Flux control in the rat gastrocnemius glycogen synthesis pathway by in vivo 13C/31P NMR spectroscopy.
To determine the relative contributions of glucose transport/hexokinase, glycogen synthase (GSase), and glycolysis to the control of insulin-stimulated muscle glycogen synthesis, we combined 13C and 31P NMR to quantitate the glycogen synthesis rate and glucose 6-phosphate (G-6-P) levels in rat (Sprague-Dawley) gastrocnemius muscle during hyperinsulinemia at euglycemic (E) and hyperglycemic (H) glucose concentrations under thiopental anesthesia. Flux control was calculated using metabolic control analysis. The combined control coefficient of glucose transport/hexokinase (GT/Hk) for glycogen synthesis was 1.1 +/- 0.03 (direct measure) and 1.14-1.16 (calculated for a range of glycolytic fluxes), whereas the control coefficient for GSase was much lower (0.011-0.448). We also observed that the increase in in vivo [G-6-P] from E to H (0.22 +/- 0.03 to 0.40 +/- 0.03 mM) effects a supralinear increase in the in vitro velocity of GSase, from 14.6 to 26.1 mU. kg(-1). min(-1) (1.8-fold). All measurements suggest that the majority of the flux control of muscle glycogen synthesis is at the GT/Hk step.
Chase
J R
JR
Department of Molecular Biophysics and Biochemistry, Yale University School of Medicine, New Haven, CT 06510, USA.
Rothman
D L
DL
Shulman
R G
RG
D000818:Animals; D002247:Carbon Isotopes; D015309:Glucose Clamp Technique; D019298:Glucose-6-Phosphate / Q000378:metabolism; D006003:Glycogen / Q000096:biosynthesis*; D006006:Glycogen Synthase / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000378:metabolism*; D010758:Phosphorus; D051381:Rats; D017207:Rats, Sprague-Dawley
D002247:Carbon Isotopes; D010758:Phosphorus; D019298:Glucose-6-Phosphate; D006003:Glycogen; D006006:Glycogen Synthase