title: Regulation of glycogen accumulation in L6 myotubes cultured under optimized differentiation conditions.
abstract: The differentiation of the L6 myogenic cell line was enhanced by the addition of dexamethasone, retinoic acid, insulin-like growth factor I (IGF-I), and creatine. Spontaneous contractions appeared from day 10 or 11 and persisted to day 14 or 15. Glucose transport was increased by insulin (100 nM) and IGF-I (5 nM) by approximately 60%. The highest level of glycogen was measured in myotubes differentiated under the influence of a combination of 5 nM dexamethasone, 100 nM retinoic acid, 5 nM IGF-I, and 10 mM creatine with glucose as substrate. The glycogen accumulation rate was constant from 0 to 2 h of incubation and decreased gradually to zero at 4 h. From 0 to 0.5 h of the glycogen accumulation, the glycogen synthase a (GSa) activity was 30-35% of the total activity, with a subsequent gradual decline to 2.5% after 6 h. The glycogen phosphorylase a (GPha) activity was constant at approximately 80% from 0 to 0.5 h, increasing to approximately 100% after 6 h. The activity ratio of GSa to GPha decreased about sixfold without significant change in the rate of glycogen accumulation. This indicates that factors other than phosphorylation/dephosphorylation play a decisive role in the regulation of glycogen metabolism in L6 myotubes. Intracellular glucose (glucosei) and glucose 6-phosphate (G-6-P) may be such factors. The observed values of these parameters may in fact explain an activation of GSa (G-6-P) and an inhibition of GPha (glucosei).
authors.affiliation: Department of Medical Biochemistry and Genetics, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen N, Denmark.
authors.forename: P
authors.initials: P
authors.lastname: Elsner
authors.forename: B
authors.initials: B
authors.lastname: Quistorff
authors.forename: T S
authors.initials: TS
authors.lastname: Hermann
authors.forename: J
authors.initials: J
authors.lastname: Dich
authors.forename: N
authors.initials: N
authors.lastname: Grunnet
mesh_terms: D001692:Biological Transport / Q000502:physiology; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D002460:Cell Line; D003401:Creatine / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism; D019298:Glucose-6-Phosphate / Q000378:metabolism; D006003:Glycogen / Q000378:metabolism*; D006006:Glycogen Synthase / Q000378:metabolism; D009119:Muscle Contraction / Q000502:physiology; D009132:Muscles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D006005:Phosphorylases / Q000378:metabolism
chemical_list: D019298:Glucose-6-Phosphate; D006003:Glycogen; D006005:Phosphorylases; D006006:Glycogen Synthase; D005947:Glucose; D003401:Creatine