Direct activation of Ca2+ channels by palmitoyl carnitine, a putative endogenous ligand

Dissecting Cellular Mechanisms of Long-Chain Acylcarnitines-Driven Cardiotoxicity: Disturbance of Calcium Homeostasis, Activation of Ca2+-Dependent Phospholipases, and Mitochondrial Energetics Collapse

Long-chain acylcarnitines (LCAC) are implicated in ischemia-reperfusion (I/R)-induced myocardial injury and mitochondrial dysfunction. Yet, molecular mechanisms underlying involvement of LCAC in cardiac injury are not sufficiently studied. It is known that in cardiomyocytes, palmitoylcarnitine (PC) can induce cytosolic Ca²⁺ accumulation, implicating L-type calcium channels, Na⁺/Ca²⁺ exchanger, and Ca²⁺-release from sarcoplasmic reticulum (SR). Alternatively, PC can evoke dissipation of mitochondrial potential (ΔΨ_m) and mitochondrial permeability transition pore (mPTP). Here, to dissect the complex nature of PC action on Ca²⁺ homeostasis and oxidative phosphorylation (OXPHOS) in cardiomyocytes and mitochondria, the methods of fluorescent microscopy, perforated path-clamp, and mitochondrial assays were used. We found that LCAC in dose-dependent manner can evoke Ca²⁺-sparks and oscillations, long-living Ca²⁺ enriched microdomains, and, finally, Ca²⁺ overload leading to hypercontracture and cardiomyocyte death. Collectively, PC-driven cardiotoxicity involves: (I) redistribution of Ca²⁺ from SR to mitochondria with minimal contribution of external calcium influx; (II) irreversible inhibition of Krebs cycle and OXPHOS underlying limited mitochondrial Ca²⁺ buffering; (III) induction of mPTP reinforced by PC-calcium interplay; (IV) activation of Ca²⁺-dependent phospholipases cPLA2 and PLC. Based on the inhibitory analysis we may suggest that simultaneous inhibition of both phospholipases could be an effective strategy for protection against PC-mediated toxicity in cardiomyocytes.

Lipid in the midst of metabolic remodeling - Therapeutic implications for the failing heart

A healthy heart relies on an intact cardiac lipid metabolism. Fatty acids represent the major source for ATP production in the heart. Not less importantly, lipids are directly involved in critical processes such as cell growth, proliferation, and cell death by functioning as building blocks or signaling molecules. In the development of heart failure, perturbations in fatty acid utilization impair cardiac energetics. Furthermore, they may affect glucose and amino acid metabolism and induce the synthesis of several lipid intermediates, whose biological functions are still poorly understood. This work outlines the pivotal role of lipid metabolism in the heart and provides a lipocentric view of metabolic remodeling in heart failure. We will also critically revisit therapeutic attempts targeting cardiac lipid metabolism in heart failure and propose specific strategies for future investigations in this regard.

Altered Calcium Handling and Ventricular Arrhythmias in Acute Ischemia

Acute ischemia results in deadly cardiac arrhythmias that are a major contributor to sudden cardiac death (SCD). The electrophysiological changes involved have been extensively studied, yet the mechanisms of ventricular arrhythmias during acute ischemia remain unclear. What is known is that during acute ischemia both focal (ectopic excitation) and nonfocal (reentry) arrhythmias occur, due to an interaction of altered electrical, mechanical, and biochemical properties of the myocardium. There is particular interest in the role that alterations in intracellular calcium handling, which cause changes in intracellular calcium concentration and to the calcium transient, play in ischemia-induced arrhythmias. In this review, we briefly summarize the known contributors to ventricular arrhythmias during acute ischemia, followed by an in-depth examination of the potential contribution of altered intracellular calcium handling, which may include novel targets for antiarrhythmic therapy.

Anaplerotic treatment of long-chain fat oxidation disorders with triheptanoin: Review of 15 years Experience

BACKGROUND

The treatment of long-chain mitochondrial β-oxidation disorders (LC-FOD) with a low fat-high carbohydrate diet, a diet rich in medium-even-chain triglycerides (MCT), or a combination of both has been associated with high morbidity and mortality for decades. The pathological tableau appears to be caused by energy deficiency resulting from reduced availability of citric acid cycle (CAC) intermediates required for optimal oxidation of acetyl-CoA. This hypothesis was investigated by diet therapy with carnitine and anaplerotic triheptanoin (TH).

METHODS

Fifty-two documented LC-FOD patients were studied in this investigation (age range: birth to 51 years). Safety monitoring included serial quantitative measurements of routine blood chemistries, blood levels of carnitine and acylcarnitines, and urinary organic acids.

RESULTS

The average frequency of serious clinical complications were reduced from ~60% with conventional diet therapy to 10% with TH and carnitine treatment and mortality decreased from ~65% with conventional diet therapy to 3.8%. Carnitine supplementation was uncomplicated.

CONCLUSION

The energy deficiency in LC-FOD patients was corrected safely and more effectively with the triheptanoin diet and carnitine supplement than with conventional diet therapy. Safe intervention in neonates and infants will permit earlier intervention following pre-natal diagnosis or diagnosis by expanded newborn screening.

Downregulation of carnitine acyl-carnitine translocase by miRNAs 132 and 212 amplifies glucose-stimulated insulin secretion

We previously demonstrated that micro-RNAs (miRNAs) 132 and 212 are differentially upregulated in response to obesity in two mouse strains that differ in their susceptibility to obesity-induced diabetes. Here we show the overexpression of miRNAs 132 and 212 enhances insulin secretion (IS) in response to glucose and other secretagogues including nonfuel stimuli. We determined that carnitine acyl-carnitine translocase (CACT; Slc25a20) is a direct target of these miRNAs. CACT is responsible for transporting long-chain acyl-carnitines into the mitochondria for β-oxidation. Small interfering RNA-mediated knockdown of CACT in β-cells led to the accumulation of fatty acyl-carnitines and enhanced IS. The addition of long-chain fatty acyl-carnitines promoted IS from rat insulinoma β-cells (INS-1) as well as primary mouse islets. The effect on INS-1 cells was augmented in response to suppression of CACT. A nonhydrolyzable ether analog of palmitoyl-carnitine stimulated IS, showing that β-oxidation of palmitoyl-carnitine is not required for its stimulation of IS. These studies establish a link between miRNA-dependent regulation of CACT and fatty acyl-carnitine-mediated regulation of IS.

International Union of Basic and Clinical Pharmacology. XC. multisite pharmacology: recommendations for the nomenclature of receptor allosterism and allosteric ligands

Allosteric interactions play vital roles in metabolic processes and signal transduction and, more recently, have become the focus of numerous pharmacological studies because of the potential for discovering more target-selective chemical probes and therapeutic agents. In addition to classic early studies on enzymes, there are now examples of small molecule allosteric modulators for all superfamilies of receptors encoded by the genome, including ligand- and voltage-gated ion channels, G protein-coupled receptors, nuclear hormone receptors, and receptor tyrosine kinases. As a consequence, a vast array of pharmacologic behaviors has been ascribed to allosteric ligands that can vary in a target-, ligand-, and cell-/tissue-dependent manner. The current article presents an overview of allostery as applied to receptor families and approaches for detecting and validating allosteric interactions and gives recommendations for the nomenclature of allosteric ligands and their properties.

Advances in G protein-coupled receptor allostery: from function to structure

It is now widely accepted that G protein-coupled receptors (GPCRs) are highly dynamic proteins that adopt multiple active states linked to distinct functional outcomes. Furthermore, these states can be differentially stabilized not only by orthosteric ligands but also by allosteric ligands acting at spatially distinct binding sites. The key pharmacologic characteristics of GPCR allostery include improved selectivity due to either greater sequence divergence between receptor subtypes and/or subtype-selective cooperativity, a ceiling level to the effect, probe dependence (whereby the magnitude and direction of the allosteric effect change with the nature of the interacting ligands), and the potential for biased signaling. Recent chemical biology developments are beginning to demonstrate how the incorporation of analytical pharmacology and operational modeling into the experimental workflow can enrich structure-activity studies of allostery and bias, and have also led to the discovery of a new class of hybrid orthosteric/allosteric (bitopic) molecules. The potential for endogenous allosteric modulators to play a role in physiology and disease remains to be fully appreciated but will likely represent an important area for future studies. Finally, breakthroughs in structural and computational biology are beginning to unravel the mechanistic basis of GPCR allosteric modulation at the molecular level.

L-Carnitine for the treatment of a calcium channel blocker and metformin poisoning

INTRODUCTION

The object of the current communication is to discuss the theory and the evidence for the use of L-carnitine in calcium channel blocker and metformin poisonings.

CASE REPORT

A 68-year-old male known for hypertension and type II diabetes was admitted to the critical care unit of a community hospital following an overdose of amlodipine and metformin. The patient was intubated, ventilated, and hemodynamically supported with vasopressors. Despite calcium, glucagon, high-dose insulin (HDI), and lipid emulsion for calcium channel blocker and bicarbonate for metabolic acidosis, the patient remained hemodynamically unstable. The patient was considered too unstable to initiate continuous renal replacement therapy; and without access to extracorporeal life support, the administration of L-carnitine was administered as a last resort. One hour after L-carnitine, the norepinephrine requirements started to decrease, the patient began to improve and was subsequently extubated successfully without apparent sequelae in less than 4 days.

DISCUSSION

L-Carnitine combined with HDI may have helped with the calcium channel blocker (CCB) poisoning by decreasing insulin resistance, promoting intracellular glucose transport, facilitating the metabolism of free fatty acids, and increasing calcium channel sensitivity. It may have also stimulated oxidative utilization of glucose instead of converting pyruvate into lactate and contributed to decrease lactate production with metformin poisoning.

Disorders of fatty acid oxidation

Recognition of fatty acid oxidation (FAO) disorders is important for the pediatric neurologist as they present with a spectrum of clinical disorders, including progressive lipid storage myopathy, recurrent myoglobinuria, neuropathy, progressive cardiomyopathy, recurrent hypoglycemic hypoketotic encephalopathy or Reye-like syndrome, seizures, and mental retardation. They constitute a critical group of diseases because they are potentially rapidly fatal and a source of major morbidity. There is frequently a family history of sudden infant death syndrome in siblings. Early recognition and prompt institution of therapy and appropriate preventive measures, and in certain cases specific therapy, may be life-saving and may significantly decrease long-term morbidity, particularly with respect to CNS sequelae. All currently known conditions are inherited as autosomal recessive traits. There are now at least 25 enzymes and specific transport proteins in the β-oxidation pathway and 18 have been associated with human disease. The most common defect is medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, which had an incidence of 1 in 8930 live births in one series. The identification of serum acylcarnitines by electrospray ionization-tandem mass spectrometry of dried blood spots on filter paper in newborn screening programs has significantly enhanced the early recognition of these disorders.

Cardiac metabolism in heart failure: implications beyond ATP production

The heart has a high rate of ATP production and turnover that is required to maintain its continuous mechanical work. Perturbations in ATP-generating processes may therefore affect contractile function directly. Characterizing cardiac metabolism in heart failure (HF) revealed several metabolic alterations called metabolic remodeling, ranging from changes in substrate use to mitochondrial dysfunction, ultimately resulting in ATP deficiency and impaired contractility. However, ATP depletion is not the only relevant consequence of metabolic remodeling during HF. By providing cellular building blocks and signaling molecules, metabolic pathways control essential processes such as cell growth and regeneration. Thus, alterations in cardiac metabolism may also affect the progression to HF by mechanisms beyond ATP supply. Our aim is therefore to highlight that metabolic remodeling in HF not only results in impaired cardiac energetics but also induces other processes implicated in the development of HF such as structural remodeling and oxidative stress. Accordingly, modulating cardiac metabolism in HF may have significant therapeutic relevance that goes beyond the energetic aspect.

L-carnitine increases survival in a murine model of severe verapamil toxicity

OBJECTIVES

L-carnitine is an essential compound involved in cellular energy production through free fatty acid metabolism. It has been theorized that severe verapamil toxicity "shifts" heart energy production away from free fatty acids and toward other sources, contributing to profound cardiogenic shock. The primary study objective was to determine whether intravenous (IV) L-carnitine affects survival in severe verapamil toxicity. Secondary objectives were to determine the effects on hemodynamic parameters. The authors hypothesized that IV L-carnitine would increase both survival and hemodynamic parameters in severe verapamil toxicity.

METHODS

This was a controlled, blinded animal investigation. Sixteen male rats were anesthetized, ventilated, and instrumented to record mean arterial pressure (MAP) and heart rate. Verapamil toxicity was achieved by a constant infusion of 5 mg/kg/hr. After 5 minutes a bolus of 50 mg/kg of either L-carnitine or normal saline was given. The experiment concluded when either 10% of baseline MAP was achieved or 150 minutes had elapsed. The data were analyzed using Kaplan-Meier analysis, log rank test, and analysis of variance.

RESULTS

The median survival for the animals in the L-carnitine group was 140.75 minutes (interquartile range [IQR] = 98.6 to 150 minutes), and for those in the normal saline group it was 49.19 minutes (IQR = 39.02 to 70.97 minutes; p = 0.0001). At 15 minutes the MAP was 20.45 mm Hg greater in the animals in the L-carnitine group than in the animals in the normal saline group (95% confidence interval [CI] = 0.25 to 40.65; p = 0.047).

CONCLUSIONS

When compared with saline, IV L-carnitine increases survival and MAP in a murine model of severe verapamil toxicity.

Agonist potency at P2X7 receptors is modulated by structurally diverse lipids

BACKGROUND AND PURPOSE

The P2X(7) receptor exhibits a high degree of plasticity with agonist potency increasing after prolonged receptor activation. In this study we investigated the ability of lipids to modulate agonist potency at P2X(7) receptors.

EXPERIMENTAL APPROACH

A variety of lipids, including lysophosphatidylcholine, sphingosylphosphorylcholine and hexadecylphosphorylcholine were studied for their effect on P2X(7) receptor-stimulated ethidium bromide accumulation in cells expressing human recombinant P2X(7) receptors and on P2X(7) receptor-stimulated interleukin-1 beta (IL1 beta) release from THP-1 cells. The effects of the lipids were also assessed in radioligand binding studies on human P2X(7) receptors.

KEY RESULTS

At concentrations (3-30 microM) below the threshold to cause cell lysis, the lipids increased agonist potency and/or maximal effects at P2X(7) receptors in both ethidium accumulation and IL1 beta release studies. There was little structure activity relationship (SAR) for this effect and sub-lytic concentrations of Triton X-100 partially mimicked the effects of the lipids. The lipids caused cell lysis and increased intracellular calcium at higher concentrations (30-100 microM) which complicated interpretation of their effects in functional studies. However, the lipids (3-100 microM) also increased agonist potency 30-100 fold in radioligand binding studies.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that a diverse range of lipids increase agonist potency at the P2X(7) receptor in functional and binding studies. The broad SAR, including the effect of Triton X-100, suggests this may reflect changes in membrane properties rather than a direct effect on the P2X(7) receptor. Since many of the lipids studied accumulate in disease states they may enhance P2X(7) receptor function under pathophysiological conditions.

Metabolic cardiomyopathies

The energy needed by cardiac muscle to maintain proper function is supplied by adenosine Ariphosphate primarily (ATP) production through breakdown of fatty acids. Metabolic cardiomyopathies can be caused by disturbances in metabolism, for example diabetes mellitus, hypertrophy and heart failure or alcoholic cardiomyopathy. Deficiency in enzymes of the mitochondrial beta-oxidation show a varying degree of cardiac manifestation. Aberrations of mitochondrial DNA lead to a wide variety of cardiac disorders, without any obvious correlation between genotype and phenotype. A completely different pathogenetic model comprises cardiac manifestation of systemic metabolic diseases caused by deficiencies of various enzymes in a variety of metabolic pathways. Examples of these disorders are glycogen storage diseases (e.g. glycogenosis type II and III), lysosomal storage diseases (e.g. Niemann-Pick disease, Gaucher disease, I-cell disease, various types of mucopolysaccharidoses, GM1 gangliosidosis, galactosialidosis, carbohydrate-deficient glycoprotein syndromes and Sandhoff's disease). There are some systemic diseases which can also affect the heart, for example triosephosphate isomerase deficiency, hereditary haemochromatosis, CD 36 defect or propionic acidaemia.

Neonatal metabolic myopathies

The primary presentations of neuromuscular disease in the newborn period are hypotonia and weakness. Although metabolic myopathies are inherited disorders that present from birth and may present with subtle to marked neonatal hypotonia, a number of these defects are diagnosed classically in childhood, adolescence, or adulthood. Disorders of glycogen, lipid, or mitochondrial metabolism may cause three main clinical syndromes in muscle, namely, (1) progressive weakness with hypotonia (e.g., acid maltase, debrancher enzyme, and brancher enzyme deficiencies among the glycogenoses; carnitine uptake and carnitine acylcarnitine translocase defects among the fatty acid oxidation (FAO) defects; and cytochrome oxidase deficiency among the mitochondrial disorders) or (2) acute, recurrent, reversible muscle dysfunction with exercise intolerance and acute muscle breakdown or myoglobinuria (with or without cramps), e.g., phosphorylase, phosphofructokinase, and phosphoglycerate kinase among the glycogenoses and carnitine palmitoyltransferase II deficiency among the disorders of FAO or (3) both (e.g., long-chain or very long-chain acyl coenzyme A (CoA) dehydrogenase, short-chain L-3-hydroxyacyl-CoA dehydrogenase, and trifunctional protein deficiencies among the FAO defects). Episodes of exercise-induced myoglobinuria tend to present in later childhood or adolescence; however, myoglobinuria in the first year of life may occur in FAO disorders during catabolic crises precipitated by fasting or infection. The following is a survey of genetic disorders of glycogen and lipid metabolism resulting in myopathy, focusing primarily on those defects, to date, that have presented in the neonatal or early infancy period. Disorders of mitochondrial metabolism are discussed in another chapter.

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.

L-carnitine supplementation in childhood epilepsy: current perspectives

In November 1996, a panel of pediatric neurologists met to update the consensus statement issued in 1989 by a panel of neurologists and metabolic experts on L-carnitine supplementation in childhood epilepsy. The panelists agreed that intravenous L-carnitine supplementation is clearly indicated for valproate (VPA)-induced hepatotoxicity, overdose, and other acute metabolic crises associated with carnitine deficiency. Oral supplementation is clearly indicated for the primary plasmalemmal carnitine transporter defect. The panelists concurred that oral L-carnitine supplementation is strongly suggested for the following groups as well: patients with certain secondary carnitine-deficiency syndromes, symptomatic VPA-associated hyperammonemia, multiple risk factors for VPA hepatotoxicity, or renal-associated syndromes; infants and young children taking VPA; patients with epilepsy using the ketogenic diet who have hypocarnitinemia; patients receiving dialysis; and premature infants who are receiving total parenteral nutrition. The panel recommended an oral L-carnitine dosage of 100 mg/kg/day, up to a maximum of 2 g/day. Intravenous supplementation for medical emergency situations usually exceeds this recommended dosage.

Hypertension, calcium channel and pyridoxine (vitamin B6)

The moderately pyridoxine (vitamin B6)-deficient male rat was introduced by us as an animal model (B6DHT) for the study of hypertension. Hypertension in this rat is associated with increased sympathetic stimulation. Arterial segments from B6DHT rats maintained a higher resting tone. The influx of 45calcium into intracellular compartment of the vascular smooth muscle of the caudate artery of B6DHT rats was also enhanced. Administration of pyridoxine attenuated the hypertension in B6DHT rats as well as in genetic or dietary-induced moderately hypertensive conditions such as in the Zucker obese rat and sucrose or low calcium-fed rats. However, pyridoxine did not have any effect on the spontaneously hypertensive rat. All classes of calcium channel blockers were effective in lowering the systolic blood pressure of B6DHT rats. The increased in vitro influx of 45calcium into intracellular compartment of artery segments of B6DHT rats as well as the BAY K 8644-induced influx of 45calcium into artery segments from normal rats were blocked by pyridoxal phosphate as well as by dihydropyridine-sensitive calcium channel blockers (DHP). Pyridoxal phosphate (PLP) in vitro enhances the binding of calcium channel antagonists to membrane preparations from vascular tissue. PLP corrects the membrane abnormality in responsive hypertensive conditions and thus, could be an endogenous modulator of DHP-sensitive calcium channels.

Metabolic myopathies

Disorders of glycogen, lipid or mitochondrial metabolism may cause two main clinical syndromes, namely (1) progressive weakness (eg, acid maltase, debrancher enzyme, and brancher enzyme deficiencies among the glycogenoses; long- and very-long-chain acyl-CoA dehydrogenase (LCAD, VLCAD), and trifunctional enzyme deficiencies among the fatty acid oxidation (FAO) defects; and mitochondrial enzyme deficiencies) or (2) acute, recurrent, reversible muscle dysfunction with exercise intolerance and acute muscle breakdown or myoglobinuria (with or without cramps) (eg, phosphorylase (PPL), phosphorylase b kinase (PBK), phosphofructokinase (PFK), phosphoglycerate kinase (PGK), phosphoglycerate mutase (PGAM), and lactate dehydrogenase (LDH) among the glycogenoses and carnitine palmitoyltransferase II (CPT II) deficiency among the disorders of FAO or (3) both (eg, PPL, PBK, PFK among the glycogenoses; LCAD, VLCAD, short-chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD), and trifunctional enzyme deficiencies among the FAO defects; and multiple mitochondrial DNA (mtDNA) deletions). Myoadenylate deaminase deficiency, a purine nucleotide cycle defect, is somewhat controversial and is characterized by exercise-related cramps leading rarely to myoglobinuria.

1H NMR spectroscopic imaging of myocardial triglycerides in excised dog hearts subjected to 24 hours of coronary occlusion

BACKGROUND

Myocardial ischemic insult causes depression of fatty-acid beta-oxidation and increased fatty-acid esterification with triglyceride (TG) accumulation. This accumulation has been demonstrated to occur in the territory with diminished blood flow surrounding an infarct, ie, the region at risk. To evaluate whether the extent of TG accumulation in the canine heart after 24 hours of ischemia could be detected, we applied myocardial 1H nuclear magnetic resonance (NMR) spectroscopic imaging (SI).

METHODS AND RESULTS

Seven adult mongrel dogs underwent 24 hours of left anterior descending coronary artery occlusion. Postmortem, the hearts were excised and the size and location of the infarct were determined. With a Philips 1.5-T clinical NMR imaging/spectroscopic system, two-dimensional (2D) 1H NMR SI was performed. TG 1H NMR chemical shift images were reconstructed from the frequency domain spectra by numerical integration. A statistically significant (P < .05) increase in TG signal intensity was demonstrated in the region at risk compared with the nonischemic control region. There was an intermediate quantity of TG in the infarct region. Biochemical determination of tissue TG content (milligrams per gram wet weight) in the control, at-risk, and infarct regions confirmed the 1H NMR measurements. Histological evaluation with oil red O staining also demonstrated graded TG accumulation in myocytes. The highest TG levels were found in the at-risk region and the lowest levels in the control region.

CONCLUSIONS

By use of 2D 1H NMR SI, the present study confirms and extends previous work that demonstrates preferential accumulation of TG in the reversibly injured myocardium after 24 hours of coronary occlusion. This study provides an important step toward the clinical application of TG imaging. When TG imaging is ultimately possible, resultant data would have diagnostic, prognostic, and therapeutic implications.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.

Clinical and neurophysiologic response of myopathy and neuropathy in long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency to oral prednisone

The purpose of this study was to evaluate the clinical and neurophysiologic responses to oral prednisone therapy in a boy with enzymatically confirmed long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency in biopsied muscle and cultured skin fibroblasts. This boy presented with progressive limb girdle myopathy, recurrent myoglobinuria, peripheral sensorimotor axonopathy, and intraventricular conduction delays. Prior to prednisone therapy, at age 8 years, he exhibited marked distal weakness greater than proximal weakness with a waddling and high-steppage gait, Gowers' maneuver (10 s to rise from the floor), fatigue after 3-20 yards of walking and the ability to climb only 2 stairs. Serum levels of creatine kinase rose from 34 to 4,124 U/L following mild exertion. Nerve conduction studies revealed progressive axonopathy with secondary demyelination. Four weeks after initiation of oral prednisone (0.75 mg/kg/day) therapy, there was approximately a 100% increase in power and endurance. He was able to walk at least 100 yards before tiring, could rise from sitting on the floor in 3-4 s, and was able to climb 20 steps in 30 s. There was concurrent improvement in nerve conduction studies. Prednisone was gradually withdrawn over the next 4 months to 0.19 mg/kg/day; lower doses of 0.08 mg/kg/day resulted in a marked deterioration in power to the prior state. Although 0.19 mg/kg/day did not maintain the peak power achieved at 0.75 mg/kg/day, it provided adequate baseline power and endurance. It is concluded that there was a significant clinical and neurophysiologic response to prednisone at a dosage > or = 0.16 mg/kg/day. Prednisone may stabilize muscle and neuronal plasma membranes, as well as the fatty acid oxidation enzyme complex in the mitochondrial membrane.

Effects of palmitoyl carnitine and related metabolites on the avian Ca(2+)-ATPase and Ca2+ release channel

1. In birds, prolonged cold exposure induces the development of a non-shivering thermogenesis (NST) of muscular origin that may result from an increase in ATP-dependent cycling of Ca2+ between the sarcoplasmic reticulum (SR) and the cytosol. 2. Because fatty acids are thought to play a significant role in NST, we investigated the effects of palmitic acid and related metabolites on skeletal SR Ca2+ uptake and release in ducklings. 3. Ca(2+)-ATPase activity, 45Ca2+ release and [3H]ryanodine-binding measurements indicated that palmitic acid was without effect on the Ca(2+)-ATPase and Ca2+ release channel. Palmitoyl carnitine and palmitoyl coenzyme A inhibited the Ca(2+)-ATPase at concentrations > 20 microM whereas both activated the Ca2+ release channel at concentrations < or = 20 microM in a dose-dependent manner. 4. Palmitoyl carnitine stimulated [3H]ryanodine binding to skeletal but not cardiac SR vesicles. Induction of 45Ca2+ release was observed with long-chain (C > or = 14) but not with short-chain acyl carnitines (C < or = 12). 5. Long-chain acyl carnitines accumulated significantly in duckling skeletal muscle during cold acclimation. Accordingly, these results suggest that long-chain acyl metabolites may modulate SR Ca2+ cycling and its associated thermogenesis in vivo.

Vasodilator action of the isopropyl ester of palmitoyl carnitine in the rat coronary circulation and mesenteric vascular bed

The vasodilator action of the isopropyl ester of palmitoyl carnitine (P1Pi) has been examined in perfused rat hearts and mesenteric vessels. The coronary vasodilator effect P1Pi was not significantly inhibited by flurbiprofen (10 microM), BW755C (10 microM), glibenclamide (10 microM) or the bradykinin B2 receptor antagonist D-Arg0[Hyp3,Thi5,8,D-Phe7]bradykinin (1 microM), indicating that the action of P1Pi is not mediated via arachidonic acid metabolites, ATP-dependent K+ channels or bradykinin B2 receptors. L-NG-Nitro arginine (100 microM) did not inhibit the vasodilator action of P1Pi whilst superoxide dismutase (20 and 50 U.ml-1) attenuated its vasodilator action. Methylene blue (10 microM) caused inhibition in three out of four hearts, while haemoglobin (1 microM) caused an irreversible inhibition of the action of P1Pi which was associated with a depression of myocardial contractility. In air-damaged mesenteric vascular beds the vasodilator action of P1Pi was not attenuated, whilst that of acetylcholine was abolished. In K(+)-depolarised mesenteric vascular beds the constrictor action of Ca2+ was attenuated by P1Pi. Therefore the vasodilator effect of P1Pi appears to be the result of a direct effect on smooth muscle.

Sites of action of carnitine and its derivatives on the cardiovascular system: interactions with membranes

Carnitine plays an essential role in the regulation of long-chain fatty acid metabolism in skeletal and cardiac muscle, a process that is mediated by well-characterized enzymatic mechanisms. Here, Irving Fritz and Edoardo Arrigoni-Martelli review the evidence that carnitine and its O-acyl derivatives also influence membrane fluidity, ion channel functions, smooth muscle contractility, membrane stability and cardiac functions. The authors present the view that direct interactions of carnitine derivatives with cell membranes are independent of reactions catalysed by carnitine acyltransferases. They propose that the novel actions discussed are implicated in the mechanisms by which carnitine and its derivatives protect perfused hearts subjected to ischaemia or to oxidative stress, and help people suffering from certain types of myocardial ischaemia or peripheral arterial disease.

Isolation, identification and synthesis of an endogenous arachidonic amide that inhibits calcium channel antagonist 1,4-dihydropyridine binding

This study was part of a broad search for endogenous regulators of L-type calcium channels. The screening for active fractions was done by measuring inhibition [3H]1,4-dihydropyridine (DHP) binding to rat cardiac and cortex membranes. An inhibitory fraction, termed lyophilized brain hexane-extractable inhibitor (LBHI), was isolated from hexane extracts of lyophilized calf brain. The active substance was purified by a series of chromatographic steps. 13C nuclear magnetic resonance (NMR), 1H coherence spectroscopy (COSY) NMR and fast atom bombardment (FAB) mass spectroscopy suggested that LBHI was N-arachidonic acid-2-hydroxyethylamide. Synthesis of this substance and subsequent high performance liquid chromatography (HPLC) and NMR analysis confirmed this structure. Synthetic LBHI (SLBHI) inhibited [3H]DHP binding to rat cortex membranes with an IC50 value of congruent to 15 microM and a Hill coefficient of congruent to 2. Saturation analysis in the presence of SLBHI showed a change in KD (equilibrium dissociation constant), but not maximal binding capacity (Bmax). SLBHI produced an increased dissociation rate, which, along with the Hill slope of > 1, suggested a non-competitive interaction with the DHP binding site. The results suggest that arachidonic acid derivatives may be endogenous modifiers of the DHP calcium antagonist binding site.

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.

Hypoxia-induced coronary flow changes in the perfused rat heart: effects of high L-carnitine concentrations

1. Hemodynamic effects of physiological (0.04 and 0.07 mM) and high (8, 15 and 25 mM) L-carnitine (LC) concentrations were tested on the normally oxygenated and hypoxic perfused rat heart. 2. No effect was detected on aerobic hearts, whereas a dose-dependent rise in coronary flow (CF) during both the hyperemic and constrictive phases was observed in hypoxic hearts with 8, 15 and 25 mM LC. This action was apparently unrelated to a resting tension (RT) improvement, which was observed only with 25 mM LC. 3. When 11 mM glucose was replaced by 11 mM mannitol in the perfusion buffer, LC effects on the hyperemic phase were abolished; however, 25 mM LC resulted in CF-values still significantly higher than those detected without the drug, though RT was similar in these glucose-free groups. 4. It may be concluded that LC is ineffective on the perfused rat heart in aerobic conditions, whereas high LC concentrations can enhance CF only during hypoxia, these effects being independent of heart function improvements and partly unrelated to glucose presence.

Palmitoyl-DL-carnitine has calcium-dependent effects on cultured neurones from rat dorsal root ganglia

1. The effects of palmitoyl-DL-carnitine (0.01 to 1 mM) on whole cell voltage-activated calcium channel currents carried by calcium or barium and Ca(2+)-activated chloride currents were studied in cultured neurones from rat dorsal root ganglia. 2. Palmitoyl-DL-carnitine applied to the extracellular environment or intracellularly via the patch solution reduced Ca2+ currents activated over a wide voltage range from a holding potential of -90 mV. Inhibition of high voltage activated Ca2+ channel currents was dependent on intracellular Ca2+ buffering and was reduced by increasing the EGTA concentration from 2 to 10 mM in the patch solution. Barium currents were significantly less sensitive to palmitoyl-DL-carnitine than Ca2+ currents. 3. The amplitude of Ca(2+)-activated Cl- tail currents was reduced by palmitoyl-DL-carnitine. However, the duration of these Cl- currents was greatly prolonged by palmitoyl-DL-carnitine, suggesting slower removal of free Ca2+ from the cytoplasm following Ca2+ entry through voltage-activated channels. 4. Palmitoyl-DL-carnitine evoked Ca(2+)-dependent inward currents which could be promoted by activation of the residual voltage-activated Ca2+ currents and attenuated by intracellular application of EGTA. 5. We conclude that palmitoyl-DL-carnitine reduced the efficiency of intracellular Ca2+ handling in cultured dorsal root ganglion neurones and resulted in enhancement of Ca(2+)-dependent events including inactivation of voltage-activated Ca2+ currents. The activation of inward currents by palmitolyl-DL-carnitine may involve Ca(2+)-induced Ca2+ release from intracellular stores, or direct interaction of palmitoyl-DL-carnitine with Ca2+ stores.

Identification of ascorbate as an endogenous substance that irreversibly inhibits binding of dihydropyridine calcium channel blockers

Endogenous material present in heat-denatured extracts of rat brain that inhibited the binding of [3H]-isopropyl-4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-metho xyca rbonyl-2,6-dimethyl-3-pyridinecarboxylate ([3H]-PN200-110) to calcium channels in brain membranes was purified. Spectrophotometric analysis of material purified by strong anion-exchange and reverse-phase chromatography showed an absorption maximum at 266 nm at pH 7.0 that shifted to 245 nm at pH 2.0. This pH-dependent spectral shift was indistinguishable from that of ascorbic acid. Samples of the purified extract contained ascorbic acid; however, the inhibition of binding by purified material was always greater than the inhibition seen with equivalent concentrations of ascorbate, implying the presence of additional inhibitory factors. Attempts to detect and identify such inhibitory substances by chromatography showed that inhibition activity was coincident with the presence of ascorbate, and the inhibitory activity of purified material was abolished after treatment with ascorbic acid oxidase. Iron enhanced the inhibition produced by ascorbate, and chemical analysis of purified preparations revealed the presence of iron. Studies comparing the potency of the purified material with that of a mixture of ascorbate plus iron showed that the content of ascorbate and iron in the purified brain extract is sufficient to explain the observed inhibition of binding of [3H]PN200-110.

Catecholamine-induced cardiac hypertrophy uncouples beta-adrenoceptors from slow calcium channels

Changes in the parameters of Ca(2+)-dependent slow action potentials (APs) and in their sensitivity to noradrenaline, forskolin, dibutyryl-cAMP and extracellular Ca2+ concentration were studied and compared in left ventricular trabeculae from normal control rats and rats with cardiac hypertrophy. Cytochemical studies were also carried out to determine changes in the activity of membrane-bound adenylate cyclase. Hypertrophy was induced by administration of 5 mg/kg isoproterenol once daily for 7 days. In hypertrophied cardiac muscle, the overshoot of the slow APs was increased by 75%, the maximum rate of rise (Vmax) increased by 76% and the AP duration at 50% repolarization (APD50) prolonged by 56%. The Vmax, an indicator of the slow inward Ca2+ current, increased, in a dose-dependent manner, in response to the beta-adrenoceptor agonist noradrenaline, the adenylate cyclase activator forskolin, the protein kinase activator cAMP and elevated Ca2+ concentration in normal control preparations, whereas in hypertrophied myocardium, the beta-agonist noradrenaline and the adenylate cyclase activator forskolin had no effect. In cytochemical studies with ATP as substrate, adenylate cyclase activity was localized in the sarcolemma, and significantly fewer reaction products appeared on the outer side of the cell membrane in hypertrophied myocytes than in control myocytes. The results suggest that catecholamine-induced cardiac hypertrophy damages the catalytic subunit of membrane-bound adenylate cyclase, thus uncoupling beta-adrenoceptors from slow Ca2+ channels in the transmembrane signalling process.

The synthesis, and structure-activity relationships of some long chain acyl carnitine esters on the coronary circulation of the rat isolated heart

The synthesis of the isopropyl ester of carnitine and a series of fatty acid derivatives with fatty acid lengths C8-C30 is described. Bolus doses of these compounds (0.03-300 nmol) showed coronary vasodilator activity in the rat isolated heart. Increasing fatty acid chain length from C8 to C16 resulted in an increased vasodilator potency. Longer lasting vasodilation was observed with the C20 compound. Increasing fatty acid chain length to C30 was associated with a small dilator response preceded by vasoconstriction.

Identification of capric acid as a potent vasorelaxant of human basilar arteries

To determine whether naturally occurring fatty acids, especially saturated ones, might act directly as vasodilators, segments of human basilar arteries and umbilical arteries were precontracted submaximally with prostaglandin F2 alpha and then exposed to different saturated fatty acids (C4 through C16) or unsaturated fatty acids (C14:1, C18:1, C18:2, and C18:3) at concentrations from 4 microM to 4 mM. The results showed caprate (C10) to be the most potent vasorelaxant and basilar arteries to be more responsive (EC50 = 63 microM) than umbilical arteries (EC50 = 780 microM). Caprate also inhibited contractions elicited by KCl, serotonin, and the thromboxane analogue U46619. The relaxation was independent of the endothelium, and potency was not related to the weak capacity of caprate to inhibit Ca(2+)-induced contractions of K(+)-depolarized basilar arteries. The pattern of potencies for the arteries differed, but among unsaturated fatty acids the monounsaturated (C14:1, C18:1) were more potent than the polyunsaturated (C18:2, C18:3). Comparing the potencies obtained with the concentrations reported for the free fatty acid content of arteries, brain, and plasma indicates that these lipids could influence vasomotion in health and disease.

Effects of some L-carnitine derivatives on heart membrane ATPases

In order to understand the role of carnitine metabolites in the genesis of cellular dysfunction and damage due to myocardial ischemia, the effects of 1-100 microM L-carnitine, acetylcarnitine, propionylcarnitine, and palmitoylcarnitine were investigated on rat heart sarcolemmal, sarcoplasmic reticular, and mitochondrial ATPase activities. Palmitoylcarnitine, unlike acetylcarnitine, propionylcarnitine and carnitine, produced marked inhibitory actions on sarcolemmal Na,K-ATPase and Ca2(+)-stimulated ATPase, as well as sarcoplasmic reticular Ca2(+)-stimulated ATPase activities; Na,K-ATPase was most sensitive. Although palmitoylcarnitine, unlike carnitine or its short-chain fatty-acid derivatives, also depressed sarcolemmal Ca2+ ATPase or Mg2+ ATPase, sarcoplasmic reticular Mg2+ ATPase, and mitochondrial Mg2+ ATPase, mitochondria were less sensitive in comparison to other organelles. Myofibrillar Ca2(+)-stimulated ATPase was slightly inhibited by very high concentrations of palmitoylcarnitine only. It is suggested that the observed depression of the sarcolemmal Na(+)-pump system by low concentrations of long-chain acyl derivatives of carnitine may contribute towards the pathogenesis of arrhythmias due to myocardial ischemia. Furthermore, the inhibition of Ca2(+)-pump mechanisms in the sarcolemmal and sarcoplasmic reticular membranes by relatively high concentrations of palmitoylcarnitine may result in the occurrence of intracellular Ca2+ overload and subsequent cell damage, as well as cardiac dysfunction due to myocardial ischemia.

Capric acid as a potent dilator of canine vessels in vitro and in vivo

1. Pharmacodynamic effects of even numbered saturated fatty acids, C4-C16, were determined on isolated canine basilar and femoral arteries precontracted with PGF2 alpha. 2. The fatty acids relaxed the precontracted vessels. 3. The basilar artery was the most sensitive vessel and caprate (C10) was the most potent acid with an EC50 of 49 microM. 4. The relaxant effect was endothelium-independent. 5. Contractions elicited by norepinephrine, serotonin, and U46619 were also inhibited. 6. Caprate (C10) given intra-arterially increased femoral blood flow in a dose-dependent manner and the dose computed to increase blood flow 50% was 1.27 microM/kg.

Bay K 8644, modifier of calcium transport and energy metabolism in rat heart mitochondria: a new intracellular site of action

1. The dihydropyridine Ca2+ channel agonist Bay K 8644 (10-200 microM) produced a concentration-dependent increase in State 4 respiration in the rat heart mitochondria with the highest concentration (200 microM) increasing the rate from 33.1 +/- 0.7 to 187.0 +/- 13.3 ng atoms O2 consumed min-1 mg-1 protein. 2. Bay K 8644 (200 microM) reduced State 3 respiration from 247.2 +/- 11.7 to 174.4 +/- 0.06 ng atoms O2 min-1 mg-1 protein, reduced the respiratory control index (RCI) from 5.3 +/- 0.45 to 1.1 +/- 0.03 and reduced the ADP:O ratio from 2.75 +/- 0.03 to 1.3 +/- 0.15. 3. A similar, but smaller, stimulation of State 4 respiration was seen with nitrendipine (25-200 microM), the rate increasing from 22.6 +/- 1.0 to 33.1 +/- 1.8 ng atoms O2 consumed min-1 mg-1 protein in the presence of 200 microM nitrendipine. 4. Bay K 8644 (10-60 microM) increased the total Ca2+ uptake into rat heart mitochondria, the total increasing from 248.8 +/- 8.4 to 406.9 +/- 17.6 ng Ca2+ mg-1 protein at 60 microM Bay K 8644 (EC50 = 18.9 +/- 1.4 microM). 5. Bay K 8644 (10-60 microM) produced a concentration-dependent reduction in the Ca2+ influx rate (IC50 = 52.5 +/- 2.8 microM). Similar effects were seen with (+)-Bay K 8644 and (-)-Bay K 8644. 6. Nitrendipine (40-120 microM) stimulated Ca2+ efflux from mitochondria preloaded with the ion; the efflux rate increasing from 2.9 +/- 0.05 to 114.2 +/- 6.2 nmol Ca2+ min-1 mg-1 protein (EC50 = 57.3 +/- 1.3 microM). 7. These data indicate dihydropyridine-induced changes in the activity of the mitochondrial Na+/Ca2 . antiporter pathway; nitrendipine causing stimulation and Bay K 8644 causing inhibition.

Interactions of palmitoyl carnitine with the endothelium in rat aorta

1. Palmitoyl carnitine (10-1000 microM) resembled Bay K 8644 (10-1000 nM) in that it directly contracted rat aortic rings which were partially depolarized with K+ (12 mM). However, the effects of Bay K 8644 were reduced in the presence of endothelium whereas the presence of the endothelium hardly affected the palmitoyl carnitine-induced contractions, which occurred at high concentrations (greater than 10 microM). 2. Lower concentrations of palmitoyl carnitine (0.3-30 microM; EC50 1.1 microM), but not Bay K 8644, carnitine or palmitic acid, antagonized the relaxant effects of acetylcholine in rat aorta. The antagonism was specific for endothelium-dependent relaxations, in that the relaxations to ATP and the calcium ionophore A23187 were also non-competitively antagonized, albeit at slightly higher concentrations, whereas the direct relaxant effects of sodium nitroprusside were unaffected. Palmitoyl carnitine therefore antagonizes the effects or the release of endothelial-derived relaxant factor (EDRF). The inhibitory effects were reversed on prolonged washout, indicating that the effects were not due to destruction of the endothelial cells. 3. In superfusion experiments, palmitoyl carnitine inhibited the release of EDRF from rat aorta but did not affect the responsiveness to exogenous EDRF, indicating a site of action at the endothelial cell. In superfusion experiments, palmitoyl carnitine, and lysophosphatidyl choline, caused direct relaxations of the aorta, indicating EDRF release, prior to inhibition of release evoked by receptor stimulation. These substances may modulate vascular responsiveness under certain conditions.

The effects of novel vasodilator long chain acyl carnitine esters in the isolated perfused heart of the rat

1. The effects of palmitoyl carnitine (PC) and novel derivatives were examined on the isolated Langendorff perfused heart of the rat. 2. Bolus injections of PC (1-300 nmol) produced coronary constriction accompanied by a cumulative irreversible depression of contractility. 3. Prior storage of PC in chloroform containing 2% ethanol in heat-sealed ampoules resulted in production of the ethyl ester of the compound (PCE). This compound was isolated and also synthesized (P1E). In contrast to PC, both PCE and P1E exhibited potent vasodilator activity. 4. Increasing the fatty acid chain length from palmitoyl to stearoyl resulted in a significant reduction in coronary dilator activity of the ester compound, whereas different ester groups did not affect the vasodilator action appreciably. Complete removal of the fatty acid chain abolished all vascular effects at the doses used. 5. The vasodilatation produced by these acyl carnitine esters was comparable to that produced by several known vasodilator drugs including verapamil, cromakalim, amyl nitrate and iloprost; however, the duration of the vasodilator response was more prolonged with the carnitate derivatives.

Activators and inactivators of calcium channels: effects in the central nervous system

The interactions of calcium antagonists or channel activators with the different classes of calcium channel are reviewed with particular emphasis on interactions with neuronal tissue; reasons for the failure of calcium antagonists to inhibit neurotransmitter release under normal circumstances are outlined. Calcium antagonists may be protective in several pathological situations and the possibilities of protection against ischaemic damage in the central nervous system are evaluated.

Palmitoyl-L-carnitine increases the release of prostacyclin from vascular endothelial cells

Prostacyclin biosynthesis is dramatically increased in patients with acute myocardial infarction. As palmitoylcarnitine accumulates during myocardial ischemia, the action of this metabolite on the endothelial production of prostacyclin was studied. Palmitoyl-L-carnitine (10-100 microM) enhanced the release of prostacyclin and free arachidonic acid from bovine aortic endothelial cells. This action was mimicked by lysophosphatidylcholine, but by none of the following compounds: acetylcarnitine, carnitine, palmitic acid, sphingosine, dihydrosphingosine and N-stearoyl-dihydrosphingosine. In addition to mobilizing free arachidonate, palmitoylcarnitine induced the release of free choline and phosphorylcholine presumably via the activation of phospholipases C and D. Palmitoyl-L-carnitine had also a cytotoxic effect on the endothelial cells. These data suggest that the increased biosynthesis of prostacyclin in myocardial infarction might be partially explained by the accumulation and release of palmitoyl-L-carnitine.

Reperfusion injury and its pharmacologic modification

Reperfusion injury includes a spectrum of events, such as reperfusion arrhythmias, vascular damage and no-reflow, and myocardial functional stunning. The concept of reperfusion injury remains controversial with many proposed mechanisms when applied to humans, whereas in animal models, there are two main proposed mechanisms: calcium over-load and formation of oxygen free radicals. To prove that reperfusion injury is specifically caused by reperfusion would require evidence that an intervention given at the time of reperfusion can diminish or abolish the injury as in the case of arrhythmias, which are thought to be mediated by excess recycling of cytosolic calcium with delayed afterdepolarizations and ventricular automaticity. In the case of myocardial stunning, the phenomenon may be mediated, at least in part, by a burst of free radicals formed within the first minute of reperfusion and improved by free radical scavengers given at the time of reperfusion. The alternate hypothesis is that cytosolic calcium overload damages mechanisms for normal intracellular calcium regulation so that the stunned myocardium responds to agents that are thought to increase intracellular cytosolic calcium, such as beta-receptor agonists. A further component of reperfusion injury, under active investigation, is microvascular damage with alterations at the level of platelets, leukocytes, and endothelial integrity. From the therapeutic point of view, the divergent results of experimental interventions and the possibility that the abrupt onset of reperfusion in animals differs from the situation in humans with thrombolysis means that the best way currently available to limit reperfusion injury is by minimizing the ischemic period by early reperfusion and by optimizing the metabolic status of the ischemic myocardium at the end of the ischemic period.

Interaction of palmitoyl carnitine with calcium antagonists in myocytes

1. Beating of aggregates of embryonic chick myocytes, in primary culture, was quantified by use of a motion-detector and video-recorder technique. Interactions of palmitoyl carnitine, a putative endogenous ligand at Ca2+ channels, with calcium antagonists were investigated. 2. Bay K 8644 (1-100 nM) and palmitoyl carnitine (0.2-30 microM) increased edge movement of the aggregates; beats fused so that there was an increase in baseline 'tone'. The concentrations required to produce a 50% increase in edge movement were 2.5 nM for Bay K 8644 and 2 microM for palmitoyl carnitine. Higher concentrations (20-30 microM) of palmitoyl carnitine caused tachycardia of abrupt onset but resulted in cessation of beating. The effects of palmitoyl carnitine were not stereo-selective in that the (+)- and (-)-isomers were equieffective. Lysophosphatidyl choline (LPC) had no effect in concentrations up to 10 microM but higher concentrations caused tachycardia followed by cessation of beating. High concentrations of both palmitoyl carnitine and LPC (100 microM) caused break-up of the aggregates, presumably as a result of detergent effects. 3. Palmitoyl carnitine (1-100 microM) reversed the inhibitory effects of nisoldipine (0.3 microM), diltiazem (10 microM) and verapamil (1 microM). Ouabain was ineffective in reversing the effects of nisoldipine, differentiating the effects of palmitoyl carnitine from those of Na+/K+ ATPase inhibition. In contrast, palmitoyl carnitine did not reverse the inhibitory effects of pimozide (2 microM) or lidoflazine (7 microM); palmitoyl carnitine showed a similar profile to Bay K 8644 in this respect. 4. These findings indicate that the effects of palmitoyl carnitine closely resemble those of Bay K 8644 and can be differentiated from those of lysophospholipids. As palmitoyl carnitine accumulates in the sarcolemma during myocardial ischaemia, the mode of action in the Ca2 + channel may have clinical relevance for the use of calcium antagonists in ischaemia.