Ischemic preconditioning: exploring the paradox

PKC-dependent activation of p46/p54 JNKs during ischemic preconditioning in conscious rabbits

A conscious rabbit model was used to study the effect of ischemic preconditioning (PC) on stress-activated kinases [c-Jun NH(2)-terminal kinases (JNKs) and p38 mitogen-activated protein kinase (MAPK)] in an environment free of surgical trauma and attending external stress. Ischemic PC (6 cycles of 4-min ischemia/4-min reperfusion) induced significant activation of protein kinase C (PKC)-epsilon in the particulate fraction, which was associated with activation of p46 JNK in the nuclear fraction and p54 JNK in the cytosolic fraction; all of these changes were completely abolised by the PKC inhibitor chelerythrine. Selective enhancement of PKC-epsilon activity in adult rabbit cardiac myocytes resulted in enhanced activity of p46/p54 JNKs, providing direct in vitro evidence that PKC-epsilon is coupled to both kinases. Studies in rabbits showed that the activation of p46 JNK occurred during ischemia, whereas that of p54 JNK occurred after reperfusion. A single 4-min period of ischemia induced a robust activation of the p38 MAPK cascade, which, however, was attenuated after 5 min of reperfusion and disappeared after six cycles of 4-min ischemia/reperfusion. Overexpression of PKC-epsilon in cardiac myocytes failed to increase the p38 MAPK activity. These results demonstrate that ischemic PC activates p46 and p54 JNKs via a PKC-epsilon-dependent signaling pathway and that there are important differences between p46 and p54 JNKs with respect to the subcellular compartment (cytosolic vs. nuclear) and the mechanism (ischemia vs. reperfusion) of their activation after ischemic PC.

Clinical effects of ischemic preconditioning

A variety of experimental studies have confirmed that preconditioning the myocardium by brief periods of ischemia represents a powerful cardioprotective effect resulting in a reduction of infarct size. After 15 years of research in the experimental laboratory, some evidence shows the existence of preconditioning in human patients with coronary artery disease: repeated balloon inflations before coronary angioplasty induce preconditioning-like effects; moreover, some studies demonstrate better clinical outcome in patients with angina before acute myocardial infarction, resembling a preconditioning effect. So far, a few drugs have been identified as potential mediators of preconditioning, e.g., adenosine, adenosine receptor agonists, and adenosine triphosphate-sensitive potassium channel openers. Before coronary angioplasty and heart surgery, these preconditioning mimetics might be used to protect myocardial tissue by means of preconditioning. Further research is required before preconditioning mimetics could be used for therapy in patients with chronic myocardial ischemia. Possible antipreconditioning effects of several drugs, e.g., sulfonylurea drugs have to be considered in the treatment of patients with coronary artery disease.

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

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

Protection of the myocardial cell during ischemia

Prevention of myocardial necrosis in acute coronary syndromes is the immediate goal of therapy. Decreasing myocardial oxygen needs is of marginal value. Improving oxygen delivery by mechanical or thrombolytic reperfusion is more successful but still leaves much to be desired in terms of time to reperfusion before damage occurs due to reperfusion itself. During ischemia, there is a metabolic mismatch between glycolysis and glucose oxidation that results in accumulation of hydrogen ions, which, in turn, activates the Na+/H+ exchange system (NHE-1), leading to Na+ and Ca2+ overload and cell death. Blocking NHE-1 is a new strategy designed to prevent or delay cell death. Cariporide, a potent inhibitor of the NHE-1 system, is currently under investigation. Other agents under investigation are designed to modify proton generation, modify proton effects, and attenuate necrosis progression. Also under study are agents designed to mediate preconditioning (adenosine agonists and adenosine triphosphate-sensitive potassium channel openers). Other approaches to minimize cell injury include use of antioxidants and free-radical scavengers, complement inhibitors, selectin blockers, and glycoprotein (GP) IIb/IIIa antagonists.

Sarcolemmal versus mitochondrial ATP-sensitive K+ channels and myocardial preconditioning

Ischemic preconditioning (IPC) is a phenomenon in which single or multiple brief periods of ischemia have been shown to protect the heart against a more prolonged ischemic insult, the result of which is a marked reduction in myocardial infarct size, severity of stunning, or incidence of cardiac arrhythmias. Although a number of substances and signaling pathways have been proposed to be involved in mediating the cardioprotective effect of IPC, the overwhelming majority of evidence suggests that the ATP-sensitive potassium channel (KATP channel) is an important component of this phenomenon and may serve as the end effector in this process. Initially, it was hypothesized that the surface or sarcolemmal KATP (sarc KATP) channel mediated protection observed after IPC; however, subsequent evidence suggested that the recently identified mitochondrial KATP channel (mito KATP) may be the potassium channel mediating IPC-induced cardioprotection. In this review, evidence will be presented supporting a role for either the sarc KATP or the mito KATP in IPC and potential mechanisms by which opening these channels may produce cardioprotection; additionally, we will address important questions that still need to be investigated to define the role of the sarc or mito KATP channel, or both, in cardiac pathophysiology.

Evidence from cultured rat cortical neurons of differences in the mechanism of ischemic preconditioning of brain and heart

Ca2+ influx and activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) during nonlethal ischemic preconditioning have been implicated in the protection of the heart against subsequent lethal ischemic injury. Thus, we determined if Ca2+ influx, PKC and MAPK also mediate ischemic preconditioning-induced protection in neurons. Preconditioning by exposure of E18 rat cortical cultures to 90 min of nonlethal oxygen-glucose deprivation (OGD) 24 h prior to 180-240 min of lethal OGD was neuroprotective. Exposure to nominally free Ca2+, or blockade of the alpha-amino-hydroxy-5-methyl-isoxazolepropionate (AMPA) receptor with CNQX did not eliminate protection. MAPK activity did not change and PKC activity decreased by 50% relative to normal baseline levels at 0 and 24 h following preconditioning. The sustained decrease in PKC activity was not due to a loss of enzyme as determined from immunoblots using pan and epsilon-, beta- and zeta-specific PKC antibodies. Neuroprotection was maintained with pharmacological inhibition of PKC activity by staurosporine, chelerythrine and calphostin C and MAPK activity by PD 98059 during preconditioning, indicating that activation of these enzymes during preconditioning was not necessary for protection. Therefore, in contrast to cardiac tissue, ischemic preconditioning of neurons does not require activation of PKC and MAP kinase, and protection is maintained with substantial removal of extracellular Ca2+ or blockade of the AMPA receptor.

Ischemic preconditioning does not acutely improve load-insensitive parameters of contractility in in vivo stunned porcine myocardium

OBJECTIVE

Ischemic preconditioning has been shown to have no beneficial effect on segment shortening in in vivo regionally stunned myocardium. The purpose of this study was to determine whether ischemic preconditioning improves the recovery of postischemic ventricular function when contractility is assessed by load-insensitive measurements including end-systolic pressure length relations, preload recruitable stroke work, and preload recruitable stroke work area in in vivo regionally stunned porcine myocardium.

METHODS

Open chest, pentobarbital-anesthetized pigs were used. Regional ventricular function was monitored by measurements of segment shortening, stroke work, end systolic pressure length relations, preload recruitable stroke work, and preload recruitable stroke work area. The control group was submitted to 15 minutes of left anterior descending coronary artery occlusion and 3 hours of reperfusion. The preconditioned group underwent 2 cycles of 5-minute left anterior descending coronary artery occlusion and 10-minute reperfusion before 15 minutes of occlusion.

RESULTS

There was no infarct in either group. The preconditioning protocol significantly depressed preischemic segment shortening but not regional stroke work. Ischemic preconditioning had no significant beneficial effect on regional stroke work, end-systolic pressure length relations, preload recruitable stroke work, or preload recruitable stroke work area.

CONCLUSIONS

These results confirm that ischemic preconditioning does not ameliorate in vivo porcine myocardial stunning and indicate that ischemic preconditioning may have a limited cardioprotective role during cardiac operation.