Loss of asymmetric distribution of sarcolemmal phosphatidylethanolamine during simulated ischemia in the isolated neonatal rat cardiomyocyte

Sarcolemmal dependence of cardiac protection and stress-resistance: roles in aged or diseased hearts

Disruption of the sarcolemmal membrane is a defining feature of oncotic death in cardiac ischaemia-reperfusion (I-R), and its molecular makeup not only fundamentally governs this process but also affects multiple determinants of both myocardial I-R injury and responsiveness to cardioprotective stimuli. Beyond the influences of membrane lipids on the cytoprotective (and death) receptors intimately embedded within this bilayer, myocardial ionic homeostasis, substrate metabolism, intercellular communication and electrical conduction are all sensitive to sarcolemmal makeup, and critical to outcomes from I-R. As will be outlined in this review, these crucial sarcolemmal dependencies may underlie not only the negative effects of age and common co-morbidities on myocardial ischaemic tolerance but also the on-going challenge of implementing efficacious cardioprotection in patients suffering accidental or surgically induced I-R. We review evidence for the involvement of sarcolemmal makeup changes in the impairment of stress-resistance and cardioprotection observed with ageing and highly prevalent co-morbid conditions including diabetes and hypercholesterolaemia. A greater understanding of membrane changes with age/disease, and the inter-dependences of ischaemic tolerance and cardioprotection on sarcolemmal makeup, can facilitate the development of strategies to preserve membrane integrity and cell viability, and advance the challenging goal of implementing efficacious 'cardioprotection' in clinically relevant patient cohorts. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

The feasibility of imaging myocardial ischemic/reperfusion injury using (99m)Tc-labeled duramycin in a porcine model

When pathologically externalized, phosphatidylethanolamine (PE) is a potential surrogate marker for detecting tissue injuries. (99m)Tc-labeled duramycin is a peptide-based imaging agent that binds PE with high affinity and specificity. The goal of the current study was to investigate the clearance kinetics of (99m)Tc-labeled duramycin in a large animal model (normal pigs) and to assess its uptake in the heart using a pig model of myocardial ischemia-reperfusion injury.

METHODS

The clearance and distribution of intravenously injected (99m)Tc-duramycin were characterized in sham-operated animals (n=5). In a closed chest model of myocardial ischemia, coronary occlusion was induced by balloon angioplasty (n=9). (99m)Tc-duramycin (10-15mCi) was injected intravenously at 1hour after reperfusion. SPECT/CT was acquired at 1 and 3hours after injection. Cardiac tissues were analyzed for changes associated with acute cellular injuries. Autoradiography and gamma counting were used to determine radioactivity uptake. For the remaining animals, (99m)Tc-tetrafosamin scan was performed on the second day to identify the infarct site.

RESULTS

Intravenously injected (99m)Tc-duramycin cleared from circulation predominantly via the renal/urinary tract with an α-phase half-life of 3.6±0.3minutes and β-phase half-life of 179.9±64.7minutes. In control animals, the ratios between normal heart and lung were 1.76±0.21, 1.66±0.22, 1.50±0.20 and 1.75±0.31 at 0.5, 1, 2 and 3hours post-injection, respectively. The ratios between normal heart and liver were 0.88±0.13, 0.80±0.13, 0.82±0.19 and 0.88±0.14. In vivo visualization of focal radioactivity uptake in the ischemic heart was attainable as early as 30min post-injection. The in vivo ischemic-to-normal uptake ratios were 3.57±0.74 and 3.69±0.91 at 1 and 3hours post-injection, respectively. Ischemic-to-lung ratios were 4.89±0.85 and 4.93±0.57; and ischemic-to-liver ratios were 2.05±0.30 to 3.23±0.78. The size of (99m)Tc-duramycin positive myocardium was qualitatively larger than the infarct size delineated by the perfusion defect in (99m)Tc-tetrafosmin uptake. This was consistent with findings from tissue analysis and autoradiography.

CONCLUSION

(99m)Tc-duramycin was demonstrated, in a large animal model, to have suitable clearance and biodistribution profiles for imaging. The agent has an avid target uptake and a fast background clearance. It is appropriate for imaging myocardial injury induced by ischemia/reperfusion.

Contribution of calpain to myoglobin efflux from cardiomyocytes during ischaemia and after reperfusion in anaesthetized rats

AIM

Calpain activation has a putative role in ischaemia-reperfusion injury of cardiomyocytes. This study clarified the in vivo contribution of calpain to disruption of cardiomyocyte sarcolemma during ischaemia and after reperfusion in anaesthetized rats.

METHODS

Using a microdialysis technique in the hearts of anaesthetized rats, we investigated the effects of the calpain inhibitors on myocardial interstitial myoglobin level in the ischaemic region during coronary occlusion and after reperfusion. The calpain inhibitors were administered locally via a dialysis probe. Two durations of coronary occlusion were tested.

RESULTS

Thirty-minute coronary occlusion: dialysate myoglobin concentration increased markedly from 385 ± 46 ng mL(-1) at baseline to 3701 ± 527 ng mL(-1) at 20-30 min of occlusion. After reperfusion, dialysate myoglobin concentration further increased, reaching a peak (12 296 ± 1564 ng mL(-1) ) at 10-20 min post-reperfusion and then declined gradually. The calpain inhibitors, MDL-28170 and SNJ-1945 did not change dialysate myoglobin concentration during occlusion but attenuated the increase after reperfusion to 6826 ± 1227 and 8130 ± 938 ng mL(-1) at 10-20 min post-reperfusion (P < 0.05), respectively. Ninety-minute coronary occlusion: dialysate myoglobin concentration increased from 516 ± 33 ng mL(-1) at baseline to 5463 ± 387 ng mL(-1) at 80-90 min after occlusion. After reperfusion, there was no significant increase in dialysate myoglobin concentration. MDL-28170 did not affect dialysate myoglobin concentration during occlusion or after reperfusion.

CONCLUSION

Calpain contributes to sarcolemmal disruption immediately after reperfusion following 30-min coronary occlusion, but has little effects during ischaemia and after reperfusion in 90-min coronary occlusion.

Potent cardioprotection from ischemia-reperfusion injury by a two-domain fusion protein comprising annexin V and Kunitz protease inhibitor

BACKGROUND

Considerable evidence suggests that coagulation proteases (tissue factor [TF]/activated factor VII [FVIIa]/FXa/thrombin) and their target protease activated receptors (PAR-1/PAR-2) play important roles in myocardial ischemia-reperfusion (I-R) injury. We hypothesized that localized inhibition of TF/FVIIa on the membrane surfaces of ischemic cells could effectively block coagulation cascade and subsequent PAR-1/PAR-2 cell signaling, thereby protecting the myocardium from I-R injury.

OBJECTIVES

We recently developed an annexin V-Kunitz inhibitor fusion protein (ANV-6L15) that could specifically bind to anionic phospholipids on the membrane surfaces of apoptotic cells and efficiently inhibit the membrane-anchored TF/FVIIa. In this study, we investigated the cardioprotective effect of ANV-6L15 in a rat cardiac I-R model in comparison with that of hirudin.

METHODS

Left coronary artery occlusion was maintained for 45 min followed by 4 h of reperfusion in anesthetized Sprague-Dawley rats. One minute before or 2 min after coronary ligation, rats received an intravenous bolus injection of ANV-6L15 (2.5-250 μg kg(-1) ), vehicle, or hirudin via bolus injection and continuous infusion.

RESULTS AND CONCLUSIONS

ANV-6L15 dose-dependently reduced infarct size by up to 87% and decreased plasma levels of cardiac troponin I, tumor necrosis factor-α, and soluble intercellular adhesion molecule-1, by up to 97%, 96%, and 66%, respectively, with little impact on the coagulation parameters. ANV-6L15 also ameliorated hemodynamic derangements, attenuated neutrophil infiltration and reduced Terminal deoxynucleotidyl transferase dUTP nick end labeling-positive apoptotic cardiomyocytes. Hirudin was less efficacious even at supraclinical dose. ANV-6L15 confers exceptionally potent cardioprotection and is a promising drug candidate for the prevention of myocardial I-R injury.

Dual role of cyclic GMP in cardiac cell survival

It is well known that cyclic guanosine 3',5'-monophosphate plays an important role in cardioprotection against ischemia/reperfusion injury through activation of protein kinase G (PKG). We found that cGMP prevents the mitochondrial permeability transition pore (mPTP) opening by inactivating glycogen synthase kinase 3β (GSK-3β) via protein kinase G (PKG) in cardiac H9c2 cells. While GSK-3β and its major upstream regulator phosphoinositide 3-kinase (PI3K)/Akt are critical for acute cardioprotection, an excessive activation of PI3K/Akt or GSK-3β inactivation can also lead to cardiac hypertrophy. Here, we show that cGMP not only inactivates GSK-3β through PKG (this leads to acute cardioprotection) but also negatively regulates Akt activity (this may lead to prevention of hypertrophy and heart failure, and the regulation of NO synthesis) in cardiac cells. We further found that the negative regulatory effect of cGMP on Akt activity is not mediated by PKG but may be through up-regulation of protein phosphatase PP2A activity. We propose that cGMP is a versatile signal with dual beneficial role in cardiac cell survival.

Left-shifted nav channels in injured bilayer: primary targets for neuroprotective nav antagonists?

Mechanical, ischemic, and inflammatory injuries to voltage-gated sodium channel (Nav)-rich membranes of axon initial segments and nodes of Ranvier render Nav channels dangerously leaky. By what means? The behavior of recombinant Nav1.6 (Wang et al., 2009) leads us to postulate that, in neuropathologic conditions, structural degradation of axolemmal bilayer fosters chronically left-shifted Nav channel operation, resulting in E(Na) rundown. This "sick excitable cell Nav-leak" would encompass left-shifted fast- and slow-mode based persistent I(Na) (i.e., I(window) and slow-inactivating I(Na)). Bilayer-damage-induced electrophysiological dysfunctions of native-Nav channels, and effects on inhibitors on those channels, should, we suggest, be studied in myelinated axons, exploiting I(Na)(V,t) hysteresis data from sawtooth ramp clamp. We hypothesize that (like dihydropyridines for Ca channels), protective lipophilic Nav antagonists would partition more avidly into disorderly bilayers than into the well-packed bilayers characteristic of undamaged, healthy plasma membrane. Whereas inhibitors using aqueous routes would access all Navs equally, differential partitioning into "sick bilayer" would co-localize lipophilic antagonists with "sick-Nav channels," allowing for more specific targeting of impaired cells. Molecular fine-tuning of Nav antagonists to favor more avid partitioning into damaged than into intact bilayers could reduce side effects. In potentially salvageable neurons of traumatic and/or ischemic penumbras, in inflammatory neuropathies, in muscular dystrophy, in myocytes of cardiac infarct borders, Nav-leak driven excitotoxicity overwhelms cellular repair mechanisms. Precision-tuning of a lipophilic Nav antagonist for greatest efficacy in mildly damaged membranes could render it suitable for the prolonged continuous administration needed to allow for the remodeling of the excitable membranes, and thus functional recovery.

Inhibition of phosphodiesterases leads to prevention of the mitochondrial permeability transition pore opening and reperfusion injury in cardiac H9c2 cells

PURPOSE

We tested if inhibition of phosphodiesterases (PDEs) with IBMX (1-methyl-3-isobutylxanthine) can modulate the mitochondrial permeability transition pore (mPTP) opening by inactivating glycogen synthase kinase 3β (GSK-3β).

METHODS

H9c2 cells were exposed to 600 μM H(2)O(2) for 20 min to cause the mPTP opening. Mitochondrial membrane potential (ΔΨm) was assessed by imaging cells loaded with tetramethylrhodamine ethyl ester (TMRE). Cell viability was measured with propidium iodide (PI) fluorometry using a fluorescence reader. Ischemia/reperfusion injury was induced by exposing cells to ischemic solution for 90 min followed by 30 min of reperfusion.

RESULTS

IBMX reduced loss of ΔΨm caused by H(2)O(2), indicating that inhibition of PDEs can prevent the mPTP opening. However, IBMX could not inhibit the pore opening in cells transfected with the constitutively active GSK-3β (GSK-3β-S9A) mutant, suggesting a critical role of GSK-3β in the action of IBMX. IBMX also reduced reperfusion injury in a GSK-3β dependent manner. In support, IBMX increased GSK-3β phosphorylation at Ser(9), an effect that was reversed by both the PKA inhibitor H89 and the PKG inhibitor KT5823. In support, IBMX activated both PKA and PKG. IBMX failed to prevent the loss of ΔΨm in the presence of H89 or PKA siRNA. Similarly, both KT5823 and PKG siRNA reversed the protective effect of IBMX.

CONCLUSION

Inhibition of PDEs prevents the mPTP opening by inactivating GSK-3β through PKA and PKG. GSK-3β is a common downstream target of PKA and PKG. Inhibition of PDEs may be a useful approach to prevent reperfusion injury.

Development of an in vitro model of myotube ischemia

Critical limb ischemia causes severe damage to the skeletal muscle. This study develops a reproducible model of myotube ischemia by simulating, in vitro, the critical parameters that occur in skeletal muscle ischemia. Monolayers of C2C12 myoblasts were differentiated into mature myotubes and exposed to nutrition depletion, hypoxia and hypercapnia for variable time periods. A range of culture media and gas mixture combinations were used to obtain an optimum ischemic environment. Nuclear staining, cleaved caspase-3 and lactate dehydrogenase (LDH) release assay were used to assess apoptosis and myotube survival. HIF-1α concentration of cell lysates, pH of conditioned media as well as partial pressures of oxygen (PO₂) and carbon dioxide (PCO₂) in the media were used to confirm ischemic simulation. Culturing myotubes in depleted media, in a gas mixture containing 20% CO+80% N₂ for 6-12 h increased the PCO₂ and decreased the pH and PO₂ of culture media. This attempts to mimic the in vivo ischemic state of skeletal muscle. These conditions were used to study the potential tissue-protective effects of erythropoietin (EPO) in C2C12 myotubes exposed to ischemia. EPO (60 ng/ml) suppressed LDH release, decreased cleaved caspase-3 and reduced the number of apoptotic nuclei, suggesting significantly decreased ischemia-induced apoptosis in myotubes (P<0.01) and a potential role in tissue protection. Additional therapeutic agents designed for tissue protection can also be evaluated using this model.

Modulation of the phase heterogeneity of aminoglycerophospholipid mixtures by sphingomyelin and monovalent cations: maintenance of the lamellar arrangement in the biological membranes

The phase behaviour of mixed molecular species of phosphatidylethanolamine, phosphatidylserine and sphingomyelin of biological origin were examined in aqueous co-dispersions using synchrotron X-ray diffraction. The co-dispersions of phospholipids studied were aimed to model the mixing of lipids populating the cytoplasmic and outer leaflets in the resting or "scrambled" activated cell membrane. Mixtures enriched with phosphatidylethanolamine and phosphatidylserine were characterized by a phase separation of non-lamellar phases (cubic and inverted hexagonal) with a lamellar gel phase comprising the most saturated molecular species. Inclusion of sphingomyelin in the mixture resulted in a suppression of the hexagonal-II phase in favour of lamellar phases at temperatures where a proportion of the phospholipid was fluid. The effect was also dependent on the total amount of sphingomyelin in ternary mixtures, and the lamellar phase dominated in mixtures containing more than 30 mol%, irrespective of the relative proportions of phosphatidylserine/sphingomyelin. A transition from gel to liquid-crystal phase was detected by wide-angle scattering during heating scans of ternary mixtures enriched in sphingomyelin and was shown by thermal cycling experiments to be coupled with a hexagonal-II phase to lamellar transition. In such samples there was evidence of a coexistence of non-lamellar phases with a lamellar gel phase. A transition of the gel phase to the fluid state on heating from 35 to 41 degrees C was evidenced by a progressive increase in the lamellar d-spacing. The presence of calcium enhanced the phase separation of a lamellar gel phase from a hexagonal-II phase in mixtures enriched in phosphatidylserine. This effect was counteracted by charge screening with 150 mM NaCl. The effect of sphingomyelin on stabilizing the lamellar phase is discussed in the context of an altered composition in the cytoplasmic/outer leaflets of the plasma membrane resulting from scrambling of the phospholipid distribution. The results suggest that a lamellar structure can be retained by the inward translocation of sphingomyelin in biological membranes. The presence of monovalent cations serves also to stabilize the bilayer in activated cells where a translocation of aminoglycerophospholipids and an influx of calcium occur simultaneously.

Sarcolemma phospholipid structure investigated by immunogold electron microscopy and (31)P NMR spectroscopy with lanthanide ions

The biological functions of plasma membranes depend greatly on the biophysical properties resulting from protein and phospholipid structure. We investigated the phospholipid structure of the normal sarcolemma membrane, which is known to be highly dysfunctional in myopathies. Combining electron microscopy and (31)P nuclear magnetic resonance (NMR) spectroscopy on isolated sarcolemma vesicles, we find that (i) the sarcolemma vesicles maintain the in-vivo cellular sidedness, (ii) the phospholipid mobility is close to that observed in model membranes (similar lateral diffusion coefficients and spin-lattice T(1) relaxation times). Using broad-band and magic angle spinning (31)P NMR spectroscopy with lanthanide ions (Pr(3+)), it is possible to quantify the distribution of phospholipids between internal and external membrane layers, showing that the trans-bilayer distribution is highly asymmetrical.

Heat shock proteins and cardiovascular pathophysiology

In the eukaryotic cell an intrinsic mechanism is present providing the ability to defend itself against external stressors from various sources. This defense mechanism probably evolved from the presence of a group of chaperones, playing a crucial role in governing proper protein assembly, folding, and transport. Upregulation of the synthesis of a number of these proteins upon environmental stress establishes a unique defense system to maintain cellular protein homeostasis and to ensure survival of the cell. In the cardiovascular system this enhanced protein synthesis leads to a transient but powerful increase in tolerance to such endangering situations as ischemia, hypoxia, oxidative injury, and endotoxemia. These so-called heat shock proteins interfere with several physiological processes within several cell organelles and, for proper functioning, are translocated to different compartments following stress-induced synthesis. In this review we describe the physiological role of heat shock proteins and discuss their protective potential against various stress agents in the cardiovascular system.

pHe, [Ca2+]e, and cell death during metabolic inhibition: role of phospholipase A2 and sarcolemmal phospholipids

This study measures cellular lactate dehydrogenase (LDH) release during metabolic inhibition as a monitor of sarcolemmal integrity as affected by variation of external pH (pHe) and Ca2+ concentration ([Ca2+]e). The sigmoidal relationship between pHe and LDH release and pHe and net Ca2+ uptake was essentially identical with the 50% maximal value occurring at pH 7.0 for both. This suggests that a process(es) sensitive to both pHe and [Ca2+]e plays a role in cell lysis during the course of metabolic inhibition. Variation of pHe during metabolic inhibition did not alter the decline in cellular ATP, nor did it affect changes in sarcolemmal phospholipid topology. Intracellular pH followed changes of pHe with a few minutes lag. Cell lysis increased in a graded manner as pHe and [Ca2+]e were increased, but pHe was the sole determinant of lysis, i.e., [Ca2+]e level had no effect, at the lowest (6.2) and the highest (8.0) pHe levels. pHe variation did not affect the release of radiolabeled arachidonic acid, nor did inhibitors of phospholipase A2 (PLA2) affect cell lysis at varying pHe. Therefore, cellular PLA2 activation could not be implicated for a role in cell lysis in the present model of metabolic inhibition. Alternatively, we propose that Ca2+ binding to the cytoplasmic leaflet, in combination with membrane alterations secondary to the metabolic insult, combine to destabilize the sarcolemma (20). This Ca2+ binding to the negatively charged phosphatidylserine results in the expression of the bilayer destabilizing effect of phosphatidylethanolamine. This Ca2+ binding is greatly diminished by lowered pH, resulting in an attenuation of cell lysis.

Redistribution of phosphatidylethanolamine and phosphatidylserine precedes reperfusion-induced apoptosis

Although cardiomyocyte death and infarction associated with ischemia-reperfusion are traditionally believed to be induced via necrosis, recent studies implicated apoptotic cell death in ischemic reperfused tissue. To examine whether myocardial ischemic reperfusion injury is mediated by apoptotic cell death, isolated perfused rat hearts were subjected to 15 and 30 min of ischemia as well as 15 min of ischemia followed by 30, 90, or 120 min of reperfusion. At the end of each experiment, hearts were processed for the evaluation of apoptosis and DNA laddering. Apoptosis was studied by visualizing the apoptotic cardiomyocytes by direct fluorescence detection of digoxigenin-labeled genomic DNA using APOPTAG in situ apoptosis detection kit. DNA laddering was evaluated by subjecting the DNA obtained from cardiomyocytes to 1.8% agarose gel electrophoresis and photographed under ultraviolet illumination. In addition, high-performance thin-layer chromatography (HPTLC) of aminophospholipids labeled with 2,4,6-trinitrobenzenesulfonate was performed to evaluate phospholipid topography in cardiomyocytes. The results of our study revealed apoptotic cells only in the 90- and 120-min reperfused hearts as demonstrated by the intense fluorescence of the immunostained digoxigenin-labeled genomic DNA when observed under fluorescence microscope. None of the ischemic hearts showed any evidence of apoptosis. These results corroborated with the findings of DNA fragmentation that showed increased ladders of DNA bands in the 120-min reperfused hearts, representing integer multiples of the internucleosomal DNA length (approximately 180 bp). Two-dimensional HPTLC of the phospholipids obtained from the cardiomyocytes and transbilayer organization of the phosphatidylethanolamine (PE) and phosphatidylserine (PS) in the myocytes indicated translocation of both PE and PS from the inner leaflet to the outer leaflet of the membrane as early as after 20 min of ischemia. These results demonstrate that the redistribution of PS and PE precedes the apototic cell death and DNA fragmentation associated with the reperfusion of ischemic myocardium, suggesting that ischemia may trigger the signal for apoptosis although it becomes evident during reperfusion.

The use of cis-parinaric acid to measure lipid peroxidation in cardiomyocytes during ischemia and reperfusion

cis-Parinaric acid (PnAc), a fluorescent, polyunsaturated fatty acid, was used to measure lipid peroxidation during simulated ischemia and reperfusion in cultured neonatal rat cardiomyocytes. PnAc was used both as free fatty acid, inserted in the membranes following cultivation of the cells, as well as constituent of the cellular complex lipids by metabolically integrating the fatty acid during growth. In the insertion experiments a pre-incubation with DL-aminocarnitine, an inhibitor of beta-oxidation, was necessary to prevent loss of fluorescent signal. Such a pre-incubation resulted in an enrichment of PnAc in the sarcolemma: In pre-treated cells 57 +/- 1.3% of total inserted PnAc is present in the sarcolemma compared to 27 +/- 5.7% in cells containing the integrated probe. Both methods to introduce PnAc into the cells were compared with respect to their sensitivity for an externally applied oxidative stress and thereafter lipid peroxidation during simulated ischemia and reperfusion was assayed. Going from normoxic to ischemic conditions lipid peroxidation did not increase and remained at a low level. When the ischemic cells were subsequently subjected to reperfusion (reintroduction of both oxygen and glucose), large scale lipid peroxidation was obvious. When, on the other hand, oxygen alone was reintroduced (reoxygenation) no increased lipid peroxidation was observed. These observations led to the conclusion that ischemia does not lead to an enhanced lipid peroxidation and that resumption of metabolic activity during reperfusion is necessary to induce lipid peroxidation.

Role of water and electrolyte influxes in anoxic plasma membrane disruption

The role of water and electrolyte influxes in anoxia-induced plasma membrane disruption was investigated using rabbit proximal tubule suspension. The results indicated that normal proximal tubule (PT) cells have a great capacity for expanding cell volume in response to water influx, whereas anoxia increases the susceptibility to water influx-induced disruption, and this was attenuated by glycine. However, resistance of anoxic plasma membranes to water influx-induced stress is not lost, although their mechanical strength was diminished, compared with normoxic membranes. Anoxic membranes did not disrupt under an intra-to-extracellular osmotic difference as great as 150 mosM. Potentiating or attenuating water influx by incubating PT cells in hypotonic or hypertonic medium, respectively, during anoxia, did not affect anoxia-induced membrane disruption. After the transmembrane electrolyte concentration gradient was eliminated by a "intracellular" buffer or by permeabilizing the plasma membrane to molecules <4 kDa using alpha-toxin, anoxia still caused further membrane disruption that was prevented by glycine or low pH. These results demonstrate that 1) water or net electrolyte influxes are probably not a primary cause for anoxia-induced membrane disruption and 2) glycine could prevent the plasma membrane disruption during anoxia independently from its effect on transmembrane electrolyte or water influxes. The present data support a biochemical rather than a mechanical alteration of the plasma membrane as the underlying cause of membrane disruption during anoxia.

Lipid peroxidation and modification of lipid composition in an endothelial cell model of ischemia and reperfusion

Among the changes that accompany the development of ischemia are alterations in the composition and turnover of membrane phospholipids. To study these effects, a cell culture model was developed to facilitate accurate measurements of lipids over varying intervals of ischemia and reperfusion (I/R). In order to mimic ischemia, rabbit aortic endothelial cells were grown to confluency on collagen coated beads and the bead cultures allowed to settle to the bottom of a conical test tube or spectrofluorometric cuvette. The cell-coated beads were then resuspended in media to simulate the process of reperfusion. Survival after ischemia/reperfusion, was determined by measurements of cellular replating efficiency, and found to decrease after periods longer than three hours of ischemia (followed by 24 h of reperfusion). Plating efficiencies were reduced to nearly 50% after 5 h of ischemia followed by reperfusion. Release of LDH inversely correlated with cell survival, and lactate production, ATP levels, and extracellular H2O2 concentration were all affected by the duration of ischemia. These changes could be directly related to rates of cellular oxygen consumption which decreased by 50% after 5 h of ischemia, while the percentage of oxygen consumption not be inhibitable by cyanide, increased. Release of esterified fatty acids, which was partly inhibited by the phospholipase A2 inhibitor, mepacrine, was stimulated by increasing periods of ischemia while the incorporation of free fatty acids into phospholipids was inhibited. The incorporation of arachidonic acid was inhibited to a lesser degree than that of oleic or linoleic acids with a resulting change in phospholipid fatty acyl composition favoring greater proportions of unsaturated fatty acids. In some experiments, the effects of vitamin E or ascorbic acid administered prior to ischemia were studied. The degree of fatty acid unsaturation, fatty acid incorporation into phospholipids, and release from phospholipids into the free fatty acid pool during ischemia/reperfusion were not affected by prior administration of vitamin E or ascorbic acid. However, the extent of lipid peroxidation during ischemia was inhibited by 100 mM ascorbic acid when present during the ischemia/reperfusion period, but not by vitamin E administered for 24 h prior to ischemia. Ascorbic acid treatment, but not vitamin E, also enabled cells to recover substantial amounts of the ATP lost following prolonged ischemia. The ATP recovery corresponded to an increased cell survival and decreased lipid peroxidation. Progressive intervals of ischemia followed by reperfusion result in compromised cell respiratory activity and decreased ATP production, and decreased phospholipid acylation leading to net hydrolysis. The associated changes in phospholipid composition, and specifically increased unsaturation appear to favor peroxidation of membrane phospholipids.

Sarcolemmal phosphatidylethanolamine reorganization during simulated ischaemia and reperfusion: reversibility and ATP dependency

Exposure of cultured neonatal rat heart cells to simulated ischaemia results in a cessation of the spontaneous contractile activity and changes at both the level of sarcolemmal phospholipid topology and the ultrastructural level. Reperfusion at a timepoint before irreversible cell damage develops leads to a recovery of contractile activity. Furthermore, the shift in transbilayer distribution of sarcolemmal phosphatidylethanolamine in favour of the outer membrane leaflet, due to the ischaemic period, is reversed during subsequent reperfusion. Also the morphological changes (mitochondrial oedema, reorganization of the mitochondrial cristae and the formation of extrusions at the sarcolemma) are reversible. At the same time total intracellular ATP levels are restored to 80% of control. The role of cellular ATP content on sarcolemmal phospholipid topology was further studied by the use of the calcium antagonist verapamil (10 microM), which preserved cellular ATP content by inhibiting cell contractility before the onset of ischaemia. After 120 min of ischaemia, cell ATP content was still 63% of control in the presence of verapamil, versus 20% of control in untreated cells. Verapamil treatment also prevented the loss of the asymmetrical distribution of phosphatidylethanolamine and sarcolemmal disruption, the latter occurring during 120 min of ischaemia in untreated cells. It is proposed that maintenance of phospholipid asymmetry of the sarcolemma of the myocytes depends on the cellular ATP concentrations, indicating the involvement of an ATP dependent aminophospholipid translocase.

Reversible and irreversible damage in reoxygenated 'ischemic' ventricular myocytes in culture

The LDH release pattern from cardiomyocytes under 'ischemia-like' conditions shows two phases. In the initial slow phase, reoxygenation immediately stops further enzyme release. Accelerated LDH release, which occurs concomitantly with Iysosomal enzyme release, characterizes the second phase of 'ischemia.' Reoxygenation at this stage does not put a stop to further enzyme release. Reoxygenation during the first phase of 'ischemia' rapidly restored ATP level, while in the second phase, ATP levels remained low even after 6 h of reoxygenation. This study as well as previous data seem to suggest that irreversible cellular damage leading to cell death, occurs by synergistic action of many effectors, each of which does not necessarily cause irreversible damage.