title: Production and release of acylcarnitines by primary myotubes reflect the differences in fasting fat oxidation of the donors.
abstract: BACKGROUND
  Acylcarnitines are biomarkers of incomplete β-oxidation and mitochondrial lipid overload but indicate also high rates of mitochondrial fatty acid oxidation. It is unknown whether the production of acylcarnitines in primary human myotubes obtained from lean, metabolically healthy subjects reflects the fat oxidation in vivo.
  
  
  OBJECTIVE
  Our objective was to quantify the acylcarnitine production in myotubes obtained from subjects with low and high fasting respiratory quotient (RQ).
  
  
  METHODS
  Fasting RQ was determined by indirect calorimetry. Muscle biopsies from the vastus lateralis muscle were taken from 6 subjects with low fasting RQ (mean 0.79 ± 0.03) and 6 with high fasting RQ (0.90 ± 0.03), and satellite cells were isolated, cultured, and differentiated to myotubes. Myotubes were cultivated with 125 μM (13)C-labeled palmitate for 30 minutes and 4 and 24 hours. Quantitative profiling of 42 intracellular and 31 extracellular acylcarnitines was performed by stable isotope dilution-based metabolomics analysis by liquid chromatography coupled to mass spectrometry.
  
  
  RESULTS
  Myotubes from donors with high fasting RQ produced and released significant higher amounts of medium-chain acylcarnitines. High (13)C8 and (13)C10 acylcarnitine levels in the extracellular compartment correlated with high fasting RQ. The decreased expression of medium-chain acyl-coenzyme A dehydrogenase (MCAD) in these myotubes can explain the higher rate of incomplete fatty acid oxidation. A lower intracellular [(13)C]acetylcarnitine to carnitine and lower intracellular (13)C16/(13)C18 acylcarnitine to carnitine ratio indicate reduced fatty acid oxidation capacity in these myotubes. Mitochondrial DNA content was not different.
  
  
  CONCLUSIONS
  Acylcarnitine production and release from primary human myotubes of donors with high fasting RQ indicate a reduced fatty acid oxidation capacity and a higher rate of incomplete fatty acid oxidation. Thus, quantitative profiling of acylcarnitine production in human myotubes can be a suitable tool to identify muscular determinants of fat oxidation in vivo.
authors: Division of Endocrinology, Diabetology, Angiology, Nephrology, Pathobiochemistry, and Clinical Chemistry, Department of Internal Medicine, University Tübingen, 72076 Tübingen, Germany.
authors: Magnus
authors: M
authors: Wolf
authors: Shili
authors: S
authors: Chen
authors: Xinjie
authors: X
authors: Zhao
authors: Mika
authors: M
authors: Scheler
authors: Martin
authors: M
authors: Irmler
authors: Harald
authors: H
authors: Staiger
authors: Johannes
authors: J
authors: Beckers
authors: Martin Hrabé
authors: MH
authors: de Angelis
authors: Andreas
authors: A
authors: Fritsche
authors: Hans-Ulrich
authors: HU
authors: Häring
authors: Erwin D
authors: ED
authors: Schleicher
authors: Guowang
authors: G
authors: Xu
authors: Rainer
authors: R
authors: Lehmann
authors: Cora
authors: C
authors: Weigert
mesh_terms: D042964:Acyl-CoA Dehydrogenase / Q000235:genetics / Q000378:metabolism; D000328:Adult; D002331:Carnitine / Q000031:analogs & derivatives* / Q000378:metabolism; D005215:Fasting / Q000378:metabolism*; D005260:Female; D006801:Humans; D050356:Lipid Metabolism; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000378:metabolism*; D010084:Oxidation-Reduction
chemical_list: C116917:acylcarnitine; D042964:Acyl-CoA Dehydrogenase; D002331:Carnitine