion creatine phosphate as well as the expression of sarcoplasmic reticulum calcium ATPase (SERCA), phospholamban (PLB), calsequestrin (CSQ), and troponin inhibitor (TnI) were determined.

RESULTS

During ischaemia creatine phosphate, after an initial reduction, recovered back to control values, and necrosis was absent (hibernation). After 90 min of ischaemia the myocardium was reperfused for 120 min but regional contractile function continued to be depressed (stunning). PLB, SERCA, CSQ, and TnI proteins were unchanged during ischaemia as well as reperfusion. Likewise, levels of PLB and SERCA mRNAs were unchanged.

CONCLUSION

It is concluded that other mechanisms than altered expression of these regulating proteins underlie the contractile dysfunction observed during acute ischaemia, short-term hibernation and stunning.

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.

[Stunned myocardium or hibernating myocardium? Apropos of a case]

The authors relate a case report of unstable angina pectoris accompanied by a well-documented stunned myocardium phenomenon. Stunned and hibernating myocardium resulting from an acute or chronic coronary ischaemia on the myocardium are notions which widely govern revascularisation indications, especially after a myocardial infarction. At present, their detection is based on isotopic methods and stress echocardiography.

Stunned myocardium, an opinionated review

Brief coronary occlusions cause stunning and ischemic preconditioning, the molecular mechanisms of which are insufficiently known. A marked reprogramming of sarcolemmal functions (including those of the sarcoplasmic reticulum) seems to occur and may be the basis for many of the observed phenomena. A bewilderingly complex pattern of gene expression emerges from mRNA-studies with reperfused tissue which at present does not permit a focussed or coherent view of mechanisms.

Altered gene transcription following brief episodes of coronary occlusions

This study was designed to elucidate whether previously observed enhanced mRNAs were due to accelerated transcription, enhanced mRNA stability or both mechanisms. We employed the nuclear run-on technique on myocardial nuclei and found the transcriptional induction of several genes, especially nuclear protooncogenes, Ca2+ regulating and heat shock protein genes.

Involvement of the sarcoplasmic reticulum calcium pump in myocardial contractile dysfunction: comparison between chronic pressure-overload and stunning

Acute as well as chronic forms of heart failure involve mechanical dysfunction during systole and/or diastole. The rapid Ca2+ release from and Ca2+ reuptake into the tubuli of the sarcoplasmic reticulum are processes that critically determine normal systolic and diastolic myocardial function, which explains why in the last fifteen years so much attention has been paid to understand the performance of the sarcoplasmic reticulum Ca2+ pump during myocardial contractile dysfunction. In this communication we have reviewed the literature data on sarcoplasmic reticulum Ca2+ pump function in the chronically pressure-overloaded hypertrophied and stunned (post-ischemic reversibly injured) myocardium in the light of some new data from our laboratory. Results on the pressure-overloaded hypertrophied myocardium provide evidence that impaired relaxation is most likely due to a low capacity of the sarcoplasmic reticulum to pump Ca2+, a consequence of a lower density of Ca(2+)-pumping sites within the sarcotubular membranes. Contractile dysfunction in stunned myocardium is accompanied by an upregulation of the sarcoplasmic reticulum Ca2+ ATPase gene resulting in a slight increase of the Ca2+ pumping activity. The latter increase is likely an adaptive response of the reversibly injured myocardium which may contribute to the slow recovery of contractile function.