L-carnitine increases glucose metabolism and mechanical function following ischaemia in diabetic rat heart

Potential protective effects of L-carnitine against myocardial ischemia/reperfusion injury in a rat model

Myocardial ischemia/reperfusion (I/R) injury is a growing concern for global public health. This study seeks to explore the potential protective effects of L-carnitine (LC) against heart ischemia-reperfusion injury in rats. To induce I/R injury, the rat hearts underwent a 30-min ligation of the left anterior descending coronary artery, followed by 24 h of reperfusion. We evaluated cardiac function through electrocardiography and heart rate variability (HRV) and conducted pathological examinations of myocardial structure. Additionally, the study investigated the influence of LC on myocardial apoptosis, inflammation, and oxidative stress in the context of I/R injury. The results show that pretreatment with LC led to improvements in the observed alterations in ECG waveforms and HRV parameters in the nontreated ischemic reperfusion model group, although most of these changes did not reach statistical significance. Similarly, although without a significant difference, LC reduced the levels of proinflammatory cytokines when compared to the values in the nontreated ischemic rat group. Furthermore, LC restored the reduced expressions of SOD1, SOD2, and SOD3. Additionally, LC significantly reduced the elevated Bax expressions and showed a nonsignificant increase in Bcl-2 expression, resulting in a favorable adjustment of the Bcl-2/Bax ratio. We also observed a significant enhancement in the histological appearance of cardiac muscles, a substantial reduction in myocardial fibrosis, and suppressed CD3 + cell proliferation in the ischemic myocardium. This small-scale, experimental, in vivo study indicates that LC was associated with enhancements in the pathological findings in the ischemic myocardium in the context of ischemia/reperfusion injury in this rat model. Although statistical significance was not achieved, LC exhibits potential and beneficial protective effects against I/R injury. It does so by modulating the expression of antioxidative and antiapoptotic genes, inhibiting the inflammatory response, and enhancing autonomic balance, particularly by increasing vagal tone in the heart. Further studies are necessary to confirm and elaborate on these findings.

Serum L-Carnitine Levels Are Associated With First Stroke in Chinese Adults With Hypertension

BACKGROUND

This study aimed to evaluate the association of serum L-carnitine with first stroke and explore potential effect modifiers.

METHODS

This is a nested, case-control study drawn from the China Stroke Primary Prevention Trial among rural Chinese adults with hypertension, including 557 first stroke cases and 557 age-matched, sex-matched, treatment group-matched, and residence-matched controls. Serum L-carnitine was measured by liquid chromatography with tandem quadrupole mass spectrometry. Multiple conditional logistic regression models were used to evaluate the association between L-carnitine and first stroke.

RESULTS

The mean level of serum L-carnitine in the stroke population was 4.7 μg/mL, which was significantly lower than that of the control group (5.7 μg/mL). When L-carnitine was assessed as quintiles, compared with the reference group (quintile 1, <3.3 μg/mL), the odds of stroke were 0.62 (95% CI, 0.39-1.00) in quintile 2, 0.66 (95% CI, 0.40-1.10) in quintile 3, 0.47 (95% CI, 0.28-0.81) in quintile 4, and 0.50 (95% CI, 0.30-0.84) in quintile 5. The trend test was significant (P=0.01). When quintiles 2 to 5 were combined, the adjusted odds ratio of first stroke was 0.58 (95% CI, 0.38-0.87) compared with quintile 1. Similar associations were found for ischemic stroke and hemorrhagic stroke. In subgroup analysis, a significant L-carnitine-stroke association was only observed in the normal folate group (P interaction, 0.039) and in the MTHFR CC genotype group (P interaction, 0.047).

CONCLUSIONS

In this study of rural Chinese adults with hypertension, serum L-carnitine had an inverse but nonlinear association with first stroke. Folate status and the MTHFR C677T variant were significant effect modifiers of the association.

Metabolic, structural and biochemical changes in diabetes and the development of heart failure

Diabetes contributes to the development of heart failure through various metabolic, structural and biochemical changes. The presence of diabetes increases the risk for the development of cardiovascular disease (CVD), and since the introduction of cardiovascular outcome trials to test diabetic drugs, the importance of improving our understanding of the mechanisms by which diabetes increases the risk for heart failure has come under the spotlight. In addition to the coronary vasculature changes that predispose individuals with diabetes to coronary artery disease, diabetes can also lead to cardiac dysfunction independent of ischaemic heart disease. The hyperlipidaemic, hyperglycaemic and insulin resistant state of diabetes contributes to a perturbed energy metabolic milieu, whereby the heart increases its reliance on fatty acids and decreases glucose oxidative rates. In addition to changes in cardiac energy metabolism, extracellular matrix remodelling contributes to the development of cardiac fibrosis, and impairments in calcium handling result in cardiac contractile dysfunction. Lipotoxicity and glucotoxicity also contribute to impairments in vascular function, cardiac contractility, calcium signalling, oxidative stress, cardiac efficiency and lipoapoptosis. Lastly, changes in protein acetylation, protein methylation and DNA methylation contribute to a myriad of gene expression and protein activity changes. Altogether, these changes lead to decreased cardiac efficiency, increased vulnerability to an ischaemic insult and increased risk for the development of heart failure. This review explores the above mechanisms and the way in which they contribute to cardiac dysfunction in diabetes.

The Contribution of Cardiac Fatty Acid Oxidation to Diabetic Cardiomyopathy Severity

Diabetes is a major risk factor for the development of cardiovascular disease via contributing and/or triggering significant cellular signaling and metabolic and structural alterations at the level of the heart and the whole body. The main cause of mortality and morbidity in diabetic patients is cardiovascular disease including diabetic cardiomyopathy. Therefore, understanding how diabetes increases the incidence of diabetic cardiomyopathy and how it mediates the major perturbations in cell signaling and energy metabolism should help in the development of therapeutics to prevent these perturbations. One of the significant metabolic alterations in diabetes is a marked increase in cardiac fatty acid oxidation rates and the domination of fatty acids as the major energy source in the heart. This increased reliance of the heart on fatty acids in the diabetic has a negative impact on cardiac function and structure through a number of mechanisms. It also has a detrimental effect on cardiac efficiency and worsens the energy status in diabetes, mainly through inhibiting cardiac glucose oxidation. Furthermore, accelerated cardiac fatty acid oxidation rates in diabetes also make the heart more vulnerable to ischemic injury. In this review, we discuss how cardiac energy metabolism is altered in diabetic cardiomyopathy and the impact of cardiac insulin resistance on the contribution of glucose and fatty acid to overall cardiac ATP production and cardiac efficiency. Furthermore, how diabetes influences the susceptibility of the myocardium to ischemia/reperfusion injury and the role of the changes in glucose and fatty acid oxidation in mediating these effects are also discussed.

The Effects of Streptozotocin-Induced Diabetes and Insulin Treatment on Carnitine Biosynthesis and Renal Excretion

Carnitine insufficiency is reported in type 1 diabetes mellitus. To determine whether this is accompanied by defects in biosynthesis and/or renal uptake, liver and kidney were obtained from male Sprague-Dawley rats with streptozotocin-induced diabetes. Diabetic rats exhibited the metabolic consequences of type 1 diabetes, including hypoinsulinemia, hyperglycemia, and increased urine output. Systemic hypocarnitinemia, expressed as free carnitine levels, was evident in the plasma, liver, and kidney of diabetic rats. Compared to control rats, the low free carnitine in the plasma of diabetic rats was accompanied by decreased expression of γ-butyrobetaine hydroxylase in liver and kidney, suggesting impaired carnitine biosynthesis. Expression of organic cation transporter-2 in kidney was also reduced, indicating impaired renal reabsorption, and confirmed by the presence of elevated levels of free carnitine in the urine of diabetic rats. Insulin treatment of diabetic rats reversed the plasma hypocarnitinemia, increased the free carnitine content in both kidney and liver, and prevented urinary losses of free carnitine. This was associated with increased expression of γ-butyrobetaine hydroxylase and organic cation transporter-2. The results of our study indicate that type 1 diabetes induced with streptozotocin disrupts carnitine biosynthesis and renal uptake mechanisms, leading to carnitine insufficiency. These aberrations in carnitine homeostasis are prevented with daily insulin treatment.

Natural and synthetic antioxidants targeting cardiac oxidative stress and redox signaling in cardiometabolic diseases

Cardiometabolic diseases (CMDs) are metabolic diseases (e.g., obesity, diabetes, atherosclerosis, rare genetic metabolic diseases, etc.) associated with cardiac pathologies. Pathophysiology of most CMDs involves increased production of reactive oxygen species and impaired antioxidant defense systems, resulting in cardiac oxidative stress (OxS). To alleviate OxS, various antioxidants have been investigated in several diseases with conflicting results. Here we review the effect of CMDs on cardiac redox homeostasis, the role of OxS in cardiac pathologies, as well as experimental and clinical data on the therapeutic potential of natural antioxidants (including resveratrol, quercetin, curcumin, vitamins A, C, and E, coenzyme Q10, etc.), synthetic antioxidants (including N-acetylcysteine, SOD mimetics, mitoTEMPO, SkQ1, etc.), and promoters of antioxidant enzymes in CMDs. As no antioxidant indicated for the prevention and/or treatment of CMDs has reached the market despite the large number of preclinical and clinical studies, a sizeable translational gap is evident in this field. Thus, we also highlight potential underlying factors that may contribute to the failure of translation of antioxidant therapies in CMDs.

L-Carnitine Stimulates In Vivo Carbohydrate Metabolism in the Type 1 Diabetic Heart as Demonstrated by Hyperpolarized MRI

The diabetic heart is energetically and metabolically abnormal, with increased fatty acid oxidation and decreased glucose oxidation. One factor contributing to the metabolic dysfunction in diabetes may be abnormal handling of acetyl and acyl groups by the mitochondria. L-carnitine is responsible for their transfer across the mitochondrial membrane, therefore, supplementation with L-carnitine may provide a route to improve the metabolic state of the diabetic heart. The primary aim of this study was to use hyperpolarized magnetic resonance imaging (MRI) to investigate the effects of L-carnitine supplementation on the in vivo metabolism of [1-¹³C]pyruvate in diabetes. Male Wistar rats were injected with either vehicle or streptozotocin (55 mg/kg) to induce type-1 diabetes. Three weeks of daily i.p. treatment with either saline or L-carnitine (3 g/kg/day) was subsequently undertaken. In vivo cardiac function and metabolism were assessed with CINE and hyperpolarized MRI, respectively. L-carnitine supplementation prevented the progression of hyperglycemia, which was observed in untreated streptozotocin injected animals and led to reductions in plasma triglyceride and ß-hydroxybutyrate concentrations. Hyperpolarized MRI revealed that L-carnitine treatment elevated pyruvate dehydrogenase flux by 3-fold in the diabetic animals, potentially through increased buffering of excess acetyl-CoA units in the mitochondria. Improved functional recovery following ischemia was also observed in the L-carnitine treated diabetic animals.

Carnitine therapy for the treatment of metabolic syndrome and cardiovascular disease: evidence and controversies

As the incidence of metabolic syndrome increases, there is also a growing interest in finding safe and inexpensive treatments to help lower associated risk factors. L-carntine, a natural dietary supplement with the potential to ameliorate atherosclerosis, has been the subject of recent investigation and controversy. A majority of studies have shown benefit of L-C supplementation in the metabolic syndrome or cardiovascular risk factors. However, recent work has suggested that dietary L-C may accelerate atherosclerosis via gut microbiota metabolites, complicating the role of L-C supplementation in health.

The effect of diabetes and poor left ventricular function on bone marrow cell-induced myocardial protection

OBJECTIVES

The myocardium of patients with diabetes and poor left ventricular (LV) function cannot be protected by interventions such as ischemic preconditioning (IP). We investigated whether these clinical conditions influence the protection elicited by the paracrine effect of bone marrow cells (BMCs) and whether the cause for loss in protection resides in the BMCs, the myocardium, or both.

METHODS

BMCs and right atrial appendage were obtained from patients with and without diabetes and from poor (EF < 30%) and preserved LV function undergoing elective cardiac surgery. Muscles (n = 6/group) were co-cultured with BMCs and subjected to 90 min ischemia/120 min reoxygenation at 37°C. The degree of protection was assessed by measuring creatine kinase (CK) released, and myocardial cell necrosis and apoptosis.

RESULTS

Ischemia-induced CK release, cell necrosis, and apoptosis in the diabetic myocardium were not significantly affected by IP or by co-incubation with autologous or non-diabetic allogenic BMCs. Conversely, significant reduction in CK release, cell necrosis, and apoptosis were observed when non-diabetic myocardium was co-incubated with allogenic diabetic BMCs. Interestingly, while allogenic BMCs from subjects with preserved LV function exerted a modest but significant reduction in CK leakage and cell necrosis, but not apoptosis, on failing myocardium, the BMCs from patients with poor LV function failed to protect their own and the allogenic myocardium from subjects with normal LV function.

CONCLUSIONS

The failure to protect the myocardium of patients with poor LV function against ischemia/reoxygenation-induced injury is mainly due to a deficit in their BMCs and the myocardium itself, whereas in patients with diabetes the deficit remains within the myocardium and not in the BMCs.

Role of carnitine in the regulation of glucose homeostasis and insulin sensitivity: evidence from in vivo and in vitro studies with carnitine supplementation and carnitine deficiency

BACKGROUND

Although carnitine is best known for its role in the import of long-chain fatty acids (acyl groups) into the mitochondrial matrix for subsequent β-oxidation, carnitine is also necessary for the efflux of acyl groups out of the mitochondria. Since intracellular accumulation of acyl-CoA derivatives has been implicated in the development of insulin resistance, carnitine supplementation has gained attention as a tool for the treatment of insulin resistance. More recent studies even point toward a causative role for carnitine insufficiency in developing insulin resistance during states of chronic metabolic stress, such as obesity, which can be reversed by carnitine supplementation.

METHODS

The present review provides an overview about data from both animal and human studies reporting effects of either carnitine supplementation or carnitine deficiency on parameters of glucose homeostasis and insulin sensitivity in order to establish the less well-recognized role of carnitine in regulating glucose homeostasis.

RESULTS

Carnitine supplementation studies in both humans and animals demonstrate an improvement of glucose tolerance, in particular during insulin-resistant states. In contrast, less consistent results are available from animal studies investigating the association between carnitine deficiency and glucose intolerance. The majority of studies dealing with this question could either find no association or even reported that carnitine deficiency lowers blood glucose and improves insulin sensitivity.

CONCLUSIONS

In view of the abovementioned beneficial effect of carnitine supplementation on glucose tolerance during insulin-resistant states, carnitine supplementation might be an effective tool for improvement of glucose utilization in obese type 2 diabetic patients. However, further studies are necessary to explain the conflicting observations from studies dealing with carnitine deficiency.

L-Carnitine rescues ketamine-induced attenuated heart rate and MAPK (ERK) activity in zebrafish embryos

Ketamine, an antagonist of the N-methyl-D-aspartate (NMDA)-type glutamate receptors, is a pediatric anesthetic. Ketamine has been shown to be neurotoxic and cardiotoxic in mammals. Here, we show that after 2 h of exposure, 5 mM ketamine significantly reduced heart rate in 26 h old zebrafish embryos. In 52 h old embryos, 1 mM ketamine was effective after 2 h and 0.5 mM ketamine at 20 h of exposure. Ketamine also induced significant reductions in activated MAPK (ERK) levels. Treatment of the embryos with the ERK inhibitor, PD 98059, also significantly reduced heart rate whereas the p38/SAPK inhibitor, SB203580, was ineffective. Ketamine is known to inhibit lipolysis and a decrease of ATP content in the heart. Co-treatment with l-carnitine that enhances fatty acid metabolism effectively rescued ketamine-induced attenuated heart rate and ERK activity. These findings demonstrate that l-carnitine counteracts ketamine's negative effects on heart rate and ERK activity in zebrafish embryos.

Pharmacological effects and clinical applications of propionyl-L-carnitine

Propionyl-L-carnitine (PLC) is a naturally occurring derivative of carnitine that plays an important role in the metabolism of both carbohydrates and lipids, leading to an increase of ATP generation. PLC, however, is not only a metabolic drug; it is also a potent antiradical agent and thus may protect tissues from oxidative damage. PLC has been demonstrated to exert a protective effect in different models of both cardiac and endothelial dysfunction, to prevent the progression of atherosclerosis, and, more recently, to improve some of the cardiometabolic alterations that frequently accompany insulin resistance. As a result, most of the clinical trials conducted in humans highlight PLC as a potential treatment option in cardiovascular diseases such as peripheral arterial disease, chronic heart failure, or stable angina, especially when type 2 diabetes mellitus or hyperglycemia (i.e., patients on hemodialysis) are also present. The aim of this review is to summarize the pharmacological effects and possible therapeutic applications of PLC, including the most recent findings to date.

Ameliorating effects of L-carnitine on diabetic podocyte injury

High glucose levels can change podocyte gene expression and subsequently induce podocyte damage through altered glucose metabolism. l-Carnitine is known to play a beneficial role in diabetes; however, there are no studies on the effects of l-carnitine on podocyte alteration under high glucose conditions. This study investigated whether l-carnitine can attenuate diabetic podocyte injury through the prevention of loss of slit diaphragm proteins. The l-carnitine treatment group showed increased glucose uptakes compared to the control group, suggesting that glucose utilization in the podocytes was increased by l-carnitine. l-Carnitine treatment also prevented decreased mRNA expressions of nephrin and podocin in the high glucose-stimulated podocytes. However, mRNA expressions of CD2AP and α-actinin-4 were not significantly changed by the high glucose conditions. When these data are taken together, l-carnitine can increase glucose uptake in podocytes under high glucose conditions, and its mechanism may be at least partly related to the up-regulation of nephrin and podocin. Our results help clarify the beneficial effects of l-carnitine in diabetic nephropathy.

Relative importance of enhanced glucose uptake versus attenuation of long-chain acyl carnitines in protecting ischemic myocardium

BACKGROUND

A number of experimental studies have shown that increasing glucose use or decreasing accumulation of long-chain acyl carnitines (LCAC) protect ischemic hearts.

METHODS

To evaluate the relative importance of these two strategies in protecting ischemic myocardium, isolated rat hearts (n = 6 in each group) were paced at 300 bpm and subjected to 50 min of low-flow ischemia followed by 60 min of reperfusion. Buffer contained 0.4 m mol/l albumin, 0.4 m mol/l palmitate, and 70 mU/l insulin, and either normal glucose (5 m mol/l) (CON), high glucose (10 m mol/l total) (HG, known to increase glucose use), 5 m mol/l glucose and niacin (10 micromol/l) (NIA, known to increase glucose use and decrease LCAC) or carnitine (10 m mol/l) (CAR, known to increase glucose use and decrease LCAC). Separate groups of hearts were perfused in the presence of 10 micromol/l cytochalasin-B (CB), an inhibitor of insulin-sensitive glucose transporters.

RESULTS

Ischemic injury, as assessed by creatine kinase (CK) release was diminished by an average of 50% in HG, NIA, and CAR hearts, and the percentage recovery of left ventricular (LV) function with reperfusion was enhanced by approximately 20% compared with CON hearts (P < 0.05 for each comparison). Cytochalasin-B abolished all of the salutary effects. Long-chain acyl carnitines levels were higher in HG hearts compared with NIA- and CAR-treated hearts ( P < 0.05), but ischemic protection and functional recovery was greater in HG hearts.

CONCLUSIONS

The data support the adjunctive use of agents that promote glucose uptake during ischemia and suggest that increasing glucose use is more important than decreasing LCAC in the protection against ischemic injury or in the recovery of contractile function.

Short-term carnitine deficiency does not alter aerobic rat heart function but depresses reperfusion recovery after ischemia

Clinical and experimental studies have shown that long-term carnitine deficiency is often associated with cardiomyopathy and ischemic failure. The present study was designed to determine whether cardiac dysfunction is seen in an experimental model of short-term carnitine deficiency. Carnitine deficiency was induced in Sprague-Dawley rats by supplementing the drinking water with sodium pivalate for a period of 2 weeks. This resulted in a 25% depletion of total myocardial carnitine content. When isolated working hearts from these animals were paced and subjected to increments in left atrial filling pressure, there were no differences in mechanical function compared with control hearts. Following no-flow ischemia, however, recovery of cardiac output and relaxation parameters was depressed in hearts from pivalate-treated animals. Under these conditions, L-carnitine prevented the depressions of function from occurring. Our results show that short-term carnitine deficiency is not associated with cardiac dysfunction under normoxic conditions. However, hearts from pivalate-treated animals are more susceptible to ischemic injury and thus may prove to be useful for the study of metabolic and functional aspects of carnitine deficiency.Key words: pivalate, carnitine deficiency, cardiac, ischemia, reperfusion.

Effects of propionyl-L-carnitine on isolated mitochondrial function in the reperfused diabetic rat heart

The effects of propionyl-L-carnitine (PLC) on isolated mitochondrial respiration in the ischemic reperfused diabetic heart were studied. Oral PLC treatment of STZ-diabetic rats was initiated for a period of 6 weeks. After treatment, isolated working hearts from diabetic rats were perfused under aerobic conditions then subjected to 25 min of no-flow ischemia followed by 15 min of aerobic reperfusion. At the end of reperfusion, heart mitochondria was isolated using differential centrifugation and respiration measured in the presence of pyruvate, glutamate, and palmitoylcarnitine. Our results indicate that diabetes was characterized by a pronounced decrease in heart function under aerobic conditions as well as during reperfusion following ischemia. Treatment with PLC resulted in a significant improvement in heart function under these conditions. The depressions in state 3 mitochondrial respiration with both pyruvate and glutamate seen in reperfused hearts from diabetic rats were prevented by PLC. State 3 respiration in the presence of palmitoylcarnitine was also improved in the ischemic reperfused diabetic rat heart. Our results show that PLC improves recovery of mechanical function following ischemia in the diabetic rat heart. The beneficial effects of PLC are associated with enhanced mitochondrial oxidation of fuels.

Synthesis, characterization, and biological relevance of hydroxypyrone and hydroxypyridinone complexes of molybdenum

We have prepared a number of complexes of the type cis-MoO2L2 where L represents a hydroxypyronato or hydroxypyridinonato ligand. Both the maltol (3-hydroxy-2-methyl-4-pyrone, Hma) and kojic acid (5-hydroxy-2-hydroxymethyl-4-pyrone, Hka) complexes, cis-MoO2(ma)2 (1) and cis-MoO2(ka)2 (2), have been characterized by X-ray diffraction studies. The pyrone ligands are bound to molybdenum in a cis bidentate fashion via the deprotonated hydroxyl groups and the ketone moieties. Crystals of 1 are orthorhombic, a = 12.107 (1), b = 8.6169 (8), c = 16.472 (1) Å, Z = 4, space group Pca21, and those of 2 are monoclinic, a = 8.4591 (5), b = 16.3453 (10), c = 10.2954 (7) Å, β = 103.0320 (10)°, Z = 4, space group P21/c. Hydroxypyridinone molybdenum complexes have been prepared for both maltol and kojic acid derivatives with the substituents Me, n-Pr, CH2Ph, Ph at the ring nitrogen. Crystals of the 3-hydroxy-2-methyl-1-phenyl-4-pyridinone (Hppp) derivative, MoO2(ppp)2 (9), are monoclinic, a = 10.9476 (6), b = 13.5353 (9), c = 17.4877 (10) Å, β = 93.465 (4)°, Z = 4, space group P21/n. Initial investigations into the effects molybdenum compounds have on diabetic hearts are presented. Both Na2MoO4 (used as a control) and 1 were effective in lowering blood glucose and free fatty acid levels. Diabetic rats treated with molybdate showed significant improvements in postischemic cardiac function.Key words: molybdenum, hydroxypyrones, hydroxypyridinones, heart function.

Intracellular pH control mechanisms in the diabetic myocardium

Intracellular pH (pHi) regulation and the consequences of this regulation may play a crucial role during ischaemia and especially on reperfusion after an ischaemic episode. A significant decrease in Na+/H+ exchange activity was first demonstrated in papillary muscles isolated from hearts of diabetic rats, and recently confirmed in diabetic rat isolated ventricular myocytes. This depressed activity of one of the major pHi regulatory mechanisms may afford some protection against ischaemia/reperfusion injury. A protection was indeed observed in perfused hearts isolated from diabetic rats, in which there was a markedly improved recovery of contractility following ischaemia, comparable to that obtained with a pharmacological block of Na+/H+ exchange in normal hearts. This was associated with a markedly slower pHi recovery.