Ischemic preconditioning preserves end-ischemic ATP, enhancing functional recovery and coronary flow during reperfusion

Sex is no determinant of cardioprotection by ischemic preconditioning in rats, but ischemic/reperfused tissue mass is for remote ischemic preconditioning

We determined the impact of sex on the magnitude of cardioprotection by local and remote ischemic preconditioning (IPC and RIPC) and of ischemic/reperfused peripheral tissue mass on protection by RIPC. Hearts of female and male Lewis rats were excised, perfused with buffer, and underwent either IPC by 3 × 5/5 min global zero‐flow ischemia/reperfusion (GI/R) or time‐matched perfusion (TP) before 30/120 min GI/R. In a second approach, anesthetized female and male Lewis rats underwent RIPC, 3 × 5/5 min ischemia/reperfusion of one or both hindlimbs (1‐RIPC or 2‐RIPC), or placebo. Thirty minutes after the RIPC/placebo protocol, hearts were excised and subjected to GI/R. In female and male hearts, infarct size was less with IPC than with TP before GI/R (IPC+GI/R_female: 12 ± 5%; IPC+GI/R_male: 12 ± 7% vs. TP+GI/R_female: 33 ± 5%; TP+GI/R_male: 37 ± 7%, P < 0.001). With 2‐RIPC, infarct size was less than with 1‐RIPC in female and male rat hearts, respectively (2‐RIPC+GI/R_female: 15 ± 5% vs. 1‐RIPC+GI/R_female: 22 ± 7%, P = 0.026 and 2‐RIPC+GI/R_male: 16 ± 5% vs. 1‐RIPC+GI/R_male: 22 ± 8%, P = 0.016). Infarct size after the placebo protocol and GI/R was not different between female and male hearts (36 ± 8% vs. 34 ± 5%). Sex is no determinant of IPC‐ and RIPC‐induced cardioprotection in isolated Lewis rat hearts. RIPC‐induced cardioprotection is greater with greater mass of ischemic/reperfused peripheral tissue.

Ischemic preconditioning protects against cardiac ischemia reperfusion injury without affecting succinate accumulation or oxidation

Ischemia-reperfusion (IR) injury occurs when blood supply to an organ is disrupted and then restored, and underlies many disorders, notably myocardial infarction and stroke. While reperfusion of ischemic tissue is essential for survival, it also initiates cell death through generation of mitochondrial reactive oxygen species (ROS). Recent work has revealed a novel pathway underlying ROS production at reperfusion in vivo in which the accumulation of succinate during ischemia and its subsequent rapid oxidation at reperfusion drives ROS production at complex I by reverse electron transport (RET). Pharmacologically inhibiting ischemic succinate accumulation, or slowing succinate metabolism at reperfusion, have been shown to be cardioprotective against IR injury. Here, we determined whether ischemic preconditioning (IPC) contributes to cardioprotection by altering kinetics of succinate accumulation and oxidation during IR. Mice were subjected to a 30-minute occlusion of the left anterior descending coronary artery followed by reperfusion, with or without a protective IPC protocol prior to sustained ischemia. We found that IPC had no effect on ischemic succinate accumulation with both control and IPC mice having profound increases in succinate compared to normoxia. Furthermore, after only 1-minute reperfusion succinate was rapidly metabolised returning to near pre-ischemic levels in both groups. We conclude that IPC does not affect ischemic succinate accumulation, or its oxidation at reperfusion.

Postischemic treatment with ethyl pyruvate prevents adenosine triphosphate depletion, ameliorates inflammation, and decreases thrombosis in a murine model of hind-limb ischemia and reperfusion

INTRODUCTION

Experiments were designed to investigate the effects of ethyl pyruvate (EP) in a murine model of hind-limb ischemia-reperfusion (IR) injury.

METHODS

C57BL6 mice underwent 90 minutes of unilateral ischemia followed by 24 hours of reperfusion using two treatment protocols. For the preischemic treatment (pre-I) protocol, mice (n=6) were given 300 mg/kg EP before ischemia, followed by 150 mg/kg of EP just before reperfusion and at 6 hours and 12 hours after reperfusion. In a postischemic treatment (post-I) protocol, mice (n=7) were treated with 300 mg/kg EP at the end of the ischemic period, then 15 minutes later, and 2 hours after reperfusion and 150 mg/kg of EP at 4 hours, 6 hours, 10 hours, 16 hours, and 22 hours after reperfusion. Controls mice for both protocols were treated with lactated Ringers alone at time intervals identical to EP. Skeletal muscle levels of adenosine triphosphate (ATP), interleukin-1β, keratinocyte chemoattractant protein, and thrombin antithrombin-3 complex were measured. Skeletal muscle architectural integrity was assessed microscopically.

RESULTS

ATP levels were higher in mice treated with EP compared with controls under the both treatment protocols (p=0.02). Interleukin-1β, keratinocyte chemoattractant protein, thrombin antithrombin-3 complex (p<0.05), and the percentage of injured fibers (p<0.0001) were significantly decreased in treated versus control mice under the both protocols.

CONCLUSION

Muscle fiber injury and markers of tissue thrombosis and inflammation were reduced, and ATP was preserved with EP in pre-I and post-I protocols. Further investigation of the efficacy of EP to modulate IR injury in a larger animal model of IR injury is warranted.

Ischemic preconditioning protects cardiomyocytes against ischemic injury by inducing GRP78

Ischemic preconditioning (IP) conferred by brief ischemia-reperfusion induces resistance to cell injury due to the following lethal ischemia. This study aimed to elucidate whether 78-kDa glucose-regulated protein (GRP78), a main ER molecular chaperone, contributes to IP-mediated protection against ischemic myocardial injury. In a rat coronary artery occlusion model, the GRP78 protein level increased to 210% of the sham level by early IP with three cycles of 4-min ischemia and 4-min reperfusion. The IP reduced infarct size in subsequent lethal ischemia. In primary cardiomyocytes, the simulated IP procedure, incubation in oxygen-glucose deprivation (OGD) medium, also increased the GRP78 expression and suppressed the cell death caused by lethal ischemia. Transfection of grp78 antisense oligonucleotide attenuated the IP-mediated resistance to ischemia. This study showed for the first time that early IP up-regulates myocardial GRP78. It was suggested that GRP78 induced by early IP contributes to protect cardiomyocytes against ischemic injury.

Adenosine preconditions against endothelin-induced constriction of coronary arterioles

Myocardial hypoperfusion is accompanied by concomitant increases in adenosine and endothelin-1 (ET-1) production, but the vasodilatory effect of adenosine prevails over that of ET-1. Therefore, we hypothesized that adenosine-induced or ischemic preconditioning reduces the vasoconstrictive effect of ET-1. Coronary arteriolar diameter in vivo was measured using fluorescence microangiography in anesthetized open-thorax dogs. ET-1 (5 ng. kg(-1). min(-1) administered intracoronary, n = 10) induced progressive constriction over 45 min [25 +/- 6% (SE)]. The constriction was blocked by preconditioning with adenosine (25 microgram. kg(-1). min(-1) administered intracoronary) for 20 min and 10 min of washout (n = 10) or attenuated by ischemic preconditioning (four 5-min periods of ischemia, 9 +/- 5% at 45 min). To investigate the receptor involved in this process, coronary arterioles (50-150 micrometer) were isolated and pressurized at 60 mmHg in vitro. The ET-1 dose-response curve (1 pM-5 nM) was rightward shifted after preconditioning with adenosine (1 microM) for 20 min and 10 min of washout (n = 11). Blockade of A(2) receptors [8-(3-chlorostyryl)caffeine, 1 microM, n = 9] but not A(1) receptors (8-cyclopentyl-1,3-dipropylxanthine, 100 nM, n = 7) prevented this shift. These results suggest that adenosine confers a vascular preconditioning effect, mediated via the A(2) receptor, against endothelin-induced constriction. This effect may offer a new protective function of adenosine in preventing excessive coronary constriction.

Preconditioning prevents myocardial stunning after cardiac transplantation

BACKGROUND

Preconditioning has been shown to reduce myocardial stunning after reversible global ischemia. To determine whether preconditioning improves functional recovery after cardiac transplantation, 16 sheep were randomly assigned to a preconditioning protocol or to a control group.

METHODS

Preconditioning was achieved with 5 minutes of global ischemia followed by 10 minutes of reperfusion. The heart was then arrested with 1 L of crystalloid cardioplegia, explanted, stored in a transport cooler, and then transplanted into recipient sheep. The total ischemia time was 2 hours. Pressure-volume loops were used to calculate preload recruitable stroke work, the maximum elastance, and diastolic compliance. Linear regression analysis was used to determine the preload recruitable stroke work, maximum elastance, and diastolic compliance-and end-diastolic volume relationship. The area under the regression curve for preload recruitable stroke work was defined as the preload recruitable stroke work area. Biopsies were taken for high-energy phosphates.

RESULTS

Systolic function, represented by preload recruitable stroke work area, was preserved after cardiac transplantation in preconditioned animals. Maximum elastance and diastolic compliance were unaffected by preconditioning or ischemia. High-energy phosphates were better preserved in preconditioned animals.

CONCLUSION

Preconditioning prevented myocardial stunning and preserved high-energy phosphates after experimental cardiac transplantation.

Bioenergetics, ischemic contracture and reperfusion injury

The mammalian heart is normally well oxygenated and anaerobic glycolysis is extremely rare except for the production of extra ATP during extreme exercise like a marathon race. Anaerobic glycolysis plays a role when there is a serious impairment in coronary blood flow such as during heart attack and open heart surgery. The control of glycolysis in ischemic myocardial tissue appears to be extremely complex. During aerobic glycolysis, phosphofructokinase is the most important regulatory enzyme that controls the energy requirements of the cell. Under anaerobic conditions, however, glyceraldehyde-3-phosphate dehydrogenase becomes the key enzyme because it responds promptly to any changes in the essential supply of co-factors for oxidation. The conversion of pyruvate to acetyl CoA (aerobic metabolism) involves a series of chain reactions primarily catalyzed by pyruvate dehydrogenase complex which is situated at the cross roads between both aerobic and anaerobic glycolysis. It is important to remember that substrate utilization is carefully controlled by substrate availability. During aerobic metabolism, control mechanisms using fatty acids, lactate and glucose as energy substrates regulate the rate of ATP production according to energy demand. This precise mechanism is upset during ischemia and post-ischemic reperfusion for reasons discussed in this review. The demand for ATP can no longer be met by its supply because of severely reduced anaerobic glycolysis and significantly inhibited beta-oxidation of fatty acids. The impairment of bioenergetics is discussed in the context of several diseases such as cardiomyopathy, heart failure, diabetes, arrhythmias, cardiac surgery, heart-lung transplantation, and also in aging and oxidative stress. The regulation of energy metabolism in preconditioned heart is also discussed. Finally, methods used to preserve energy in ischemic myocardium are summarized and quantitation of the high-energy phosphates is discussed. This review challenges scientists to discover drugs which will stimulate energy supply during myocardial ischemia.

Does aprotinin increase the myocardial damage in the setting of ischemia and preconditioning?

BACKGROUND

Aprotinin reduces postoperative bleeding in cardiac operations, but its association with perioperative myocardial infarction remains controversial. Ischemic preconditioning is a novel method of myocardial protection.

METHODS

To answer whether aprotinin increases postischemic myocardial damage and also to characterize the effect of aprotinin on ischemic preconditioning, four groups of sheep were fully heparinized to keep activated clotting time readings greater than 750 seconds and subjected to 60 minutes of normothermic regional ischemia (diagonal artery occlusion) with 3 hours of reperfusion. Group I was the control with no treatment, group II received aprotinin (1 million KIU load followed by 250,000 KIU/h), group III underwent ischemic preconditioning (three 5-minute intervals of ischemia and reperfusion) before prolonged 1-hour ischemia, and group IV underwent similar ischemic preconditioning and received aprotinin. Area at risk was delineated by monastryl blue pigment, and infarction size by tetrazolium staining.

RESULTS

The ratios of weight of area at risk to left ventricular weight and left ventricular weight to body weight were constant between groups. Infarction size to area at risk ratio data demonstrated that aprotinin increases infarction size by 60% (infarction size to area at risk ratio from 52% +/- 10% to 84% +/- 10% for I versus II; p < 0.001). Aprotinin also attenuates the protective effect of ischemic preconditioning (infarction size to area at risk ratio from 25% +/- 4% to 41% +/- 6%; p < 0.001).

CONCLUSIONS

In the setting of ischemia, aprotinin increases myocardial damage. If, however, the heart is provided with protective preconditioning, then the deleterious effect of aprotinin may be neutralized. From these data we suggest that aprotinin should not be used routinely in cardiac operations unless extensive blood loss is anticipated, such as in redo open heart operations.

Biological perspective on "myocardial preconditioning"

There has been an upsurge of research on myocardial preconditioning because of its potential clinical application in areas such as cardiology, cardiac surgery, and transplantation. From a broad biological standpoint, a conceptual framework may help in both promoting understanding and suggesting future research paths. The living organism's tendency toward developing evolutionarily advantageous strategies has led to a fight or flight response, for which the authors consider preconditioning a component. Added to preconditioning, the production of stress proteins and altered myocardial states (especially that of "hibernating myocardium") can be seen as a series of biological strategies developed and maintained during evolution. An increased understanding of the mechanisms involved in the body's self-defense strategies should lead to better approaches, those in which we can help the cells, including those comprising the myocardium, to preserve themselves.