The effects of lysophosphatidylcholine, a toxic metabolite of ischemia, on the components of cardiac excitability in sheep Purkinje fibers

iPLA2β Contributes to ER Stress-Induced Apoptosis during Myocardial Ischemia/Reperfusion Injury

Both calcium-independent phospholipase A2 beta (iPLA2β) and endoplasmic reticulum (ER) stress regulate important pathophysiological processes including inflammation, calcium homeostasis and apoptosis. However, their roles in ischemic heart disease are poorly understood. Here, we show that the expression of iPLA2β is increased during myocardial ischemia/reperfusion (I/R) injury, concomitant with the induction of ER stress and the upregulation of cell death. We further show that the levels of iPLA2β in serum collected from acute myocardial infarction (AMI) patients and in samples collected from both in vivo and in vitro I/R injury models are significantly elevated. Further, iPLA2β knockout mice and siRNA mediated iPLA2β knockdown are employed to evaluate the ER stress and cell apoptosis during I/R injury. Additionally, cell surface protein biotinylation and immunofluorescence assays are used to trace and locate iPLA2β. Our data demonstrate the increase of iPLA2β augments ER stress and enhances cardiomyocyte apoptosis during I/R injury in vitro and in vivo. Inhibition of iPLA2β ameliorates ER stress and decreases cell death. Mechanistically, iPLA2β promotes ER stress and apoptosis by translocating to ER upon myocardial I/R injury. Together, our study suggests iPLA2β contributes to ER stress-induced apoptosis during myocardial I/R injury, which may serve as a potential therapeutic target against ischemic heart disease.

Mechanisms of ranolazine pretreatment in preventing ventricular tachyarrhythmias in diabetic db/db mice with acute regional ischemia-reperfusion injury

Studies have demonstrated that diabetic (db/db) mice have increased susceptibility to myocardial ischemia-reperfusion (IR) injury and ventricular tachyarrhythmias (VA). We aimed to investigate the antiarrhythmic and molecular mechanisms of ranolazine in db/db mouse hearts with acute IR injury. Ranolazine was administered for 1 week before coronary artery ligation. Diabetic db/db and control db/+ mice were divided into ranolazine-given and -nongiven groups. IR model was created by 15-min left coronary artery ligation and 10-min reperfusion. In vivo electrophysiological studies showed that the severity of VA inducibility was higher in db/db mice than control (db/ +) mice. Ranolazine suppressed the VA inducibility and severity. Optical mapping studies in Langendorff-perfused hearts showed that ranolazine significantly shortened action potential duration, Ca_i transient duration, Ca_i decay time, ameliorated conduction inhomogeneity, and suppressed arrhythmogenic alternans induction. Western blotting studies showed that the expression of pThr¹⁷-phospholamban, calsequestrin 2 and voltage-gated sodium channel in the IR zone was significantly downregulated in db/db mice, which was ameliorated with ranolazine pretreatment and might play a role in the anti-arrhythmic actions of ranolazine in db/db mouse hearts with IR injury.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Phospholipid alterations in the brain and heart in a rat model of asphyxia-induced cardiac arrest and cardiopulmonary bypass resuscitation

Cardiac arrest (CA) induces whole-body ischemia, causing damage to multiple organs. Ischemic damage to the brain is mainly responsible for patient mortality. However, the molecular mechanism responsible for brain damage is not understood. Prior studies have provided evidence that degradation of membrane phospholipids plays key roles in ischemia/reperfusion injury. The aim of this study is to correlate organ damage to phospholipid alterations following 30 min asphyxia-induced CA or CA followed by cardiopulmonary bypass (CPB) resuscitation using a rat model. Following 30 min CA and CPB resuscitation, rats showed no brain function, moderately compromised heart function, and died within a few hours; typical outcomes of severe CA. However, we did not find any significant change in the content or composition of phospholipids in either tissue following 30 min CA or CA followed by CPB resuscitation. We found a substantial increase in lysophosphatidylinositol in both tissues, and a small increase in lysophosphatidylethanolamine and lysophosphatidylcholine only in brain tissue following CA. CPB resuscitation significantly decreased lysophosphatidylinositol but did not alter the other lyso species. These results indicate that a decrease in phospholipids is not a cause of brain damage in CA or a characteristic of brain ischemia. However, a significant increase in lysophosphatidylcholine and lysophosphatidylethanolamine found only in the brain with more damage suggests that impaired phospholipid metabolism may be correlated with the severity of ischemia in CA. In addition, the unique response of lysophosphatidylinositol suggests that phosphatidylinositol metabolism is highly sensitive to cellular conditions altered by ischemia and resuscitation.

Examination of physiological function and biochemical disorders in a rat model of prolonged asphyxia-induced cardiac arrest followed by cardio pulmonary bypass resuscitation

Background

Cardiac arrest induces whole body ischemia, which causes damage to multiple organs particularly the heart and the brain. There is clinical and preclinical evidence that neurological injury is responsible for high mortality and morbidity of patients even after successful cardiopulmonary resuscitation. A better understanding of the metabolic alterations in the brain during ischemia will enable the development of better targeted resuscitation protocols that repair the ischemic damage and minimize the additional damage caused by reperfusion.

Method

A validated whole body model of rodent arrest followed by resuscitation was utilized; animals were randomized into three groups: control, 30 minute asphyxial arrest, or 30 minutes asphyxial arrest followed by 60 min cardiopulmonary bypass (CPB) resuscitation. Blood gases and hemodynamics were monitored during the procedures. An untargeted metabolic survey of heart and brain tissues following cardiac arrest and after CPB resuscitation was conducted to better define the alterations associated with each condition.

Results

After 30 min cardiac arrest and 60 min CPB, the rats exhibited no observable brain function and weakened heart function in a physiological assessment. Heart and brain tissues harvested following 30 min ischemia had significant changes in the concentration of metabolites in lipid and carbohydrate metabolism. In addition, the brain had increased lysophospholipid content. CPB resuscitation significantly normalized metabolite concentrations in the heart tissue, but not in the brain tissue.

Conclusion

The observation that metabolic alterations are seen primarily during cardiac arrest suggests that the events of ischemia are the major cause of neurological damage in our rat model of asphyxia-CPB resuscitation. Impaired glycolysis and increased lysophospholipids observed only in the brain suggest that altered energy metabolism and phospholipid degradation may be a central mechanism in unresuscitatable brain damage.

Inhibition of MMP-2 expression affects metabolic enzyme expression levels: proteomic analysis of rat cardiomyocytes

In this study we examined the effect of inhibition of MMP-2 expression, using siRNA, on the cardiomyocyte proteome. Isolated cardiomyocytes were transfected with MMP-2 siRNA and incubated for 24h. Control cardiomyocytes from the same heart were transfected with scrambled siRNA following the same protocol. Comparison of control cardiomyocyte proteomes with proteomes from MMP-2 suppressed cardiomyocytes revealed 13 protein spots of interest (9 protein spots increased; 4 decreased). Seven protein spots were identified as mitochondrial enzymes involved in energy production and represent: ATP synthase beta subunit, dihydrolipoyllysine-residue succinyltransferase component of 2-oxoglutarate dehydrogenase complex, cytochrome c oxidase subunit 5A, electron transfer flavoprotein subunit beta, NADH dehydrogenase (ubiquinone) 1 alpha subcomplex subunit 5 and a fragment of mitochondrial precursor of long-chain specific acyl-CoA dehydrogenase. Furthermore, precursor of heat shock protein 60 and Cu-Zn superoxide dismutase were identified. Two protein spots corresponding to MLC1 were also detected. In addition, ATP synthase activity was measured and was increased by approximately 30%. Together, these results indicate that MMP-2 inhibition represents a novel cardioprotective therapy by promoting alterations in the levels of mitochondrial enzymes for improved energy metabolism and by preventing degradation of contractile proteins needed for normal excitation-contraction coupling.

BIOLOGICAL SIGNIFICANCE

During ischemia and reperfusion of cardiomyocytes, abnormality in excitation-contraction coupling and decreased energy metabolism often lead to myocardial infarction, but the cellular mechanisms are not fully elucidated. We show for the first time that intracellular inhibition of MMP-2 in cardiomyocytes increases contractility of aerobically perfused myocytes, which was accompanied by increased expression of contractile proteins (e.g., MLC-1). We also showed that MMP-2 inhibition produced a cardiomyocyte proteome that is consistent with improved mitochondrial energy metabolism (e.g., increased expression and activity of mitochondrial beta ATP synthase). Thus, MMP-2 appears to be involved in homeostatic regulation of protein turnover. Our results are significant since they point to targeting MMP-2 activity as a novel therapeutic option to limit myocardial damage by decreasing proteolytic degradation of mitochondrial metabolic enzymes and myocardial contractile proteins during ischemia. In addition, the development of novel pharmacological agents that selectively targets cardiac MMP-2 represents a novel approach to treat and prevent other heart diseases.

Reduced conduction reserve in the diabetic rat heart: role of iPLA2 activation in the response to ischemia

Hearts from streptozotocin (STZ)-induced diabetic rats have previously been shown to have impaired intercellular electrical coupling, due to reorganization (lateralization) of connexin43 proteins. Due to the resulting reduction in conduction reserve, conduction velocity in diabetic hearts is more sensitive to conditions that reduce cellular excitability or intercellular electrical coupling. Diabetes is a known risk factor for cardiac ischemia, a condition associated with both reduced cellular excitability and reduced intercellular coupling. Activation of Ca(2+)-independent phospholipase A(2) (iPLA(2)) is known to be part of the response to acute ischemia and may contribute to the intercellular uncoupling by causing increased levels of arachidonic acid and lysophosphatidyl choline. Normally perfused diabetic hearts are known to exhibit increased iPLA(2) activity and may thus be particularly sensitive to further activation of these enzymes. In this study, we used voltage-sensitive dye mapping to assess changes in conduction velocity in response to acute global ischemia in Langendorff-perfused STZ-induced diabetic hearts. Conduction slowing in response to ischemia was significantly larger in STZ-induced diabetic hearts compared with healthy controls. Similarly, slowing of conduction velocity in response to acidosis was also more pronounced in STZ-induced diabetic hearts. Inhibition of iPLA(2) activity using bromoenol lactone (BEL; 10 μM) had no effect on the response to ischemia in healthy control hearts. However, in STZ-induced diabetic hearts, BEL significantly reduced the amount of conduction slowing observed beginning 5 min after the onset of ischemia. BEL treatment also significantly increased the time to onset of sustained arrhythmias in STZ-induced diabetic hearts but had no effect on the time to arrhythmia in healthy control hearts. Thus, our results suggest that iPLA(2) activation in response to acute ischemia in STZ-induced diabetic hearts is more pronounced than in control hearts and that this response is a significant contributor to arrhythmogenic conduction slowing.

Late sodium current is a new therapeutic target to improve contractility and rhythm in failing heart

Most cardiac Na+ channels open transiently within milliseconds upon membrane depolarization and are responsible for the excitation propagation. However, some channels remain active during hundreds of milliseconds, carrying the so-called persistent or late Na+ current (I(NaL)) throughout the action potential plateau. I(NaL) is produced by special gating modes of the cardiac-specific Na+ channel isoform. Experimental data accumulated over the past decade show the emerging importance of this late current component for the function of both normal and especially failing myocardium, where I(NaL) is reportedly increased. Na+ channels represent a multi-protein complex and its activity is determined not only by the pore-forming alpha subunit but also by its auxiliary beta subunits, cytoskeleton, and by Ca2+ signaling and trafficking proteins. Remodeling of this protein complex and intracellular signaling pathways may lead to alterations of I(NaL) in pathological conditions. Increased I(NaL) and the corresponding Na+ influx in failing myocardium contribute to abnormal repolarization and an increased cell Ca2+ load. Interventions designed to correct I(NaL) rescue normal repolarization and improve Ca2+ handling and contractility of the failing cardiomyocytes. New therapeutic strategies to target both arrhythmias and deficient contractility in HF may not be limited to the selective inhibition of I(NaL) but also include multiple indirect, modulatory (e.g. Ca(2+)- or cytoskeleton- dependent) mechanisms of I(NaL) function.

Late sodium current in failing heart: friend or foe?

Most cardiac Na+ channels open transiently upon membrane depolarization and then are quickly inactivated. However, some channels remain active, carrying the so-called persistent or late Na+ current (INaL) throughout the action potential (AP) plateau. Experimental data and the results of numerical modeling accumulated over the past decade show the emerging importance of this late current component for the function of both normal and failing myocardium. INaL is produced by special gating modes of the cardiac-specific Na+ channel isoform. Heart failure (HF) slows channel gating and increases INaL, but HF-specific Na+ channel isoform underlying these changes has not been found. Na+ channels represent a multi-protein complex and its activity is determined not only by the pore-forming alpha subunit but also by its auxiliary beta subunits, cytoskeleton, calmodulin, regulatory kinases and phosphatases, and trafficking proteins. Disruption of the integrity of this protein complex may lead to alterations of INaL in pathological conditions. Increased INaL and the corresponding Na+ flux in failing myocardium contribute to abnormal repolarization and an increased cell Ca2+ load. Interventions designed to correct INaL rescue normal repolarization and improve Ca2+ handling and contractility of the failing cardiomyocytes. This review considers (1) quantitative integration of INaL into the established electrophysiological and Ca2+ regulatory mechanisms in normal and failing cardiomyocytes and (2) a new therapeutic strategy utilizing a selective inhibition of INaL to target both arrhythmias and impaired contractility in HF.

Peroxynitrite formation mediates LPC-induced augmentation of cardiac late sodium currents

Lysophosphatidylcholine (LPC) accumulates in the ischaemic myocardium and is arrhythmogenic. We have examined the mechanisms underlying the effects of LPC on the late cardiac Na(+) current (I(L)Na). Na(+) currents were recorded in HEK293 cells expressing Na(V)1.5 and isolated rat ventricular myocytes. LPC enhanced recombinant I(L)Na, while it reduced peak Na(+) current. Computer modeling of human ventricular myocyte action potentials predicted a marked duration prolonging effect and arrhythmogenic potential due to these effects of LPC on peak and late currents. Enhancement of recombinant I(L)Na was suppressed by the antioxidant ascorbic acid and by the NADPH oxidase inhibitor DPI. Inhibitors of the mitochondrial electron transport chain (rotenone, TTFA and myxothiazol) were without effect on LPC responses. The superoxide donor pyrogallol was without effect on I(L)Na. Enhancement of I(L)Na was abrogated by the NOS inhibitors l-NAME and 7-nitroindazole, while LPC induced an l-NAME-sensitive production of NO, measured as enhanced DAF-FM fluorescence, in both HEK293 cells and ventricular myocytes. Despite this, the NO donors SNAP and SNP caused no change in I(L)Na. However, SNAP enhanced TTX-sensitive recombinant and native I(L)Na in the presence of pyrogallol, suggesting peroxynitrite formation as a mediator of the response to LPC. In support of this, the peroxynitrite scavenger FeTPPS prevented the response of I(L)Na to LPC. Peroxynitrite formation provides a novel mechanism by which LPC regulates the late cardiac Na(+) current.

Effects of arrhythmogenic lipid metabolites on the L-type calcium current of diabetic vs. non-diabetic rat hearts

Accumulation of lipid metabolites, such as palmitoylcarnitine and lysophosphatidylcholine, is thought to be a major contributor to the development of cardiac arrhythmias during myocardial ischemia. This arrhythmogenicity is likely due to the effects of these metabolites on various ion channels. Diabetic hearts have been shown to accumulate much higher concentrations of these lipid metabolites during ischemia, which may be an important factor in the enhanced incidence of arrhythmias in diabetic hearts. However, it is not known whether these metabolites have similar effects on the ion channels of diabetic hearts as in non-diabetic hearts. Previous studies on myocytes from non-diabetic hearts have reported either enhancement or inhibition of L-type calcium current (I(Ca)) by these lipid metabolites. Thus, it is not clear whether the effects of palmitoylcarnitine and/or lysophosphatidlycholine on I(Ca) contribute to the enhanced arrhythmogenicity of diabetic hearts or protect against arrhythmias. We determined the effect of exogenous palmitoylcarnitine and lysophosphatidylcholine on the (I(Ca)) in ventricular myocytes from streptozotocin-diabetic and non-diabetic rat hearts under identical conditions. We found that palmitoylcarnitine and lysophosphatidylcholine exhibited a dose-dependent inhibition of I(Ca), which was virtually identical in diabetic and non-diabetic cardiac myocytes. Thus, we conclude that these arrhythmogenic lipid metabolites have similar actions on calcium channels in diabetic and non-diabetic hearts. Therefore, the greater susceptibility of diabetic hearts to arrhythmias during myocardial ischemia is not due to an altered sensitivity of the L-type calcium channels to lipid metabolites, but may be explained, in large part, by the greater accumulation of these metabolites during ischemia.

Selective impairment of HCO3(-)-dependent pHi regulation by lysophosphatidylcholine in guinea pig ventricular myocardium

OBJECTIVE

The aim was to examine the effects of lysophosphatidylcholine (LPC), an amphiphilic lipid metabolite in ischemic myocardium, on intracellular pH (pH(i)) regulatory systems in guinea pig papillary muscles.

METHODS

In CO2/HCO(3-)-buffered Tyrode solution, pH(i), intracellular Na+ activity (aNai) and membrane potential of isolated guinea pig papillary muscles were measured using ion-selective microelectrode and conventional microelectrode. Standard ammonium prepulsing with 20 mM NH4Cl was used to produce an intracellular acid load, and effects of LPC on the pH(i) recovery from acidosis were evaluated in the absence and presence of a transport inhibitor.

RESULTS

LPC acidified the resting pH(i) by 0.03 +/- 0.01 pH units (n = 15, p < 0.01) concomitantly with a slight decrease in resting membrane potential and an increase in aNai in quiescent preparations. The pH(i) recovery rate from an intracellular acid load was decreased to 83 +/- 4% of the control value by 30 microM LPC (n = 8, P < 0.05) but not by 30 microM phosphatidylcholine (PC). In the presence of 10 microM 5-(N,N-hexamethylene) amiloride (HMA), a Na(+)-H+ exchange inhibitor, LPC still slowed pH(i) recovery from an intracellular acid load to 77 +/- 4% of the control (n = 5, P < 0.05). However, LPC failed to alter the pH(i) recovery rate in the presence of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 0.5 mM), a Na(+)-HCO3- symport inhibitor.

CONCLUSION

LPC impairs Na(+)-HCO3- symport but not Na(+)-H+ exchange, and LPC may potentiate its arrhythmogenic action by intensifying the intracellular acidosis in ischemic myocardium.

Flecainide and the electrophysiologic matrix: the effects of flecainide acetate on the determinants of cardiac excitability in sheep Purkinje fibers

Flecainide was associated with excess mortality distributed virtually equally throughout the period of the Cardiac Arrhythmia Suppression Trial, suggesting the intersection of two events, drug effect and perhaps ischemia. Flecainide's effect on active properties has been studied extensively, but nothing is known of its effects on passive properties or on the balance among active and passive cellular properties that determines cardiac excitability. The multiple microelectrode method of intracellular current application and transmembrane voltage recording was used in sheep Purkinje fibers to determines strength- and charge-duration as well as constant current-voltage relationships and to estimate active properties, liminal length, and cable properties at a normal [K+]o and in a setting of hyperkalemia analogous to that of ischemia. A computer tracked in time the alterations in the active and passive properties relevant to excitability. Flecainide slightly decreased excitability at a normal [K+]o, primarily by depressing the sodium system with some contributory effect of passive properties. At high [K+]o, flecainide caused a frequency-dependent decrease in excitability and conduction, the latter best interpreted as a failure of the fiber to attain the liminal length requirements to produce a local action potential due primarily to an effect on sodium conductance. Together, the observations suggest that the action potential is the local phenomenon and that the propagated event is the sequential fulfillment of liminal length requirements. The data were interpreted in terms of the electrophysiologic matrix first proposed in detail in this Journal, which indicated that the electrophysiologic universe moved as a system in response to the drug and a change in [K+]o, the presumed antiarrhythmic and proarrhythmic electrophysiologic matrices for flecainide were quite similar, and the matrical configuration shared characteristics with the matrices of other drugs with known proarrhythmic potential.

Protective effects of free polyunsaturated fatty acids on arrhythmias induced by lysophosphatidylcholine or palmitoylcarnitine in neonatal rat cardiac myocytes

Cultured, spontaneously beating, neonatal rat cardiac myocytes were used to examine the effects of various free fatty acids added to the medium perfusing the cells on lysophosphatidylcholine (LPC)- or acylcarnitine-induced arrhythmias. Perfusion of the cells with LPC or palmitoylcarnitine (2-10 microM) induced sustained tachyrhythmia with episodes of spasmodic contractures and fibrillation. Free PUFA (10-15 microM) including eicosapentaenoic acid (EPA, 20:5n-3), docosahexaenoic acid (DHA, 22:6n-3), alpha-linolenic acid (18:3n-3), arachidonic acid (AA, 20:4n-6) and linoleic acid (18:2n-6) were able to effectively prevent as well as terminate the LPC or acylcarnitine-induced arrhythmias. In contrast, monounsaturated oleic acid (18:1n-9) and saturated stearic acid (18:0) did not have such effects. The protective effects of the polyunsaturated fatty acids (PUFA) could be reversed by cell perfusion with delipidated bovine serum albumin. To determine the potential primary action by which the PUFA exert the antiarrhythmic effects, measurements of intracellular Ca2+ levels and the response of the cells to electrical pacing in the absence or presence of the PUFA were performed and the effects of verapamil (a L-type Ca2+ channel blocker), tetrodotoxin (a Na+ channel blocker) and Ca2+ ionophore A23187 on the cell contraction and the cytosolic Ca2+ levels were compared with that of the PUFA. Results suggest that an inhibitory effect on the electrical automaticity/excitability of the cardiac myocyte rather than a reduction in cytosolic Ca2+ underlie the protective effects of PUFA. In conclusion, free PUFAs are able to effectively protect the cardiac myocytes against the arrhythmias induced by low concentrations of lysophosphatidylcholine or palmitoylcarnitine.

Arrhythmias caused by platelet activating factor

INTRODUCTION

Both ischemia and reperfusion are associated with ventricular arrhythmias. In both instances, neutrophils migrate into the ischemic zone, are activated by locally released factors, and bind to myocytes. The activated neutrophils liberate platelet activating factor (PAF). We have studied the arrhythmogenic actions of PAF on transmembrane potentials of isolated canine cardiac myocytes.

METHODS AND RESULTS

Cardiac myocytes were prepared from normal canine hearts by standard methods and studied in vitro by recording transmembrane potentials under control conditions and during exposure to graded doses of PAF, usually 0.25 to 1.25 micrograms (0.25 to 1.2 microM). Myocytes were superfused with Tyrode's solution (2.0 mL/min), paced at a cycle length of 1000 msec, and maintained at a temperature between 36 degrees and 38 degrees C. PAF caused a consistent and dose-dependent set of alterations in the transmembrane potential, including increased action potential duration, runs of early afterdepolarizations (EADs), and transient arrest of repolarization (PA). In addition, in some myocytes PAF caused intermittent small depolarizations both at the plateau voltage and resting potential. The effects of PAF were transient: only some residual action potential prolongation was noted after Tyrode's washout for 5 minutes. Effects of PAF were blocked in a dose-dependent manner by the PAF receptor antagonist, CV-6209. Both tetrodotoxin (1.2 x 10(-6) M) and xylocaine (5 x 10(-5) M) antagonized the ability of PAF to cause EADs and PA.

CONCLUSIONS

PAF consistently exerts arrhythmogenic effects on the membrane of ventricular myocytes. Since PAF is liberated by activated neutrophils and since activated neutrophils migrate into ischemic myocardium on reperfusion, we judge that PAF liberated by such neutrophils is an important arrhythmogenic factor for reperfusion arrhythmias. The same mechanism may be a cause of arrhythmias during the evolution of infarction.

Reversibility of electrophysiological abnormalities in subacute ischemia

Twenty-four hours after occlusion of the left anterior descending coronary artery in the dog, ventricular tachycardia is the predominant rhythm. At this time, records of transmembrane potentials from the subendocardial Purkinje fibers adjacent to the infarct show a low maximum diastolic potential, prominent phase 4 depolarization, and slow response action potentials. Exposure of the fibers to pinacidil, 25-100 microM, increases resting potential to the estimated value of EK, abolishes the phase 4 depolarization, and restores action potential amplitude and Vmax toward normal. Perfusion of the bed of the occluded coronary artery with Tyrode's solution prior to isolation of the subendocardial tissues results in similar normalization of transmembrane potentials. These findings indicate: (a) that the major cause of the abnormal transmembrane potentials of the subendocardial tissues is the loss of resting potential; and (b) that abnormalities of the transmembrane potentials are caused by some substance that can be washed out by perfusion and not by a direct effect of ischemia.

Reversibility of electrophysiologic abnormalities of subendocardial Purkinje fibers induced by ischemia

INTRODUCTION

During the subacute phase of infarction in the canine heart, the subendocardial Purkinje fibers subtended by the infarct show depolarization greater than can be accounted for by the decrease in [K+]i, and generate abnormal action potentials and spontaneous rhythms due to abnormal automaticity. We have used pinacidil to hyperpolarize these fibers and evaluate the extent to which an increase in resting potential can normalize action potential generation.

METHODS AND RESULTS

Twenty-four hours after two-stage ligation of the canine left anterior descending coronary artery, preparations of subendocardial Purkinje fibers were studied in vitro by recording transmembrane potentials through standard microelectrodes and exposing the preparation to pinacidil and increases in [K+]o. Pinacidil increased resting potential to the estimated value of EK, abolished the abnormal automaticity, and restored action potentials of normal amplitude with normal values of Vmax. This effect often persisted after washout of pinacidil. Elevation of [K+]o from 4.0 to 20.0 mM slightly increased maximum diastolic potential, suggesting that the excess (over the change in EK) depolarization was caused by a decrease in gK1.

CONCLUSION

The ventricular arrhythmias seen during the subacute stage of infarction probably are caused by abnormal automaticity. Our findings support the conclusion that this abnormal automaticity arises in partially depolarized subendocardial Purkinje fibers. This loss of resting potential is due in large part to a decrease in gK1. Restoration of resting potential to the value of EK permits the Purkinje fibers to develop essentially normal action potentials. An agent capable of reversing the partial block of IK,1 thus might be an effective drug for some types of arrhythmias.

The effect of quinidine and myocardial ischemia on the isolated rat heart with fat-free diet

Fat-free diet changes the lipid content and the electrophysiological properties of the rat myocardium. Five percent fat supplementation to the diet does not alter the basic electrophysiological properties but still has a biochemical effect on the lipid content of the myocardium. The purpose of this work was to determine whether these biochemical alterations affect the response of the myocardium to quinidine and ischemia, both of which interact with the lipid component of the membrane. We used strength-duration, strength-interval and threshold of ventricular fibrillation to measure the electrophysiological properties of the isolated rat heart at baseline and after 30 minutes of quinidine perfusion or coronary artery ligation. The fatty acid composition of the myocardium was analyzed. We found that a fat-free diet caused essential fatty-acid deficiency, while 5% fat supplementation had a partial protective effect. Quinidine decreased excitability and increased refractoriness in both groups but had more effect on the fat-free diet hearts group. There was no difference in the ventricular fibrillation threshold. Ischemia increased myocardial excitability in the fat-free diet hearts group and had no effect on refractoriness or ventricular fibrillation threshold. These results support the theory that the lipid composition of the myocardial membrane affects its response to lipophilic drugs and ischemia.

Differential mechanism of block of palmitoyl lysophosphatidylcholine and of palmitoylcarnitine on inward rectifier K+ channels of guinea-pig ventricular myocytes

We investigated the effect of lysophosphatidylcholine (lysoPtdCho) and palmitoylcarnitine (PamCar), ischemia-induced amphipathic lipid metabolites, on the inward rectifier K+ channel in guinea-pig ventricular cells, under whole-cell and cell-attached configurations with patch-clamp techniques. (a) Both lysoPtdCho (10-50 microM) and PamCar (10-50 microM) depolarized the resting membrane potential (RP), retarded the repolarization of action potential, provoked spontaneous action potential discharges from oscillatory afterpotentials, and eventually caused a sudden rise of the RP to plateau levels. (b) These lysoPtdCho- or PamCar-induced depolarizations of RP were due to a decrease in the inward rectifier K+ current (IK1), and the sudden rise of the RP could be accounted for by a crossover of N-shaped current-voltage relationship on the voltage axis (zero current line) more than once. (c) Single-channel studies in the cell-attached mode revealed that lysoPtdCho (5-100 microM) decreased the conductance of the single IK1 channel with little change in its open probability, whereas PamCar (10-50 microM) did so by decreasing the open probability, with the channel conductance unaltered. (d) A short-chain acylcarnitine, l-propionylcarnitine (PpCar, 100 microM), prevented the depressant effect of lysoPtdCho (50 microM), but not of PamCar (50 microM), on the IK1. (e) Both lysoPtdCho and PamCar produced identical electrophysiological alterations on the membrane potential and IK1 in whole-cell recordings. However, molecular mechanisms involved in the effects of these toxic metabolites on single IK1 channels differ.

Recent insights pertaining to sarcolemmal phospholipid alterations underlying arrhythmogenesis in the ischemic heart

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations that occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on two amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC), which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mole%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. The pathophysiological effects of the accumulation of these amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect upon ion channels. Inhibition of carnitine acyltransferase I (CAT-I) in the ischemic cat heart was found to prevent the increase in long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the electrophysiologic derangements that are observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to these changes. The potential contribution of these amphiphiles to the increases in extracellular potassium and intracellular calcium are examined. Finally, recent data pertaining to the accumulation of long-chain acylcarnitines on cell-to-cell uncoupling are presented. In addition to the events reviewed here, there are many other alterations that occur during early myocardial ischemia, but the results from multiple studies over the past two decades indicate that the accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of CAT-I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.

Inward sodium current at resting potentials in single cardiac myocytes induced by the ischemic metabolite lysophosphatidylcholine

To investigate possible ionic current mechanisms underlying ischemic arrhythmias, we studied single Na+ channel currents in rat and rabbit cardiac myocytes treated with the ischemic metabolite lysophosphatidylcholine (LPC) using the cell-attached and excised inside-out patch-clamp technique at 22 degrees C. LPC has been reported previously to reduce open probability and to induce sustained open channel activity at depolarized potentials. We now report two new observations for Na+ currents in LPC-treated patches: 1) The activation-voltage relation of the peak of the ensemble currents is shifted in the negative (hyperpolarizing) direction by approximately 20 mV compared with control currents. This effect was observed in all patches for depolarizations from a holding potential of -150 mV to different test potentials. 2) In some LPC-treated patches, Na+ channels exhibited sustained bursting activity at potentials as negative as -150 mV, giving a nondecaying inward current. This bursting activity was accompanied by double and triple simultaneous openings and closings, suggesting tight cooperativity in channel gating. These LPC-modified channels were identified as Na+ channels, because their unitary conductance was the same as Na+ channels in control solutions, because the single channel current-voltage relation was extrapolated to reverse at the Na+ Nernst potential, and because the current was blocked by the local anesthetic QX-222. This novel depolarizing current may play a role in the electrophysiological abnormalities in ischemia, including abnormal automaticity and reentrant arrhythmias, and could be a target for antiarrhythmic drugs.

Lysoplasmenylethanolamine accumulation in ischemic/reperfused isolated fatty acid-perfused hearts

Lysophospholipid accumulation has been implicated in the pathogenesis of irreversible injury during myocardial ischemia and reperfusion. Plasmalogens (phospholipids with a vinyl-ether bond in the sn-1 position) account for more than 50% of total myocardial sarcolemmal and sarcoplasmic reticulum phospholipids. Accumulation of plasmalogen choline and ethanolamine lysophospholipids (lysoplasmenylcholine and lysoplasmenylethanolamine) or the effects of exogenous fatty acids on lysoplasmalogen accumulation during ischemia and reperfusion have not been examined. Isolated working rat hearts perfused with buffer containing either 11 mM glucose or 11 mM glucose plus 1.2 mM palmitate were subjected to aerobic, ischemic, or ischemia/reperfusion protocols. Levels of lysoplasmenylcholine and lysoplasmenylethanolamine were quantified using a two-stage high-performance liquid chromatographic technique. In hearts perfused with glucose alone, no significant differences in levels of lysoplasmenylcholine or lysoplasmenylethanolamine were seen during ischemia or reperfusion. In fatty acid-perfused hearts, however, significant accumulation of lysoplasmenylethanolamine occurred during reperfusion but not during ischemia (723 +/- 112, 734 +/- 83, and 1,394 +/- 193 nmol/g dry wt for aerobic, ischemic, and ischemic/reperfused hearts, respectively; p less than 0.05 for ischemic/reperfused hearts versus aerobic or ischemic hearts). Lysoplasmenylcholine levels after ischemia and reperfusion did not differ significantly from aerobic values, regardless of whether fatty acids were present or absent from the perfusate. Aerobic and ischemic/reperfused rabbit hearts, in the presence of fatty acid, showed a similar profile in their lysoplasmalogen content. We conclude that differential lysoplasmenylethanolamine accumulation occurs during myocardial reperfusion when exogenous fatty acid concentrations are high. This may reflect the selective action of fatty acid intermediates on the metabolism of lysoplasmenylethanolamines.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of palmitoylcarnitine and lysophosphatidylcholine on the sodium current of cardiac ventricular cells

We investigated the effects of ischemia-related amphipathic compounds, palmitoylcarnitine (PamCar, 0.5-50 microM) and lysophosphatidylcholine (lysoPtdCho, 5-50 microM) on sodium current (INa) of guinea-pig ventricular myocytes. The cells were perfused with low-Na+ (60 mM) Tyrode's solution, and Ca2+ and K+ currents were blocked by external Co2+ (3 mM) and internal Cs+ (140 mM), respectively. INa was elicited by depolarizing voltage steps from a holding potential of -100 mV at a temperature of 33 degrees C. PamCar (5 microM) decreased the peak INa (attained at -20 mV or -30 mV) from 6.1 +/- 2.1 nA to 3.9 +/- 1.4 nA (n = 11), or by 36.1% within 2 min, and shifted the curve of steady-state INa inactivation by 5.4 mV in the positive direction (from -76.3 +/- 4.6 mV, control to -70.9 +/- 4.0 mV, in PamCar, n = 4). Partial restoration of the amplitude and the shift of the steady-state inactivation curve of INa was attained after washout of PamCar. In contrast, lysoPtdCho at concentrations over 10 microM irreversibly depressed the INa within 0.5-3 min and the reduction of INa was followed by cell contracture or cell death (n = 9). The survival time, defined as a period from the start of lysoPtdCho application to the time of the last successful recording of the INa (before evolution of sudden changes in the holding current), depended on the concentrations of lysoPtdCho. Both PamCar and lysoPtdCho retarded the time course of activation and inactivation of INa.(ABSTRACT TRUNCATED AT 250 WORDS)

The contribution of nonreentrant mechanisms to malignant ventricular arrhythmias

Evidence obtained from experimental animals and man indicates that reentry is a major mechanism underlying arrhythmogenesis. However, focal or nonreentrant mechanisms also appear to be operative under a wide variety of pathophysiologic conditions. For example, results obtained using three-dimensional (3D) mapping from 232 simultaneous sites in the feline heart in vivo revealed that nonreentrant or focal mechanisms were prominent during both ischemia and reperfusion. During early ischemia, nonreentrant mechanisms were responsible for initiation of ventricular tachycardia (VT) in 25% of cases and, in cases where VT was initiated by reentry, it often could be maintained by a nonreentrant mechanism. During reperfusion of ischemic myocardium, nonreentrant mechanisms were responsible for initiation of VT in 75% of cases. Most importantly, the transition from VT to ventricular fibrillation in response to reperfusion was secondary to acceleration of a nonreentrant mechanism in either the subendocardium or subepicardium. Potential cellular mechanisms include: 1) sarcolemmal accumulation of amphiphiles such as long-chain acylcarnitines and lysophosphatidylcholine; 2) alpha- and beta-adrenergic mediated effects of catecholamines on the transient inward current (ITI) secondary to an increase in intracellular Ca2+; and 3) alpha-adrenergic receptor-induced decrease in IK mediated by activation of protein kinase C. Recent findings obtained using 3D intraoperative mapping in patients with refractory VT and a previous myocardial infarction also indicate that both reentrant and nonreentrant or focal mechanisms contribute. For example, in 13 selected patients, mapping was of a sufficient resolution to define the mechanisms of 10 runs of VT. Intraoperative mapping indicated that five runs of VT were initiated by intramural reentry, whereas five runs of VT were initiated by a focal or nonreentrant mechanism. The mechanisms underlying ventricular arrhythmias associated with ischemic cardiomyopathy have recently been delineated in dogs after multiple sequential intracoronary embolizations with microspheres (with a decrease in mean ejection fraction from 64% to 25%). Spontaneous VT initiated by focal mechanisms from the subendocardium in 82% and epicardium in 18%, with no evidence of macroreentry. Thus, in divergent pathophysiologic settings, nonreentrant mechanisms appear to contribute importantly to the genesis of lethal ventricular arrhythmias, suggesting that development of novel therapeutic approaches should be directed at inhibition of not only reentrant circuits, but also nonreentrant mechanisms, including triggered activity.

Effects of lysophosphatidylcholine on electrophysiological properties and excitation-contraction coupling in isolated guinea pig ventricular myocytes

Lysophosphoglyceride accumulation in ischemic myocardium has been implicated as a cause of arrhythmias. We examined the effects of lysophosphatidylcholine (LPC) in isolated guinea pig ventricular myocytes. In paced myocytes loaded with the Ca2+ indicator Indo-1-AM and studied at room temperature, 20 microM LPC caused an initial positive inotropic effect followed by spontaneous automaticity, a decline in active cell shortening, and progressive diastolic shortening (contracture) leading to cell death. These changes were accompanied by a progressive increase in cytosolic [Ca2+]i. In patch-clamped myocytes dialyzed internally with high EGTA concentrations, LPC caused membrane depolarization, shortening of the action potential duration, and abnormal automaticity as seen in multicellular preparations. Voltage clamp experiments revealed the appearance of a nonselective leak conductance without significant changes in the delayed rectifier K+ current, inward rectifier K+ current, L-type Ca2+ current, and T-type Ca2+ current. Pretreatment with 20 mM caffeine and [Ca2+]o-free solution did not prevent the leak current. In patch clamped myocytes loaded with 0.1 mM Fura-2 salt, the [Ca2+]i transient induced by either voltage clamps or brief caffeine exposure remained normal until the nonselective leak current developed. The Na(+)-Ca2+ exchange current elicited during caffeine-induced [Ca2+]i transients also did not appear to be altered by LPC. Qualitatively similar results were obtained in myocytes studied at 35 degrees C. The membrane detergent saponin (0.005% wt/wt) mimicked all of the effects of LPC. We conclude that under these experimental conditions the effects of LPC are most compatible with a detergent action causing membrane leakiness with resultant depolarization, [Ca2+]i overload, and contracture.

Electrophysiologic mechanisms for ventricular arrhythmias in left ventricular dysfunction: electrolytes, catecholamines and drugs

Cardiac arrhythmias are generated as the result of disorders of automaticity or of impulse conduction. Regardless of the mechanism, calcium is likely to be involved, although calcium antagonists are rarely useful antiarrhythmics in ventricular arrhythmias. Myocardial cells that do not ordinarily initiate action potentials may do so when they are partially depolarized, giving rise to an ectopic focus. Early afterdepolarizations (EADs) are also induced in cardiac cells by partial depolarization, whereas delayed afterdepolarizations (DADs) are induced by Ca++ overloading. EADs may be the initiating mechanism of torsade de pointes, a complication of QT prolongation associated with quinidine therapy. Both in the animal model and in humans, treatment with magnesium, isoproterenol, or pacing, all of which suppress EADs, will also suppress torsade de pointes. Ventricular tachycardia is a manifestation of ordered re-entry, and may be exacerbated by antiarrhythmics, especially class 1c drugs. In the individual patient, prediction of proarrhythmia is not possible. The risk of proarrhythmia is increased in patients with episodes of sustained ventricular tachycardia or with significant left ventricular dysfunction.

Lysophosphatidylcholine and sodium-calcium exchange in cardiac sarcolemma: comparison with ischemia

Lysophosphoglyceride accumulation in ischemic myocardium has been hypothesized to be a mechanism for altered sarcolemmal properties that underlie electrophysiological changes and Ca2+ accumulation in ischemia. We find that in vitro application of lysophosphatidylcholine to normal canine sarcolemmal vesicles at a concentration of 0.3 mumol/mg sarcolemmal protein inhibits Na(+)-Ca2+ exchange. Both maximum velocity (Vmax) for Ca2+ transport and Ca2+ affinity are reduced by lysophosphatidylcholine, whereas in ischemia only Vmax is reduced [M. M. Bersohn, K. D. Philipson, and J. Y. Fukushima. Am. J. Physiol. 242 (Cell Physiol. 11): C288-C295, 1982]. This amount of lysophosphatidylcholine does not affect sarcolemmal passive permeability to either Ca2+ or Na+. Treatment of sarcolemma with phospholipase A2 sufficient to inhibit Na(+)-Ca2+ exchange velocity by 50% causes large increases in sarcolemmal lysophosphatidylcholine and lysophosphatidylethanolamine. On the other hand, 1 h of ischemia in rabbit hearts does not affect sarcolemmal phospholipid composition. Thus, although in vitro treatment with lysophosphatidylcholine or phospholipase A2 has profound effects on sarcolemmal properties, sarcolemmal accumulation of lysophosphatidylcholine cannot account for the effects of ischemia as measured in highly purified sarcolemmal vesicles from ischemic hearts.

Uncoupling of cardiac cells by fatty acids: structure-activity relationships

The permeability and conductance of gap junctions between pairs of neonatal rat heart cells were rapidly and reversibly decreased by oleic acid in a dose- and time-dependent manner. Other unsaturated fatty acids (C-18: cis 6, 9, or 11, and C-18, 16, and 14, cis 9), saturated fatty acids (C-10, 12, and 14), and saturated fatty alcohols (C-8, 10, and 12) also caused uncoupling. The most effective compounds of the unsaturated and saturated fatty acid and saturated fatty alcohol series caused essentially complete uncoupling at comparable aqueous concentrations. However, oleic acid uncoupled cells at membrane concentrations as low as 1 mol%, whereas decanoic acid required upwards of 35 mol%. The channels that support the action potential remained functional at these same membrane concentrations. The data are discussed in terms of the possible mechanism by which these compounds cause uncoupling and the possible role of uncoupling by nonesterified free fatty acids in the initiation of arrhythmias during and after ischemic insults.

Effect of palmitoylcarnitine on the passive electrical properties of isolated guinea pig ventricular myocytes

The effects of palmitoylcarnitine (PC) on the passive electrical properties of isolated guinea pig ventricular myocytes were studied using conventional microelectrode techniques. The amphiphile depolarized the cell membrane, and at low concentration (10(-6) M), increased the input resistance (Rin) from 12.4 to 17.9 M omega and the time constant (tau m) from 1.38 to 2.20 ms. A higher concentration of PC (10(-5) M) decreased Rin to 8.5 M omega and tau m to 1.14 ms. No significant change was observed in the membrane capacitance and in the capacitative membrane area. The results suggest that PC incorporates into the cell membrane and in low concentration increases the membrane resistance, while its high concentration increases the membrane permeability probably by causing serious damages in the membrane structure.

Protective effects of D,L-carnitine against arrhythmias induced by lysophosphatidylcholine or reperfusion

Electrophysiological effects of lysophosphatidylcholine (50 or 100 microM) and D,L-carnitine (100 microM) were studied under control conditions and in response to simulated ischaemia and reperfusion using the superfused right ventricular free wall preparation from the guinea pig heart. Lysophosphatidylcholine, 100 microM, induced a significant depolarization of the maximum diastolic potential (MDP) in the epicardium, as well as the development of ventricular premature beats, salvos and ventricular tachycardia. Both coupled beats and abnormal automaticity were observed in lysophosphatidylcholine (100 microM)-treated preparations. Carnitine (100 microM) alone had no effect on preparations superfused with normal Tyrode solution. However, it delayed the time to onset and reduced the cumulative duration of lysophosphatidylcholine-induced arrhythmias (P less than 0.05). The incidence of lysophosphatidylcholine-induced abnormal automaticity and salvos was also significantly decreased in the presence of carnitine. Twenty minutes of simulated ischaemia caused depolarization of MDP as well as prolongation followed by block of transmural conduction. Lysophosphatidylcholine (100 microM) did not alter this response however, carnitine significantly reduced ischaemia-induced depolarization in the epicardium. All control preparations developed arrhythmic activity during 30 min of reperfusion. Carnitine accelerated recovery of MDP in the epicardium upon reperfusion, prolonged the time to onset of arrhythmic activity and reduced both its cumulative duration and incidence. In contrast, reperfusion in the presence of lysophosphatidylcholine (100 microM) significantly increased the incidence of arrhythmic activity. Carnitine exerted only minimal antiarrhythmic action when preparations were exposed to reperfusion in the presence of lysophosphatidylcholine. In conclusion, this study demonstrates that carnitine can modify various cellular mechanisms of arrhythmia induced by lysophosphatidylcholine or by reperfusion but is much less effective when lysophosphatidylcholine and reperfusion are combined.

The association of lysophosphatidylcholine with isolated cardiac myocytes

The ability of exogenous lysophosphatidylcholine to produce electrophysiological derangements and cardiac arrhythmias in the heart has been documented. The action of lysophosphatidylcholine is thought to be mediated via its association with the membrane. The present study examined the nature of the association of lysophosphatidylcholine with isolated rat myocyte membrane. The association was studied by incubating myocytes in a lysophosphatidylcholine-containing medium. The association of lysophosphatidylcholine with the myocyte sarcolemma was not affected by palmitic acid and glycerophosphocholine but was reduced by platelet-activating factor (PAF). The addition of albumin (5 mg/mL) at the end of the incubation period effectively removed the lysophosphatidylcholine from the myocytes. Our results suggest that most of the lysophosphatidylcholine in isolated myocytes was associated preferentially with the outer leaflet of the myocyte sarcolemma. This type of association might be responsible for the lysophosphatidylcholine-induced electrophysiological alterations in the heart.

Mechanisms underlying the development of ventricular fibrillation during early myocardial ischemia

The mechanisms underlying the development of ventricular fibrillation (VF) during early myocardial ischemia were assessed by use of a computerized three-dimensional mapping system capable of recording simultaneously from 232 intramural recording sites throughout the entire feline heart in vivo. Occlusion of the proximal left anterior descending coronary artery led to ventricular tachycardia (VT), which degenerated to VF in 1-5 minutes in four of 15 animals. Normal sinus beats immediately preceding the initiation of VT leading to VF demonstrated delayed activation (total activation time 133 +/- 14 msec), which was not significantly different from the activation time for normal sinus beats immediately preceding nonsustained VT (149 +/- 7 msec). Most of the conduction delay occurred in the subendocardial and midmyocardial regions in both groups. Initiation of VT leading to VF occurred by intramural reentry in three of the four cases. In one case, a mechanism responsible for the initiation of VT could not be assigned. The coupling interval of the initiating beats of VT ultimately leading to VF (210 +/- 15 msec) did not differ from that of nonsustained VT. Maintenance of the VT that led to VF was due primarily to intramural reentry (84% of cases) involving multiple activation sites in and around the border region of the ischemic zone. Nonreentrant mechanisms, arising in the subendocardium and subepicardium, also contributed to the maintenance of VT before development of VT. The transition from VT to VF was due exclusively to intramural reentry with initiation of the reentrant beats in the subendocardium and, occasionally, the subepicardium. Acceleration of the tachycardia by intramural reentry, along with very rapid and inhomogeneous recovery of excitability (as low as 50-60 msec), led to increased functional block and conduction delay. As a result, the total activation time for a given beat exceeded the coupling interval for that beat and led to the multiple reentrant circuits and multiple simultaneous activations characteristic of VF. Thus, the initiation and maintenance of VT leading to VF during early ischemia is due to intramural reentry, although nonreentrant mechanisms also contribute. However, the development of VF is due to continued intramural reentry and rapid recovery of excitability.

A sensitive, radiometric assay for lysophosphatidylcholine

To facilitate investigation of the metabolism of lysophosphatidylcholine and choline lysoplasmalogen in small quantities of tissue, a method for the quantification of these phospholipid species that is capable of accurate and reproducible analysis in samples which contain less than 1 nmol of total choline lysophospholipid was developed. The procedure employs chloroform and methanol extraction of phospholipids from isolated tissue with subsequent separation of the choline lysophospholipid fraction by high-performance liquid chromatography. The choline lysophospholipids are then acetylated with [3H]acetic anhydride and the [3H]acetyl-lysophosphatidylcholine product is isolated by thin-layer chromatography and quantified by liquid scintillation counting. The choline lysophospholipid content in the sample is determined from a standard curve constructed from samples containing a known amount of synthetic lysophosphatidylcholine with correction for recovery based on the inclusion of [14C]lysophosphatidylcholine as an internal standard.

Electrical properties of canine subendocardial Purkinje fibers surviving in 1-day-old experimental myocardial infarction

The passive electrical properties of subendocardial Purkinje fibers surviving in infarcted regions of canine ventricle 24 hours after coronary ligation were studied by using microelectrode techniques and cable theory. In normal hearts, cells within the subendocardial Purkinje fiber strands were found to be well coupled to each other but electrically isolated from neighboring myocardium. Voltage response to intracellular current injection was consistent with one-dimensional cable behavior and yielded estimates of passive electrical properties in general agreement with previous work on free-running Purkinje strands (membrane length constant, 1.2 +/- 0.1 mm; membrane time constant, 7.3 +/- 0.8 msec; input resistance, 67.4 +/- 7.4 K omega; membrane resistance, 8.2 +/- 0.7 K omega.cm; axial resistance, 0.52 +/- 0.06 M omega/cm; membrane capacitance, 960 +/- 102 nF/cm) (n = 21). On the day after coronary ligation, subendocardial Purkinje fiber action potentials were prolonged and slightly depolarized. Significant increases were measured in input resistance (+40.5%), membrane resistance (+43.9%), and axial resistance (+47.5%), whereas membrane capacitance was found to be significantly decreased (-24.3%) (n = 19). Conduction velocity, membrane length constant, membrane time constant, and the time constant and capacitance for the foot of the action potential remained unchanged. These results are consistent with electrical uncoupling between adjacent cells, which will increase internal resistivity, accompanied by changes in cellular phospholipid content, which can increase membrane resistance and alter membrane capacitance. Alternatively, the results can be explained by a simple model in which the apparent electrical structure is altered by changes in electrical coupling alone, with specific electrical properties remaining constant. Although the mechanisms underlying the observed changes remain uncertain, the present study indicates that myocardial infarction is associated with alterations in the passive electrical structure of surviving subendocardial Purkinje fibers, which, together with changes in action potential configuration, may provide a substrate for the generation of ventricular arrhythmias 24 hours after coronary ligation.

Effect of lysophosphatidylcholine on atherosclerotic rabbit arteries

We report here on the effect of an endothelium-dependent vascular smooth muscle relaxant, lysophosphatidylcholine (LPC) on rabbit aortic strips and on hemodynamic changes by LPC in atherosclerotic animals. Cyclic GMP changes induced by LPC in atherosclerotic vessels were also determined. Atherosclerosis was produced by feeding a high cholesterol and saturated fatty acid diet. LPC was injected into the left atrium and coronary flow was measured by radioactive microspheres; in vitro, relaxation of precontracted aortic strips by lysophosphatidylcholine was also recorded. LPC failed to increase coronary flow in the presence of atherosclerosis. In isolated aortic strips, dose-response curves with acetylcholine and LPC showed diminished relaxation in atherosclerotic preparations, and cyclic GMP production following LPC was reduced. The results demonstrate that vascular relaxation by LPC, together with its ability to activate guanylate cyclase is dependent on the functional and morphological integrity of the vascular wall.

Ischemic poison lysophosphatidylcholine modifies heart sodium channels gating inducing long-lasting bursts of openings

The effects of lysophosphatidylcholine (LPC) on Na channels in inside-out patches of adult rat ventricular cells using the patch-clamp technique have been investigated. Application of LPC (9-25 microM) from the inner side of membrane for 4-15 min caused a reduction of averaged Na current (INa) peak and prolonged the time course of inactivation in the potential range of -50 to -10 mV. Analysis of single channel behaviour revealed that after 30-50 min of exposure, in addition to normally functioning Na channels with short openings, LPC induced long-lasting bursts of Na channel openings (up to the 300 ms duration of the test pulses). This resulted in an appearance of noninactivated component of INa. The slope conductance of these modified channels remained the same as in control (11.3 pS - control; 11.6 pS - LPC-treated). The dwell time for modified channels increased significantly.

Cardiac excitability and antiarrhythmic drugs: a different perspective

A matrix of active and passive cellular properties determines net cardiac excitability. The hypothesis of altered excitability suggests that for cardiac arrhythmias to arise, the normal matrix must be perturbed by arrhythmogenic influences to produce a proarrhythmic matrical configuration to permit rhythm disturbances caused by abnormalities of propagation, abnormal automaticity, or altered excitability. Antiarrhythmic drugs may act with one or more components of the normal or proarrhythmic matrix to normalize or to create new antiarrhythmic or, perhaps, proarrhythmic matrices. Traditionally, antiarrhythmic drug classifications have been based on predominant drug actions. These classifications have clinical and some experimental utility but fail to consider the complicated effects that pathophysiologic influences and pharmacologic actions may have on active and passive cellular properties. Cluster analysis may allow the development of new classifications of arrhythmogenesis and antiarrhythmic drugs. The matrical concept has important clinical implications and suggest strategies for treating patients with cardiac rhythm disturbances.

Prophylaxis of early ventricular fibrillation by inhibition of acylcarnitine accumulation

Hypoxia in isolated myocytes results in accumulation of long-chain acylcarnitines (LCA) in sarcolemma. Inhibition of carnitine acyltransferase I (CAT-I) with sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (POCA) prevents both the accumulation of LCA in the sarcolemma and the initial electrophysiologic derangements associated with hypoxia. Another amphiphilic metabolite, lysophosphatidylcholine (LPC), accumulates in the ischemic heart in vivo, in part because of inhibition of its catabolism by accumulating LCA. It induces electrophysiologic alterations in vitro analogous to early changes induced by ischemia in vivo. The present study was performed to determine whether POCA could prevent accumulation of both LCA and LPC induced by ischemia in vivo and if so, whether attenuation of early arrhythmogenesis would result. LAD coronary artery occlusions were induced for 5 min in chloralose-anesthetized cats. Coronary occlusion in untreated control animals elicited prompt, threefold increases of LCA (73 +/- 8 to 286 +/- 60 pmol/mg protein) and twofold increase of LPC (3.3 +/- 0.4 to 7.5 +/- 0.9 nmol/mg protein) selectively in the ischemic zone, associated with ventricular tachycardia (VT) or ventricular fibrillation (VF) occurring within the 5-min interval before acquisition of myocardial samples in 64% of the animals. POCA prevented the increase of both LCA and LPC. It also prevented the early occurrence of VT or VF (within 5 min of occlusion) in all animals studied. The antiarrhythmic effect of POCA was not attributable to favorable hemodynamic changes or to changes in myocardial perfusion measured with radiolabeled microspheres. Thus, inhibition of CAT-I effectively reduced the incidence of lethal arrhythmias induced early after the onset of ischemia. Accordingly, pharmacologic inhibition of this enzyme provides a promising approach for prophylaxis of sudden cardiac death, that typically occurs very soon after the onset of acute ischemia, in man.

Effects of L-propionylcarnitine on electrical and mechanical alterations induced by amphiphilic lipids in isolated guinea pig ventricular muscle

We examined the effects of L-propionylcarnitine (Prop. C), a short-chain acylcarnitine, on amphiphile (L-lysophosphatidylcholine or L-palmitoylcarnitine)-induced electrophysiological and ultrastructural changes in isolated guinea pig ventricular papillary muscles, under acidic conditions (pH 6.9). Conventional microelectrode, tension-recording, and electron microscope techniques were used. Both amphiphiles, at a concentration of 10(-4) M, significantly decreased the resting membrane potential, action potential amplitude, and action potential duration, but increased the developed and resting tension. Such amphiphile-induced electrical changes were not observed in muscles pretreated with the beta-blocker, atenolol, although the mechanical changes remained unaffected. The application of Prop. C (10(-2) M), in the continued presence of the amphiphiles caused a return of the action potential duration and the developed tension to the control level. However, the resting potential and action potential amplitude remained unaffected; in fact, the maximum upstroke velocity (Vmax) of the action potential tended to decrease further. Pretreatment with Prop. C prevented all the amphiphile-induced electrophysiological and mechanical changes, except for Vmax. Electron microscopic studies revealed that amphiphile-induced ultrastructural changes were prevented, at least in part, in the presence of Prop. C. Thus, Prop. C antagonizes some of deleterious effects of amphiphiles, such as lysophosphatidylcholine and palmitoylcarnitine, upon the electrical and mechanical activities of the ventricular muscle, under acidic conditions.

Importance of adenosine triphosphate in phospholipase A2-induced rabbit renal proximal tubule cell injury

The pathogenesis of ischemic renal tubular cell injury involves a complex interaction of different processes, including membrane phospholipid alterations and depletion of high-energy phosphate stores. To assess the role of membrane phospholipid changes due to activation of phospholipases in renal tubule cell injury, suspensions enriched in rabbit renal proximal tubule segments were incubated with exogenous phospholipase A2 (PLA2). Exogenous PLA2 did not produce any significant change in various metabolic parameters reflective of cell injury in control nonhypoxic preparations despite a significant decrease in phosphatidylethanolamine (PE) and moderate increases in lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE). In contrast, exogenous PLA2 treatment of hypoxic tubules resulted in a severe degree of cell injury, as demonstrated by marked declines in tubule K+ and ATP contents and significant decreases in tubule uncoupled respiratory rates, and was associated with significant phospholipid alterations, including marked declines in phosphatidylcholine (PC) and PE and significant rises in LPC, LPE, and free fatty acids (FFA). The injurious metabolic effects of exogenous PLA2 on hypoxic tubules were reversed by addition of ATP-MgCl2 to the tubules. The protective effect of ATP-MgCl2 was associated with increases in tubule PC and PE contents and declines in LPC, LPE, and FFA contents. These experiments thus indicate that an increase in exogenous PLA2 activity produces renal proximal tubule cell injury when cell ATP levels decline, at which point phospholipid resynthesis cannot keep pace with phospholipid degradation with resulting depletion of phospholipids and accumulation of lipid by-products. High-energy phosphate store depletion appears to be an important condition for exogenous PLA2 activity to induce renal tubule cell injury.

Lysophosphatidylcholine-induced arrhythmias and its accumulation in the rat perfused heart

1. The tissue level of lysophosphatidylcholine (LPC) was determined in rat hearts perfused with a solution containing 5 microM LPC. The relationship between LPC accumulation and the severity of arrhythmias produced was examined. 2. The accumulation of LPC was dependent on the perfusion time and this accumulation was associated with the occurrence of severe arrhythmias. A positive correlation between the tissue LPC content and the arrhythmia score was found (P less than 0.01). 3. No consistent alteration in total phospholipid, phosphatidylcholine or cholesterol content was found. This suggests that LPC-induced arrhythmias are not associated with alterations of major lipid components in the heart. 4. When severe arrhythmias occurred in the presence of LPC in the rat perfused heart, less than 2% of total tissue phospholipid was in the form of LPC. 5. The positive correlation between LPC accumulation and the occurrence of arrhythmias suggests a cause and effect relationship of LPC with cardiac arrhythmias in the rat perfused heart. However, in the ischaemic heart, other biochemical factors can contribute, to different degrees, to ischaemia-induced cardiac arrhythmias.

Lysophosphatidylcholine accumulation in the ischemic canine heart

The production of cardiac arrhythmias and the elevation of lysophosphatidylcholine level in the ischemic myocardium have been well-documented in a number of studies. However, the relationship between the production arrhythmias and the elevation of tissue lysophosphatidylcholine level was not reported. In this study, the lysophosphatidylcholine level and the occurrence of cardiac arrhythmias in the ischemic canine heart were monitored. A temporal relationship between the accumulation of lysophosphatidylcholine and the occurrence of arrhythmias was established after five hr of ischemia. A significant elevation of lysophosphatidylcholine was detected at three hr of ischemia without the occurrence of arrhythmias. The results indicate that cardiac arrhythmias did not cause the elevation of lysophosphatidylcholine and if lysophospholipids are causally related to the arrhythmias that a critical level of the lysophospholipid must accumulate in order to elicit electrophysiological alterations.