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

A Distinctive Metabolomics Profile and Potential Biomarkers for Very Long Acylcarnitine Dehydrogenase Deficiency (VLCADD) Diagnosis in Newborns

Very long-chain acylcarnitine dehydrogenase deficiency (VLCADD) is a rare inherited metabolic disorder associated with fatty acid β-oxidation and characterized by genetic mutations in the ACADVL gene and accumulations of acylcarnitines. VLCADD, developed in neonates or later adults, can be diagnosed using newborn bloodspot screening (NBS) or genetic sequencing. These techniques have limitations, such as a high false discovery rate and variants of uncertain significance (VUS). As a result, an extra diagnostic tool is needed to deliver improved performance and health outcomes. As VLCADD is linked with metabolic disturbance, we postulated that newborn patients with VLCADD could display a distinct metabolomics pattern compared to healthy newborns and other disorders. Herein, we applied an untargeted metabolomics approach using liquid chromatography-high resolution mass spectrometry (LC-HRMS) to measure the global metabolites in dried blood spot (DBS) cards collected from VLCADD newborns (n = 15) and healthy controls (n = 15). Two hundred and six significantly dysregulated endogenous metabolites were identified in VLCADD, in contrast to healthy newborns. Fifty-eight and one hundred and eight up- and down-regulated endogenous metabolites were involved in several pathways such as tryptophan biosynthesis, aminoacyl-tRNA biosynthesis, amino sugar and nucleotide sugar metabolism, pyrimidine metabolism and pantothenate, and CoA biosynthesis. Furthermore, biomarker analyses identified 3,4-Dihydroxytetradecanoylcarnitine (AUC = 1), PIP (20:1)/PGF1alpha) (AUC = 0.982), and PIP2 (16:0/22:3) (AUC = 0.978) as potential metabolic biomarkers for VLCADD diagnosis. Our findings showed that compared to healthy newborns, VLCAADD newborns exhibit a distinctive metabolic profile, and identified potential biomarkers that can be used for early diagnosis, which improves the identification of the affected patients earlier. This allows for the timely administration of proper treatments, leading to improved health. However, further studies with large independent cohorts of VLCADD patients with different ages and phenotypes need to be studied to validate our potential diagnostic biomarkers and their specificity and accuracy during early life.

Role of ranolazine in heart failure: From cellular to clinic perspective

Ranolazine was approved by the US Food and Drug Administration as an antianginal drug in 2006, and has been used since in certain groups of patients with stable angina. The therapeutic action of ranolazine was initially attributed to inhibitory effects on fatty acids metabolism. As investigations went on, however, it developed that the main beneficial effects of ranolazine arise from its action on the late sodium current in the heart. Since late sodium currents were discovered to be involved in various heart pathologies such as ischemia, arrhythmias, systolic and diastolic dysfunctions, and all these conditions are associated with heart failure, ranolazine has in some way been tested either directly or indirectly on heart failure in numerous experimental and clinical studies. As the heart continuously remodels following any sort of severe injury, the inhibition by ranolazine of the underlying mechanisms of cardiac remodeling including ion disturbances, oxidative stress, inflammation, apoptosis, fibrosis, metabolic dysregulation, and neurohormonal impairment are discussed, along with unresolved issues. A projection of pathologies targeted by ranolazine from cellular level to clinical is provided in this review.

Long-Chain Acylcarnitines and Cardiac Excitation-Contraction Coupling: Links to Arrhythmias

A growing number of metabolomic studies have associated high circulating levels of the amphiphilic fatty acid metabolites, long-chain acylcarnitines (LCACs), with cardiovascular disease (CVD) risk. These studies show that plasma LCAC levels can be correlated with the stage and severity of CVD and with indices of cardiac hypertrophy and ventricular function. Complementing these recent clinical associations is an extensive body of basic research that stems mostly from the twentieth century. These works, performed in cardiomyocyte and multicellular preparations from animal and cell models, highlight stereotypical derangements in cardiac electrophysiology induced by exogenous LCAC treatment that promote arrhythmic muscle behavior. In many cases, this is coupled with acute inotropic modulation; however, whether LCACs increase or decrease contractility is inconclusive. Linked to the electromechanical alterations induced by LCAC exposure is an array of effects on cardiac excitation-contraction coupling mechanisms that overload the cardiomyocyte cytosol with Na⁺ and Ca²⁺ ions. The aim of this review is to revisit this age-old literature and collate it with recent findings to provide a pathophysiological context for the growing body of metabolomic association studies that link circulating LCACs with CVD.

Vaginal lipidomics of women with vulvovaginal candidiasis and cytolytic vaginosis: A non-targeted LC-MS pilot study

Objective

To characterize the lipid profile in vaginal discharge of women with vulvovaginal candidiasis, cytolytic vaginosis, or no vaginal infection or dysbiosis.

Design

Cross-sectional study.

Setting

Genital Infections Ambulatory, Department of Tocogynecology, University of Campinas, Campinas, São Paulo–Brazil.

Sample

Twenty-four women were included in this study: eight with vulvovaginal candidiasis, eight with cytolytic vaginosis and eight with no vaginal infections or dysbiosis (control group).

Methods

The lipid profile in vaginal discharge of the different study groups was determined by liquid chromatography-mass spectrometry and further analyzed with MetaboAnalyst 3.0 platform.

Main outcome measures

Vaginal lipids concentration and its correlation with vulvovaginal candidiasis and cytolytic vaginosis.

Results

PCA, PLS-DA and hierarchical clustering analyses indicated 38 potential lipid biomarkers for the different groups, correlating with oxidative stress, inflammation, apoptosis and integrity of the vaginal epithelial tissue. Among these, greater concentrations were found for Glycochenodeoxycholic acid-7-sulfate, O-adipoylcarnitine, 1-eicosyl-2-heptadecanoyl-glycero-3-phosphoserine, undecanoic acid, formyl dodecanoate and lipoic acid in the vulvovaginal candidiasis group; N–(tetradecanoyl)-sphinganine, DL-PPMP, 1-oleoyl-cyclic phosphatidic, palmitic acid and 5-aminopentanoic acid in the cytolytic vaginosis group; and 1-nonadecanoyl-glycero-3-phosphate, eicosadienoic acid, 1-stearoyl-cyclic-phosphatidic acid, 1-(9Z,12Z-heptadecadienoyl)-glycero-3-phosphate, formyl 9Z-tetradecenoate and 7Z,10Z-hexadecadienoic acid in the control group.

Conclusions

Lipids related to oxidative stress and apoptosis were found in higher concentrations in women with vulvovaginal candidiasis and cytolytic vaginosis, while lipids related to epithelial tissue integrity were more pronounced in the control group. Furthermore, in women with cytolytic vaginosis, we observed higher concentrations of lipids related to bacterial overgrowth.

Acylcarnitines--old actors auditioning for new roles in metabolic physiology

Perturbations in metabolic pathways can cause substantial increases in plasma and tissue concentrations of long-chain acylcarnitines (LCACs). For example, the levels of LCACs and other acylcarnitines rise in the blood and muscle during exercise, as changes in tissue pools of acyl-coenzyme A reflect accelerated fuel flux that is incompletely coupled to mitochondrial energy demand and capacity of the tricarboxylic acid cycle. This natural ebb and flow of acylcarnitine generation and accumulation contrasts with that of inherited fatty acid oxidation disorders (FAODs), cardiac ischaemia or type 2 diabetes mellitus. These conditions are characterized by very high (FAODs, ischaemia) or modestly increased (type 2 diabetes mellitus) tissue and blood levels of LCACs. Although specific plasma concentrations of LCACs and chain-lengths are widely used as diagnostic markers of FAODs, research into the potential effects of excessive LCAC accumulation or the roles of acylcarnitines as physiological modulators of cell metabolism is lacking. Nevertheless, a growing body of evidence has highlighted possible effects of LCACs on disparate aspects of pathophysiology, such as cardiac ischaemia outcomes, insulin sensitivity and inflammation. This Review, therefore, aims to provide a theoretical framework for the potential consequences of tissue build-up of LCACs among individuals with metabolic disorders.

Myocardial impulse propagation is impaired in right ventricular tissue of Zucker diabetic fatty (ZDF) rats

Background

Diabetes increases the risk of cardiovascular complications including arrhythmias, but the underlying mechanisms remain to be established. Decreased conduction velocity (CV), which is an independent risk factor for re-entry arrhythmias, is present in models with streptozotocin (STZ) induced type 1 diabetes. Whether CV is also disturbed in models of type 2 diabetes is currently unknown.

Methods

We used Zucker Diabetic Fatty (ZDF) rats, as a model of type 2 diabetes, and their lean controls Zucker Diabetic Lean (ZDL) rats to investigate CV and its response to the anti-arrhythmic peptide analogue AAP10. Gap junction remodeling was examined by immunofluorescence and western blotting. Cardiac histomorphometry was examined by Masson`s Trichrome staining and intracellular lipid accumulation was analyzed by Bodipy staining.

Results

CV was significantly slower in ZDF rats (56±1.9 cm/s) compared to non-diabetic controls (ZDL, 66±1.6 cm/s), but AAP10 did not affect CV in either group. The total amount of Connexin43 (C×43) was identical between ZDF and ZDL rats, but the amount of lateralized C×43 was significantly increased in ZDF rats (42±12 %) compared to ZDL rats (30±8%), p<0.04. Judged by electrophoretic mobility, C×43 phosphorylation was unchanged between ZDF and ZDL rats. Also, no differences in cardiomyocyte size or histomorphometry including fibrosis were observed between groups, but the volume of intracellular lipid droplets was 4.2 times higher in ZDF compared to ZDL rats (p<0.01).

Conclusion

CV is reduced in type 2 diabetic ZDF rats. The CV disturbance may be partly explained by increased lateralization of C×43, but other factors are likely also involved. Our data indicates that lipotoxicity potentially may play a role in development of conduction disturbances and arrhythmias in type 2 diabetes.

Long-chain acylcarnitines regulate the hERG channel

Background and purpose

In some pathological conditions carnitine concentration is high while in othersitis low.In bothcases,cardiac arrhythmiascan occur and lead to sudden cardiac death. It has been proposed that in ischaemia, acylcarnitine (acyl-CAR), but not carnitine, is involved in arrhythmiasthrough modulation of ionic currents. We studied the effects of acyl-CARs on hERG, K_IR2.1 and K_v7.1/minKchannels (channels responsible for I_KR, I_K1 and I_KS respectively).

Experimental approach

HEK293 cells stably expressing hERG, K_IR2.1 or Kv7.1/minK were studied using the patch clamp technique. Free carnitine (CAR) and acyl-CAR derivatives from medium- (C8 and C10) and long-chain (C16 and C18∶1) fatty acids were applied intra- and extracellularly at different concentrations. Forstudies onhERG, C16 and C18∶1 free fatty acid were also used.

Key results

Extracellular long-chain (LCAC), but not medium-chain, acyl-CAR,induced an increase of I_hERG amplitude associated with a dose-dependent speeding of deactivation kinetics. They had no effect on K_IR2.1 or Kv7.1/minK currents.Computer simulations of these effects wereconsistent with changes in action potential profile.

Conclusions and applications

Extracellular LCAC tonically regulates I_hERG amplitude and kineticsunder physiological conditions. This modulation maycontribute tothe changes in action potential duration thatprecede cardiac arrhythmias in ischaemia, diabetes and primary systemic carnitine deficiency.

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.

Lysophosphatidylcholine augments Ca(v)3.2 but not Ca(v)3.1 T-type Ca(2+) channel current expressed in HEK-293 cells

Lysophosphatidylcholine (LPC) has been shown to induce electrophysiological disturbances to arrhythmogenesis. However, the effects of LPC on the low-voltage-activated T-type Ca(2+) channels in the heart are not understood yet. We found that LPC increases the T-type Ca(2+) channel current (I(Ca.T)) in neonatal rat cardiomyocytes. To further investigate the underlying modulatory mechanism of LPC on T-type Ca(2+) channels, we utilized HEK-293 cells stably expressing alpha1G and alpha1H subunits (HEK-293/alpha1G and HEK-293/alpha1H), by use of patch-clamp techniques. A low concentration of LPC (10 micromol/l) significantly increased Ca(v)3.2 I(Ca.T) (alpha1H) that were similar to those observed in neonatal rat cardiomyocytes. Activation and steady-state inactivation curves were shifted in the hyperpolarized direction by 5.1 +/- 0.2 and 4.6 +/- 0.4 mV, respectively, by application of 10 micromol/l LPC. The pretreatment of cells with a protein kinase C inhibitor (chelerythrine) attenuated the effects of LPC on I(Ca.T) (alpha1H). However, the application of LPC failed to modify Ca(v)3.1 (alpha1G) I(Ca.T) at concentrations of 10-50 micromol/l. In conclusion, these data demonstrate that extracellularly applied LPC augments Ca(v)3.2 I(Ca.T) (alpha1H) but not Ca(v)3.1 I(Ca.T) (alpha1G) in a heterologous expression system, possibly by modulating protein kinase C signaling.

Physiological mechanisms of lysophosphatidylcholine-induced de-ramification of murine microglia

Activation of microglial cells, the resident macrophages of the brain, occurs rapidly following brain injury. De-ramification, i.e. transformation from ramified into amoeboid morphology is one of the earliest manifestations of microglial activation. In the present study, we identified the physiological mechanisms underlying microglial de-ramification induced by lysophosphatidylcholine (LPC). Patch-clamp experiments revealed activation of non-selective cation currents and Ca(2+)-dependent K(+) currents by extracellular LPC. LPC-activated non-selective cation channels were permeable for monovalent and divalent cations. They were inhibited by Gd(3+), La(3+), Zn(2+) and Grammostola spatulata venom, but were unaffected by diltiazem, LOE908MS, amiloride and DIDS. Ca(2+) influx through non-selective cation channels caused sustained increases in intracellular Ca(2+) concentration. These Ca(2+) increases were sufficient to elicit charybdotoxin-sensitive Ca(2+)-dependent K(+) currents. However, increased [Ca(2+)](i) was not required for LPC-induced morphological changes. In LPC-stimulated microglial cells, non-selective cation currents caused transient membrane depolarization, which was followed by sustained membrane hyperpolarization induced by Ca(2+)-dependent K(+) currents. Furthermore, LPC elicited K(+) efflux by stimulating electroneutral K(+)-Cl(-) cotransporters, which were inhibited by furosemide and DIOA. LPC-induced microglial de-ramification was prevented by simultaneous inhibition of non-selective cation channels and K(+)-Cl(-) cotransporters, suggesting their functional importance for microglial activation.

Alterations in Ca2+ cycling by lysoplasmenylcholine in adult rabbit ventricular myocytes

We previously reported that lysoplasmenylcholine (LPlasC) altered the action potential (AP) and induced afterdepolarizations in rabbit ventricular myocytes. In this study, we investigated how LPlasC alters excitation-contraction coupling using edge-motion detection, fura-PE3 fluorescent indicator, and perforated and whole cell patch-clamp techniques. LPlasC increased contraction, myofilament Ca(2+) sensitivity, systolic and diastolic free Ca(2+) levels, and the magnitude of Ca(2+) transients concomitant with increases in the maximum rates of shortening and relaxation of contraction and the rising and declining phases of Ca(2+) transients. In some cells, LPlasC induced arrhythmias in a pattern consistent with early and delayed aftercontractions. LPlasC also augmented the caffeine-induced Ca(2+) transient with a reduction in the decay rate. Furthermore, LPlasC enhanced L-type Ca(2+) channel current (I(Ca,L)) and outward currents. LPlasC-induced alterations in contraction and I(Ca,L) were paralleled by its effect on the AP. Thus these results suggest that LPlasC elicits distinct, potent positive inotropic, lusitropic, and arrhythmogenic effects, resulting from increases in Ca(2+) influx, Ca(2+) sensitivity, sarcoplasmic reticular (SR) Ca(2+) release and uptake, SR Ca(2+) content, and probably reduction in sarcolemmal Na(+)/Ca(2+) exchange.

Phospholipid metabolite 1-palmitoyl-lysophosphatidylcholine enhances human ether-a-go-go-related gene (HERG) K(+) channel function

BACKGROUND

Lysophosphatidylcholine (LPC), a naturally occurring phospholipid metabolite, accumulates in the ischemic heart and causes extracellular K(+) accumulation and action potential shortening. LPC has been incriminated as a biochemical trigger of lethal cardiac arrhythmias, but the underlying mechanisms remain poorly understood.

METHODS AND RESULTS

We studied the effect of 1-palmitoyl-LPC (Pal-LPC) on currents resulting from human ether-a-go-go-related gene (HERG) expression in human embryonic kidney (HEK) cells using whole-cell patch-clamp techniques. Bath application of Pal-LPC consistently and reversibly increased HERG current (I(HERG)). The effects of Pal-LPC were apparent as early as 3 minutes after application of the drug, reached maximum within 10 minutes, and were reversible on washout. Pal-LPC increased I(HERG) at voltages between -20 and +30 mV, with greater effects at stronger depolarization. However, Pal-LPC did not affect the voltage-dependence of I(HERG) activation. In contrast, Pal-LPC significantly shifted the inactivation curve toward more positive potentials, causing a mean 20.0+/-2.2 mV shift in half-inactivation voltage relative to control.

CONCLUSIONS

Our results indicate that apart from being a well-recognized target for drug inhibition, I(HERG) can also be enhanced by natural substances. An increase in I(HERG) by Pal-LPC may contribute to K(+) loss, abnormal electrophysiology, and arrhythmia occurrence in the ischemic heart.

Long-chain acylcarnitine induces Ca2+ efflux from the sarcoplasmic reticulum

Long-chain acylcarnitines increase intracellular Ca2+ (Ca2+i) and induce electrophysiologic alterations that likely contribute to the genesis of malignant ventricular arrhythmias induced during myocardial ischemia. The mechanisms by which long-chain acylcarnitines increase Ca2+i are not known, although it occurs in the presence of Ca2+ channel blockade and inhibition of Na+/Ca2+ exchange. Long-chain acylcarnitines activate Ca2+ release channels from skeletal muscle sarcoplasmic reticulum (SR), but their effect on cardiac SR is unclear. To test the hypothesis that long-chain acylcarnitines increase Ca2+i from the SR, SR-enriched membrane fractions were prepared from rabbit left ventricular myocardium using sucrose density-gradient centrifugation and characterized by marker enzyme analysis. 45Ca2+ efflux was assessed in the presence or absence of long-chain acylcarnitines. Palmitoylcarnitine and stearoylcarnitine produced concentration-dependent efflux of 45Ca2+, whereas shorter chain acylcarnitines, palmitate, and palmitoyl-coenzyme A did not. Pretreatment of cardiac SR vesicles with ryanodine did not prevent palmitoylcarnitine-induced Ca2+ release. In addition, palmitoylcarnitine did not influence specific [3H]ryanodine binding, suggesting a mechanism independent of alterations in ryanodine receptor/Ca2+ release channel binding. In summary, long-chain acylcarnitines enhance Ca2+ release from cardiac SR vesicles and may thereby mobilize Ca2+i to induce electrophysiologic derangements under conditions, such as ischemia, in which these amphiphiles accumulate.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Effects of Palmitoyl Carnitine on Perfused Heart and Papillary Muscle

BACKGROUND: Palmitoyl carnitine accumulation during ischemia causes profound electrophysiological changes, resulting in arrhythmias. We studied the electrophysiological and contractile effects of palmitoyl carnitine. METHODS AND RESULTS: Extracellular recordings made by using the endocardial unipolar paced evoked response (PER) in isolated perfused rabbit hearts were compared with action potentials (AP) recorded from septal artery perfused rabbit papillary muscle. Left ventricular pressure was monitored in isolated hearts. In perfused hearts palmitoyl carnitine (30 µmol/L, 30 minutes) significantly (P <.001) increased the latency of activation (St-R interval) by 58% +/- 8% and reduced repolarization time (R-E interval) by 39% +/- 4%. PER duration (St-E interval), was reduced by 30% +/- 3%. Palmitoyl carnitine (30 µmol/L) significantly (P <.001) decreased resting membrane potential (19 +/- 2 mV) of AP, reduced peak amplitude (33.5 +/- 8 mV) and rate of rise of phase 0 (41 +/- 8 V/s). Significant reductions (P <.001) in the action potential duration 50% (129.4 +/- 28 ms) and 90% (139.8 +/- 32 ms) were also observed. An initial positive inotropic effect, which declined as irreversible contracture developed, was also observed. Verapamil (1 µmol/L), nifedipine (1 µmol/L), and caffeine (10 mmol/L) failed to abolish the positive inotropy. CONCLUSIONS: We suggest that palmitoyl carnitine disrupts intracellular calcium homeostasis leading to disturbances in electrical and contractile activity. Its accumulation during myocardial ischemia could contribute to calcium overloading and initiate lethal arrhythmias.

K+ current inhibition by amphiphilic fatty acid metabolites in rat ventricular myocytes

Fatty acid metabolites accumulate in the heart under pathophysiological conditions that affect beta-oxidation and can elicit marked electrophysiological changes that are arrhythmogenic. The purpose of the present study was to determine the impact of amphiphilic fatty acid metabolites on K+ currents that control cardiac refractoriness and excitability. Transient outward (Ito) and inward rectifier (IK1) K+ currents were recorded by the whole cell voltage-clamp technique in rat ventricular myocytes, and the effects of two major fatty acid metabolites were examined: palmitoylcarnitine and palmitoyl-coenzyme A (palmitoyl-CoA). Palmitoylcarnitine (0.5-10 microM) caused a concentration-dependent decrease in Ito density in myocytes internally dialyzed with the amphiphile; 10 microM reduced mean Ito density at +60 mV by 62% compared with control (P < 0.05). In contrast, external palmitoylcarnitine at the same concentrations had no effect, nor did internal dialysis significantly alter IK1. Dialysis with palmitoyl-CoA (1-10 microM) produced a smaller decrease in Ito density compared with that produced by palmitoylcarnitine; 10 microM reduced mean Ito density at +60 mV by 37% compared with control (P < 0.05). Both metabolites delayed recovery of Ito from inactivation but did not affect voltage-dependent properties. Moreover, the effects of palmitoylcarnitine were relatively specific, as neither palmitate (10 microM) nor carnitine (10 microM) alone significantly influenced Ito when added to the pipette solution. These data therefore suggest that amphiphilic fatty acid metabolites downregulate Ito channels by a mechanism confined to the cytoplasmic side of the membrane. This decrease in cardiac K+ channel activity may delay repolarization under pathophysiological conditions in which amphiphile accumulation is postulated to occur, such as diabetes mellitus or myocardial infarction.

Plasmalogen-derived lysolipid induces a depolarizing cation current in rabbit ventricular myocytes

Plasmalogen rather than diacyl phospholipids are the preferred substrate for the cardiac phospholipase A2 (PLA2) isoform activated during ischemia. The diacyl metabolite, lysophosphatidylcholine, is arrhythmogenic, but the effects of the plasmalogen metabolite, lysoplasmenylcholine (LPLC), are essentially unknown. We found that 2.5 and 5 micromol/L LPLC induced spontaneous contractions of intact isolated rabbit ventricular myocytes (median times, 27.4 and 16.4 minutes, respectively) significantly faster than lysophosphatidylcholine (>60 and 37.8 minutes, respectively). Whole-cell recordings revealed that LPLC depolarized the resting membrane potential from -83.5+/-0.2 to -21.5+/-1.0 mV. Depolarization was due to a guanidinium toxin-insensitive Na+ influx. The LPLC-induced current reversed at -18.5+/-0.9 mV and was shifted 26.7+/-4.2 mV negative by a 10-fold reduction of bath Na+ (Na+/K+ permeability ratio, approximately 0.12+/-0.06). In contrast, block of Ca2+ channels with Cd2+ and reducing bath Cl failed to affect the current. The actions of LPLC were opposed by lanthanides. Gd3+ and La3+ were equally effective inhibitors of the LPLC-induced current and equally delayed the onset of spontaneous contractions. However, the characteristics of lanthanide block imply that Gd3+-sensitive, poorly selective, stretch-activated channels were not involved. Instead, the data are consistent with the view that lanthanides increase phospholipid ordering and may thereby oppose membrane perturbations caused by LPLC. Plasmalogens constitute a significant fraction of cardiac sarcolemmal choline phospholipids. In light of their subclass-specific catabolism by phospholipase A2 and the present results, it is suggested that LPLC accumulation may contribute to ventricular dysrhythmias during ischemia.

Inhibition and kinetic properties of membrane-bound carbonic anhydrases in rabbit skeletal muscles

It was the aim of this study to investigate whether the carbonic anhydrases associated with the sarcoplasmic reticulum (SR) and sarcolemmal membranes differ in their kinetic and inhibitory properties. To this end, sarcolemmal and SR membrane vesicle fractions were prepared from rabbit white and red skeletal muscles, the white muscle sarcolemmal fraction (WSL), the red muscle sarcolemmal fraction (RSL), the white muscle SR fraction (WSR), and the red muscle SR fraction (RSR). WSL displayed a specific carbonic anhydrase activity of 22.1 U . ml/mg and RSL of 7.5 U . ml/mg, whereas the SR fractions showed a much lower activity of 0.5 U . ml/mg for WSR and of 2.4 U . ml/mg for RSR. In both SR fractions phase separation experiments with Triton X-114 demonstrated that the carbonic anhydrase activity is due to a membrane-bound enzyme and not due to a cytosolic isozyme. The kinetic properties of carbonic anhydrase from the four distinct membane fractions were evaluated by determination of the Michaelis constant, Km, and of the catalytic centre activity kcat. Km appears to be somewhat lower for SR than for SL. Inhibition constants of SR and SL carbonic anhydrases were determined applying six carbonic anhydrase inhibitors: chlorzolamide, ethoxzolamide, methazolamide, benzolamide, and acetazolamide, and also cyanate. The inhibition constants of the SR fractions were significantly different from those of the corresponding sarcolemmal fractions, indicating that the carbonic anhydrase measured in the SR fractions does not originate from contaminating sarcolemmal membrane vesicles, but appears to represent a distinct carbonic anhydrase associated with the SR membrane.

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.

Lysophosphatidylcholine and Cellular Potassium Loss in Isolated Rabbit Ventricle

BACKGROUND: Lysophospholipids such as lysophosphatidylcholine (LPC) have many direct electrophysiological effects on cardiac muscle and have been implicated as a cause of lethal ventricular arrhythmias during acute myocardial ischemia. Because extracellular K(+) accumulation is also a key arrhythmogenic factor during acute ischemia, we examined the effects of LPC on cellular K(+) balance, including its interaction with adenosine triphosphate-sensitive K(+) (K(ATP)) channels. METHODS AND RESULTS: Isolated rabbit interventricular septa paced at 75 beats/min were loaded with (42)K(+) to measure unidirectional K(+) efflux rate (in (42)K(+) washout experiments) or tissue K(+) content ((42)K(+) uptake experiments) and action potential duration (APD) during exposure to 20 µM LPC for 30 minutes. LPC caused tissue K(+) content to decrease by 15 +/- 2% (n = 4) at a steady rate over 30 minutes, associated with gradual APD shortening and a delayed increase in unidirectional K(+) efflux rate. Pretreatment with 12 µM cromakalim to selectively activate K(ATP) channels shortened APD by 44 +/- 66% and had no effect on net tissue K(+) content during control aerobic perfusion. However, cromakalim increased net K(+) loss during exposure to LPC to 22 +/- 4%, a 47% increase. CONCLUSIONS: LPC induced net K(+) loss in heart, which was potentiated by the K(ATP) channel agonist cromakalim. This ATP finding suggests that if LPC accumulates to similar levels during myocardial ischemia and hypoxia, it may be an important mechanism in net K(+) loss.

Lysophosphatidylcholine modulates cardiac I(Na) via multiple protein kinase pathways

Lysophosphatidylcholine (LPC) is a naturally occurring intracellular phospholipid metabolite that has been implicated in arrhythmogenesis during ischemia. LPC has been shown to affect the cardiac Na+ current (I(Na)), but the mechanism of modulation remains undescribed. Recently, low concentrations of LPC have been shown to activate protein kinase C (PKC) independent of the receptor-delineated pathway. The purposes of this study were to describe the effects of intracellularly introduced LPC on I(Na) and to determine if these effects were mediated by kinases. Modulation of I(Na) was studied in ventricular cells with LPC (1 nmol/L to 1 micromol/L) internally applied using whole-cell patch-clamp techniques. Intracellular LPC caused a dose-dependent depolarizing shift of steady state inactivation that was accompanied by a change in slope factor. The development of resting inactivation from closed states was delayed 40%, whereas the recovery from inactivation was significantly accelerated. These results were mimicked by another bioactive lipid, lysophosphatidylethanolamine, or by a peptide analogue of PKC, which is a potent stimulator of endogenous PKC activity. Maximal recruitable current was significantly increased by LPC but not by PKC activation. Some of the effects of LPC on I(Na) could be partially inhibited by the specific PKC inhibitor chelerythrine chloride or by downregulation of PKC with phorbol ester pretreatment. However, genistein, a specific tyrosine kinase inhibitor, completely inhibited all the modulation of I(Na) caused by LPC. These data suggest that LPC modulates I(Na) in cardiac myocytes by a pathway that involves both PKC-dependent and tyrosine kinase- dependent phosphorylation.

Differential effects of Ca2+ channel blockers on Ca2+ overload induced by lysophosphatidylcholine in cardiomyocytes

The effects of Ca2+ channel blockers (verapamil, diltiazem, nicardipine, bepridil and flunarizine) on Ca2+ overload induced by lysophosphatidylcholine were examined in rat isolated cardiomyocytes. Addition of lysophosphatidylcholine (15 microM) produced Ca2+ overload as evidenced by a marked increase in the concentration of intracellular Ca2+ and hypercontracture of cells. Verapamil, flunarizine and bepridil concentration dependently inhibited the lysophosphatidylcholine-induced Ca2+ overload, whereas diltiazem and nicardipine did not. Lysophosphatidylcholine increased the release of creatine kinase, which was significantly attenuated by verapamil, flunarizine or bepridil (5 microM for each), but not by diltiazem or nicardipine (20 microM for each). Verapamil, flunarizine, bepridil (which attenuated the lysophosphatidylcholine-induced Ca2+ overload) and nicardipine (which did not) inhibited the veratridine-induced increase in the concentration of intracellular Na+ (indicated by the increase in fluorescence ratio of Na(+)-binding benzofuran isophthalate) and cell contracture, whereas diltiazem did not. These results suggest that verapamil, bepridil and flunarizine attenuate the Ca2+ overload induced by lysophosphatidylcholine, and that the Ca2+ channel blocking action of these drugs does not contribute substantially to this effect. The Na+ channel inhibition together with high lipophilicity of these drugs may be important for the attenuation of the lysophosphatidylcholine-induced Ca2+ overload.

Mechanisms for depolarization by l-palmitoylcarnitine in single guinea pig ventricular myocytes

INTRODUCTION

The changes of the resting potential (RP) and of the current-voltage (I-V) relationship induced by l-palmitoylcarnitine (l-PC) in the presence of the IKI blocker, cesium, or in the presence of the INa/K blocker, ouabain, were tested in guinea pig ventricular myocytes to ascertain the relative contributions of IKI and INa/K suppression to the membrane depolarization caused by this amphiphile.

METHODS AND RESULTS

Ramp voltages were applied to myocytes with the whole cell, patch clamp technique. l-PC (10 microM) produced additional membrane depolarization in the presence of either 10 mM Cs+ or 30 microM ouabain. In the presence of Cs+, l-PC, like 3 microM ouabain, shifted current inward at potentials negative to -20 mV as a result of INa/K blockade. In the presence of 30 microM ouabain, l-PC, like Cs+, shifted current inward at potentials between -27 and -88 mV and outward at potentials negative to -88 mV. This is attributed to IKI block because the current was inwardly rectifying, with a reversal potential near EK. When l-PC or ouabain inhibited INa/K, the presence of an Ni(2+)-sensitive component attributed to INa/Ca distorted the membrane I-V relationship, particularly in the presence of Cs+. The relative contributions of IKI and INa/K block by l-PC were voltage dependent. At the RP, l-PC produced a greater block of INa/K than of IKI.

CONCLUSION

l-PC depolarizes the resting membrane by inhibiting both IKI and INa/K. It is concluded that suppression of INa/K by l-PC predominates over block of IKI to depolarize the membrane at the RP.

Exogenous lysophosphatidylcholine increases non-selective cation current in guinea-pig ventricular myocytes

Whole cell, patch-clamp studies were performed to examine the effect of lysophosphatidylcholine (LPC) on the membrane current in guinea-pig ventricular myocytes. The addition of 10 microM LPC to the external solution induced a membrane current which had a reversal potential of 0 mV. When Na+, the main cation in the external solution, was replaced by either K+, N-methyl-D-glucamine (NMG) or 90 mM Ca2+, LPC induced a current with the reversal potential near 0 mV, indicating that the current passed through a Ca2+-permeable non-selective cation channel. The order of the cationic permeability calculated from the reversal potential of the current was Cs+ > K+ > NMG > Na+ > Ca2+. Cl- did not pass through the LPC-induced channel. The LPC-induced current was not blocked by Gd3+ in the external solution, nor by the absence of Ca2+ in the pipette solution. In conclusion, LPC induces a Ca2+-permeable non-selective cation channel in guinea-pig ventricular myocytes.

Diminished Ca2+ and Ba2+ currents in myocytes surviving in the epicardial border zone of the 5-day infarcted canine heart

Ventricular arrhythmias frequently occur in patients suffering from ischemic heart disease. In a canine model developed to understand the pathoelectrophysiological mechanisms of the ischemia-related arrhythmias, electrical stimulation can initiate and terminate reentrant ventricular tachyarrhythmias, which arise in surviving subepicardial muscle fibers (epicardial border zone [EBZ] fibers) of the left ventricle 5 days after coronary artery occlusion. Both the structural and electrical changes occurring in the EBZ provide the important substrate for generation of reentrant ventricular tachyarrhythmias. In this study, we tested the hypothesis that abnormalities exist in the electrophysiological properties of macroscopic Ca2+ currents in myocytes isolated from the EBZ of the 5-day infarcted canine heart (IZs). We recorded the T-type (ICa,T) and L-type (ICa,L) Ca2+ currents by using the whole-cell voltage-clamp technique with either Ca2+ or Ba2+ (5 mmol/L) as the charge carrier and under experimental conditions (Na(+)- and K(+)-free solutions, 10 mmol/L intracellular EGTA) that eliminated contamination by other currents. When Ca2+ served as the charge carrier, the density of peak ICa,T in IZs (0.89 +/- 0.5 pA/pF, n = 28) was similar to that in myocytes from normal noninfarcted hearts (NZs) (1.1 +/- 0.5 pA/pF, n = 32). Although no changes existed in the properties of ICa,T, dramatic changes occurred in the density and function of ICa.L in IZs compared with NZs. Density of peak ICa,L at a holding potential of -40 mV (8-second clamp-step interval) was significantly reduced in IZs (4.6 +/- 1.5 pA/pF, n = 40) compared with NZs (7.2 +/- 1.6 pA/pF, n = 53). The reduction in peak ICa,L density was not attributable to altered steady state inactivation relations or a delay in recovery of ICa,L from inactivation. The time course of decay of peak ICa,: was described by a biexponential function in both cell types, with the fast and slow time constants (tau 1 and tau 2, respectively) of decay being significantly faster in IZs (tau 1 12.3 +/- 3.6 ms; tau 2, 55.1 +/- 31.1 ms) than in NZs (tau 1, 16.1 +/- 4.1 ms; tau 2, 85.2 +/- 51.7 ms). In addition, rapid clamp stimulation (at 1-s intervals) of cells produced a larger frequency-dependent decrease of peak ICa,L density in IZs than NZs, suggesting that at more physiologically relevant rates, little ICA.L may be activated. Finally, a significant reduction and acceleration of decay of the ICa,L persisted even when Ca2+ was substituted by equimolar Ba2+ as the charge carrier.(ABSTRACT TRUNCATED AT 400 WORDS)

Acylcarnitines in intermediary metabolism

From the time of its discovery in 1905 until the first description of its deficiency in 1973, the role of carnitine in intermediary metabolism was decidedly vague. Identification of carnitine acyl transferases and their products, acylcarnitines, have paved the way to the confirmation of the importance of carnitine in the transfer of fatty acid CoAs into the mitochondrion for beta-oxidation and energy production. The elucidation of defects in fatty acid oxidation together with the concept of carnitine therapy in certain organoacidaemias have given a new meaning to the term acylcarnitine. Not only are these compounds of diagnostic importance, their formation may be part of a secondary carnitine depletion which may be brought about as a result of various medications. Recent evidence suggests that long-chain acylcarnitines are responsible for cardiac arrhythmias and other effects, both good and bad, will certainly be found. This review will attempt to highlight the importance of acylcarnitines, from their production, the difficulties in analysis, the diagnostic possibilities and their positive and negative effects on intermediary metabolism.

Lysophospholipids modulate channel function by altering the mechanical properties of lipid bilayers

Lipid metabolites, free fatty acids and lysophospholipids, modify the function of membrane proteins including ion channels. Such alterations can occur through signal transduction pathways, but may also result from "direct" effects of the metabolite on the protein. To investigate possible mechanisms for such direct effects, we examined the alterations of gramicidin channel function by lysophospholipids (LPLs): lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), and lysophosphatidylinositol (LPI). The experiments were done on planar bilayers formed by diphytanoylphosphatidylcholine in n-decane a system where receptor-mediated effects can be excluded. At aqueous concentrations below the critical micelle concentration (CMC), LPLs can increase the dimerization constant for membrane-bound gramicidin up to 500-fold (at 2 microM). The relative potency increases as a function of the size of the polar head group, but does not seem to vary as a function of head group charge. The increased dimerization constant results primarily from an increase in the rate constant for channel formation, which can increase more than 100-fold (in the presence of LPC and LPI), whereas the channel dissociation rate constant decreases only about fivefold. The LPL effect cannot be ascribed to an increased membrane fluidity, which would give rise to an increased channel dissociation rate constant. The ability of LPC to decrease the channel dissociation rate constant varies as a function of channel length (which is always less than the membrane's equilibrium thickness): as the channel length is decreased, the potency of LPC is increased. LPC has no effect on membrane thickness or the surface tension of monolayers at the air/electrolyte interface. The bilayer-forming glycerolmonooleate does not decrease the channel dissociation rate constant. These results show that LPLs alter gramicidin channel function by altering the membrane deformation energy, and that the changes in deformation energy can be related to the molecular "shape" of the membrane-modifying compounds. Similar alterations in the mechanical properties of biological membranes may form a general mechanism by which one can alter membrane protein function.

Cellular uncoupling induced by accumulation of long-chain acylcarnitine during ischemia

Long-chain acylcarnitines (LCACs) increase rapidly within minutes after the onset of ischemia in vivo or hypoxia in vitro and produce a time-dependent reversible reduction in gap junctional conductance in isolated myocyte pairs. The present study was performed to assess whether LCACs contribute to cellular uncoupling in response to ischemia in isolated blood-perfused rabbit papillary muscles by use of simultaneous measurements of transmembrane action potentials, extracellular electrograms, extracellular K+, and tissue LCACs and ATP. LCACs increased threefold in response to 20 minutes of no-flow ischemia from 127 +/- 5 to 397 +/- 113 pmol/mg protein (P < .01), concomitant with the onset of cellular uncoupling, extracellular K+ accumulation, and a marked reduction in conduction velocity and action potential duration. To assess whether inhibition of the accumulation of LCACs modified the electrophysiological alterations during ischemia, muscles were pretreated with either sodium 2-(5-(4-chlorophenyl)-pentyl)-oxirane-2-carboxylate (POCA, 10 mumol/L) or oxfenicine (100 mumol/L), inhibitors of carnitine acyltransferase I. Both POCA and oxfenicine completely prevented the increase in LCACs even with 40 minutes of ischemia (138 +/- 37 and 56 +/- 4 pmol/mg protein, respectively), associated with a marked delay in the onset and progression of cellular uncoupling and ischemic contracture. Although POCA and oxfenicine did not affect either the initial early rise in extracellular K+ or the initial fall in conduction velocity, both agents markedly delayed the secondary rise in extracellular K+ as well as the secondary fall in conduction velocity, independent of the level of tissue ATP. Thus, LCACs accumulate during myocardial ischemia and contribute substantially to the initiation of cell-to-cell uncoupling. Inhibition of carnitine acyltransferase I and prevention of the increase in LCACs markedly delays cellular uncoupling and development of ischemic contracture in response to ischemia.

Elevated levels of glucose and L-fucose reduce 22Na+ uptake and whole cell Na+ current in cultured neuroblastoma cells

Na+ flux was studied in cultured neuroblastoma cells grown in medium containing increased glucose or L-fucose concentrations. Chronic exposure of neuroblastoma cells to 30 mM glucose or 30 mM L-fucose caused a decrease in ouabain-sensitive and veratridine-stimulated 22Na+ uptake compared with cells cultured in unsupplemented medium. The Na+ current, determined by using whole-cell configuration of the patch clamp, was also decreased in these cells. Tetrodotoxin (3 microM), which blocked whole cell Na+ currents, also blocked veratridine-stimulated 22Na+ accumulation. Culturing cells in medium containing 30 mM fructose as an osmotic control had no effect on Na+ flux. Specific [3H]saxitoxin binding was not affected by 30 mM glucose or 30 mM L-fucose compared with cells grown in unsupplemented medium, suggesting that the number of Na+ channels was not decreased. These studies suggest that exposing cultured neuronal cells to conditions that occur in the diabetic milieu alters Na+ transport and Na(+)-channel activity.

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.

Inhibition of gap junctional conductance by long-chain acylcarnitines and their preferential accumulation in junctional sarcolemma during hypoxia

Electrophysiological and biochemical sequelae of myocardial ischemia occur within minutes of the onset of myocardial ischemia in vivo. Both conduction delay and conduction block occur rapidly within the same time interval as the accumulation of long-chain acylcarnitines. In the present study, double whole-cell voltage-clamp procedures were used to assess the influence of long-chain acylcarnitines on gap junctional conductance in isolated pairs of canine ventricular myocytes. Long-chain acylcarnitine (5 microM) decreased gap junctional conductance from 153 to 48 nS in a time-dependent and reversible manner. Although the amplitude of junctional current was reduced by 68%, the current continued to demonstrate a linear current-voltage relation. The extent of endogenous accumulation of long-chain acylcarnitines in junctional regions of the sarcolemma was assessed in isolated myocytes in which endogenous free, short-chain, and long-chain acylcarnitine pools had been equilibrated with [3H]carnitine. Under normoxic conditions, long-chain acylcarnitines were not detectable in junctional sarcolemma of myocytes as assessed using electron microscopic autoradiography. Exposure of myocytes to hypoxia (PO2, < 15 mm Hg) for 10 minutes resulted in the preferential accumulation of endogenous long-chain acylcarnitines in junctional sarcolemma (173 +/- 5 x 10(5) molecules/microns 3), a concentration that was sevenfold greater than that found in nonjunctional sarcolemma. Therefore, endogenous long-chain acylcarnitines accumulate preferentially in junctional regions of the sarcolemma during short intervals of hypoxia. Exogenously supplied long-chain acylcarnitines can markedly decrease cellular coupling in a reversible manner, suggesting that this amphiphile may contribute to the marked slowing in conduction velocity in the ischemic heart in vivo, not only by suppressing the rapid Na+ inward current directly, as has been shown previously, but also by decreasing cellular coupling.

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.