title: Myocellular creatine and creatine transporter serine phosphorylation after starvation.
abstract: BACKGROUND
  Myocellular creatine, which is critically important for normal energy metabolism, increases in rat gastrocnemius muscle after starvation via unknown mechanisms. Creatine (Cr) uptake across plasma membranes is governed by a single, specific transporter (CrTr) that shares 50% amino acid sequence identity with GABA/choline/betaine transporters whose functions are modulated by phosphorylation.
  
  
  METHODS
  Gastrocnemius muscle was collected from adult male Sprague-Dawley (225-250 g) rats that were randomized to receive normal rat chow and distilled water ad libitum (CTL) or distilled water alone for 4 days (STV). Total Cr, phosphocreatine (PCr), free Cr, and ATP were measured luminometrically. CrTr protein expression and protein serine and tyrosine phosphorylation and mRNA expression were determined using immunoprecipitation and quantitative Western blotting and reverse transcription polymerase chain reaction (RT-PCR) analyses, respectively. Guanidinoacetate methyltransferase (GAMT) activity, guanidinoacetic acid (GAA) content, creatine kinase (CK) activity, and creatinine (Crn) content were assayed luminometrically or spectrophotometrically. Creatine transporter uptake activity was also measured in skeletal muscle membrane vesicles. Data were analyzed by t test.
  
  
  RESULTS
  Total Cr and free Cr increased 26 and 280% in STV (32.3 +/- 1.0 and 12.9 +/- 1.4 vs 25.7 +/- 1.1 and 3.4 +/- 0.9 micromol/g wet wt, mean +/- SEM, respectively, P < 0.01) whereas PCr content decreased 18% (18.6 +/- 0.8 vs 22.8 +/- 0.9 micromol/g wet wt, STV vs CTL P < 0.05). CrTr protein and mRNA expression, ATP, GAA, CK, GAMT, and protein tyrosine phosphorylation of CrTr were not significantly different between the two groups. However, protein serine phosphorylation of CrTr was significantly reduced by 30% (P < 0.05) and creatine uptake activity was significantly increased (P < 0.05) in starved animals.
  
  
  CONCLUSIONS
  Increases in myocellular creatine content after starvation are associated with reduced serine phosphorylation of the creatine transporter.
authors.affiliation: Surgical Research Laboratory, School of Medicine, Omaha, Nebraska 68131, USA.
authors.forename: Chun-Rui
authors.initials: CR
authors.lastname: Zhao
authors.forename: Lihong
authors.initials: L
authors.lastname: Shang
authors.forename: Weiyang
authors.initials: W
authors.lastname: Wang
authors.forename: Danny O
authors.initials: DO
authors.lastname: Jacobs
mesh_terms: D000818:Animals; D003401:Creatine / Q000493:pharmacokinetics*; D003402:Creatine Kinase / Q000378:metabolism; D015870:Gene Expression / Q000502:physiology; D005998:Glycine / Q000031:analogs & derivatives* / Q000378:metabolism; D050937:Guanidinoacetate N-Methyltransferase; D008297:Male; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D008780:Methyltransferases / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism; D010725:Phosphocreatine / Q000378:metabolism; D010766:Phosphorylation; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D017208:Rats, Wistar; D013047:Specific Pathogen-Free Organisms; D013217:Starvation / Q000378:metabolism*
chemical_list: D026901:Membrane Transport Proteins; D012333:RNA, Messenger; C080679:creatine transporter; D010725:Phosphocreatine; D008780:Methyltransferases; C494508:Gamt protein, rat; D050937:Guanidinoacetate N-Methyltransferase; D003402:Creatine Kinase; C004946:glycocyamine; D003401:Creatine; D005998:Glycine