A NOVEL C-13 NMR METHOD TO ASSESS INTRACELLULAR GLUCOSE-CONCENTRATIONIN MUSCLE, IN-VIVO

Intracellular glucose concentration in skeletal muscle of awake rats w as determined under conditions of hyperglycemic (10.2 +/-: 0.6 mM) hyp erinsulinemia (similar to 1,200 pM) and hyperglycemic (20.8 +/- 1.5 mM ) hypoinsulinemia (<12 pM) by use of C-13 nuclear magnetic resonance ( NMR) spectroscopy during a prime-constant infusion of[1-C-13]glucose a nd [1-C-13]mannitol with either insulin (10 mU.kg(-1).min(-1)) or soma tostatin (1.0 mu g.kg(-1).min(-1)). Intracellular glucose was calculat ed as the difference between the concentrations of total tissue glucos e (calculated from the in vivo C-13 NMR spectrum with mannitol as an i nternal concentration standard) and extracellular glucose, corrected b y the ratio of intra- and extracellular water space. Extracellular con centration was corrected for an interstitial fluid-to-plasma glucose c oncentration gradient of 0.83 +/- 0.07, determined by open-flow microp erfusion. The mean ratio of intra-to extracellular glucose space, dete rmined from the relative NMR signal intensities and concentrations of mannitol and total creatine, was 9.2 +/- 1.1 (hyperglycemic hyperinsul inemia, n = 10), and 9.0 +/- 1.7 (hyperglycemic hypoinsulinemia, n = 7 ). Mean muscle intracellular glucose concentration was <0.07 mM under hyperglycemic-hyperinsulinemic conditions (n = 10) and 0.32 +/- 0.06 m M under hyperglycemic-hypoinsulinemic conditions (n = 7). This method is noninvasive and should prove useful for resolving the question of w hether glucose transport or phosphorylation is responsible for the red uced rate of muscle glycogen synthesis observed in diabetic subjects.