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.

Increased tricarboxylic acid cycle flux in rat brain during forepaw stimulation detected with 1H[13C]NMR.

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-(13)C]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 [(V(TCA)] increased from 0.49 +/- 0.03 at rest to 1.48 +/- 0.82 micromol/g/min during stimulation (P < 0.01). The minimum fraction of C4-glutamate derived from C1-glucose was approximately 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 (CMRglc) equals the percentage increases in V(TCA) and cerebral metabolic rate of oxygen consumption (CMRO2). Comparison with previous work for the same rat model, which measured total CMRglc [Ueki, M., Linn, F. & Hossman, K. A. (1988) J. Cereb. Blood Flow Metab. 8, 486-4941, indicates that oxidative CMRglc supplies the majority of energy during sustained brain activation.

Statistical analysis of results of carcinogenicity studies of synthetic vitreous fibres at Research and Consulting Company, Geneva.

Five inhalation studies of synthetic vitreous fibres have recently investigated experimental tumorigenic responses to four different refractory ceramic fibres (RCF), two fibre glasses, one stone (rock) wool and one slag wool. Except for one RCF, the source materials were typical commercial products. Three studies included positive control groups exposed to chrysotile or crocidolite asbestos. The studies were conducted using state-of-the-art technologies for fibre size separation, fibre lofting and nose-only inhalation exposure. The target average fibre size was 20 microns long by 1 micron diameter. Hamsters exposed to a kaolin RCF yielded a mesothelioma rate of 38%, but no lung cancers. There were no tumours among the chrysotile-exposed hamsters. At the highest dose of 30 mg m-3 in rat studies, the commercial RCF all produced significant numbers of lung tumours, and some mesotheliomas. The fourth RCF, which had been heat-treated to simulate an after-service fibre, did not produce a significant excess of lung cancers, but did produce one mesothelioma. A rat multi-dose experiment with three lower doses of the kaolin RCF yielded one mesothelioma among 379 rats, but no excess of lung tumours. The overall dose-response relation for lung cancer did not appear to be linear, consistent with the possibility of a threshold close to the Maximum Tolerated Dose. No insulation wool (glass, stone or slag) exposure group had a lung tumour rate that differed statistically significantly from the tumour rate for the respective concurrent control groups, sham-exposed to filtered air. There was no significant difference in the total tumour rates between the four insulation wool groups and the control animals, and no significant dose-response relation above the respective sham-exposed control tumour rates. The total lung tumour rates for rats in both chrysotile and crocidolite exposure groups were significantly raised. One animal in each asbestos-exposed group developed a mesothelioma, whereas no air control or insulation wool-exposed animal did so.

In vivo regulation of rat muscle glycogen resynthesis after intense exercise.

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.

In vivo 31P NMR measurement of glucose-6-phosphate in the rat muscle after exercise.

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.