Measurement of stable isotopic enrichment and concentration of long-chain fatty acyl-carnitines in tissue by HPLC-MS

Characterisation of Aberrant Metabolic Pathways in Hepatoblastoma Using Liquid Chromatography and Tandem Mass Spectrometry (LC-MS/MS)

Hepatoblastoma (HB) is a rare childhood tumour with an evolving molecular landscape. We present the first comprehensive metabolomic analysis using untargeted and targeted liquid chromatography coupled to high-resolution tandem mass spectrometry (LC-MS/MS) of paired tumour and non-tumour surgical samples in HB patients (n = 8 pairs). This study demonstrates that the metabolomic landscape of HB is distinct from that of non-tumour (NT) liver tissue, with 35 differentially abundant metabolites mapping onto pathways such as fatty acid transport, glycolysis, the tricarboxylic acid (TCA) cycle, branched-chain amino acid degradation and glutathione synthesis. Targeted metabolomics demonstrated reduced short-chain acylcarnitines and a relative accumulation of branched-chain amino acids. Medium- and long-chain acylcarnitines in HB were similar to those in NT. The metabolomic changes reported are consistent with previously reported transcriptomic data from tumour and non-tumour samples (49 out of 54 targets) as well as metabolomic data obtained using other techniques. Gene set enrichment analysis (GSEA) from RNAseq data (n = 32 paired HB and NT samples) demonstrated a downregulation of the carnitine metabolome and immunohistochemistry showed a reduction in CPT1a (n = 15 pairs), which transports fatty acids into the mitochondria, suggesting a lack of utilisation of long-chain fatty acids in HB. Thus, our findings suggest a reduced metabolic flux in HB which is corroborated at the gene expression and protein levels. Further work could yield novel insights and new therapeutic targets.

Metabolomic exploration of the effects of habituation to livestock trailer and extended transportation in goats

Goats raised for meat production are often transported long distances. Twelve-month-old male Spanish goats were used to determine the effects of habituation to trailers on plasma metabolomic profiles when transported for extended periods. In a split-plot design, 168 goats were separated into two treatment (TRT; whole plot) groups and maintained on two different paddocks. Concentrate supplement was fed to one group inside two livestock trailers (habituated group, H), while the other group received the same quantity of concentrate, but not inside the trailers (non-habituated, NH). Goats were subjected to a 10-h transportation stress in 4 replicates (n = 21 goats/replicate/TRT) after 4 weeks of habituation period. Blood samples were collected prior to loading, 20 min after loading (0 h), and at 2, 4, 6, 8, and 10 h of transportation (Time; subplot). A targeted quantitative metabolomics approach was employed to analyze the samples. The data were analyzed using R software and MIXED procedures in SAS. Several amino acids (alanine, serine, glycine, histidine, glutamate, trans-hydroxyproline, asparagine, threonine, methylhistidine, ornithine, proline, leucine, tryptophan) were higher (p < 0.05) in the H group compared to the NH group. Six long-chain acylcarnitines were higher (p < 0.05), while free (C0) and short-chain (C3, C5) carnitines were lower (p < 0.05) in the NH goats compared to the H goats. In general, amino acid concentrations decreased and long-chain acylcarnitine (>C10) levels increased with transportation time (p < 0.05). Butyric acid, α-ketoglutaric acid, and α-aminoadipic acid concentrations were lower (p < 0.05) and β-hydroxybutyric acid concentrations were higher in the NH goats compared to the H goats. Plasma glucose, non-esterified fatty acid (NEFA) and urea nitrogen concentrations were significantly influenced by Time (p < 0.01). Plasma NEFA concentrations were significantly lower (p < 0.01) in the H group than the NH group. Habituation to trailers can be beneficial in enhancing stress coping abilities in goats due to higher concentrations of metabolites such as butyrate and certain amino acids that support antioxidant activities and immune function. Plasma long-chain acylcarnitines may be good indicators of stress during long-distance transportation in goats.

A New Peritoneal Dialysis Solution Containing L-Carnitine and Xylitol for Patients on Continuous Ambulatory Peritoneal Dialysis: First Clinical Experience

Peritoneal dialysis (PD) is a feasible and effective renal replacement therapy (RRT) thanks to the dialytic properties of the peritoneal membrane (PM). Preservation of PM integrity and transport function is the key to the success of PD therapy, particularly in the long term, since the prolonged exposure to unphysiological hypertonic glucose-based PD solutions in current use is detrimental to the PM, with progressive loss of peritoneal ultrafiltration capacity causing technique failure. Moreover, absorbing too much glucose intraperitoneally from the dialysate may give rise to a number of systemic metabolic effects. Here we report the preliminary results of the first clinical experience based on the use in continuous ambulatory PD (CAPD) patients of novel PD solutions obtained through partly replacing the glucose load with other osmotically active metabolites, such as L-carnitine and xylitol. Ten CAPD patients were treated for four weeks with the new solutions. There was good tolerance to the experimental PD solutions, and no adverse safety signals were observed. Parameters of dialysis efficiency including creatinine clearance and urea Kt/V proved to be stable as well as fluid status, diuresis, and total peritoneal ultrafiltration. The promising tolerance and local/systemic advantages of using L-carnitine and xylitol in the PD solution merit further research.

Advances in stable isotope tracer methodology part 1: hepatic metabolism via isotopomer analysis and postprandial lipolysis modeling

Stable isotope tracers have been used to gain an understanding of integrative animal and human physiology. More commonly studied organ systems include hepatic glucose metabolism, lipolysis from adipose tissue, and whole body protein metabolism. Recent improvements in isotope methodology have included the use of novel physiologic methods/models and mathematical modeling of data during different physiologic states. Here we review some of the latest advancements in this field and highlight future research needs. First we discuss the use of an oral [U-13C3]-glycerol tracer to determine the relative contribution of glycerol carbons to hepatic glucose production after first cycling through the tricarboxylic acid cycle, entry of glycerol into the pentose phosphate pathway or direct conversion of glycerol into the glucose. Second, we describe an adaptation of the established oral minimal model used to define postprandial glucose dynamics to include glycerol dynamics in an oral glucose tolerance test with a [2H5]-glycerol tracer to determine dynamic changes in lipolysis. Simulation results were optimized when parameters describing glycerol flux were determined with a hybrid approach using both tracer-based calculations and constrained parameter optimization. Both of these methodologies can be used to expand our knowledge of not only human physiology, but also the effects of various nutritional strategies and medications on metabolism.

The Impact of OMEGA-3 Fatty Acids Supplementation on Insulin Resistance and Content of Adipocytokines and Biologically Active Lipids in Adipose Tissue of High-Fat Diet Fed Rats

It has been established that OMEGA-3 polyunsaturated fatty acids (PUFAs) may improve lipid and glucose homeostasis and prevent the "low-grade" state of inflammation in animals. Little is known about the effect of PUFAs on adipocytokines expression and biologically active lipids accumulation under the influence of high-fat diet-induced obesity. The aim of the study was to examine the effect of fish oil supplementation on adipocytokines expression and ceramide (Cer) and diacylglycerols (DAG) content in visceral and subcutaneous adipose tissue of high-fat fed animals. The experiments were carried out on Wistar rats divided into three groups: standard diet-control (SD), high-fat diet (HFD), and high-fat diet + fish oil (HFD+FO). The fasting plasma glucose and insulin concentrations were examined. Expression of carnitine palmitoyltransferase 1 (CPT1) protein was determined using the Western blot method. Plasma adipocytokines concentration was measured using ELISA kits and mRNA expression was determined by qRT-PCR reaction. Cer, DAG, and acyl-carnitine (A-CAR) content was analyzed by UHPLC/MS/MS. The fish oil supplementation significantly decreased plasma insulin concentration and Homeostatic Model Assesment for Insulin Resistance (HOMA-IR) index and reduced content of adipose tissue biologically active lipids in comparison with HFD-fed subjects. The expression of CPT1 protein in HFD+FO in both adipose tissues was elevated, whereas the content of A-CAR was lower in both HFD groups. There was an increase of adiponectin concentration and expression in HFD+FO as compared to HFD group. OMEGA-3 fatty acids supplementation improved insulin sensitivity and decreased content of Cer and DAG in both fat depots. Our results also demonstrate that PUFAs may prevent the development of insulin resistance in response to high-fat feeding and may regulate the expression and secretion of adipocytokines in this animal model.

Function, Detection and Alteration of Acylcarnitine Metabolism in Hepatocellular Carcinoma

Acylcarnitines play an essential role in regulating the balance of intracellular sugar and lipid metabolism. They serve as carriers to transport activated long-chain fatty acids into mitochondria for β-oxidation as a major source of energy for cell activities. The liver is the most important organ for endogenous carnitine synthesis and metabolism. Hepatocellular carcinoma (HCC), a primary malignancy of the live with poor prognosis, may strongly influence the level of acylcarnitines. In this paper, the function, detection and alteration of acylcarnitine metabolism in HCC were briefly reviewed. An overview was provided to introduce the metabolic roles of acylcarnitines involved in fatty acid β-oxidation. Then different analytical platforms and methodologies were also briefly summarised. The relationship between HCC and acylcarnitine metabolism was described. Many of the studies reported that short, medium and long-chain acylcarnitines were altered in HCC patients. These findings presented current evidence in support of acylcarnitines as new candidate biomarkers for studies on the pathogenesis and development of HCC. Finally we discussed the challenges and perspectives of exploiting acylcarnitine metabolism and its related metabolic pathways as a target for HCC diagnosis and prognosis.

Quantification of muscle triglyceride synthesis rate requires an adjustment for total triglyceride content

Intramyocellular triglyceride (imTG) in skeletal muscle plays a significant role in metabolic health, and an infusion of [¹³C₁₆]palmitate can be used to quantitate the in vivo fractional synthesis rate (FSR) and absolute synthesis rate (ASR) of imTGs. However, the extramyocellular TG (emTG) pool, unless precisely excised, contaminates the imTG pool, diluting the imTG-bound tracer enrichment and leading to underestimation of FSR. Because of the difficulty of excising the emTGs precisely, it would be advantageous to be able to calculate the imTG synthesis rate without dissecting the emTGs from each sample. Here, we tested the hypothesis that the ASR of total TGs (tTGs), a combination of imTGs and emTGs, calculated as "FSR × tTG pool," reasonably represents the imTG synthesis. Muscle lipid parameters were measured in nine healthy women at 90 and 170 min after the start of [¹³C₁₆]palmitate infusion. While the measurements of tTG content, enrichment, and FSR did not correlate (P > 0.05), those of the tTG ASR were significantly correlated (r = 0.947, P < 0.05). These results demonstrate that when imTGs and emTGs are pooled, the resulting underestimation of imTG FSR is balanced by the overestimation of the imTG content. We conclude that imTG metabolism is reflected by the measurement of the tTG ASR.

The effect of high-fat diet and inhibition of ceramide production on insulin action in liver

Liver, as one of the most important organs involved in lipids and glucose metabolism, is perceived as a key tissue for pharmacotherapy of insulin resistance (IRes) and type 2 diabetes. Ceramides (Cer) are biologically active lipids, which accumulation is associated with the induction of muscle IRes. We sought to determine the role of intrahepatic bioactive lipids production on insulin action in liver of insulin-resistant rats and after myriocin administration. The experiments were conducted on male Wistar rats divided into three groups: Control, fed high-fat diet (HFD), and fed HFD and treated with myriocin (HFD/Myr). Before sacrifice, the animals were infused with a [U-¹³ C]palmitate to calculate lipid synthesis rate by means of tracer incorporation technique in particular lipid groups. Liver Cer, diacylglycerols (DAG), acyl-carnitine concentration, and isotopic enrichment were analyzed by LC/MS/MS. Proteins involved in lipid metabolism and insulin pathway were analyzed by western blot analysis. An OGTT and ITT was also performed. HFD-induced IRes and increased both the synthesis rate and the content of DAG and Cer, which was accompanied by inhibition of an insulin pathway. Interestingly, myriocin treatment reduced synthesis rate not only of Cer but also DAG and improved insulin sensitivity. We conclude that the insulin-sensitizing action of myriocin in the liver is a result of the lack of inhibitory effect of lipids on the insulin pathway, due to the reduction of their synthesis rate. This is the first study showing how the synthesis rate of individual lipid groups in liver changes after myriocin administration.

The effect of high fat diet and metformin treatment on liver lipids accumulation and their impact on insulin action

We sought to determine whether metformin treatment reverses a high-fat diet (HFD)-induced hepatic insulin resistance (IRes) and to identify lipid intermediates involved in induction of liver IRes. The experiments were conducted on male Wistar rats divided into three groups: 1. Control, 2. fed HFD and 3. fed HFD and treated with metformin. The animals were infused with a [U-¹³C]palmitate to measure fractional lipid synthesis rate. This allowed for the calculation of fractional synthesis rate of signaling lipids (FSR) through the estimation of their isotopic enrichment. Liver ceramide (Cer), diacylglycerol (DAG) and acyl-carnitine concentration and enrichment were analyzed by LC/MS/MS. The content of proteins involved in lipid metabolism and insulin signaling were analyzed by Western Blot. HFD treatment increased the content and FSR of DAG and Cer in the liver which was accompanied by systemic insulin resistance and inhibition of hepatic insulin signaling pathway under insulin stimulation. Metformin treatment ameliorated systemic insulin resistance and augmented the hepatic insulin signaling cascade. It reduced both the concentration and FSR of Cer, DAG, and increased acyl-carnitine content and the expression of mitochondrial markers. We postulate, that in liver, the insulin sensitizing effect of metformin depends on augmentation of mitochondrial β-oxidation, which protects from hepatic accumulation of both the Cer and DAG and preserves insulin sensitivity under HFD consumption. Moreover, we showed that hepatic content of Cer and DAG corresponds with their respective FSR.

Comprehensive Analysis of Acylcarnitine Species in db/db Mouse Using a Novel Method of High-Resolution Parallel Reaction Monitoring Reveals Widespread Metabolic Dysfunction Induced by Diabetes

Acylcarnitines are exerting a variety of biological functions depending on the differences in lengths, saturation levels, and conjugation groups, which to a great extent contribute to the challenges of acylcarnitines quantifications due to various kinds of isomers. Here, we describe a novel method by using high-resolution parallel reaction monitoring (PRM) liquid chromatography-tandem mass spectrometry (LC-MS/MS). Both reversed-phase and normal-phase column were used in order to get accurate, reliable, widespread quantification of acylcarnitines, and without tedious sample preparation procedure. The method provided the most comprehensive acylcarnitine profile with high-resolution MS and MS/MS confirmation to date. A total of 117 acylcarnitines were detected from plasma and urine samples. The application of targeted profiling of acylcarnitines in db/m+ control and db/db diabetic mice indicated incomplete amino acid and fatty acid oxidation on diabetic mice. Interestingly, the reduction of medium odd-numbered chain acylcarnitines in urine samples was first observed between db/m+ and db/db mice. The high-resolution PRM method makes it possible to monitor the widespread metabolic changes of the acylcarnitines in response to stimuli. Besides, the accurate MS and MS/MS spectra data of the 117 acylcarnitines could be used as mass spectrometric resources for the identification of acylcarnitines.

Effect of metformin on bioactive lipid metabolism in insulin-resistant muscle

Intramuscular accumulation of bioactive lipids leads to insulin resistance and type 2 diabetes (T2D). There is lack of consensus concerning which of the lipid mediators has the greatest impact on muscle insulin action in vivo Our aim was to elucidate the effects of high-fat diet (HFD) and metformin (Met) on skeletal muscle bioactive lipid accumulation and insulin resistance (IR) in rats. We employed a [U-¹³C]palmitate isotope tracer and mass spectrometry to measure the content and fractional synthesis rate (FSR) of intramuscular long-chain acyl-CoA (LCACoA), diacylglycerols (DAG) and ceramide (Cer). Eight weeks of HFD-induced intramuscular accumulation of LCACoA, DAG and Cer accompanied by both systemic and skeletal muscle IR. Metformin treatment improved insulin sensitivity at both systemic and muscular level by the augmentation of Akt/PKB and AS160 phosphorylation and decreased the content of DAG and Cer and their respective FSR. Principal component analysis (PCA) of lipid variables revealed that altered skeletal muscle IR was associated with lipid species containing 18-carbon acyl-chain, especially with C18:0-Cer, C18:1-Cer, 18:0/18:2-DAG and 18:2/18:2-DAG, but not palmitate-derived lipids. It is concluded that the insulin-sensitizing action of metformin in skeletal muscle is associated with decreased 18-carbon acyl-chain-derived bioactive lipids.

Accumulation of Palmitoylcarnitine and Its Effect on Pro-Inflammatory Pathways and Calcium Influx in Prostate Cancer

BACKGROUND

Acylcarnitines are intermediates of fatty acid oxidation and accumulate as a consequence of the metabolic dysfunction resulting from the insufficient integration between β-oxidation and the tricarboxylic acid (TCA) cycle. The aim of this study was to investigate whether acylcarnitines accumulate in prostate cancer tissue, and whether their biological actions could be similar to those of dihydrotestosterone (DHT), a structurally related compound associated with cancer development.

METHODS

Levels of palmitoylcarnitine (palcar), a C16:00 acylcarnitine, were measured in prostate tissue using LC-MS/MS. The effect of palcar on inflammatory cytokines and calcium (Ca(2+) ) influx was investigated in in vitro models of prostate cancer.

RESULTS

We observed a significantly higher level of palcar in prostate cancerous tissue compared to benign tissue. High levels of palcar have been associated with increased gene expression and secretion of the pro-inflammatory cytokine IL-6 in cancerous PC3 cells, compared to normal PNT1A cells. Furthermore, we found that high levels of palcar induced a rapid Ca(2+) influx in PC3 cells, but not in DU145, BPH-1, or PNT1A cells. This pattern of Ca(2+) influx was also observed in response to DHT. Through the use of whole genome arrays we demonstrated that PNT1A cells exposed to palcar or DHT have a similar biological response.

CONCLUSIONS

This study suggests that palcar might act as a potential mediator for prostate cancer progression through its effect on (i) pro-inflammatory pathways, (ii) Ca(2+) influx, and (iii) DHT-like effects. Further studies need to be undertaken to explore whether this class of compounds has different biological functions at physiological and pathological levels. Prostate 76:1326-1337, 2016. © 2016 The Authors. The Prostate published by Wiley Periodicals, Inc.

Palmitoyl-carnitine production by blood cells associates with the concentration of circulating acyl-carnitines in healthy overweight women

BACKGROUND

Circulating acyl-carnitines (acyl-CNTs) are associated with insulin resistance (IR) and type 2 diabetes (T2D) in both rodents and humans. However, the mechanisms whereby circulating acyl-CNTs are increased in these conditions and their role in whole-body metabolism remains unknown. The purpose of this study was to determine if, in humans, blood cells contribute in production of circulating acyl-CNTs and associate with whole-body fat metabolism.

METHODS AND RESULTS

Eight non-diabetic healthy women (age: 47 ± 19 y; BMI: 26 ± 1 kg·m^-2) underwent stable isotope tracer infusion and hyperinsulinemic-euglycemic clamp study to determine in vivo whole-body fatty acid flux and insulin sensitivity. Blood samples collected at baseline (0 min) and after 3 h of clamp were used to determine the synthesis rate of palmitoyl-carnitine (palmitoyl-CNT) in vitro. The fractional synthesis rate of palmitoyl-CNT was significantly higher during hyperinsulinemia (0.788 ± 0.084 vs. 0.318 ± 0.012%·hr^-1, p = 0.001); however, the absolute synthesis rate (ASR) did not differ between the periods (p = 0.809) due to ∼30% decrease in blood palmitoyl-CNT concentration (p = 0.189) during hyperinsulinemia. The ASR of palmitoyl-CNT significantly correlated with the concentration of acyl-CNTs in basal (r = 0.992, p < 0.001) and insulin (r = 0.919, p = 0.001) periods; and the basal ASR significantly correlated with plasma palmitate oxidation (r = 0.764, p = 0.027).

CONCLUSION

In women, blood cells contribute to plasma acyl-CNT levels and the acyl-CNT production is linked to plasma palmitate oxidation, a marker of whole-body fat metabolism. Future studies are needed to confirm the role of blood cells in acyl-CNT and lipid metabolism under different physiological (i.e., in response to meal) and pathological (i.e., hyperlipidemia, IR and T2D) conditions.

Triglycerides produced in the livers of fasting rabbits are predominantly stored as opposed to secreted into the plasma

OBJECTIVE

The liver plays a central role in regulating fat metabolism; however, it is not clear how the liver distributes the synthesized triglycerides (TGs) to storage and to the plasma.

MATERIALS AND METHODS

We have measured the relative distribution of TGs produced in the liver to storage and the plasma by means of U-(13)C(16)-palmitate infusion in anesthetized rabbits after an overnight fast.

RESULTS

The fractional synthesis rates of TGs stored in the liver and secreted into the plasma were not significantly different (stored vs. secreted: 31.9 ± 0.8 vs. 27.7 ± 2.6%∙h(-1), p > 0.05). However, the absolute synthesis rates of hepatic stored and secreted TGs were 543 ± 158 and 27 ± 7 nmol∙kg(-1)∙min(-1) respectively, indicating that in fasting rabbits the TGs produced in the liver were predominately stored (92 ± 3%) rather than secreted (8 ± 3%) into the plasma. This large difference was mainly due to the larger pool size of the hepatic TGs which was 21 ± 9-fold that of plasma TGs. Plasma free fatty acids (FFAs) contributed 47 ± 1% of the FA precursor for hepatic TG synthesis, and the remaining 53 ± 1% was derived from hepatic lipid breakdown and possibly plasma TGs depending on the activity of hepatic lipase. Plasma palmitate concentration significantly correlated with hepatic palmitoyl-CoA and TG synthesis.

CONCLUSION

In rabbits, after an overnight fast, the absolute synthesis rate of hepatic stored TGs was significantly higher than that of secreted due to the larger pool size of hepatic TGs. The net synthesis rate of TG was approximately half the absolute rate. Plasma FFA is a major determinant of hepatic TG synthesis, and therefore hepatic TG storage.

Fish oil omega-3 fatty acids partially prevent lipid-induced insulin resistance in human skeletal muscle without limiting acylcarnitine accumulation

Intravenous infusion of lipid into healthy males caused insulin resistance. Addition of fish oil omega-3 polyunsaturated fatty acids to the lipid infusion partially prevented the insulin resistance. This effect was not due to differences in muscle acylcarnitine content.

Separation of carnitine and acylcarnitines in biological samples: a review

Carnitine and its acylesters are a family of compounds that can be used in the early diagnosis of many diseases. Carnitine and acylcarnitines have a crucial role in fatty acid transportation. The increased level of free carnitine, total carnitine, or the acylesters can act as biomarkers for many metabolic disorders, including diabetes, encephalopathy and cardiomyopathy. The determination of these compounds is difficult owing to the simple aliphatic structure, the chiral center and the permanent positive charge. Although MS detection can be enough to differentiate between some carnitine derivatives, closely related structural isomers of the acylcarnitines must be separated before detection because they form the same base peak and second most abundant ion peak. Different separation methods are discussed in this review, including reversed-phase, hydrophilic interaction, ion exchange, ion pairing, mixed mode liquid chromatography, gas chromatography and electrophoresis. Representative example chromatograms are shown. The sample preparation and the different derivatization reactions are also covered. A table that summarizes the most important analytical methods by detailing the analyte mixture, the sample matrix, the separation mode and the detection method is provided.

Effect of an L-carnitine-containing peritoneal dialysate on insulin sensitivity in patients treated with CAPD: a 4-month, prospective, multicenter randomized trial

BACKGROUND

In peritoneal dialysis, the high glucose load absorbed from dialysis fluid contributes to several metabolic abnormalities, including insulin resistance. We evaluate the efficacy of a peritoneal dialysis solution containing l-carnitine as an additive to improve insulin sensitivity.

STUDY DESIGN

Multicenter parallel randomized controlled trial.

SETTING & PARTICIPANTS

Nondiabetic uremic patients on continuous ambulatory peritoneal dialysis enrolled in 8 peritoneal dialysis centers.

INTERVENTION

Patients were randomly assigned to receive peritoneal dialysis diurnal exchanges with either a standard glucose-based solution (1.5% or 2.5% according to the patient's need) or a glucose-based solution (identical glucose amount) enriched with l-carnitine (0.1%, weight/volume; 2 g/bag) for 4 months, the nocturnal exchange with icodextrin being unmodified.

OUTCOMES & MEASUREMENTS

The primary outcome was insulin sensitivity, measured by the magnitude of change from baseline in glucose infusion rate (in milligrams per kilogram of body weight per minute) during a euglycemic hyperinsulinemic clamp. Secondary outcomes were safety and tolerability, body fluid management, peritoneal dialysis efficiency parameters, and biochemistry tests.

RESULTS

35 patients were randomly assigned, whereas 27 patients (standard solution, n=12; experimental solution, n = 15) were analyzed. Adverse events were not attributable to treatment. Glucose infusion rates in the l-carnitine-treated group increased from 3.8 ± 2.0 (SD) mg/kg/min at baseline to 5.0 ± 2.2 mg/kg/min at day 120 (P = 0.03) compared with 4.8 ± 2.4 mg/kg/min at baseline and 4.7 ± 2.4 mg/kg/min at day 120 observed in the control group (P = 0.8). The difference in glucose infusion rates between groups was 1.3 (95% CI, 0.0-2.6) mg/kg/min. In patients treated with l-carnitine-containing solution, urine volume did not change significantly (P = 0.1) compared to a significant diuresis reduction found in the other group (P = 0.02). For peritoneal function, no differences were observed during the observation period.

LIMITATIONS

Small sample size.

CONCLUSIONS

The use of l-carnitine in dialysis solutions may represent a new approach to improving insulin sensitivity in nondiabetic peritoneal dialysis patients.

Acute hyperinsulinemia and reduced plasma free fatty acid levels decrease intramuscular triglyceride synthesis

OBJECTIVE

To investigate the effect of acute hyperinsulinemia and the resulting decrease in plasma free fatty acid (FFA) concentrations on intramuscular TG synthesis.

MATERIALS/METHODS

U-(13)C(16)-palmitate was infused for 3 h in anesthetized rabbits after overnight food deprivation. Arterial blood and leg muscle were sampled during the tracer infusion. Plasma samples were analyzed for free and TG-bound palmitate enrichments and concentrations. The enrichments and concentrations of palmitoyl-CoA and palmitoyl-carnitine as well as the enrichment of palmitate bound to TG were measured in muscle samples. Fractional synthetic rate (FSR) of intramuscular TG was calculated using the tracer incorporation method. The rabbits were divided into a control group and a hyperinsulinemic euglycemic clamp group. Insulin infusion decreased the rate of appearance of plasma free palmitate (2.00±0.15 vs 0.68±0.20 μmol⋅kg(-1)⋅min(-1); P<.001), decreased plasma FFA concentration (327±61 vs 72±25 nmol/mL; P<.01), decreased the total concentration of intramuscular fatty acyl-CoA plus fatty acyl-carnitine (12.1±1.6 vs 7.0±0.7 nmol/g; P<.05), and decreased intramuscular TG FSR (0.48±0.05 vs 0.21±0.06%/h; P<.01) in comparison with the control group. Intramuscular TG FSR was correlated (P<.01) with both plasma FFA concentrations and intramuscular fatty acyl-CoA concentrations.

CONCLUSIONS

Fatty acid availability is a determinant of intramuscular TG synthesis. Insulin infusion decreases plasma and intramuscular fatty acid availability and thereby decreases TG synthesis.

Measurement of precursor enrichment for calculating intramuscular triglyceride fractional synthetic rate

Our goal was to assess the validity of the enrichments of plasma free palmitate and intramuscular (IM) fatty acid metabolites as precursors for calculating the IM triglyceride fractional synthetic rate. We infused U-¹³C₁₆-palmitate in anesthetized rabbits for 3 h and sampled adductor muscle of legs using both freeze-cut and cut-freeze approaches. We found that IM free palmitate enrichment (0.70 ± 0.07%) was lower (P < 0.0001) than IM palmitoyl-CoA enrichment (2.13 ± 0.17%) in samples taken by the freeze-cut approach. The latter was close (P = 0.33) to IM palmitoyl-carnitine enrichment (2.42 ± 0.16%). The same results were obtained from the muscle samples taken by the cut-freeze approach, except the enrichment of palmitoyl-CoA (2.21 ± 0.08%) was lower (P = 0.02) than that of palmitoyl-carnitine (2.77 ± 0.17%). Plasma free palmitate enrichment was ∼2-fold that of IM palmitoyl-CoA enrichment and palmitoyl-carnitine enrichment (P < 0.001). These findings indicate that plasma free palmitate overestimated IM precursor enrichment owing to in vivo IM lipid breakdown, whereas IM free palmitate enrichment underestimated the precursor enrichment because of lipid breakdown during muscle sampling and processing. IM palmitoyl-carnitine enrichment was an acceptable surrogate of the precursor enrichment because it was less affected by in vitro lipid breakdown after sampling.

L-carnitine is an osmotic agent suitable for peritoneal dialysis

Excessive intraperitoneal absorption of glucose during peritoneal dialysis has both local cytotoxic and systemic metabolic effects. Here we evaluate peritoneal dialysis solutions containing L-carnitine, an osmotically active compound that induces fluid flow across the peritoneum. In rats, L-carnitine in the peritoneal cavity had a dose-dependent osmotic effect similar to glucose. Analogous ultrafiltration and small solute transport characteristics were found for dialysates containing 3.86% glucose, equimolar L-carnitine, or combinations of both osmotic agents in mice. About half of the ultrafiltration generated by L-carnitine reflected facilitated water transport by aquaporin-1 (AQP1) water channels of endothelial cells. Nocturnal exchanges with 1.5% glucose and 0.25% L-carnitine in four patients receiving continuous ambulatory peritoneal dialysis were well tolerated and associated with higher net ultrafiltration than that achieved with 2.5% glucose solutions, despite the lower osmolarity of the carnitine-containing solution. Addition of L-carnitine to endothelial cells in culture increased the expression of AQP1, significantly improved viability, and prevented glucose-induced apoptosis. In a standard toxicity test, the addition of L-carnitine to peritoneal dialysis solution improved the viability of L929 fibroblasts. Thus, our studies support the use of L-carnitine as an alternative osmotic agent in peritoneal dialysis.

Relationship between plasma free fatty acid, intramyocellular triglycerides and long-chain acylcarnitines in resting humans

We hypothesized that plasma non-esterified fatty acids (NEFA) are trafficked directly to intramyocellular long-chain acylcarnitines (imLCAC) rather than transiting intramyocellular triglycerides (imTG) on the way to resting muscle fatty acid oxidation. Overnight fasted adults (n = 61) received intravenous infusions of [U-(13)C]palmitate (0400-0830 h) and [U-(13)C]oleate (0800-1400 h) labelling plasma NEFA, imTG, imLCAC and im-non-esterified FA (imNEFA). Two muscle biopsies (0830 and 1400 h) were performed following 6 h, overlapping, sequential palmitate/oleate tracer infusions. Enrichment of plasma palmitate was approximately 15 times greater than enrichment of imTG, imNEFA-palmitate and im-palmitoyl-carnitine. Fatty acid enrichment in LCAC was correlated with imTG and imNEFA; there was a significant correlation between imTG concentrations and imLCAC concentrations in women (r = 0.51, P = 0.005), but not men (r = 0.30, P = 0.11). We estimated that approximately 11% of NEFA were stored in imTG. imTG NEFA storage was correlated only with NEFA concentrations (r = 0.52, P = 0.004) in women and with V(O(2),peak) (r = 0.45, P = 0.02) in men. At rest, plasma NEFA are trafficked largely to imTG before they enter LCAC oxidative pools; thus, imTG are an important, central pool that regulates the delivery of fatty acids to the intracellular environment. Factors relating to plasma NEFA storage into imTG differ in men and women.

The synthetic rate of muscle triglyceride but not phospholipid is increased in obese rabbits

Fat is a major energy source for skeletal muscle, and disruption of normal trafficking of fatty acids in muscle is linked to insulin resistance. We quantified muscle triglyceride (TG) and phospholipid (PL) synthesis in lean and obese rabbits by means of l-[U-(13)C(16)]palmitate infusion. Intramyocellular palmitoyl-coenzyme A was used as the precursor for rates of TG and PL synthesis, which were compared with the rates calculated using plasma nonesterified palmitate as the precursor. The muscle of obese rabbits had a greater (P < .05) combined pool of fatty acyl-coenzyme A plus fatty acyl-carnitine than lean rabbits (40.9 +/- 3.7 vs 28.6 +/- 5.3 nmol/g). Although the fractional synthetic rates of muscle TG were almost identical (0.095%/h +/- 0.016%/h vs 0.092%/h +/- 0.019%/h), the absolute synthetic rates were greater (P < .01) in the obese than in lean rabbits (39.7 +/- 9.5 vs 10.1 +/- 2.5 nmol g(-1) h(-1)) because of greater TG content in the muscle of obese rabbits. Plasma nonesterified fatty acids and TG accounted for 51% to 55% of the true precursor pool for muscle lipid synthesis in both groups, and the rest was derived from fatty acids recycled within the muscle. In contrast, the fractional and absolute synthetic rates of muscle PL as well as PL contents were comparable in the 2 groups. In conclusion, the content and synthetic rate of muscle TG rather than PL were increased in obese rabbits, which might be linked to insulin resistance. Plasma lipids and muscle lipolysis were the 2 predominate contributors to the intramyocellular fatty acyl-coenzyme A pool for lipid synthesis.

Lipidomics: practical aspects and applications

Lipidomics is the characterization of the molecular species of lipids in biological samples. The polar lipids that comprise the bilayer matrix of the constituent cell membranes of living tissues are highly complex and number many hundreds of distinct lipid species. These differ in the nature of the polar group representing the different classes of lipid. Each class consists of a range of molecular species depending on the length, position of attachment and number of unsaturated double bonds in the associated fatty acids. The origin of this complexity is described and the biochemical processes responsible for homeostasis of the lipid composition of each morphologically-distinct membrane is considered. The practical steps that have been developed for the isolation of membranes and the lipids there from, their storage, separation, detection and identification by liquid chromatography coupled to mass spectrometry are described. Application of lipidomic analyses and examples where clinical screening for lipidoses in collaboration with mass spectrometry facilities are considered from the user point of view.

PPAR-alpha agonism improves whole body and muscle mitochondrial fat oxidation, but does not alter intracellular fat concentrations in burn trauma children in a randomized controlled trial

Background

Insulin resistance is often associated with increased levels of intracellular triglycerides, diacylglycerol and decreased fat β-oxidation. It was unknown if this relationship was present in patients with acute insulin resistance induced by trauma.

Methods

A double blind placebo controlled trial was conducted in 18 children with severe burn injury. Metabolic studies to assess whole body palmitate oxidation and insulin sensitivity, muscle biopsies for mitochondrial palmitate oxidation, diacylglycerol, fatty acyl Co-A and fatty acyl carnitine concentrations, and magnetic resonance spectroscopy for muscle and liver triglycerides were compared before and after two weeks of placebo or PPAR-α agonist treatment.

Results

Insulin sensitivity and basal whole body palmitate oxidation as measured with an isotope tracer increased significantly (P = 0.003 and P = 0.004, respectively) after PPAR-α agonist treatment compared to placebo. Mitochondrial palmitate oxidation rates in muscle samples increased significantly after PPAR-α treatment (P = 0.002). However, the concentrations of muscle triglyceride, diacylglycerol, fatty acyl CoA, fatty acyl carnitine, and liver triglycerides did not change with either treatment. PKC-θ activation during hyper-insulinemia decreased significantly following PPAR-α treatment.

Conclusion

PPAR-α agonist treatment increases palmitate oxidation and decreases PKC activity along with reduced insulin sensitivity in acute trauma, However, a direct link between these responses cannot be attributed to alterations in intracellular lipid concentrations.

High-precision isotopic analysis of palmitoylcarnitine by liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry

Single quadrupole gas chromatography/mass spectrometry (GC/MS) has been widely used for isotopic analysis in metabolic investigations using stable isotopes as tracers. However, its inherent shortcomings prohibit it from broader use, including low isotopic precision and the need for chemical derivatization of the analyte. In order to improve isotopic detection power, liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry (LC/ESI-itMS2) has been evaluated for its isotopic precision and chemical sensitivity for the analysis of [13C]palmitoylcarnitine. Over the enrichment range of 0.4-10 MPE (molar % excess), the isotopic response of LC/ESI-itMS2 to [13C]palmitoylcarnitine was linear (r = 1.00) and the average isotopic precision (standard deviation, SD) was 0.11 MPE with an average coefficient of variation (CV) of 5.6%. At the lower end of isotopic enrichments (0.4-0.9 MPE), the isotopic precision was 0.05 MPE (CV = 8%). Routine analysis of rat skeletal muscle [13C4]palmitoylcarnitine demonstrated an isotopic precision of 0.03 MPE for gastrocnemius (n = 16) and of 0.02 MPE for tibialis anterior (n = 16). The high precision enabled the detection of a small (0.08 MPE) but significant (P = 0.01) difference in [13C4]palmitoylcarnitine enrichments between the two muscles, 0.51 MPE (CV = 5.8%) and 0.43 MPE (CV = 4.6%), respectively. Therefore, the system demonstrated an isotopic lower detection limit (LDL) of < or =0.1 MPE (2 x SD) that has been impossible previously with other organic mass spectrometry instruments. LC/ESI-itMS2 systems have the potential to advance metabolic investigations using stable isotopes to a new level by significantly increasing the isotopic solving power.