Injury to the Ca2+ ATPase of the sarcoplasmic reticulum in anesthetized dogs contributes to myocardial reperfusion injury

Oral administration of L-arginine in patients with angina or following myocardial infarction may be protective by increasing plasma superoxide dismutase and total thiols with reduction in serum cholesterol and xanthine oxidase

Administration of L-arginine has been shown to control ischemic injury by producing nitric oxide which dilates the vessels and thus maintains proper blood flow to the myocardium. In the present study attempt has been made to determine whether oral administration of L-arginine has any effect on oxidant/antioxidant homeostasis in ischemic myocardial patients [represented by the patients of acute angina (AA) and acute myocardial infarction (MI)]. L-arginine has antioxidant and antiapoptotic properties, decreases endothelin-1 expression and improves endothelial function, thereby controlling oxidative injury caused during myocardial ischemic syndrome. Effect of L-arginine administration on the status of free radical scavenging enzymes, pro-oxidant enzyme and antioxidants viz. total thiols, carbonyl content and plasma ascorbic acid levels in the patients has been evaluated. We have observed that L-arginine administration (three grams per day for 15 days) resulted in increased activity of free radical scavenging enzyme superoxide dismutase (SOD) and increase in the levels of total thiols (T-SH) and ascorbic acid with concomitant decrease in lipid per-oxidation, carbonyl content, serum cholesterol and the activity of proxidant enzyme, xanthine oxidase (XO). These findings suggest that the supplementation of L-arginine along with regular therapy may be beneficial to the patients of ischemic myocardial syndromes.

High yield heterologous expression of wild-type and mutant Cu+-ATPase (ATP7B, Wilson disease protein) for functional characterization of catalytic activity and serine residues undergoing copper-dependent phosphorylation

ATP7B is a P-type ATPase required for copper homeostasis and related to Wilson disease of humans. In addition to various domains corresponding to other P-type ATPases, ATP7B includes an N terminus extension (NMBD) with six copper binding sites. We obtained high yield expression of WT and mutant ATP7B in COS1 cells infected with adenovirus vector. ATP7B, isolated with the microsomal fraction of cell homogenates, accounts for 10–20% of the total protein. Copper-dependent, steady-state ATPase yields 30 nmol of P_i/mg of protein/min at 37 °C, pH 6.0. ATP7B phosphorylation with ATP occurs with diphasic kinetics and is totally copper-dependent. Alkali labile phosphoenzyme (catalytic intermediate of P-ATPases) accounts for a small fraction of the total phosphoprotein and is prevented by D1027N (P domain) or C983A/C985A (CXC copper binding motif in TM6) mutations. Decay of [³²P]phosphoenzyme following chase with non-radioactive ATP occurs with an initial burst involving alkali labile phosphoenzyme (absent in D1027N and C983A/C985A mutants) and continues at a slow rate involving alkali-resistant phosphoenzyme. If a copper chelator is added with the ATP chase, the initial burst is smaller, and further cleavage is totally inhibited. Analysis by proteolysis and mass spectrometry demonstrates that the alkali stable phosphoenzyme involves Ser⁴⁷⁸ and Ser⁴⁸¹ (NMBD), Ser¹¹²¹ (“N” domain) and Ser¹⁴⁵³ (C terminus), and occurs with the same pattern ex vivo (COS-1) and in vitro (microsomes). The overall copper dependence of phosphorylation and hydrolytic cleavage suggests long range conformational effects, including interactions of NMBD and headpiece domains, with strong influence on catalytic turnover.

Sasanquasaponin protects rat cardiomyocytes against oxidative stress induced by anoxia-reoxygenation injury

Reactive oxygen species can play an important role in the pathogenesis of anoxia-reoxygenation injury. Sasanquasaponin (SQS) is a biologically active ingredient extracted from the Chinese medicinal plant Camellia oleifera Abel. Some studies have shown that SQS possesses potent antioxidant activities. However, it has not been elucidated whether SQS diminishes reactive oxygen species stress induced by anoxia-reoxygenation injury in cardiomyocytes. In this work, neonatal rat cardiomyocytes pretreated with the test compound were subjected to anoxia-reoxygenation. The extent of cellular damage was accessed by cell viability and the amount of released lactate dehydrogenase (LDH). Superoxide dismutase, catalase and glutathione peroxidase activities, reduced (GSH) and oxidized glutathione (GSSG) levels, and malondialdehyde contents were measured by a colorimetric method. The levels of intracellular reactive oxygen species and calcium were determined by flow cytometry. The results showed that SQS reduced LDH release and increased cell viability in a dose-dependent manner up to 10 microM and concomitantly decreased malondialdehyde and GSSG contents, while significantly increased GSH contents and the activities of superoxide dismutase, catalase and glutathione peroxidase. Moreover, treatment with SQS decreased intracellular reactive oxygen species levels and alleviated calcium accumulation in cardiomyocytes undergoing anoxia-reoxygenation. It is suggested that SQS could protect cardiomyocytes against oxidative stress induced by anoxia-reoxygenation by attenuating reactive oxygen species generation and increasing activities of endogenous antioxidants.

Caldaret, an intracellular Ca2+ handling modulator, limits infarct size of reperfused canine heart

The cardioprotective effect of caldaret, a novel intracellular Ca(2+) handling modulator that acts through reverse-mode Na(+)/Ca(2+) exchanger inhibition and potential sarcoplasmic reticulum (SR) Ca(2+) uptake enhancement, against reperfusion injury was investigated. We employed a canine model of myocardial infarction induced by 90-min occlusion of left circumflex (LCX) coronary artery followed by 4 h of reperfusion. Intravenously infused caldaret (3 or 30 microg/kg per hour) for 30 min at LCX-reperfusion markedly reduced infarct size (by 51.3% or 71.9%, respectively). This cardioprotection was accompanied by an acceleration of left ventricular (LV) contraction and relaxation during reperfusion, but not by an increase in ischemic regional transmural myocardial blood flow (TMBF) or endocardial/epicardial blood flow ratio (Endo/Epi ratio) or a reduction in double-product throughout the protocol. Diltiazem (2000 microg/kg per hour) also reduced infarct size (by 36.1%), but unlike caldaret, was accompanied by the significant increase in Endo/Epi ratio in the ischemic region and decrease in double-product. There were significant inverse relationships between infarct size and ischemic regional TMBF in all groups. Caldaret, but not diltiazem shifted the regression line downward with a flatter slope. These results suggest that the amelioration of intracellular Ca(2+) handling dysfunction achieved by caldaret leads to cardioprotective effects against reperfusion injury following prolonged ischemia.

Cellular abnormalities linked to endoplasmic reticulum dysfunction in cerebrovascular disease—therapeutic potential

Unfolded proteins accumulate in the lumen of the endoplasmic reticulum (ER) as part of the cellular response to cerebral hypoxia/ischemia and also to the overexpression of the mutant genes responsible for familial forms of degenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyothrophic lateral sclerosis, and Huntington's disease, as well as other disorders that are caused by an expanded CAG repeat. This accumulation arises from an imbalance between the load of proteins that need to be folded and processed in the ER lumen and the ER folding/processing capacity. To withstand such potentially lethal conditions, stress responses are activated that includes the shutdown of translation to reduce the ER work load and the activation of the expression of genes coding for proteins involved in the folding and processing reactions, to increase folding/processing capacity. In transient cerebral ischemia, ER stress-induced suppression of protein synthesis is believed to be too severe to permit sufficient activation of the genetic arm of the ER stress response. Mutations associated with Alzheimer's disease down-regulate the ER stress response and make cells more vulnerable to conditions associated with ER stress. When the functioning of the ER is severely impaired and affected cells can no longer withstand these stressful conditions, programmed cell death is induced, including a mitochondria-driven apoptotic pathway. Raising the resistance of cells to conditions that interfere with ER functions and activating the degradation and refolding of unfolded proteins accumulated in the ER lumen are possible strategies for blocking the pathological process leading to cell death at an early stage.

Role of adenosine receptor activation in antioxidant enzyme regulation during ischemia-reperfusion in the isolated rat heart

The aim of the present study was to investigate the protective role of pharmacological preconditioning on antioxidant enzymes using A(1) and A(3) adenosine receptor agonists in the recovery of the isolated myocardium after cardioplegic ischemia. Two different modes of preconditioning were studied: isolated rat hearts were perfused with A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) or A(3) 2-chloro-N(6)-(3-iodobenzyl) adenosine-5'-N-methyluronamide (Cl-IB-MECA) (1 nM), followed by cardioplegic ischemia and reperfusion (30 min each) (perfusion mode), or CCPA or Cl-IB-MECA (100 micro g/kg) were injected intravenously 24 h before the experiment (injection mode). Hearts treated with CCPA improved in terms of mechanical function, infarct size, ATP levels, superoxide dismutase, and catalase (p < 0.005) in both modes of administration. Cl-IB-MECA was beneficial mainly in the injected group. Reduced damage to the mitochondria in the CCPA-treated hearts was observed using electron microscopy evaluation. In the Cl-IB-MECA-injected hearts, mitochondrial damage was moderate. CCPA in both modes of treatment and Cl-IB-MECA in the injected mode were beneficial in protecting the perfused isolated rat heart, subjected to normothermic cardioplegic ischemia. This protection was partially related to the higher myocardial activity of superoxide dismutase and catalase.

Cardioprotective effects of lowering oxygen tension after aortic unclamping on cardiopulmonary bypass during coronary artery bypass grafting

The effect on myocardial reperfusion injury of reducing oxygen tension during reperfusion on cardiopulmonary bypass (CPB) in coronary artery bypass grafting (CABG) was examined at the same time as the influence of diltiazem during CPB was evaluated. A prospective, randomized trial evaluated the hemodynamic and myocardial metabolic recovery in 3 groups of patients undergoing elective CABG; subjects were randomly allocated on the basis of oxygen tension during reperfusion after aortic unclamping: group 1 (n=10) hyperoxic reperfusion (oxygen tension [PO2]=450-550 mmHg); group 2 (n=10): hyperoxic reperfusion and subsequent continuous infusion of diltiazem (0.5 microg/kg); group 3 (n=10): lowering reperfusate PO2 (PO2=200-250 mmHg). Hemodynamic and myocardial metabolic measurements were taken at 6 preset times: before starting the surgical procedure and at 30 min and 3, 9, 21, and 45 h after discontinuation of CPB. The cardiac index in the lowering reperfusate PO2 group was higher than that of the hyperoxic reperfusion groups at 30 min and 3 h after CPB, and malondialdehyde and troponin-T were significantly lower at 30 min and 3 h, respectively. In comparison with the hyperoxic + diltiazem group, the hemodynamic and myocardial recovery in the lowering reperfusate PO2 group was improved for about 3 h after CPB. Reduced oxygen tension during reperfusion after aortic unclamping on CPB is more effective against myocardial injury than a calcium antagonist in the short term. It is a convenient and safe management technique that can reduce morbidity and mortality, especially in the severely compromised heart.

Attenuation of oxidant stress during reoxygenation by AMP 579 in cardiomyocytes

AMP 579, an adenosine A(1)/A(2) receptor agonist, has a strong anti-infarct effect when administered just before reperfusion. Because oxidative stress has been proposed to contribute to myocardial reperfusion injury, we tested whether AMP 579 can reduce the production of reactive oxidant species (ROS) during reoxygenation in cultured chick embryonic cardiomyocytes. The intracellular fluorescent probe 2',7'-dichlorofluorescin diacetate (DCFH) was used to detect ROS. The cells were subjected to 60 min of simulated ischemia, followed by either 15 min or 3 h of reoxygenation. AMP 579 (0.5 and 1 microM), when started 10 min before reoxygenation, significantly reduced ROS generation from 4.86 +/- 0.30 (arbitrary units) in untreated cells to 2.72 +/- 0.31 and 1.85 +/- 0.14, respectively (P < 0.05). Cell death that was assessed by propidium iodide uptake was markedly reduced by AMP 579 (49.6 +/- 4.7% of control cells vs. 25.4 +/- 2.4%, P < 0.05). In contrast, adenosine did not alter ROS generation or cell death. Attenuation of ROS production by AMP 579 was completely prevented by simultaneous exposure of cells to the selective adenosine A(2) antagonist 8-(13-chlorostyryl) caffeine. These results indicate that AMP 579 directly protects cardiomyocytes from reperfusion injury by a mechanism that attenuates intracellular oxidant stress. Furthermore, adenosine could not duplicate these effects.

Chronic unloading by left ventricular assist device reverses contractile dysfunction and alters gene expression in end-stage heart failure

BACKGROUND

Left ventricular (LV) assist devices (LVADs) can improve contractile strength and normalize characteristics of the Ca(2+) transient in myocytes isolated from failing human hearts. The purpose of the present study was to determine whether LVAD support also improves contractile strength at different frequencies of contraction (the force-frequency relationship [FFR]) of intact myocardium and alters the expression of genes encoding for proteins involved in Ca(2+) handling.

METHODS AND RESULTS

The isometric FFRs of LV trabeculae isolated from 15 patients with end-stage heart failure were compared with those of 7 LVAD-supported patients and demonstrated improved contractile force at 1-Hz stimulation, with reversal of a negative FFR after LVAD implantation. In 20 failing hearts, Northern blot analysis for sarcoplasmic endoreticular Ca(2+)-ATPase subtype 2a (SERCA2a), the ryanodine receptor, and the sarcolemmal Na(+)-Ca(2+) exchanger was performed on LV tissue obtained before and after LVAD implantation. These paired data demonstrated an upregulation of all 3 genes after LVAD support. In tissue obtained from subsets of these patients, Western blot analysis was performed, and oxalate-supported Ca(2+) uptake by isolated sarcoplasmic reticular membranes was determined. Despite higher mRNA for all genes after LVAD support, only SERCA2a protein was increased. Functional significance of increased SERCA2a was confirmed by augmented Ca(2+) uptake by sarcoplasmic reticular membranes isolated from LVAD-supported hearts.

CONCLUSIONS

LVAD support can improve contractile strength of intact myocardium and reverse the negative FFR associated with end-stage heart failure. The expression of genes encoding for proteins involved in Ca(2+) cycling is upregulated (reverse molecular remodeling), but only the protein content of SERCA2a is increased.

Calcium antagonist protects the myocardium from reperfusion injury by interfering with mechanisms directly related to reperfusion: an experimental study with the ultrashort-acting calcium antagonist clevidipine

To test the hypothesis that calcium antagonists protect the myocardium from reperfusion-induced damage by local myocardial mechanisms just at the time of reperfusion, the myocardioprotective effects of the dihydropyridine clevidipine were investigated, taking advantage of its ultrashort-acting effect. Pigs were subjected to 45 min of myocardial ischemia by occlusion of the left anterior descending coronary artery followed by 4 h of reperfusion. Either clevidipine (0.3 nmol/kg/min, n = 6) or the corresponding amount of vehicle (n = 6) was administered to the ischemic myocardium by retrograde coronary venous infusion over a 30-min period starting 10 min before reperfusion. Hemodynamic variables (heart rate, left ventricular systolic and end-diastolic pressure, max dP/dt, and mean arterial blood pressure) as well as coronary blood flow were measured throughout the experiment. At the end of reperfusion, the area at risk (percentage of left ventricle) was determined by infusion of Evans blue into the left atrium, and the infarct size, by triphenyl tetrazolium chloride (TTC) staining. The plasma level of endothelin-like immunoreactivity (ET-LI) was analyzed in blood from the aorta and the anterior coronary vein before ischemia and at different times during reperfusion. The area at risk was similar in the vehicle and the clevidipine groups. The infarct size, expressed as a percentage of the area at risk, was 80 +/- 9.2 in the vehicle group, whereas it was significantly reduced to 51 +/- 9.2% in the clevidipine group (p < 0.01). Clevidipine did not influence any of the hemodynamic variables measured throughout the study. A nonsignificant trend toward decreased total ET-LI overflow during 4-h reperfusion was observed in the clevidipine-treated pigs compared with vehicle-treated ones (5.3 +/- 1.4 vs. 7.1 +/- 3.4 pmol). These results demonstrate that, in this model of ischemia/reperfusion-induced myocardial infarction, clevidipine reduced the damage to the myocardium when given in association with reperfusion. The local administration of the compound together with its short blood half-life shows that clevidipine reduces reperfusion-induced damage by local mechanisms within the ischemic tissue rather than by peripheral mechanisms.

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.

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.

Disturbances of the functioning of endoplasmic reticulum: a key mechanism underlying neuronal cell injury?

Cerebral ischemia leads to a massive increase in cytoplasmic calcium activity resulting from an influx of calcium ions into cells and a release of calcium from mitochondria and endoplasmic reticulum (ER). It is widely believed that this increase in cytoplasmic calcium activity plays a major role in ischemic cell injury in neurons. Recently, this concept was modified, taking into account that disturbances occurring during ischemia are potentially reversible: it then was proposed that after reversible ischemia, calcium ions are taken up by mitochondria, leading to disturbances of oxidative phosphorylation, formation of free radicals, and deterioration of mitochondrial functions. The current review focuses on the possible role of disturbances of ER calcium homeostasis in the pathologic process culminating in ischemic cell injury. The ER is a subcellular compartment that fulfills important functions such as the folding and processing of proteins, all of which are strictly calcium dependent. ER calcium activity is therefore relatively high, lying in the lower millimolar range (i.e., close to that of the extracellular space). Depletion of ER calcium stores is a severe form of stress to which cells react with a highly conserved stress response, the most important changes being a suppression of global protein synthesis and activation of stress gene expression. The response of cells to disturbances of ER calcium homeostasis is almost identical to their response to transient ischemia, implying common underlying mechanisms. Many observations from experimental studies indicate that disturbances of ER calcium homeostasis are involved in the pathologic process leading to ischemic cell injury. Evidence also has been presented that depletion of ER calcium stores alone is sufficient to activate the process of programmed cell death. Furthermore, it has been shown that activation of the ER-resident stress response system by a sublethal form of stress affords tolerance to other, potentially lethal insults. Also, disturbances of ER function have been implicated in the development of degenerative disorders such as prion disease and Alzheimer's disease. Thus, disturbances of the functioning of the ER may be a common denominator of neuronal cell injury in a wide variety of acute and chronic pathologic states of the brain. Finally, there is evidence that ER calcium homeostasis plays a key role in maintaining cells in their physiologic state, since depletion of ER calcium stores causes growth arrest and cell death, whereas cells in which the regulatory link between ER calcium homeostasis and protein synthesis has been blocked enter a state of uncontrolled proliferation.