NMR spectroscopy was used to test recent proposals that the additional energy required for brain activation is provided through nonoxidative glycolysis. Using localized NMR spectroscopic methods, the rate of C4-glutamate isotopic turnover from infused [1-_3C]glucose was measured in the somatosensory cortex of rat brain both at rest and during forepaw stimulation. Analysis of the glutamate turnover data using a mathematical model of cerebral glucose metabolism showed that the tricarboxylic acid cycle flux (VTCA) increased from 0.49 + 0.03 at rest to 1.48 ± 0.82 ,umol/g/min during stimulation (P < 0.01). The minimum fraction of C4-glutamate derived from Cl-glucose was -75%, and this fraction was found in both the resting and stimulated rats. Hence, the percentage increase in oxidative cerebral metabolic rate of glucose use (CMR51I) equals the percentage increases in VTCA and cerebral metabolic rate of oxygen consumption (CMRo2 It has been generally believed that, under normal physiological conditions in the adult mammalian brain, almost all the energy required for ATP generation is supplied by oxidation of glucose through the tricarboxylic acid (TCA) cycle (1). However, more recent human studies using positron emission tomography during visual stimulation (2, 3) reported a greater localized increase of the cerebral metabolic rate of glucose use (CMRgic) compared with that of oxygen consumption (CMRo2). This loss of stoichiometry between CMRgic and CMR02 has been qualitatively consistent with the finding of elevated lactate during somatosensory stimulation of the rat cortex (4) and visual stimulation of the human cortex (5, 6).However, the increase in lactate is small and/or transient in contrast to the large change predicted from the positron emission tomography findings (7). To date, it remains unclear whether nonoxidative glycolysis is a major energy source for sustained cortical activation (7).In a study by [14C]deoxyglucose autoradiography, Ueki and coworkers (4) reported that, during electrical stimulation of the rat forepaw, both CMRglc and cerebral blood flow (CBF) increased in the contralateral somatosensory area. The additional finding that the concentration of lactate increased led them to conclude that glucose oxidation was incomplete during brain activation (4). However, CMRo2 was not measured in this study (4), so the mismatch between CMRglc and CMR02 was not determined quantitatively.NMR spectroscopy can be used to determine the TCA cycle flux (VTCA) in the brain by measuring the rate of 13C-label flow from Cl-glucose to C4-glutamate (8-10). Here, we present the rate of glutamate 13C isotopic turnover data localized by functional MRI (fMRI) to the-region activated during forepaw stimulation (11). We have shown by fMRI that activation during this stimulation is confined to the contralateral motor and somatosensory areas (11), in agreement with the autoradiography study (4). A mathematical model of cerebral glucose metabolism (12, 13) was fitted to the present glutamate 13C isotopic data to cal...

To examine the impact of insulin resistance on the insulin-dependent and insulin-independent portions of muscle glycogen synthesis during recovery from exercise, we studied eight young, lean, normoglycemic insulin-resistant (IR) offspring of individuals with non-insulin-dependent diabetes mellitus and eight age-weight matched control (CON) subjects after plantar flexion exercise that lowered muscle glycogen to -25% of resting concentration. After '20 min of exercise, intramuscular glucose 6-phosphate and glycogen were simultaneously monitored with 31P and 13C NMR spectroscopies. The postexercise rate of glycogen resynthesis was nonlinear. Glycogen synthesis rates during the initial insulin independent portion (0-1 hr of recovery) were similar in the two groups (IR, 15.5 ± 1.3 mM/hr and CON, 15.8 + 1.7 mM/hr); however, over the next 4 hr, insulin-dependent glycogen synthesis was significantly reduced in the IR group [IR, 0.1 + 0.5 mM/hr and CON, 2.9 ± 0.2 mM/hr; (P ' 0.001)]. After exercise there was an initial rise in glucose 6-phosphate concentrations that returned to baseline after the first hour of recovery in both groups. In summary, we found that following muscle glycogen-depleting exercise, IR offspring of parents with non-insulin-dependent diabetes mellitus had (i) normal rates of muscle glycogen synthesis during the insulin-independent phase of recovery from exercise and (ii) severely diminished rates of muscle glycogen synthesis during the subsequent recovery period (2-5 hr), which has previously been shown to be insulin-dependent in normal CON subjects. These data provide evidence that exercise and insulin stimulate muscle glycogen synthesis in humans by different mechanisms and that in the IR subjects the early response to stimulation by exercise is normal.After intense exercise that depletes muscle glycogen concentrations to <35 mM glycogen, resynthesis proceeds in an approximately biphasic manner in both animal and human skeletal muscles (1-4). In normal healthy humans, there is an initial phase of rapid glycogen resynthesis (12-30 mM/hr) lasting -45 min that is insulin independent (1). The subsequent period of glycogen resynthesis (beyond -35 mM glycogen) is much slower (-3 mM/hr) and insulin dependent (5).Exercise and insulin are both known to stimulate muscle glucose uptake and subsequent glycogen synthesis in an independent and additive manner (6-10). Under resting conditions, the effect of insulin stimulation on glycogen synthesis has been compared in healthy control (CON) subjects and in subjects with non-insulin-dependent diabetes mellitus (NIDDM) by 13C NMR (11). In both CON subjects and NIDDM subjects placed under hyperglycemic-hyperinsulinemic conditions, the major pathway of insulin-dependent glucose metabolism was muscle glycogen synthesis (11). However, in the NIDDM subjects the rate of muscle glycogen synthesis was significantly impaired (11).31P NMR has been used to measure concentrations of glucose 6-phosphate (G6P), an intermediate of glycogen synthesis, under hyperglycemic-...

Time courses of the glycogen synthesis rate and of the glucose 6-phosphate (G-6-P) concentration after an electrically induced exercise were followed in the anesthetized rat gastrocnemius by in vivo 13C and 31P nuclear magnetic resonance (NMR) spectroscopy, respectively. The ratio of glycogen synthase I to glycogen synthase I and D (I/I+D) and allosteric activation by G-6-P were also studied in vitro on muscles sampled at rest and 10 min (early recovery) and 100 min (late recovery) after exercise. From early recovery to late recovery, the in vivo glycogen synthesis rate dropped from 0.46 +/- 0.06 to 0.11 +/- 0.04 mmol.kg wet tissue-1.min-1, the G-6-P concentration from 0.83 +/- 0.08 to 0.32 +/- 0.05 mmol/kg wet tissue, and I/I+D from 83 +/- 4 to 47 +/- 1%. The combination of the changes in G-6-P concentration and in I/I+D quantitatively describes the fourfold decrease in glycogen synthesis rate from early to late recovery. These results demonstrate that phosphorylation, determining glycogen synthase I/I+D, and allosteric control of glycogen synthase by G-6-P contribute approximately equally to the regulation of the postexercise in vivo glycogen synthesis rate.

Comparison of 31P NMR spectra of the rat gastrocnemius, obtained in vivo and from PCA extracts, after electrically induced contractions, demonstrates that glucose-6-phosphate (G6P) is the major metabolite in the low-field part of the PME spectral region. In vivo 31P NMR can thus be used to measure the muscle G6P concentration after exercise.

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.