Ischemic but not mechanical preconditioning attenuates ischemia/reperfusion induced myocardial apoptosis in anaesthetized rabbits: the role of Bcl-2 family proteins and ERK1/2

OBJECTIVE

Recent studies suggest that ischemic preconditioning (IPC) inhibits myocardial apoptosis after ischemia and reperfusion. This study aimed first, to examine whether short mechanical stretch with acute pressure overload (MPC), which has been shown to reduce infarct size after ischemia/reperfusion, mimics IPC in attenuating myocardial apoptosis and second, to evaluate whether induced cardioprotection involves modulation of the expression of the Bcl-2 family proteins and phosphorylation of prosurvival kinases.

METHODS AND RESULTS

A model of anaesthetized rabbit was used and the preconditioning protocol included one cycle of short ischemia/reperfusion, or short mechanical stretch with acute pressure overload. Preconditioning stimuli were equally effective in reducing the infarct size, determined after 4 h reperfusion. However, IPC but not MPC attenuated myocardial apoptosis. IPC restored the decreased expression of Bcl-2 and Bcl-xL observed in hearts subjected to ischemia and reperfusion only. Bax levels were not different among the groups. ERK1/2 were activated during reperfusion in both IPC and MPC groups.

CONCLUSIONS

The data provide further evidence that apoptosis and necrosis contribute independently to infarct size after ischemia and reperfusion. Inhibition of the myocardial apoptotic processes by IPC may involve modulation of the expression of anti-apoptotic proteins, Bcl-2 and Bcl-xL. ERK1/2 may be involved in the inhibition of both apoptosis and necrosis.

Protection from postconditioning depends on the number of short ischemic insults in anesthetized pigs

Post-conditioning in the early reperfusion period confers protection to the heart after a potentially lethal episode of prolonged ischemia. Protection from this novel intervention has been documented in rat, rabbit and canine hearts, but one group has reported that it is ineffective in pigs, a large-animal species that should be most relevant to humans. We hypothesized that this negative result was related to an inappropriate post-conditioning protocol rather than the species. The present study, therefore, tested whether an effective post-conditioning protocol could be identified that limits infarct size in anesthetized pigs. Domestic Landrace pigs weighing 25-29 kg were anesthetized, and after a mid-sternal thoracotomy and pericardiotomy the left anterior descending coronary artery was ligated for 60 min followed by 3 h of reperfusion. Three groups were studied: control group (n = 5) with no other intervention, 4-30 PostC group (n = 5) with 4 cycles of 30-s reperfusion/30-s ischemia, and 8-30 PostC group (n = 6) with 8 cycles of 30-s reperfusion/30-s ischemia. The two post-conditioning protocols started immediately after termination of the 60-min coronary occlusion. Region at risk and infarct size were delineated with the aid of pre-mortem monastral blue injection and postmortem staining with triphenyltetrazolium chloride, respectively. In control hearts 33.5 +/- 7.6% of the risk zone infarcted and 36.7 +/- 3.7% in the 4-30 PostC group (P = NS). But there was only 10.5 +/- 0.5% infarction in the 8-30 PostC group (P < 0.01 vs. the other two groups). Post-conditioning confers protection in pigs but requires more than 4 ischemia/reperfusion cycles. Post-conditioning may protect by inhibiting mitochondrial permeability transition pore formation by keeping the heart acidotic as it is reoxygenated. If true, then it would be difficult to employ too many occlusion cycles.

Differential activation of mitogen-activated protein kinases in ischemic and nitroglycerin-induced preconditioning

Previous studies have shown that the cardioprotective effect of ischemic preconditioning (IPC) can be mimicked pharmacologically with clinically relevant agents, including nitric oxide (NO) donors. However, whether pharmacological preconditioning shares the same molecular mechanism with IPC is not fully elucidated. The present study aimed to determine the activation of mitogen-activated protein kinases (MAPKs) (ERK1/2, p38 MAPK and p46/p54 JNKs) during ischemia and at reperfusion in nitroglycerin-induced preconditioning as compared to IPC and to correlate this with the conferred cardioprotection in anesthetized rabbits. Sixty minutes of intravenous administration of nitroglycerin was capable of inducing both early and late phase preconditioning in anesthetized rabbits, as it was expressed by the reduction of infarct size. Despite the cardioprotective effect conferred by both ischemic and nitroglycerin-induced preconditioning, there was a differential phosphorylation of MAPKs between the studied groups. p38 MAPK was activated early in ischemia in both ischemic and the early nitroglycerin-induced preconditioning while JNKs were markedly increased only after IPC. Furthermore, in these groups, ERK1/2 were activated during reperfusion. A different profile was observed in the late preconditioning induced by nitroglycerin with increased p38 MAPK and ERK1/2 phosphorylation during late ischemia. No activation of JNKs was observed at any time point in this group. It seems that activation of individual MAPK subfamilies depends on the nature of preconditioning stimulus.

Intravenous atenolol and esmolol maintain the protective effect of ischemic preconditioning in vivo

Catecholamines bind to alpha- and beta-adrenoreceptors and are capable of preconditioning ischemic myocardium. Our purpose was to investigate the effect of acute either short or prolonged i.v. administration of beta-adrenoreceptor antagonists on ischemic preconditioning in vivo. Fifty-five anesthetized rabbits were divided into 10 groups (n=5-7 per group) and were subjected to 30-min regional ischemia of the heart after ligation of a prominent left coronary artery and 3-h reperfusion after releasing the snare. Ischemic preconditioning was obtained by three cycles of 5-min ischemia separated by 10-min reperfusion. beta-Adrenoreceptor blockade was obtained by the long acting beta-adrenoreceptor antagonist atenolol or by the short acting esmolol, which were given as a short 5-min infusion or as a prolonged 45-min infusion, starting respectively 20 min before and ending 15 min before the beginning of sustained ischemia, or starting 45 min before and ending immediately before the beginning of sustained ischemia. Atenolol was given at a rate of 0.2 mg min(-1) during 5 min or at a rate of 0.088 mg min(-1) as a 45-min infusion. Esmolol was given as an initial dose of 500 microg kg(-1) within 1 min, followed by a 4-min infusion at a rate of 50 microg kg(-1) min(-1) or as an initial dose of 3.4 mg within 1 min, followed by a 44-min infusion at a rate of 0.15 mg min(-1). Blood pressure and heart rate were continuously monitored. The infarcted and risk areas were delineated with the aid of tetrazolium chloride staining and fluorescent Zn-Cd particles. Infarct size was expressed in percent of the area at risk. All the animals without preconditioning developed an infarct size ranging between 36.3+/-2.4% and 49.6+/-7.6% (P=NS) and all the preconditioning groups developed an infarct size ranging between 14.9+/-1.2% and 21.0+/-2.2% (P=NS). All the preconditioning groups, independently of the use of beta-adrenoreceptor antagonists, had a smaller infarct size than the control group, which developed an infarct size of 47.3+/-2.5% (P<0.01). Intravenous atenolol and esmolol, independent of timing and mode of administration, does not seem to interfere with protection afforded by ischemic preconditioning in vivo.

Melatonin does not prevent the protection of ischemic preconditioning in vivo despite its antioxidant effect against oxidative stress

Free radicals are involved in the protective mechanism of preconditioning (PC), whereas antioxidant compounds abolish this benefit. Melatonin is a hormone with antioxidant properties. The aim of our study was to evaluate the effect of melatonin on infarct size in ischemic preconditioning in vivo. We randomly divided 33 male rabbits into four groups and subjected them to 30 min of myocardial ischemia and 3 h of reperfusion with the following prior interventions: (i) no intervention, (ii) iv melatonin at a total dose of 50 mg/kg, (iii) PC with two cycles of 5 min ischemia and 10 min reperfusion, and (iv) combined melatonin and PC. In a second series of experiments, another antioxidant agent N-acetylcysteine (NAC) was used in a control and in a PC group. Myocardial infarct size was determined and blood samples were drawn at different time points for the determination of lipid peroxidation products, total superoxide dismutase (SOD) activity, and (1)H-NMR spectra to evaluate the changes in the metabolic profile. Melatonin showed no effect on myocardial infarct size in the group of sustained ischemia (42.9 +/- 3.6% vs 47.4 +/- 4.9%) and it did not attenuate the reduction of myocardial infarct size in the PC group (13.6 +/- 2.4% vs 14.0 +/- 1.7%). A similar effect was found in NAC-treated groups (44.8 +/- 3.4% vs 14.3 +/- 1.3%). Lipid peroxidation product levels were significantly elevated in the control and PC groups, whereas melatonin decreased them in both groups. The SOD activity was enhanced in the PC group compared to controls; melatonin kept SOD activity unchanged during ischemia/reperfusion and enhanced its activity when it was combined with PC. Melatonin did not change the metabolic profile of the control and PC groups. Melatonin does not prevent the beneficial effect of ischemic PC on infarct size despite its antioxidant properties.

Estrogen alone or combined with medroxyprogesterone but not raloxifene reduce myocardial infarct size

We investigated whether estrogen protects the ischemic myocardium in oophorectomized female rabbits fed with a cholesterol-enriched diet, whether the addition of a progestin compound attenuates the beneficial effect of estrogen and whether raloxifene also limits myocardial necrosis. We treated 32 female oophorectomized hypercholesterolemic rabbits with (a) placebo (N=8, group I), (b) conjugated estrogens alone (N=8, group II), (c) conjugated estrogens combined continuously with medroxyprogesterone acetate (N=8, group III) and (d) raloxifene (N=8, group IV) all for 4 weeks. All rabbits underwent 30 min of ischemia and 120 min of reperfusion. Both infarct size (0.38+/-0.08 and 0.45+/-0.05 in groups II and III, respectively, vs. 0.78+/-0.07 in group I, P<0.005) and infarct size/risk zone% (26.34+/-4.18 and 35.01+/-4.39 in groups II and III, respectively, vs. 52.18+/-7.84 in group I, P<0.05) were significantly smaller in the estrogen treatment groups compared to placebo. No significant difference was observed between groups II and III. There was no significant difference between groups I and IV for infarct size (0.78+/-0.07 vs. 0.69+/-0.08, respectively) or for infarct size/risk zone% (52.18+/-7.84 vs. 47.17+/-4.3). Short-term estrogen protects ischemic myocardium in hypercholesterolemic oophorectomized female rabbits; this effect is not attenuated by the addition of a progestin compound. Raloxifene, however, does not decrease infarct size compared to placebo.

Dissociation of stress-activated protein kinase (p38-MAPK and JNKs) phosphorylation from the protective effect of preconditioning in vivo

The aim of the present study was to examine and compare the role of the stress-activated protein kinases in ischemic and stretch-induced preconditioning. A model of anesthetized rabbits was used, and the preconditioning protocol included one or three cycles of short ischemia/reperfusion, or short mechanical stretch with acute pressure overload without or with the addition of the stretch blocker gadolinium. Infarct size was determined after 2h reperfusion and p38 MAPK and JNKs phosphorylation was determined after 20 min of prolonged ischemia. Preconditioning stimuli were equally effective in reducing the infarct size (14.2+/-3.4%, 12.9+/-3.0%, 15.9+/-3.3%, P<0.01 vs control). The addition of the stretch channel blocker gadolinium abrogated the effect of stretch preconditioning only, without any effect on ischemic preconditioning. Comparing p38-MAPK and p46/p54 JNKs phosphorylation in the ischemic and non-ischemic regions of the heart at the time of sustained ischemia, activation was observed in the ischemic or mechanically preconditioned groups compared with the control. The addition of gadolinium abolished this activation. The above results indicate that the phosphorylation of p38-MAPK and p46/p54 JNKs is increased in preconditioning but this effect can be dissociated from the protective effect of ischemic preconditioning. Activation of the stress-activated protein kinases may be related to the increased contracture, a characteristic of ischemic preconditioning.

Antioxidant activity of novel indole derivatives and protection of the myocardial damage in rabbits

Novel indole derivatives containing a triazole moiety (1a-d, 2a-c) were synthesized as lead compounds with interesting pharmacological profiles. Their antioxidant activity was investigated on in vitro non-enzymatic rat hepatic microsomal lipid peroxidation. All compounds showed significant effect in the above assay. The effect depended mainly on the attachment position of the triazole moiety on the indole nucleus. The most potent antioxidant derivatives la, 1c and 1d were tested for their protective ability against the oxidative damage of the myocardium after ischemia-reperfusion, in male rabbits which were subjected to 30 min regional ischemia followed by reperfusion. The tested antioxidant compounds 1a, 1c and 1d were continuously infused for 30 min starting at 10th min of ischemia and lasted at 10th min of reperfusion. The concentration of malondialdehyde (MDA, a marker of lipid peroxidation) and hemodynamic parameters (blood pressure and heart rate) were measured in the baseline, at 20th min of the sustained ischemia, 1st and 20th min of reperfusion. It was found that the examined compounds la, 1c and 1d reduced significantly the level of MDA in rabbits under ischemia-reperfusion and proved to be promising substances for further evaluation of anti-ischemic properties.

Angiotensin II fails to induce preconditioning in vivo--the protective role of myocardial stretch due to pressure overload

BACKGROUND

Angiotensin II (Ang) has been successfully used as a preconditioning analogue in isolated rabbit hearts. It is also known that local concentrations of Ang accelerate ischemic injury in vivo, while activation of stretch receptors protects ischemic hearts.

OBJECTIVES

First, to investigate further whether Ang can mimic preconditioning in vivo. Second, to test the hypothesis that there is an activation of stretch receptors, and that the larger infarct from the left atrium Ang compared with that from the intravenous Ang may be associated with the ischemic injury caused by local administration.

METHODS

Male rabbits were divided into four groups - a control group, an ischemic preconditioning group with 5 min ischemia, a left atrial group and an intravenous group with 5 min Ang infusion. All animals were subjected to prolonged ischemia and reperfusion. A second series of experiments was also performed with five groups that had a 5 min mechanical obstruction of the aorta (Ao clamp), also used as a preconditioning analogue with or without the stretch receptor blocker gadolinium (Gd).

RESULTS

Contrary to what was expected from the ex vivo experiments, Ang failed to mimic preconditioning (infarct size 39.6 6.1%, 13.7 4.1%, 52.2% 6.9% and 31.2 4.8%, respectively for the above groups). Interestingly, however, when Ang was infused intravenously, it produced a significantly smaller infarct compared with that observed after the same dose was infused into the left atrium (P<0.05). The infarct size was 17.0 3.7% in the Ao clamp group, which was an effect completely prevented by Gd (45.8 4.2%, P<0.01). Although Gd did not alter infarct size in the control and ischemic preconditioning groups, it increased infarct size when added to the intravenous Ang group (Gd-intravenous Ang 48.6 3.3%, P<0.05 compared with intravenous Ang).

CONCLUSIONS

Ang fails to mimic preconditioning in vivo, but salvage of ischemic myocardium can be emanated from pressure overload.

Endogenous endothelin maintains coronary artery tone by endothelin type A receptor stimulation in patients undergoing coronary arteriography

OBJECTIVE

To examine the contribution of endothelin type A (ET(A)) receptor stimulation by endogenously generated endothelin-1 (ET-1) to the maintenance of coronary vascular tone in humans.

DESIGN

Controlled clinical study.

SETTING

Tertiary cardiovascular referral centre.

PATIENTS

14 subjects were studied, seven with normal coronary arteries and seven with coronary artery disease, mean (SEM) age, 53 (2) years.

INTERVENTIONS

After diagnostic coronary arteriography, BQ-123 (a selective ET(A) receptor antagonist; 100 nmol/min) in 0.9% saline, was infused into the left coronary artery at a rate of 1 ml/min for 60 minutes. Eight control subjects received saline alone.

MAIN OUTCOME MEASURES

Blood flow velocity in the left anterior descending coronary artery, measured using a Doppler flow guidewire; coronary arteriography performed at baseline and immediately at the end of the BQ-123 or saline infusion to measure the diameter of proximal and distal left anterior descending coronary artery segments.

RESULTS

The diameter of the proximal segment increased by 6 (2)%, while that of the distal segment increased by 12 (3)% after BQ-123 (both p < 0.05 v baseline). Coronary blood flow increased from 75 (10) to 92 (10) ml/min and coronary vascular resistance decreased from 1.99 (0.36) to 1.44 (0. 22) mm Hg/ml/min after BQ-123 (both p < 0.05 v baseline). The response to BQ-123 of patients with and without coronary artery disease was similar. There was no effect of saline in the controls.

CONCLUSIONS

Endogenously produced ET-1 contributes to the maintenance of basal coronary artery tone in humans by ET(A) receptor stimulation. The role of ET(B) receptors remains to be defined.

Preconditioning limits myocardial infarct size in hypercholesterolemic rabbits

BACKGROUND

Hypercholesterolemia predisposes to coronary artery disease and causes endothelial dysfunction; some reports suggest that endothelial derived substances are involved in ischemic preconditioning.

OBJECTIVE

Our aim was to examine the possibility that preconditioning maybe attenuated in a clinically relevant animal model of hypercholesterolemia with atherosclerosis.

METHODS

Male rabbits were fed with cholesterol enriched diet and then divided into two groups (A and B) without and with preconditioning, respectively. A second series of rabbits fed a normal diet were similarly divided into two groups (C and D) without and with preconditioning, respectively. All the animals were subjected to 30 min ischemia and 180 min reperfusion. Blood samples were collected for cholesterol assessment; arterial and heart samples were harvested at the end for histopathological examination. Infarct (I) and risk areas (R) were delineated with Zn-Cd particles and TTC staining.

RESULTS

Cholesterol in groups A and B was 58.3+/-8.7 mg% at baseline and 1402+/-125 mg% at 8 weeks (P<0.0001) and in groups C and D 57.5+/-5.8 mg% before the surgical procedure. I/R% was 39. 3+/-6.3% in group A, 16.7+/-3.9% in B (P<0.01), 41.4+/-7.5% in C and 10.8+/-3.3% in D (P<0.01).

CONCLUSION

We conclude that preconditioning is unlikely to be attenuated by hypercholesterolemia.

Enhanced protection of heat shock in myocardial infarction: inhibition of detrimental effect of systemic hyperthermia

We have shown that isolated blood-perfused heat-stressed hearts are protected only when the blood donor animal has not been exposed to hyperthermia. Systematic hyperthermia results in larger infarction of both isolated control and heat-stressed hearts. In this study we investigated whether indomethacin inhibits in vivo the detrimental effect of hyperthermia. Male rabbits were divided into four groups, that is A(30), B(30), C(30), and D(30), representing hearts that ultimately received 30 minutes of ischemia. In a second series, rabbits were divided into groups A(45), B(45), (C45), and D(45) representing hearts that ultimately received 45 minutes of ischemia, and in a third series were divided into groups A(HSP), B(HSP), C(HSP), and D(HSP) representing animals that were heat shocked and their hearts were used to measure heat shock proteins. All the A groups (heat shocked) were subjected to 42 degrees C hyperthermia, all the B groups to the same procedure but with the addition of indomethacin (heat shocked + indomethacin), all the C groups served as controls, and all the D groups were treated with indomethacin only (control + indomethacin). Twenty-four hours later, all (30) and (45) groups were subjected to ischemia, whereas hearts from all (HSP) groups were harvested for heat shock protein measurements. When the animals were exposed to 30-minute ischemia, a significant difference in the infarcted to risk zone ratio (%I/R) was observed: A(30): 33.0 +/- 5.2, B(30): 16.1 +/- 4.4 [conferring a 51.2% reduction in infarct size, P < 0.05], C(30): 48.9 +/- 4.0, and D(30): 47.8 +/- 3.8 [P < 0.001 vs. B (30) and P < 0.05 vs. A(30)]. However, the %I/R did not differ among any of the (45) groups. Heat shock proteins themselves were seen to increase in A(HSP) and B(HSP) groups. Indomethacin enhances the beneficial effect of heat shock after 30-minute ischemia in vivo, reducing the infarct size by 51.2% in comparison with heat shock.

Short-term estrogen reduces myocardial infarct size in oophorectomized female rabbits in a dose-dependent manner

17 beta-estradiol, administered acutely, protects ischemic myocardium in male rabbits. In the present study we investigated the effect of short-term estrogen on myocardial infarct size in oophorectomized female rabbits. We oophorectomized 24 sexually mature New Zealand white female rabbits. Twelve animals were left untreated and 12 received oral conjugated estrogens, 0.15 mg/day, for 4 weeks. At a second stage, a third group of 12 oophorectomized female rabbits was treated with intramuscular conjugated estrogens, 1 mg/day, also for 4 weeks. All rabbits underwent 30 minutes of coronary artery occlusion and 2 hours of reperfusion while on anesthesia with i.v. pentobarbital. Infarct and risk area were delineated by Zn-Cd fluorescent particles and tetrazolium chloride staining. The infarct size was expressed as a percentage of the risk zone (I/R %). Data are reported on 26 animals that survived the treatment period and the experiment. Heart rate, systolic, and mean blood pressure and double product did not differ between the three groups at baseline, ischemia, and reperfusion. The infarct size of the risk zone was significantly smaller in the intramuscular group compared with both the oral and the placebo group (18.5 +/- 3.5% vs. 41.3 +/- 9.2% and 43 +/- 8.4%, respectively, P = 0.03). Conjugated estrogens, administered intramuscularly at a high dose, protect ischemic myocardium in oophorectomized female rabbits.

CGMP levels following ANP challenge are markers of subsequent successful reversion of lone atrial fibrillation to sinus rhythm

The aim of the present study was to assess whether cGMP release to ANP stimulation can be a biochemical marker of subsequent successful electrical cardioversion of lone atrial fibrillation to sinus rhythm. For this purpose, we studied 13 patients with chronic, lone atrial fibrillation of less than one year's duration who presented to our laboratory for electrical therapy of their arrhythmia. Prior to electrical cardioversion, peripheral venous cGMP levels were assessed at baseline and following an intravenous challenge of 50 Ug human ANP. Venous blood samples for cGMP assessment were taken a) at baseline, b) 5 and 10 mins after the end of ANP infusion. ANOVA of repeated measures was used for statistical analysis. Eight of the study patients were successfully cardioverted to sinus rhythm, while the remaining 5 were not. Although no difference was noted between the two groups regarding the mean time of arrhythmia duration as well as left atrial and ventricular dimensions, ANP stimulation provoked significantly greater cGMP release in patients whose arrhythmia reverted to sinus rhythm, when compared with that of patients whose arrhythmia persisted (p < 0.001). Therefore, cGMP levels following ANP challenge might discriminate between patients with chronic AF who are going to be successfully cardioverted and those who are not. These findings imply that the underlying atrial disease might be different in extent/nature between patients with lone AF responsive to cardioversion and those with resistant arrhythmia.