Effect of carnitine, acetyl-, and propionylcarnitine supplementation on the body carnitine pool, skeletal muscle composition, and physical performance in mice.

PURPOSE: Pharmacokinetics and effects on skeletal muscle and physical performance of oral acetylcarnitine and propionylcarnitine are not well characterized. We therefore investigated the influence of oral acetylcarnitine, propionylcarnitine, and carnitine on body carnitine homeostasis, energy metabolism, and physical performance in mice and compared the findings to non-supplemented control animals. METHODS: Mice were supplemented orally with 2 mmol/kg/day carnitine, acetylcarnitine, or propionylcarnitine for 4 weeks and studied either at rest or after exhaustive exercise. RESULTS: In the supplemented groups, total plasma and urine carnitine concentrations were significantly higher than in the control group receiving no carnitine, whereas the skeletal muscle carnitine content remained unchanged. The supplemented acylcarnitines were hydrolyzed in intestine and liver and reached the systemic circulation as carnitine. Bioavailability of carnitine and acylcarnitines, determined as the urinary excretion of total carnitine, was in the range of 19 %. Skeletal muscle morphology, including fiber-type composition, was not affected, and oxygen consumption by soleus or gastrocnemius fibers was not different between the groups. Supplementation with carnitine or acylcarnitines had no significant impact on the running capacity, but was associated with lower plasma lactate levels and a higher glycogen content in white skeletal muscle after exhaustive exercise. CONCLUSIONS: Oral supplementation of carnitine, acetylcarnitine, or propionylcarnitine in mice is associated with increased plasma and urine total carnitine concentrations, but does not affect the skeletal muscle carnitine content. Despite better preservation of skeletal muscle glycogen and lower plasma lactate levels, physical performance was not improved by carnitine or acylcarnitine supplementation.

Quantification of plasma carnitine and acylcarnitines by high-performance liquid chromatography-tandem mass spectrometry using online solid-phase extraction.

Carnitine is an amino acid derivative that plays a key role in energy metabolism. Endogenous carnitine is found in its free form or esterified with acyl groups of several chain lengths. Quantification of carnitine and acylcarnitines is of particular interest for screening for research and metabolic disorders. We developed a method with online solid-phase extraction coupled to high-performance liquid chromatography and tandem mass spectrometry to quantify carnitine and three acylcarnitines with different polarity (acetylcarnitine, octanoylcarnitine, and palmitoylcarnitine). Plasma samples were deproteinized with methanol, loaded on a cation exchange trapping column and separated on a reversed-phase C8 column using heptafluorobutyric acid as an ion-pairing reagent. Considering the endogenous nature of the analytes, we quantified with the standard addition method and with external deuterated standards. Solid-phase extraction and separation were achieved within 8 min. Recoveries of carnitine and acylcarnitines were between 98 and 105 %. Both quantification methods were equally accurate (all values within 84 to 116 % of target concentrations) and precise (day-to-day variation of less than 18 %) for all carnitine species and concentrations analyzed. The method was used successfully for determination of carnitine and acylcarnitines in different human samples. In conclusion, we present a method for simultaneous quantification of carnitine and acylcarnitines with a rapid sample work-up. This approach requires small sample volumes and a short analysis time, and it can be applied for the determination of other acylcarnitines than the acylcarnitines tested. The method is useful for applications in research and clinical routine.

Effect of short- and long-term treatment with valproate on carnitine homeostasis in humans.

AIMS: The aim of this study was to identify the mechanisms of hypocarnitinemia in patients treated with valproate. METHODS: Plasma concentrations and urinary excretion of carnitine, acetylcarnitine, propionylcarnitine, valproylcarnitine, and butyrobetaine were determined in a patient starting valproate treatment and in 10 patients on long-term valproate treatment. Transport of carnitine and valproylcarnitine by the proximal tubular carnitine transporter OCTN2 was assessed in vitro. RESULTS: In the patient starting valproate, the plasma carnitine and acetylcarnitine levels dropped for 1-3 weeks and had recovered after 3-5 weeks, whereas the plasma levels of propionyl and valproylcarnitine increased steadily over 5 weeks. The renal excretion and excretion fractions (EFs) of carnitine, acetylcarnitine, propionylcarnitine, and butyrobetaine decreased substantially after starting valproate. Compared with controls, patients on long-term valproate treatment had similar plasma levels of carnitine, acetylcarnitine, and propionylcarnitine, whereas valproylcarnitine was found only in patients. Urinary excretion and renal clearance of carnitine, acetylcarnitine, propionylcarnitine, and butyrobetaine were decreased in valproate-treated compared with that in control patients, reaching statistical significance for carnitine. The EFs of carnitine, acetylcarnitine, and propionylcarnitine were <5% of the filtered load in controls and were lower in valproate-treated patients. In contrast, the EF for valproylcarnitine approached 100%, resulting from a low affinity of valproylcarnitine for the carnitine transporter OCTN2 and competition with concomitantly filtered carnitine. CONCLUSIONS: The initial drop in plasma carnitine levels of valproate-treated patients is most likely due to impaired carnitine biosynthesis, whereas the recovery of the plasma carnitine levels is explainable by an increased renal expression of OCTN2. Renally excreted valproylcarnitine does not affect renal handling of carnitine in vivo.

Quantification of plasma lactate concentrations using capillary electrophoresis with contactless conductivity detection.

The quantification of plasma lactate and evaluation of the lactate threshold by CE with capacitively coupled contactless conductivity is demonstrated. The only sample preparation needed was deproteinization with a ACN/methanol mixture. A solution of 10 mmol/L 2-morpholinoethanesulfonic acid monohydrate, 10 mmol/L DL-histidine, 70 µmol/L hexadecyltrimethylammonium bromide, pH 6.0 was found suitable as running buffer. Linearity was achieved for the concentration range of 10-1000 µmol/L with a correlation coefficient of 0.9994. The limit of detection (3 S/N) was determined as 3.2 µmol/L. Intra- and inter-day variabilities were less than 7% RSD. The suitability of the method could be demonstrated by analyzing various clinical samples, where the results correlated satisfactorily with those of an established enzymatic method.

Hepatic toxicity of dronedarone in mice: role of mitochondrial ß-oxidation.

Dronedarone is an amiodarone-like antiarrhythmic drug associated with severe liver injury. Since dronedarone inhibits mitochondrial respiration and ß-oxidation in vitro, mitochondrial toxicity may also explain dronedarone-associated hepatotoxicity in vivo. We therefore studied hepatotoxicity of dronedarone (200mg/kg/day for 2 weeks or 400mg/kg/day for 1 week by intragastric gavage) in heterozygous juvenile visceral steatosis (jvs(+/-)) and wild-type mice. Jvs(+/-) mice have reduced carnitine stores and are sensitive for mitochondrial ß-oxidation inhibitors. Treatment with dronedarone 200mg/kg/day had no effect on body weight, serum transaminases and bilirubin, and hepatic mitochondrial function in both wild-type and jvs(+/-) mice. In contrast, dronedarone 400mg/kg/day was associated with a 10-15% drop in body weight, and a 3-5-fold increase in transaminases and bilirubin in wild-type mice and, more accentuated, in jvs(+/-) mice. In vivo metabolism of intraperitoneal (14)C-palmitate was impaired in wild-type, and, more accentuated, in jvs(+/-) mice treated with 400mg/kg/day dronedarone compared to vehicle-treated mice. Impaired ß-oxidation was also found in isolated mitochondria ex vivo. A likely explanation for these findings was a reduced activity of carnitine palmitoyltransferase 1a in liver mitochondria from dronedarone-treated mice. In contrast, dronedarone did not affect the activity of the respiratory chain ex vivo. We conclude that dronedarone inhibits mitochondrial ß-oxidation in and ex vivo, but not the respiratory chain. Jvs(+/-) mice are slightly more sensitive for the effect of dronedarone on mitochondrial ß-oxidation than wild-type mice. The results suggest that inhibition of mitochondrial ß-oxidation is an important mechanism of hepatotoxicity associated with dronedarone.

Determination of free and total valproic acid in human plasma by capillary electrophoresis with contactless conductivity detection.

A new approach for the determination of free and total valproic acid in small samples of 140 µL human plasma based on capillary electrophoresis with contactless conductivity detection is proposed. A dispersive liquid-liquid microextraction technique was employed in order to remove biological matrices prior to instrumental analysis. The free valproic acid was determined by isolating free valproic acid from protein-bound valproic acid by ultrafiltration under centrifugation of 100 µL sample. The filtrate was acidified to turn valproic acid into its protonated neutral form and then extracted. The determination of total valproic acid was carried out by acidifying 40 µL untreated plasma to release the protein-bound valproic acid prior to extraction. A solution consisting of 10 mM histidine, 10 mM 3-(N-morpholino)propanesulfonic acid and 10 µM hexadecyltrimethylammonium bromide of pH 6.5 was used as background electrolyte for the electrophoretic separation. The method showed good linearity in the range of 0.4-300 µg/mL with a correlation coefficient of 0.9996. The limit of detection was 0.08 µg/mL, and the reproducibility of the peak area was excellent (RSD=0.7-3.5%, n=3, for the concentration range from 1 to 150 µg/mL). The results for the free and total valproic acid concentration in human plasma were found to be comparable to those obtained with a standard immunoassay. The corresponding correlation coefficients were 0.9847 for free and 0.9521 for total valproic acid.

Determination of creatine and phosphocreatine in muscle biopsy samples by capillary electrophoresis with contactless conductivity detection.

A capillary electrophoresis method with contactless conductivity detection was evaluated as a new approach for quantification of creatine and phosphocreatine in human quadriceps femoris biopsy samples. The running buffers employed consisted of 1 M acetic acid at a pH of 2.3 for the determination of creatine and 50 mM 3-(N-morpholino)propanesulfonic acid/30 mM histidine at a pH of 6.4 for the determination of phosphocreatine in the centrifuged muscle extracts. The limits of detection for creatine and phosphocreatine were found to be 2.5 and 1.0 µM, respectively. Creatine and phosphocreatine were determined in six human muscle biopsy samples and the results were found comparable to those of a standard enzymatic assay. The procedures developed for creatine and phosphocreatine also allow the determination of creatinine as well as adenosine diphosphate and adenosine triphosphate.

Capillary electrophoresis with contactless conductivity detection for the determination of carnitine and acylcarnitines in clinical samples.

A capillary electrophoresis method with contactless conductivity detection was developed for the quantification of carnitine and six acylcarnitines in plasma and urine samples. The running buffer employed consisted of 500 mmol/L acetic acid, 1.0 mmol/L hydroxypropyl-ß-cyclodextrin and 0.05% Tween at a pH of 2.6. Under these conditions, the isomeric valproyl- and octanoyl-carnitines could be distinguished. The linearity was in the range from 5.0 to 200.0 µmol/L with correlation coefficients between 0.9992 and 0.9997. The limits of detection were between 1.0 and 3.2 µmol/L. Intra- and inter-day precisions as %RSD were better than 10%. The method allows for direct determination without derivatisation or extraction processes. The method was applied for the quantification of carnitine and acetylcarnitine in plasma pre- and post-exercise, and to measure valproylcarnitine in plasma and urine of patients undergoing valproate therapy.

The role of CYP3A4 in amiodarone-associated toxicity on HepG2 cells.

Amiodarone is a class III antiarrhythmic drug with potentially life-threatening hepatotoxicity. Recent in vitro investigations suggested that the mono-N-desethyl (MDEA) and di-N-desethyl (DDEA) metabolites may cause amiodarone's hepatotoxicity. Since cytochrome P450 (CYP) 3A4 is responsible for amiodarone N-deethylation, CYP3A4 induction may represent a risk factor. Our aim was therefore to investigate the role of CYP3A4 in amiodarone-associated hepatotoxicity. First, we showed that 50µM amiodarone is more toxic to primary human hepatocytes after CYP induction with rifampicin. Second, we overexpressed human CYP3A4 in HepG2 cells (HepG2 cells/CYP3A4) for studying the interaction between CYP3A4 and amiodarone in more detail. We also used HepG2 wild type cells (HepG2 cells/wt) co-incubated with human CYP3A4 supersomes for amiodarone activation (HepG2 cells/CYP3A4 supersomes). Amiodarone (10-50µM) was cytotoxic for HepG2 cells/CYP3A4 or HepG2 cells/CYP3A4 supersomes, but not for HepG2 cells/wt or less toxic for HepG2 cells/wt incubated with control supersomes without CYP3A4. Co-incubation with ketoconazole, attenuated cytotoxicity of amiodarone incubated with HepG2 cells/CYP3A4 or HepG2 cells/CYP3A4 supersomes. MDEA and DDEA were formed only in incubations containing HepG2 cells/CYP3A4 or HepG2 cells/CYP3A4 supersomes but not by HepG2 cells/wt or HepG2 cells/wt with control supersomes. Metabolized amiodarone triggered the production of reactive oxygen species, induced mitochondrial damage and cytochrome c release, and promoted apoptosis/necrosis in HepG2 cells/CYP3A4, but not HepG2 cells/wt. This study supports the hypothesis that a high CYP3A4 activity is a risk factor for amiodarone's hepatotoxicity. Since CYP3A4 inducers are used frequently and amiodarone-associated hepatotoxicity can be fatal, our observations may be clinically relevant.