Cardiac preconditioning with calcium: clinically accessible myocardial protection

NCX and EAAT transporters in ischemia: At the crossroad between glutamate metabolism and cell survival

Energy metabolism impairment is a central event in the pathophysiology of ischemia. The limited availability of glucose and oxygen strongly affects mitochondrial activity, thus leading to ATP depletion. In this setting, the switch to alternative energy sources could ameliorate cells survival by enhancing ATP production, thus representing an attractive strategy for ischemic treatment. In this regard, some studies have recently re-evaluated the metabolic role of glutamate and its potential to promote cell survival under pathological conditions. In the present review, we discuss the ability of glutamate to exert an "energizing role" in cardiac and neuronal models of hypoxia/reoxygenation (H/R) injury, focusing on the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺-dependent excitatory amino acid transporters (EAATs) as key players in this metabolic pathway.

Calcium phosphate buffer formed in the mitochondrial matrix during preconditioning supports ΔpH formation and ischemic ATP production and prolongs cell survival -A hypothesis

Ischemic preconditioning makes cells less sensitive to oxygen deprivation. A similar effect can be achieved by increasing the calcium concentration and applying potassium channel openers. A hypothetical mechanism of preconditioning is presented. In the mitochondrial matrix, there is a calcium hydroxide buffer consisting of a few insoluble calcium phosphate minerals. During ischemia, calcium ions stored in the matrix buffer start to leak out, forming an electric potential difference, while hydroxyl ions remain in the matrix, maintaining its pH and the matrix volume. Preconditioning factors increase the matrix buffer capacity. Production of ATP during ischemia might be the relic of a pre-endosymbiotic past.

Involvement of Endothelin 1 in Remote Preconditioning-Induced Cardioprotection through connexin 43 and Akt/GSK-3β Signaling Pathway

The present study was aimed to explore the role of endothelins in remote preconditioning (RP)-induced myocardial protection in ischemia-reperfusion (IR) injury. RP stimulus was given by subjecting hind limb to four cycles of ischemia and reperfuion (5 minutes each) using blood pressure cuff in male rats. Following RP, hearts were isolated and subjected to 30 minutes of ischemia and 120 minutes of reperfusion on Langendorff apparatus. The extent of myocardial injury was determined by measuring the levels of LDH-1, CK-MB and cardiac troponin T (cTnT) in coronary effluent; caspase-3 activity and Bcl 2 expression in heart (apoptosis); infarct size by triphenyl tetrazolium chloride and contractility parameters including left ventricular developed pressure, dp/dt_max dp/dt_min and heart rate. RP reduced ischemia reperfusion-induced myocardial injury, increased the levels of endothelin 1 (in blood), Akt-P, GSK-3β-P and P-connexin 43 (in hearts). Pretreatment with ET_A receptor antagonist, BQ 123 (1 and 2 mg/kg), ET_B receptor antagonist, BQ 788 (1 and 3 mg/kg) and dual inhibitor of ET_A and ET_B receptor, bonsentan (25 and 50 mg/kg) abolished these effects of RP. However, the effects of bonsentan were more pronounced in comparison to BQ 123 and BQ 788. It is concluded that RP stimulus may release endothelin 1 in the blood, which may activate myocardial ET_A and ET_B receptors to trigger cardioprotection through connexin 43 and Akt/GSK-3β pathway.

Non-linear actions of physiological agents: Finite disarrangements elicit fitness benefits

Finite disarrangements of important (vital) physiological agents and nutrients can induce plethora of beneficial effects, exceeding mere attenuation of the specific stress. Such response to disrupted homeostasis appears to be universally conserved among species. The underlying mechanism of improved fitness and longevity, when physiological agents act outside their normal range is similar to hormesis, a phenomenon whereby toxins elicit beneficial effects at low doses. Due to similarity with such non-linear response to toxins described with J-shaped curve, we have coined a new term “mirror J-shaped curves” for non-linear response to finite disarrangement of physiological agents. Examples from the clinical trials and basic research are provided, along with the unifying mechanisms that tie classical non-linear response to toxins with the non-linear response to physiological agents (glucose, oxygen, osmolarity, thermal energy, calcium, body mass, calorie intake and exercise). Reactive oxygen species and cytosolic calcium seem to be common triggers of signaling pathways that result in these beneficial effects. Awareness of such phenomena and exploring underlying mechanisms can help physicians in their everyday practice. It can also benefit researchers when designing studies and interpreting growing number of scientific data showing non-linear responses to physiological agents.

[Cardioprotection in cardiac surgical patients : Everything good comes from the heart]

BACKGROUND

Despite substantial success in the anesthetic and surgical management of cardiac surgery, patients frequently show postoperative complications and organ dysfunctions. This is highly relevant for mid- to long-term outcomes.

OBJECTIVES

To evaluate cardioprotective strategies that may offer effective protection in vulnerable cardiac surgery patients.

METHODS

To demonstrate recent cardioprotective approaches for cardiac surgery patients, aiming to modulate the body's own protective mechanisms in cardiac surgery patients.

RESULTS

Both cardioplegia and hypothermia belong to the well-established protective strategies during myocardial ischemia. Volatile anesthetics have been repeatedly shown to improve the left ventricular function and reduce the extent of myocardial injury compared to a control group with intravenous anesthesia. Furthermore, patients receiving volatile anesthetics showed a significantly shortened stay in the ICU and in hospital after cardiac surgery. In contrast, numerous other protective strategies failed translation into the clinical practice. Despite the published reduction of troponin release after remote ischemic preconditioning, two recent large-scale randomized multicenter trials were unable to demonstrate a clinical benefit.

CONCLUSIONS

Beside the use of cardioplegia and hypothermia, the use of volatile anesthetics is well-established during cardiac surgery because of its conditioning and protective properties. Regardless of the promising results derived from experimental studies and small clinical trials, the majority of other approaches failed to translate their findings into the clinic. Therefore, systematic experimental studies are needed to identify potential confounding factors that may affect the protective effects.

Molecular basis of cardioprotection: signal transduction in ischemic pre-, post-, and remote conditioning

Reperfusion is mandatory to salvage ischemic myocardium from infarction, but reperfusion per se contributes to injury and ultimate infarct size. Therefore, cardioprotection beyond that by timely reperfusion is needed to reduce infarct size and improve the prognosis of patients with acute myocardial infarction. The conditioning phenomena provide such cardioprotection, insofar as brief episodes of coronary occlusion/reperfusion preceding (ischemic preconditioning) or following (ischemic postconditioning) sustained myocardial ischemia with reperfusion reduce infarct size. Even ischemia/reperfusion in organs remote from the heart provides cardioprotection (remote ischemic conditioning). The present review characterizes the signal transduction underlying the conditioning phenomena, including their physical and chemical triggers, intracellular signal transduction, and effector mechanisms, notably in the mitochondria. Cardioprotective signal transduction appears as a highly concerted spatiotemporal program. Although the translation of ischemic postconditioning and remote ischemic conditioning protocols to patients with acute myocardial infarction has been fairly successful, the pharmacological recruitment of cardioprotective signaling has been largely disappointing to date.

A role for the RISK pathway and K(ATP) channels in pre- and post-conditioning induced by levosimendan in the isolated guinea pig heart

BACKGROUND AND PURPOSE

Myocardial reperfusion injury prevents optimal salvage of the ischaemic myocardium, and adjunct therapy that would significantly reduce reperfusion injury is still lacking. We investigated whether (1) the heart could be pre- and/or post-conditioned using levosimendan (levosimendan pre-conditioning (LPC) and levosimendan post-conditioning (LPostC)) and (2) the prosurvival kinases and/or the sarcolemmal or mitochondrial K(ATP) channels are involved.

EXPERIMENTAL APPROACH

Isolated guinea pig hearts were treated with two 5 min cycles of levosimendan (0.1 microM) interspersed with vehicle perfusion, or two 5 min cycles of ischaemia/reperfusion, before coronary artery ligation (CAL) for 40 min at 36.5 degrees C. Hearts were treated with mitochondrial or sarcolemmal K(ATP) channel blockers before LPC or LPostC. For post-conditioning, hearts received three 30 s cycles of ischaemia/reperfusion or levosimendan/vehicle. Hearts were pretreated with levosimendan immediately before CAL (without washout). Cardiac function, infarct size and reperfusion injury salvage kinase activity was assessed.

KEY RESULTS

LPC and LPostC halved the infarct size compared with controls (P<0.05). Treatment with K(ATP) channel blockers before LPC or LPostC reversed this decrease. Pretreating hearts with levosimendan increased activity of extracellular signal-regulated kinase (ERK) 42/44 on reperfusion and had the most marked infarct-lowering effect (P<0.05).

CONCLUSIONS AND IMPLICATIONS

(1) Hearts could be pharmacologically pre- and post-conditioned with levosimendan; (2) levosimendan pretreatment is the most effective way to reduce infarct size, possibly by increasing ERK 42/44 activity; (3) benefits of LPC and LPostC were abolished by both K(ATP) channel blockers and (4) LPC may be useful before elective cardiac surgery, whereas LPostC may be used after acute coronary artery events.

Hypercholesterolemia abrogates an increased resistance of diabetic rat hearts to ischemia-reperfusion injury

Both, diabetes mellitus (DM) and hypercholesterolemia (HCH) are known as risk factors of ischemic heart disease, however, the effects of experimental DM, as well as of HCH alone, on ischemia/reperfusion-induced myocardial injury are not unequivocal. We have previously demonstrated an enhanced resistance to ischemia-induced arrhythmias in rat hearts in the acute phase of DM. Our objectives were thus to extend our knowledge on how DM in combination with HCH, a model that is relevant to diabetic patients with altered lipid metabolism, may affect the size of myocardial infarction and susceptibility to arrhythmias. A combination of streptozotocin (STZ; 80 mg/kg, i.p.) and the fat-cholesterol diet (1% cholesterol, 1% coconut oil; FCHD) was used as a double-disease model mimicking DM and HCH simultaneosly occurring in humans. Following 5 days after STZ injection and FCHD leading to increased blood glucose and cholesterol levels, anesthetized open-chest diabetic, diabetic-hypercholesterolemic (DM-HCH) and age-matched control rats were subjected to 6-min ischemia (occlusion of LAD coronary artery) followed by 10 reperfusion to test susceptibility to ventricular arrhythmias in the in vivo experiments and to 30-min ischemia and subsequent 2-h reperfusion for the evaluation of the infarct size (IS) in the Langendorff-perfused hearts. The incidence of the most life-threatening ventricular arrhythmia, ventricular fibrillation, was significantly increased in the DM-HCH rats as compared with non-diabetic control animals (100% vs. 50%; p<0.05). Likewise, arrhythmia severity score (AS) was significantly higher in the DM-HCH rats than in the controls (4.9+/-0.2 vs. 3.5+/-0.5; p<0.05), but was not increased in the diabetic animals (AS 3.7+/-0.9; p>0.05 vs. controls). Diabetic hearts exhibited a reduced IS (15.1+/-3.0% of the area at risk vs. 37.6+/-2.8% in the control hearts; p<0.05), however, a combination of DM and HCH increased the size of myocardial infarction to that observed in the controls. In conclusion, HCH abrogates enhanced resistance to ischemia-reperfusion injury in the diabetic rat heart.

Protection by 6-aminonicotinamide against oxidative stress in cardiac cells

Oxidative stress at the time of reperfusion is a major aspect of ischemia-reperfusion injury in heart as well as in other organs. There is a continuing interest in development of pharmacological approaches to alleviate this injury. 6-Aminonicotinamide (6AN) has been shown to diminish myocardial necrosis following global ischemia in an isolated rat heart, apparently by limiting the oxidative injury component. We therefore explored the antioxidative potential of 6AN in a model using H9C2(2-1) rat cardiac myoblasts exposed to H2O2 stress. Dependent on the specific protocol, 6AN pretreatment for 6-23 h resulted in a strongly increased cell survival: from 11% to 16% in untreated cells to 56-75% following 6AN treatment. This 6AN-mediated protection was associated with a modest increase (up to 55%) of the cytosolic free Ca2+, and was blocked by ryanodine, but not by verapamil or nifedipine. The protective effect of 6AN was associated with a decrease in total cell content of the reduced glutathione (GSH) by 15-44%, indicative of an oxidative shift in the GSH/GSSG system redox potential. We propose that this redox shift caused an increased Ca2+ leak through ryanodine receptors, reflecting their known sensitivity to redox modulation. In turn, this Ca2+ redistribution appeared to trigger a state of an enhanced antioxidative resistance, somewhat analogous to the phenomenon of Ca2+ preconditioning. Similar to some of the cases of Ca2+ preconditioning, this protected state involved the activity of Ca2+ -independent, but not of Ca2+ -dependent, isoform(s) of protein kinase C.

Protein kinase Cepsilon interacts with cytochrome c oxidase subunit IV and enhances cytochrome c oxidase activity in neonatal cardiac myocyte preconditioning

We have previously identified a phorbol ester-induced PKCepsilon (protein kinase Cepsilon) interaction with the ( approximately 18 kDa) COIV [CO (cytochrome c oxidase) subunit IV] in NCMs (neonatal cardiac myocytes). Since PKCepsilon has been implicated as a key mediator of cardiac PC (preconditioning), we examined whether hypoxic PC could induce PKCepsilon-COIV interactions. Similar to our recent study with phorbol esters [Ogbi, Chew, Pohl, Stuchlik, Ogbi and Johnson (2004) Biochem. J. 382, 923-932], we observed a time-dependent increase in the in vitro phosphorylation of an approx. 18 kDa protein in particulate cell fractions isolated from NCMs subjected to 1-60 min of hypoxia. Introduction of a PKCepsilon-selective translocation inhibitor into cells attenuated this in vitro phosphorylation. Furthermore, when mitochondria isolated from NCMs exposed to 30 min of hypoxia were subjected to immunoprecipitation analyses using PKCepsilon-selective antisera, we observed an 11.1-fold increase in PKCepsilon-COIV co-precipitation. In addition, we observed up to 4-fold increases in CO activity after brief NCM hypoxia exposures that were also attenuated by introducing a PKCepsilon-selective translocation inhibitor into the cells. Finally, in Western-blot analyses, we observed a >2-fold PC-induced protection of COIV levels after 9 h index hypoxia. Our studies suggest that a PKCepsilon-COIV interaction and an enhancement of CO activity occur in NCM hypoxic PC. We therefore propose novel mechanisms of PKCepsilon-mediated PC involving enhanced energetics, decreased mitochondrial reactive oxygen species production and the preservation of COIV levels.

Preconditioning: Gender Effects1

Preconditioning is injury induced protection from subsequent injury. During preconditioning protective cellular responses to injury are up regulated resulting in acute and delayed defense against further damage. Several studies indicate that females experience a protective advantage after acute insult compared to males. Despite evidence of gender differences in acute injury, relatively few studies have evaluated whether there are sex differences in preconditioning. Variations in patients' pre-morbid preconditioning status may explain outcome variations that are not apparent in small animal studies. This review discusses the differences in response to acute injury experienced by males and females, the basic mechanisms of preconditioning, and the sex differences in the mechanisms of preconditioning.

Preconditioning: evolution of basic mechanisms to potential therapeutic strategies

Preconditioning describes the phenomenon by which a traumatic or stressful stimulus confers protection against subsequent injury. Originally recognized in dog heart subjected to ischemic challenges, preconditioning has been demonstrated in multiple species, can be induced by various stimuli, and is applicable in different organ systems. Tremendous progress has been made elucidating the signal transduction cascade of preconditioning. Preconditioning represents a potent tissue-protective condition, and mechanistic understanding may allow safe clinical application. This review recalls the history of preconditioning and how it relates to the history of the investigation of endogenous adaptation; summarizes the current mechanistic understanding of acute preconditioning; outlines the signal transduction cascade leading to the development of delayed preconditioning; discusses preconditioning in noncardiac tissue; and explores the potential of using preconditioning clinically.

The role of estrogen in cardiovascular disease

Cardiovascular disease is the number one cause of death among women, accounting for nearly 50% of female deaths. Statistics show that women on average develop cardiovascular disease 10 to 15 years later in life than men, and that the risk may increase after menopause. This observation has led to much speculation as to what physiological change(s) associated with menopause is responsible for the higher risk of atherosclerosis. Estrogen, with its potential as a cardioprotective agent and as an immunomodulator of the inflammatory response in atherosclerosis, has received the most attention. Understanding the mechanisms that lead to these differences may allow beneficial therapeutic intervention to enhance this effect in females and evoke this protection in males. This review will do the following: (1) characterize mechanisms of atherosclerosis, (2) explore the role of estrogen-replacement therapy, (3) define the effect of gender on inflammation, (4) compare and contrast the effects of estrogen and testosterone on endothelial functional, and (5) suggest mechanistic based therapeutic opportunities.

Mitochondrial potassium transport: the role of the mitochondrial ATP-sensitive K(+) channel in cardiac function and cardioprotection

Coronary artery disease and its sequelae-ischemia, myocardial infarction, and heart failure-are leading causes of morbidity and mortality in man. Considerable effort has been devoted toward improving functional recovery and reducing the extent of infarction after ischemic episodes. As a step in this direction, it was found that the heart was significantly protected against ischemia-reperfusion injury if it was first preconditioned by brief ischemia or by administering a potassium channel opener. Both of these preconditioning strategies were found to require opening of a K(ATP) channel, and in 1997 we showed that this pivotal role was mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). This paper will review the evidence showing that opening mitoK(ATP) is cardioprotective against ischemia-reperfusion injury and, moreover, that mitoK(ATP) plays this role during all three phases of the natural history of ischemia-reperfusion injury preconditioning, ischemia, and reperfusion. We discuss two distinct mechanisms by which mitoK(ATP) opening protects the heart-increased mitochondrial production of reactive oxygen species (ROS) during the preconditioning phase and regulation of intermembrane space (IMS) volume during the ischemic and reperfusion phases. It is likely that cardioprotection by ischemic preconditioning (IPC) and K(ATP) channel openers (KCOs) arises from utilization of normal physiological processes. Accordingly, we summarize the results of new studies that focus on the role of mitoK(ATP) in normal cardiomyocyte physiology. Here, we observe the same two mechanisms at work. In low-energy states, mitoK(ATP) opening triggers increased mitochondrial ROS production, thereby amplifying a cell signaling pathway leading to gene transcription and cell growth. In high-energy states, mitoK(ATP) opening prevents the matrix contraction that would otherwise occur during high rates of electron transport. MitoK(ATP)-mediated volume regulation, in turn, prevents disruption of the structure-function of the IMS and facilitates efficient energy transfers between mitochondria and myofibrillar ATPases.

Cardioprotection by St Thomas' solution is mediated by protein kinase C and tyrosine kinase

BACKGROUND

Intracellular signaling pathways, specifically the activation of protein kinase C and tyrosine kinase, are essential to the cardioprotection of ischemic preconditioning. We proposed that activation of PKC and TK contribute to the myocardial protection of St. Thomas' No. 2 cardioplegia solution (STC).

MATERIALS AND METHODS

Isolated rat hearts were exposed to 40 min of global ischemia followed by 120 min of reperfusion. Before ischemia, hearts received no treatment (control; n = 7), STC (n = 7), phorbol 12-myristate 13-acetate (PMA; n = 6), PMA + chelerythrine (n = 6), anisomycin (n = 6), anisomycin + genistein (n = 7), STC + chelerythrine (n = 7), STC + genistein (n = 7), PMA + genistein (n = 7) or anisomycin + chelerythrine (n = 7). Left ventricular developed pressure (LVDP) recovery, myocardial infarct size, and lactate dehydrogenase release were measured.

RESULTS

STC as well as PMA (protein kinase C activator) and anisomycin (tyrosine kinase activator) significantly reduced infarct size (6.9 +/- 2.9%, 9.6 +/- 2.1%, 14.0 +/- 4.4%) compared with controls (42.4 +/- 2.9%, P < 0.05). The infarct reduction of PMA and anisomycin were blocked by their inhibitors chelerythrine and genistein, respectively. Both chelerythrine (29.2 +/- 4.1%, P < 0.05) and genistein (40.4 +/- 4.3%, P < 0.05) attenuated the reduction of infarct size provided by STC. The recovery of LVDP improved with STC, PMA and anisomycin (72.6 +/- 1.4%, 60.4 +/- 4.7%, 57.2 +/- 4.6%) compared with control (33.8 +/- 3.6%, P < 0.05). Addition of chelerythrine or genistein to STC impaired recovery of LVDP (52.3 +/- 4.4%, 35.1 +/- 2.5%, P < 0.05) compared with STC treatment.

CONCLUSION

Administration of the pharmacologic inhibitors chelerythrine and genistein blunts the cardioprotection caused by STC treatment.

Ischemic tolerance of rat hearts in acute and chronic phases of experimental diabetes

Different from clinical studies of diabetes mellitus (DM), experimental data reveal both, higher and lower vulnerability of the heart to ischemic injury. We have previously demonstrated an enhanced resistance to ischemia-induced arrhythmias in isolated rat hearts in the acute phase of DM. Our objectives were thus to extend our knowledge to the effects of DM of different duration on myocardial infarction, in conjunction with susceptibility to arrhythmias, in the in vivo model. DM was induced by streptozotocin (45 mg/kg, i.v.) and following 1 week (acute phase) and 8 weeks (chronic phase), anesthetized open-chest diabetic and age-matched control rats were subjected to 30-min regional ischemia (occlusion of LAD coronary artery) followed by 4-h reperfusion for the evaluation of the infarct size (tetrazolium staining). In the control rats, ventricular tachycardia (VT) represented 45.4% of total arrhythmias and occurred in 90% of the animals. In the acute phase of DM, arrhythmia profile was similar to that in the control animals, and the incidence and severity of arrhythmias were not enhanced. On the other hand, the size of infarct area normalized to the size of area at risk was significantly smaller in the diabetics than in the controls (47.2 +/- 2.8 vs. 70.2 +/- 2.1%, respectively; p < 0.05). In the chronic phase, only 17.7% of arrhythmias occurred as VT in 44% of the diabetics (p < 0.05 vs. controls). Severity of arrhythmias was also lower (arrhythmia score: 2.1 +/- 0.3 vs. 2.9 +/- 0.3 in the controls, respectively; p < 0.05). This effect was not due to a smaller infarct size, since the latter did not differ from that in the controls.

IN CONCLUSION

diabetic rat hearts exhibit rather lower, than higher sensitivity to ischemia. In acute phase of DM, diabetic hearts are more resistant to irreversible cell damage, whereas in the chronic phase they exhibit reduced susceptibility to arrhythmias; these discrepancies might reflect different pathogenesis of arrhythmias and myocardial infarction.

Pharmacological and ischemic preconditioning of the human myocardium: mitoK(ATP) channels are upstream and p38MAPK is downstream of PKC

BACKGROUND

These studies investigate the role of mitoK(ATP) channels, protein kinase C (PKC) and Mitogen activated protein kinase (p38MAPK) on the cardioprotection of ischemic (IP) and pharmacological preconditioning (PP) of the human myocardium and their sequence of activation.

RESULTS

Right atrial appendages from patients undergoing elective cardiac surgery were equilibrated for 30 min and then subjected to 90 min of simulated ischemia followed by 120 min reoxygenation. At the end of each protocol creatinine kinase leakage (CK U/g wet wt) and the reduction of MTT to formazan dye (mM/g wet wt) were measured. Similar protection was obtained with alpha1 agonist phenylephrine, adenosine and IP and their combination did not afford additional cardioprotection. Blockade of mitoK(ATP) channels with 5-hydroxydecanoate, PKC with chelerythrine, or p38MAPK with SB203580 abolished the protection of IP and of PP. In additional studies, the stimulation of mitoK(ATP) channels with diazoxide or activation of PKC with PMA or p38MAPK with anisomycin induced identical protection to that of IP and PP. The protection induced by diazoxide was abolished by blockade of PKC and by blockade of p38MAPK. Furthermore, the protection induced by PMA was abolished by SB203580 but not by 5-hydroxydecanoate, whereas the protection induced by anisomycin was unaffected by either 5-hydroxydecanoate or chelerythrine.

CONCLUSIONS

Opening of mitoK(ATP) channels and activation of PKC and p38MAPK are obligatory steps in the signal transduction cascade of IP and PP of the human myocardium with PKC activation being downstream of the opening of mitoK(ATP) channels and upstream of p38MAPK activation.

Protein kinase C isoform expression and activity in the mouse heart

The expression of protein kinase C (PKC) isoforms in the developing murine ventricle was studied using Western blotting, assays of PKC activity, and immunoprecipitations. The abundance of two Ca2+-dependent isoforms, PKCalpha and PKCbetaII, as well as two Ca2+-independent isoforms, PKCdelta and PKCepsilon, decreased during postnatal development to <15% of the levels detected at embryonic day 18. The analysis of the subcellular distribution of the four isoforms showed that PKCdelta and PKCepsilon were associated preferentially with the particulate fraction in fetal ventricles, indicating a high intrinsic activation state of these isoforms at this developmental time point. The expression of PKCalpha in cardiomyocytes underwent a developmental change. Although preferentially expressed in neonatal cardiomyocytes, this isoform was downregulated in adult cardiomyocytes. In fast-performance liquid chromatography-purified ventricular extracts, the majority of PKC activity was Ca2+-independent in both fetal and adult ventricles. Immunoprecipitation assays indicated that PKCdelta and PKCepsilon were responsible for the majority of the Ca2+-independent activity. These studies indicate a prominent role for Ca2+-independent PKC isoforms in the mouse heart.

Post-ischemic PKC inhibition impairs myocardial calcium handling and increases contractile protein calcium sensitivity

OBJECTIVE

Protein kinase C (PKC) activation impairs contractility in the normal heart but is protective during myocardial ischemia. We hypothesized that PKC remains activated post-ischemia and modulates myocardial excitation-contraction coupling during early reperfusion.

METHODS

Langendorff-perfused rabbit hearts where subjected to 25 min unmodified ischemia and 30 min reperfusion. Total PKC activity was measured, and the intracellular translocation pattern of PKC-alpha, -delta, -epsilon, and -eta assessed by immunohistochemistry and fractionated Western immunoblotting. The PKC-inhibitors chelerythrine and GF109203X were added during reperfusion and also given to non-ischemic hearts. Measurements included left ventricular function, intracellular calcium handling measured by Rhod-2 spectrofluorometry, myofibrillar calcium responsiveness in beating and tetanized hearts, and metabolic parameters.

RESULTS

Total PKC activity was increased at end-ischemia and remained elevated after 30 min of reperfusion. The translocation pattern indicated PKC-epsilon as the main active isoform during reperfusion. Post-ischemic PKC inhibition affected mainly diastolic relaxation, with lesser effect on contractility. Both PKC inhibitors increased the Ca(2+) responsiveness of the myofilaments as indicated by a leftward shift of the calcium-to-force relationship and increased maximum calcium activated tetanic pressure. Diastolic Ca(2+) removal was delayed and the post-ischemic [Ca(2+)](i) overload further exacerbated. Depressed systolic function was associated with a lower amplitude of [Ca(2+)](i) transients.

CONCLUSION

PKC is activated during ischemia and remains activated during early reperfusion. Inhibition of PKC activity post-ischemia impairs functional recovery, delays diastolic [Ca(2+)](i) removal, and increases Ca(2+) sensitivity of the contractile apparatus, resulting in impaired diastolic relaxation. Thus, post-ischemic PKC activity may serve to restore post-ischemic Ca(2+) homeostasis and attenuate contractile protein calcium sensitivity during the period of post-ischemic [Ca(2+)](i) overload.

alpha1-Adrenoceptors during simulated ischemia and reoxygenation of the human myocardium: effect of the dose and time of administration

OBJECTIVE

We sought to investigate the effect of alpha1-adrenoceptor activity on the ischemic and reoxygenated human myocardium.

METHODS

Right atrial appendages (n = 6 per group) obtained during elective cardiac operations were sliced and stabilized in normoxic normothermic buffer solution for 30 minutes and then subjected to 90 minutes of simulated ischemia, followed by 120 minutes of reoxygenation. In study 1 the dose responses to the alpha1-adrenoceptor agonist phenylephrine (0.01, 0.1, 1, 10, and 100 micromol/L) and to the alpha1-adrenoceptor antagonist prazosin (0.1, 1, 10, and 100 micromol/L) when administered for 10 minutes before ischemia, during ischemia, and during reoxygenation were examined. The influence of the time of administration (ie, before ischemia, during ischemia, or during reoxygenation) of phenylephrine (0.1 micromol/L) and prazosin (10 micromol/L) was then investigated in study 2. In study 3 the effect of the combined administration of phenylephrine given before ischemia and prazosin given during ischemia was investigated. In study 4 the protective effect of phenylephrine given before ischemia (for 10 minutes or for 5 minutes with a 5-minute washout period) was compared with that of ischemic preconditioning (5 minutes of ischemia and 5 minutes of reoxygenation). At the end of each protocol, the leakage of creatine kinase (in units per gram of wet weight) and the reduction of 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide to insoluble formazan dye (in millimoles per gram of wet weight) were measured.

RESULTS

Phenylephrine is maximally beneficial at 0.1 and 1 micromol/L (creatinine kinase, 0.97 +/- 0.06 and 0.95 +/- 0.03 U/g, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 153.0 +/- 7.8 and 156.2 +/- 6.7 mmol/g, respectively) compared with ischemic control (creatine kinase, 1.87 +/- 0.03 U/g; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 108.5 +/- 6.8 mmol/g; P <.05) but prazosin is detrimental at concentrations above 10 micromol/L (creatine kinase, 5.22 +/- 0.29 U/g; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 69.8 +/- 2.9 mmol/g; P <.05 vs ischemic control). In addition, phenylephrine (0.1 micromol/L) is protective when given before ischemia (creatine kinase, 2.06 +/- 0.21 U/g; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 148.5 +/- 4.5 mmol/g; P <.05 vs ischemic control) but is detrimental when given during ischemia alone (creatine kinase, 4.49 +/- 0.98 U/g; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 70.5 +/- 6.1 mmol/g; P <.05 vs ischemic control) and has no significant effect during reoxygenation. In contrast, prazosin (10 micromol/L) is beneficial when given during ischemia alone (creatine kinase, 1.34 +/- 0.10 U/g; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 148.5 +/- 4.5 mmol/g; P <.05 vs ischemic control), is detrimental when given during reoxygenation alone (creatine kinase, 1.5 +/- 0.16 U/g; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 85.0 +/- 4.7 mmol/g; P <.05 vs ischemic control), and has no effect when given before ischemia. The use of phenylephrine before ischemia alone is as protective as prazosin given during ischemia alone, but the combination of the two drugs does not cause additional benefit. Interestingly, the protection afforded by phenylephrine when given before ischemia is similar to that obtained with ischemic preconditioning.

CONCLUSIONS

In the human myocardium activation of alpha1-adrenoceptors before ischemia is protective but is detrimental during ischemia, whereas blockade of alpha1-adrenoceptors is beneficial during ischemia but detrimental during reoxygenation. The degree of protection achieved by activation of the alpha1-adrenoceptors before ischemia is similar to that obtained with blockade of alpha1-adrenoceptors during ischemia and that of ischemic preconditioning.

Protein kinase C isoform-dependent myocardial protection by ischemic preconditioning and potassium cardioplegia

OBJECTIVE

Ischemic preconditioning combined with potassium cardioplegia does not always confer additive myocardial protection. This study tested the hypothesis that the efficacy of ischemic preconditioning under potassium cardioplegia is dependent on protein kinase C isoform.

METHODS

Isolated and crystalloid-perfused rat hearts underwent 5 cycles of 1 minute of ischemia and 5 minutes of reperfusion (low-grade ischemic preconditioning) or 3 cycles of 5 minutes of ischemia and 5 minutes of reperfusion (high-grade ischemic preconditioning) or time-matched continuous perfusion. These hearts received a further 5 minutes of infusion of normal buffer or oxygenated potassium cardioplegic solution. The isoform nonselective protein kinase C inhibitor chelerythrine (5 micromol/L) was administered throughout the preischemic period. All hearts underwent 35 minutes of normothermic global ischemia followed by 30 minutes of reperfusion. Isovolumic left ventricular function and creatine kinase release were measured as the end points of myocardial protection. Distribution of protein kinase C alpha, delta, and epsilon in the cytosol and the membrane fractions were analyzed by Western blotting and quantified by a densitometric assay.

RESULTS

Low-grade ischemic preconditioning was almost as beneficial as potassium cardioplegia in improving functional recovery; left ventricular developed pressure 30 minutes after reperfusion was 70 +/- 15 mm Hg (P <.01) in low-grade ischemic preconditioning and 77 +/- 14 mm Hg (P <.001) in potassium cardioplegia compared with values found in unprotected control hearts (39 +/- 12 mm Hg). Creatine kinase release during reperfusion was also equally inhibited by low-grade ischemic preconditioning (18.2 +/- 10.6 IU/g dry weight, P <.05) and potassium cardioplegia (17.6 +/- 6.7 IU/g, P <.01) compared with control values. However, low-grade ischemic preconditioning in combination with potassium cardioplegia conferred no significant additional myocardial protection; left ventricular developed pressure was 80 +/- 17 mm Hg, and creatine kinase release was 14.8 +/- 11.0 IU/g. In contrast, high-grade ischemic preconditioning with potassium cardioplegia conferred better myocardial protection than potassium cardioplegia alone; left ventricular developed pressure was 121 +/- 16 mm Hg (P <.001), and creatine kinase release was 8.3 +/- 5.8 IU/g (P <.05). Chelerythrine itself had no significant effect on functional recovery and creatine kinase release in the control hearts, but it did inhibit the salutary effects not only of low-grade and high-grade ischemic preconditioning but also those of potassium cardioplegia. Low-grade ischemic preconditioning and potassium cardioplegia enhanced translocation of protein kinase C alpha to the membrane, whereas high-grade ischemic preconditioning also enhanced translocation of protein kinase C delta and epsilon. Chelerythrine inhibited translocation of all 3 protein kinase C isoforms.

CONCLUSIONS

These results suggest that myocardial protection by low-grade ischemic preconditioning and potassium cardioplegia are mediated through enhanced translocation of protein kinase C alpha to the membrane. It is therefore suggested that activation of the novel protein kinase C isoforms is necessary to potentiate myocardial protection under potassium cardioplegia.

The adenosine-induced mechanism for the acquisition of ischemic tolerance in primary rat neuronal cultures

Neurons can be preconditioned by various procedures to resist ischemic insult. The preconditioning mechanism induced by adenosine ("the adenosine mechanism") was characterized in primary rat neuronal cultures, employing a model of chemical ischemia. The protective mechanism, initiated by activation of adenosine receptors, consists of a signal transduction pathway, involving activation of protein kinase C (PKC) and opening of ATP-sensitive potassium (K(ATP)) channels. Direct activation (and inhibition) of PKC, as well as opening of K(ATP) channels, also confers protection. The opening of the K(ATP) channels mediates the signal activated by the adenosine receptors, and probably also that activated by PKC. The acquired ischemic resistance lasts up to 5 days, depending on the activating substance. The adenosine-activated cascade of events leading to ischemic tolerance in neurons is similar to that operating in cardiomyocytes.

Acute diabetes modulates response to ischemia in isolated rat heart

Diabetic hearts are suggested to exhibit either increased or lower sensitivity to ischemia. Detrimental effects of prolonged ischemia can be attenuated by preconditioning, however, relatively little is known about its effects in the diseased myocardium. This study was designed to test the susceptibility to ischemia-induced arrhythmias and the effect of preconditioning in the diabetic heart. Rats were made diabetic with streptozotocin (45 mg/kg, i.v.). After 1 week, isolated Langendorff-perfused hearts were subjected to 30 min occlusion of LAD coronary artery without or with preceding preconditioning induced by one cycle of 5 min ischemia and 10 min reperfusion. Glycogen and lactate contents were estimated in the preconditioned and non-preconditioned hearts before and after ischemia. Diabetic hearts were more resistant to ischemia-induced arrhythmias: incidence of ventricular tachycardia (VT) decreased to 42% and only transient ventricular fibrillation (VF) occurred in 17% of the hearts as compared to the non-diabetic controls (VT 100% and VF 70% including sustained VF 36%; p < 0.05). Preconditioning effectively suppressed the incidence and severity of arrhythmias (VT 33%, VF 0%) in the normal hearts. However, this intervention did not confer any additional protection in the diabetic hearts. Despite higher glycogen content in the diabetic myocardium and greater glycogenolysis during ischemia, production of lactate in these hearts was significantly lower than in the controls. Preconditioning caused a substantial decrease in the accumulation of lactate in the normal hearts, whereby in the diabetic hearts, this intervention did not cause any further reduction in the level of lactate. In conclusion, diabetic rat hearts exhibit lower susceptibility to ischemic injury and show no additional response to preconditioning. Reduced production of glycolytic metabolites during ischemia can account for the enhanced resistance of diabetic hearts to ischemia as well as for the lack of further protection by preconditioning.

ATP-Sensitive potassium channels: a review of their cardioprotective pharmacology

ATP-sensitive potassium channels (K(ATP)) have been thought to be a mediator of cardioprotection for the last ten years. Significant progress has been made in learning the pharmacology of this channel as well as its molecular regulation with regard to cardioprotection. K(ATP)openers as a class protect ischemic/reperfused myocardium and appear to do so by conservation of energy. The reduced rate of ATP hydrolysis during ischemia exerted by these openers is not due to a cardioplegic effect and is independent of action potential shortening. Compounds have been synthesized which retain the cardioprotective effects of first generation K(ATP)openers, but are devoid of vasodilator and cardiac sarcolemmal potassium outward currents. These results suggest receptor or channel subtypes. Recent pharmacologic and molecular biology studies suggest the activation of mitochondrial K(ATP)as the relevant cardioprotective site. Implications of these results for future drug discovery and preconditioning are discussed.

Preconditioning: can nature's shield be raised against surgical ischemic-reperfusion injury?

Endogenous myocardial protection refers to the natural defense mechanisms available to the heart to withstand an ischemic injury. So far, these mechanisms have been shown to encompass two phenomena most likely interrelated: ischemic preconditioning and stress protein synthesis. Ischemic preconditioning can be defined as the adaptive mechanism induced by a brief period of reversible ischemia increasing the heart's resistance to a subsequent longer period of ischemia. The therapeutic exploitation of these natural adaptive mechanisms in cardiac surgery is an appealing prospect, as preconditioning could be used before aortic cross-clamping to enhance the current methods of myocardial protection. Two major conclusions emerge from the bulk of experimental data on preconditioning: First, the adaptive phenomenon reduces infarct size after regional ischemia in animal preparations across a wide variety of species but its effects on arrhythmias and on preservation of function after global ischemia are less consistent. This is relevant to cardiac surgery where postbypass pump failure is more often due to stunning than to discrete necrosis. Second, regardless of the various components of the intracellular signaling pathway elicited by the preconditioning stimulus, it seems that the major mechanisms by which this pathway leads to a cardioprotective effect are a slowing of adenosine triphosphate depletion and a limitation of acidosis during the protracted period of ischemia. If the latter is true, then it can reasonably be predicted that these energy-sparing and acidosis-limiting effects may become redundant to those of cardioplegia. From these observations, it can be inferred that preconditioning may find an elective indication in situations where the potential for suboptimal protection increases the risk of necrosis (extensive coronary artery disease, severe left ventricular hypertrophy, long ischemic time, and beating heart operations where occlusion of the target vessels leads to unprotected distal ischemia). Since an ischemic preconditioning stimulus could be clinically undesirable, it is critically important to identify the endogenous mediators of the phenomenon in order to use them therapeutically. One of the most important of these mediators seems to be the adenosine triphosphate-dependent potassium channel. Currently, however, the clinical application of these drugs is hampered by their poor cardioselectivity which could result in untoward systemic vasodilatory effects before cardioprotection becomes manifest. Thus, although the modalities of pharmacologically induced preconditioning still remain to be determined, the concept of therapeutic exploitation of the endogenous adaptive mechanisms of the heart could potentially represent an important adjunct to our current techniques of myocardial protection.

Exogenous calcium preconditions myocardium from patients taking oral sulfonylurea agents

We have previously reported that atrial trabeculae from patients taking oral sulfonylurea hypoglycemic agents cannot be preconditioned by transient ischemia, which may, in part, explain the increased cardiovascular mortality historically associated with the use of these agents (J. C. Cleveland et al., 1997, Circulation 96, 29-32). Recently, we reported that clinically accessible and acceptable exogenous Ca(2+) pretreatment protects human atrial trabeculae from subsequent ischemia (B. S. Cain et al., 1998, Ann. Thoracic Surg. 65, 1065-1070). It remains unknown whether this preconditioning strategy could confer protection to trabeculae from patients taking oral sulfonylurea drugs. We therefore hypothesized that exogenous Ca(2+) confers ischemic protection to trabeculae from patients taking oral sulfonylureas. Human atrial trabeculae were suspended in organ baths and field stimulated at 1 Hz, and force development was recorded. Following 90 min equilibration, trabeculae from patients taking oral sulfonylurea agents (n = 6 patients) were subjected to ischemia/reperfusion (I/R; 45/120 min) with or without Ca(2+) (1 mM increase x 5 min) 10 min prior to I/R. I/R decreased postischemic human myocardial contractility in trabeculae from patients on oral hypoglycemics to 15.3 +/- 2.0% baseline developed force (%BDF). Ca(2+) pretreatment increased postischemic human myocardial developed force to 35.3 +/- 2.9 %BDF in these patients (P < 0.05 vs I/R, ANOVA and Bonferroni/Dunn). We conclude that atrial muscle from patients taking oral hypoglycemic agents can be preconditioned with exogenous Ca(2+). This therapy may offer a clinically relevant means to precondition the myocardium of diabetics taking oral hypoglycemic agents prior to clinical interventions such as coronary angioplasty or cardiac bypass.

Calcium preconditioning, but not ischemic preconditioning, bypasses the adenosine triphosphate-dependent potassium (KATP) channel

BACKGROUND

Recent evidence has implicated the KATP channel as an important mediator of ischemic preconditioning (IPC). Indeed, patients taking oral sulfonylurea hypoglycemic agents (i.e., KATP channel inhibitors) for treatment of diabetes mellitus are resistant to the otherwise profoundly protective effects of IPC. Unfortunately, many cardiopulmonary bypass patients, who may benefit from IPC, are chronically exposed to these agents. Calcium preconditioning (CPC) is a potent form of similar myocardial protection which may or may not utilize the KATP channel in its mechanism of protection. The purpose of this study was to determine whether CPC may bypass the KATP channel in its mechanism of action. If so, CPC may offer an alternative to IPC in patients chronically exposed to these agents.

METHODS

Isolated rat hearts (n = 6-8/group) were perfused (Langendorff) and received KATP channel inhibition (glibenclamide) or saline vehicle 10 min prior to either a CPC or IPC preconditioning stimulus or neither (ischemia and reperfusion, I/R). Hearts were subjected to global warm I/R (20 min/40 min). Postischemic myocardial functional recovery was determined by measuring developed pressure (DP), coronary flow (CF), and compliance (end diastolic pressure, EDP) with a MacLab pressure digitizer.

RESULTS

Both CPC and IPC stimuli protected myocardium against postischemic dysfunction (P < 0.05 vs I/R; ANOVA with Bonferroni/Dunn): DP increased from 52 +/- 4 (I/R) to 79 +/- 2 and 83 +/- 4 mmHg; CF increased from 11 +/- 0.7 to 17 +/- 2 and 16 +/- 1 ml/min; and EDP decreased (compliance improved) from 50 +/- 7 to 27 +/- 5 and 31 +/- 7 mmHg. However, KATP channel inhibition abolished protection in hearts preconditioned with IPC (P < 0.05 vs IPC alone), but not in those preconditioned with CPC (P > 0.05 vs CPC alone).

CONCLUSIONS

(1) Both IPC and CPC provide similar myocardial protection; (2) IPC and CPC operate via different mechanisms; i.e., IPC utilizes the KATP channel whereas CPC does not; and (3) CPC may offer a means of bypassing the deleterious effects of KATP channel inhibition in diabetic patients chronically exposed to oral sulfonylurea hypoglycemic agents.

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.

Human SERCA2a levels correlate inversely with age in senescent human myocardium

OBJECTIVES

This study sought to characterize functional impairment after simulated ischemia-reperfusion (I/R) or Ca2+ bolus in senescent human myocardium and to determine if age-related alterations in myocardial concentrations of SERCA2a, phospholamban, or calsequestrin participate in senescent myocardial dysfunction.

BACKGROUND

Candidates for elective cardiac interventions are aging, and an association between age and impairment of relaxation has been reported in experimental animals. Function of the sarcoplasmic reticulum resulting in diastolic dysfunction could be dysregulated at the level of cytosolic Ca2+ uptake by SERCA2a, its inhibitory subunit (phospholamban), or at the level of Ca2+ binding by calsequestrin.

METHODS

Human atrial trabeculae from 17 patients (45-75 years old) were suspended in organ baths, field simulated at 1 Hz, and force development was recorded during I/R (45/120 min). Trabeculae from an additional 12 patients (53-73 years old) were exposed to Ca2+ bolus (2-3 mmol/L bath concentration). Maximum +/- dF/dt and the time constant of force decay (tau) were measured before and after I/R or Ca2+ bolus and related to age. SERCA2a, phospholamban, and calsequestrin from 12 patients (39-77 years old) were assessed by immunoblot.

RESULTS

Functional results indicated that maximum +/-dF/dt and tau were prolonged in senescent (>60 years) human myocardium after I/R (p < 0.05). Calcium bolus increased the maximum +/-dF/dt and decreased tau in younger, but not older patients (p < 0.05). SERCA2a and the ratio of SERCA2a to either phospholamban or calsequestrin were decreased in senescent human myocardium (p < 0.05).

CONCLUSIONS

Senescent human myocardium exhibits decreased myocardial SERCA2a content with age, which may, in part, explain impaired myocardial function after either I/R or Ca2+ exposure.

Diacylglycerol-induced protection against injury during ischemia and reperfusion in the rat heart: comparative studies with ischemic preconditioning

The role of protein kinase C (PKC) activation in ischemic preconditioning remains controversial. Since diacylglycerol is the endogenous activator of PKC and as such might be expected cardioprotective, we have investigated whether: (i) the diacylglycerol analog 1,2-dioctanoyl-sn-glycerol (DOG) can protect against injury during ischemia and reperfusion; (ii) any effect is mediated via PKC activation; and (iii) the outcome is influenced by the time of administration. Isolated rat hearts were perfused with buffer at 37 degrees C and paced at 400 bpm. In Study 1, hearts (n=6/group) were subjected to one of the following: (1) 36 min aerobic perfusion (controls); (2) 20 min aerobic perfusion plus ischemic preconditioning (3 min ischemia/3 min reperfusion+5 min ischemia/5 min reperfusion); (3) aerobic perfusion with buffer containing DOG (10 microM) given as a substitute for ischemic preconditioning; (4) aerobic perfusion with DOG (10 microM) during the last 2 min of aerobic perfusion. All hearts then were subjected to 35 min of global ischemia and 40 min reperfusion. A further group (5) were perfused with DOG (10 microM) for the first 2 min of reperfusion. Ischemic preconditioning improved postischemic recovery of LVDP from 24+/-3% in controls to 71+/-2% (P < 0.05). Recovery of LVDP also was enhanced by DOG when given just before ischemia (54+/-4%), however, DOG had no effect on the recovery of LVDP when used as a substitute for ischemic preconditioning (22+/-5%) or when given during reperfusion (29+/-6%). In Study 2, the first four groups of study were repeated (n=4-5/group) without imposing the periods of ischemia and reperfusion, instead hearts were taken for the measurement of PKC activity (pmol/min/mg protein+/-SEM). PKC activity after 36 min in groups (1), (2), (3) and (4) was: 332+/-102, 299+/-63, 521+/-144, and 340+/-113 and the membrane:cytosolic PKC activity ratio was: 5.6+/-1.5, 5.3+/-1.8, 6.6+/-2.7, and 3.9+/-2.1 (P=NS in each instance). In conclusion, DOG is cardioprotective but under the conditions of the present study is less cardioprotective than ischemic preconditioning, furthermore the protection does not appear to necessitate PKC activation prior to ischemia.

Ischemic preconditioning: exploring the paradox

Brief transient episodes of nonlethal myocardial ischemia protect or "precondition" the heart and render the myocardium resistant to a subsequent more sustained ischemic insult. The hallmark of this phenomenon--documented in virtually all species and experimental models evaluated to date in countless laboratories worldwide--is the profound reduction in infarct size seen in preconditioned groups versus time-matched controls. Efforts to identify the cellular mechanisms responsible for this paradoxical ischemia-induced cardioprotection, to expand the definition of ischemic preconditioning beyond infarct size reduction, and, perhaps most importantly, to evaluate the efficacy of preconditioning in disease models and in the clinical setting, are all topics of intensive ongoing investigation.

Calcium preconditioning in human myocardium

BACKGROUND

Ischemic stress and other protein kinase C (PKC)-linked receptor stimuli can induce rapid cardiac protection against ischemia-reperfusion injury. We and others have demonstrated that exogenous calcium (Ca2+) pretreatment confers PKC-mediated cardiac functional and infarct protection in animal models, but it remains unknown whether Ca2+ preconditioning confers similar postischemic functional protection in human myocardium, and, if so, whether the mechanism is mediated by PKC. We postulated that Ca2+ preconditioning confers ischemic tolerance to human myocardium by a PKC-dependent mechanism.

METHODS

Human atrial trabeculae were suspended in organ baths and paced at 1 Hz, and force development was recorded. After 90 minutes of equilibration, all trabeculae were subjected to ischemia (45 minutes) and reperfusion (120 minutes). Exogenous CaCl2 (3.0 mmol/L for 5 minutes) or vehicle (saline solution) was administered before simulated ischemia, with or without concurrent PKC inhibition (bisindolylmaleimide I, 150 nmol/L).

RESULTS

Ischemia-reperfusion resulted in decreased postischemic developed force, Ca2+ preconditioning protected human myocardium against ischemia-reperfusion injury (p < 0.05 versus control ischemia-reperfusion), and concurrent PKC inhibition abolished the salutary effect of Ca2+ preconditioning in human myocardium (p < 0.05 versus Ca2+ preconditioning).

CONCLUSIONS

Preconditioning with Ca2+ represents a potent means of accessing PKC-mediated protection of the human myocardium against ischemia-reperfusion injury.

Activated protein kinase C alpha associates with annexin VI from skeletal muscle

We have previously detected a number of protein kinase C (PKC) alpha-binding proteins in skeletal muscle cytosol by blot overlay assay, and now identify the major, 69 kDa binding protein as annexin VI by immunoblotting and overlay assay of hydroxyapatite chromatography fractions. Annexin VI was also detected in immunoprecipitates of PKC alpha. Annexin VI and PKC alpha are both calcium-dependent phospholipid-binding proteins, and detection of the interaction was dependent on the presence of calcium and phosphatidylserine (PS). The association probably involves specific protein-protein interactions rather than mere bridging by lipid molecules: firstly, detection of PKC alpha-annexin VI complexes by overlay assay was not diminished when PS concentrations were increased over a 10-fold range, while that of other PKC alpha-binding protein complexes was reduced or abolished; secondly, the presence in the overlay assay of a PKC pseudosubstrate peptide, analogous to a PKC sequence previously found to be involved in PKC binding activity, reduced complex formation; thirdly, we were also able to detect annexin VI interaction with PKC beta by overlay of skeletal muscle cytosol, but not with PKC theta, the major novel PKC in this tissue, suggesting sequences specific to calcium-dependent PKC isoenzymes are involved. While other annexin isoforms may be PKC substrates or inhibitors, annexin VI phosphorylation by PKC alpha could not be detected after co-purification, while phosphorylation of subsequently-added histone IIIS was readily observed. Annexin VI is a major skeletal muscle protein and our data are consistent with a role for this isoform in the control of calcium-dependent PKC.

Tumor necrosis factor in the heart

The heart is a tumor necrosis factor (TNF)-producing organ. Both myocardial macrophages and cardiac myocytes themselves synthesize TNF. Accumulating evidence indicates that myocardial TNF is an autocrine contributor to myocardial dysfunction and cardiomyocyte death in ischemia-reperfusion injury, sepsis, chronic heart failure, viral myocarditis, and cardiac allograft rejection. Indeed, locally (vs. systemically) produced TNF contributes to postischemic myocardial dysfunction via direct depression of contractility and induction of myocyte apoptosis. Lipopolysaccharide or ischemia-reperfusion activates myocardial P38 mitogen-activated protein (MAP) kinase and nuclear factor kappa B, which lead to TNF production. TNF depresses myocardial function by nitric oxide (NO)-dependent and NO-independent (sphingosine dependent) mechanisms. TNF activation of TNF receptor 1 or Fas may induce cardiac myocyte apoptosis. MAP kinases and TNF transcription factors are feasible targets for anti-TNF (i.e., cardioprotective) strategies. Endogenous anti-inflammatory ligands, which trigger the gp130 signaling cascade, heat shock proteins, and TNF-binding proteins, also control TNF production and activity. Thus modulation of TNF in cardiovascular disease represents a realistic goal for clinical medicine.

Increased myocardial tumor necrosis factor-alpha in a crystalloid-perfused model of cardiac ischemia-reperfusion injury

BACKGROUND

The heart is a tumor necrosis factor-alpha (TNF-alpha)-producing organ. Recent basic experimental and clinical evidence suggests that TNF-alpha is an important mediator of myocardial injury during acute myocardial infarction, chronic heart failure, cardiac allograft rejection, and cardiopulmonary bypass operations. Although it is known that the myocardium itself is capable of producing TNF-alpha in response to endotoxin, it is unknown whether there is an increase in myocardial tissue TNF-alpha levels after ischemia-reperfusion injury. We hypothesized that ischemia-reperfusion induces the production of TNF-alpha by the heart.

METHODS

To avoid blood-borne TNF-alpha as a potentially confounding variable, we examined myocardial TNF-alpha production in a crystalloid-perfused model of cardiac ischemia-reperfusion injury. Isolated rat hearts were perfused with crystalloid solution and subjected to ischemia-reperfusion. Postischemic myocardial TNF-alpha was measured using an enzyme-linked immunosorbent assay and correlated with developed pressure, coronary flow, end-diastolic pressure, and creatine kinase loss (assay of activity in coronary effluent).

RESULTS

Ischemia-reperfusion induced a marked increase in myocardial TNF-alpha that was associated with decreased myocardial contractility and coronary flow and with increased end-diastolic pressure and postischemic creatine kinase loss.

CONCLUSIONS

The heart produces TNF-alpha in response to ischemia-reperfusion. Ischemia-induced TNF-alpha production may contribute to postischemic myocardial stunning, necrosis, or both. Strategies designed to limit ischemia-induced myocardial TNF-alpha production may have therapeutic utility in the settings of planned myocardial ischemic events.

Activation and inhibition of protein kinase C protect rat neuronal cultures against ischemia-reperfusion insult

The effect of activation and inhibition of protein kinase C (PKC) on the capacity of neurons to resist subsequent ischemic and ischemia-reperfusion-induced cell injury, was studied in a model of primary rat neuronal cultures, subjected to chemical ischemia. Activation of PKC by 1,2 dioctanoyl-rac-glycerol (DOG; 1 microM), or phorbol 12-myristate 13-acetate (PMA; 1 microM), as well as inhibition of the enzyme by chelerythrine (10 microM), or by calphostin C (0.2 microM), 10 min before the ischemic insult, resulted in acquisition of resistance against the two insults. The length of the 'time window of protection' induced by exposure to DOG and to chelerythrine was studied and found to last for several days. The results demonstrate an apparently 'paradoxical' phenomenon, in which both activation and inhibition of PKC in the same tissue induce protection. This may be explained by differential activation of various PKC isoforms.

Adaptive and maladaptive mechanisms of cellular priming

OBJECTIVE

The mechanisms of cellular priming resulting in both adaptive and maladaptive responses to subsequent injury and strategies for manipulating this priming to constructive therapeutic advantage are explored.

BACKGROUND DATA

A cell is prepared or educated by an initial insult (priming stimulus). Investigations in both laboratory animals and humans indicate that cells, organs, and perhaps even whole patients respond differently to a proximal second insult ("second hit") by virtue of this prior environmental history. The opportunity to achieve the primed state appears to be conserved across almost all cell types. The initial stimulus transmits a message to the cellular machinery that influences the cell's response to a subsequent challenge. This response may result in an exaggerated inflammatory response in the case of the neutrophil (an often maladaptive process) or an improved tolerance to injury by the myocyte (adaptive response). Our global hypothesis is that cellular priming is a conserved, receptor-dependent process that invokes common intracellular targets across multiple cell types. We further postulate that these targets create a language based on the transient phosphorylation and dephosphorylation of intracellular enzymes that is therapeutically accessible.

CONCLUSIONS

Priming is a conserved, receptor-dependent process transduced by means of intracellular targets across multiple cell types. The potential therapeutic strategies outlined involve the receptor-mediated manipulation of cellular events. These events are transmitted through an intracellular language that instructs the cell regarding its behavior in response to subsequent stimulation. Understanding these intracellular events represents a realistic goal of priming and preconditioning biology and will likely lead to clinical control of the primed state.

Mechanisms of cardiac preconditioning: ten years after the discovery of ischemic preconditioning

Cardiac preconditioning describes the phenomenon by which transient ischemia induces myocardial protection against subsequent ischemia and reperfusion injury. Ten years have passed since the original description of this potent cardiac protective strategy and within this period tremendous progress has been made elucidating the mechanisms of preconditioning. Mechanistic understanding may allow safe clinical application. This review (1) recalls the history of preconditioning and how it relates to the history of the investigation of endogenous adaptation; (2) summarizes the current mechanistic understanding of early preconditioning; (3) compares and contrasts the mechanisms of early versus delayed preconditioning; (4) suggests potential anti-inflammatory aspects of preconditioning; (5) examines limitations in laboratory models of preconditioning; and (6) explores the potential of using preconditioning clinically.

Is calcium a mediator of infarct size reduction with preconditioning in canine myocardium?

BACKGROUND

The cellular mechanisms by which brief episodes of ischemia protect or "precondition" the heart and limit infarct size caused by a later period of sustained coronary artery occlusion remain unresolved. We propose that calcium may be an important mediator in eliciting this cardioprotection.

METHODS AND RESULTS

To test this hypothesis, anesthetized dogs received a 15-minute intracoronary infusion of 20 mmol/L CaCl2 or saline before undergoing 1 hour of coronary occlusion and 4 hours of reperfusion (protocol 1). Collateral blood flow during occlusion was measured with radiolabeled microspheres, area at risk of infarction (AR) was delineated by injection of blue dye, and area of necrosis (AN) was determined by tetrazolium staining. AN/AR was reduced from 20+/-5% in the saline-treated controls to 9+/-3% in CaCl2-treated dogs (P<.05). Additional animals underwent 10 minutes of preconditioning ischemia or a comparable waiting period before the 1-hour test occlusion (protocol 2). Administration of 5-(N,N-dimethyl)-amiloride (an inhibitor of calcium influx via Na+-H+ and Na+-Ca2+ exchange) before the preconditioning stimulus attenuated the protective effect of ischemic preconditioning: AN/AR was 12+/-1%, larger than the value of 4+/-1% observed in preconditioned dogs that received saline (P<.05) and comparable to the values of 12+/-3% and 14+/-3% seen in saline- and dimethylamiloride-treated controls.

CONCLUSIONS

Brief intracoronary infusion of CaCl2 mimicked, whereas treatment with dimethylamiloride blocked, infarct size reduction with preconditioning, thereby implicating calcium as a mediator of preconditioning in this canine model.

Protein kinase C isoform diversity in preconditioning

Protein kinase C (PKC) appears to be a common intracellular effector and signal collector during cardiac preconditioning; however, it remains unknown whether agonists that activate different PKC isoforms are also linked to select aspects of myocardial protection. Using agonists that are known to activate unique combinations of PKC isoforms, we interrogated the relationship between isoform activation and the different aspects (pH, function, and viability) of endogenous myocardial protection. To study this, isolated rat hearts were subjected to ischemia-reperfusion (I/R) (20 min/40 min), without (control = Ctrl) or with receptor-dependent [phenylephrine (PE), 50 microM; adenosine (ADO), 125 microM] or -independent [phorbol myristate acetate (PMA), 100 nM] activation of PKC. Function, pH, and viability were assessed by rate pressure product (%RPP) and coronary flow (CF; ml/min), by 31P NMR, and by CF creatine kinase (CK; U/liter) leak, respectively. PMA, which activates PKC delta but not eta, resulted in intracellular pH (pHi) and viability protection, but did not protect against postischemic myocardial stunning. ADO, which activates PKC eta but not delta, protects against stunning, but not acidosis or necrosis. PE, which activates PKC delta and eta, provided global myocardial protection against necrosis, acidosis, and stunning. Different PKC isoforms may be linked to distinct aspects of myocardial protection. Targeted activation of PKC isoforms may allow precise mechanistic application of preconditioning-like myocardial protection.