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Biomedical subjects

R G Shulman

Publications and source records attributed to R G Shulman.

At least 55 records · Page 3Linked to original sources
PubMed1995

Decreased muscle glucose transport/phosphorylation is an early defect in the pathogenesis of non-insulin-dependent diabetes mellitus.

Recent studies have demonstrated that reduced insulin-stimulated muscle glycogen synthesis is the major cause of insulin resistance in patients with non-insulin-dependent diabetes mellitus (NIDDM). This reduced rate has been assigned to a defect in either glucose transport or hexokinase activity. However it is unknown whether this is a primary or acquired defect in the pathogenesis of NIDDM. To examine this question, we measured the rate of muscle glycogen synthesis and the muscle glucose 6-phosphate (G6P) concentration using 13C and 31P NMR spectroscopy as well as oxidative and nonoxidative glucose metabolism in six lean, normoglycemic offspring of parents with NIDDM and seven age/weight-matched control subjects under hyperglycemic (approximately 11 mM)-hyperinsulinemic (approximately 480 pM) clamp conditions. The offspring of parents with NIDDM had a 50% reduction in total glucose metabolism, primarily due to a decrease in the nonoxidative component. The rate of muscle glycogen synthesis was reduced by 70% (P < 0.005) and muscle G6P concentration was reduced by 40% (P < 0.003), which suggests impaired muscle glucose transport/hexokinase activity. These changes were similar to those previously observed in subjects with fully developed NIDDM. When the control subjects were studied at similar insulin levels (approximately 440 pM) but euglycemic plasma glucose concentration (approximately 5 mM), both the rate of glycogen synthesis and the G6P concentration were reduced to values similar to the offspring of parents with NIDDM. We conclude that insulin-resistant offspring of parents with NIDDM have reduced nonoxidative glucose metabolism and muscle glycogen synthesis secondary to a defect in muscle glucose transport/hexokinase activity prior to the onset of overt hyperglycemia. The presence of this defect in these subjects suggests that it may be the primary factor in the pathogenesis of NIDDM.

Adenosine Diphosphate↗
PubMed1995

Simultaneous determination of the rates of the TCA cycle, glucose utilization, alpha-ketoglutarate/glutamate exchange, and glutamine synthesis in human brain by NMR.

13C isotopic tracer data previously obtained by 13C nuclear magnetic resonance in the human brain in vivo were analyzed using a mathematical model to determine metabolic rates in a region of the human neocortex. The tricarboxylic acid (TCA) cycle rate was 0.73 +/- 0.19 mumol min-1 g-1 (mean +/- SD; n = 4). The standard deviation reflects primarily intersubject variation, since individual uncertainties were low. The rate of alpha-ketoglutarate/glutamate exchange was 57 +/- 26 mumol min-1 g-1 (n = 3), which is much greater than the TCA cycle rate; the high rate indicates that alpha-ketoglutarate and glutamate are in rapid exchange and can be treated as a single combined kinetic pool. The rate of synthesis of glutamine from glutamate was 0.47 mumol min-1 g-1 (n = 4), with 95% confidence limits of 0.139 and 3.094 mumol min-1 g-1; individual uncertainties were biased heavily toward high synthesis rates. From the TCA cycle rate the brain oxygen consumption was estimated to be 2.14 +/- 0.48 mumol min-1 g-1 (5.07 +/- 1.14 ml 100 g-1 min-1; n = 4), and the rate of brain glucose consumption was calculated to be 0.37 +/- 0.08 mumol min-1 g-1 (n = 4). The sensitivity of the model to the assumptions made was evaluated, and the calculated values were found to be unchanged as long as the assumptions remained near reported physiological values.

Brain↗
PubMed1994

Functional magnetic resonance imaging of human prefrontal cortex activation during a spatial working memory task.

High-speed magnetic resonance (MR) imaging was used to detect activation in the human prefrontal cortex induced by a spatial working memory task modeled on those used to elucidate neuronal circuits in nonhuman primates. Subjects were required to judge whether the location occupied by the current stimulus had been occupied previously over a sequence of 14 or 15 stimuli presented in various locations. Control tasks were similar in all essential respects, except that the subject's task was to detect when one of the stimuli presented was colored red (color detection) or when a dot briefly appeared within the stimulus (dot detection). In all tasks, two to three target events occurred randomly. The MR signal increased in an area of the middle frontal gyrus corresponding to Brodmann's area 46 in all eight subjects performing the spatial working memory task. Right hemisphere activation was greater and more consistent than left. The MR signal change occurred within 6-9 sec of task onset and declined within a similar period after task completion. An increase in MR signal was also noted in the control tasks, but the magnitude of change was less than that recorded in the working memory task. These differences were replicated when testing was repeated in five of the original subjects. The localization of spatial working memory function in humans to a circumscribed area of the middle frontal gyrus supports the compartmentalization of working memory functions in the human prefrontal cortex and the localization of spatial memory processes to comparable areas in humans and nonhuman primates.

Color Perception↗
PubMed1994

Proton NMR observation of glycogen in vivo.

Our previous solution studies of the proton relaxation properties of glycogen H1 have shown significant dipolar cross-relaxation with intra-ring H2 and inter-ring H4' protons characterized by a correlation time tau c = 2.7 x 10(-9) s. This leads to a significant negative Nuclear Overhauser Enhancement (NOE) of glycogen H1 following either transient or steady state perturbations of the longitudinal magnetization of dipolar coupled protons, especially H2 and H4'. Here we use the NOE to edit selectively the H1 resonance of glycogen in the rat liver in vivo using a surface coil probe. The approach shows the possibility of measuring glycogen in vivo with high sensitivity using 1H NMR.

Animals↗
PubMed1994

Validation of 13C NMR measurements of liver glycogen in vivo.

The natural abundance 13C NMR intensity of the glycogen C1 resonance was measured in the surgically exposed liver of rabbits in vivo (n = 17) by integration from 98 to 104 ppm and compared double blindedly to the subsequent biochemical measurement. Coil loading was measured each time from a reference sphere at the coil center and the NMR intensity was normalized accordingly. For quantification, the normalized NMR intensity was calibrated using aqueous glycogen solutions ranging from 110 to 1100 mumol glucosyl units/g (n = 14). An in vivo range from 110 to 800 mumol glucosyl units/g wet weight was measured with a highly linear correlation with concentration (r = 0.85, P < 0.001). The in vivo NMR concentration was 0.95 +/- 0.05 (mean +/- standard error, n = 17) of the concomitant enzymatic measurement of glycogen content. We conclude that the 13C NMR signal of liver glycogen C1 is essentially 100% visible in vivo and that natural abundance 13C NMR spectroscopy can provide reliable noninvasive estimates of in vivo glycogen content over the physiological range of liver glycogen concentrations when using adequate localization and integration procedures.

Animals↗
PubMed1994

Implementation of echo-planar imaging on an unmodified spectrometer at 2.1 Tesla for functional imaging.

The high speed magnetic resonance imaging technique of Echo-Planar imaging (EPI) has been developing since 1977 using specialized imaging systems. Recent experiments in functional magnetic resonance imaging (FMRI) using EPI on specialized MR systems have generated increased interest in the EPI sequence. Presented here is an implementation of the EPI sequence on a large bore, human scale spectrometer using widely available standard hardware. Existing EPI methods and concepts were amalgamated to obtain images without hardware modifications. Spatial distortion of Echo-Planar images by internal field gradients within the brain were minimized by active shimming on each subject. Nonlinear sampling artifacts caused by nonideal gradient switching were addressed by oversampling the signal and simple extraction of data points corresponding to the ideal sampling pattern. Multislice Echo-Planar images with functional contrast have been obtained at 6 x 3 mm resolution using these methods.

Brain↗
PubMed1994

NMR studies of human brain function.

Recent advances in magnetic resonance imaging and spectroscopy make it possible to measure localized changes in human brain activity and metabolism in single subjects during sensory stimulation and cognition. Differences between stimulated and unstimulated subjects can be visualized to a resolution of mm3 in less than 1s, a significant improvement over the more established method, positron emission tomography. Magnetic resonance spectroscopy of the human brain, measuring fluxes in several cm3, has followed changes in metabolic rates during visual stimulation.

Biological Transport↗
PubMed1994

Dynamic magnetic resonance imaging of the rat brain during forepaw stimulation.

A magnetic resonance (MR) imaging brain mapping method was used to localize an activated volume of brain tissue in chloralose-anesthetized rats during electrical stimulation of the forepaw. Physiologically-induced changes are characterized by alterations of the magnetic properties of blood as determined by the oxygenation state of hemoglobin. Stimulation of the left forepaw led to an increase in MR signal intensity of the contralateral frontal and parietal cortices, which corresponded to forelimb motor and somatosensory areas. The activation was contiguous in coronal planes between +5 and +2 mm anterior to the bregma, and its volume was calculated to be 20-30 mm3. Each activated region was revealed using a paired t-test statistical analysis method and the activated volume was calculated from regions exposed by thresholding at p < 0.005. Physiologically-induced fractional signal changes, delta S/S, in the motor and somatosensory areas were 0.06 +/- 0.04 and 0.17 +/- 0.06, respectively.

Animals↗
PubMed1994

Localized 13C NMR spectroscopy in the human brain of amino acid labeling from D-[1-13C]glucose.

Cerebral metabolism of D[1-13C]glucose was studied with localized 13C NMR spectroscopy during intravenous infusion of enriched [1-13C]glucose in four healthy subjects. The use of three-dimensional localization resulted in the complete elimination of triacylglycerol resonance that originated in scalp and subcutaneous fat. The sensitivity and resolution were sufficient to allow 4 min of time-resolved observation of label incorporation into the C3 and C4 resonances of glutamate and C4 of glutamine, as well as C3 of aspartate with lower time resolution. [4-13C]Glutamate labeled rapidly reaching close to maximum labeling at 60 min. The label flow into [3-13C]glutamate clearly lagged behind that of [4-13C]-glutamate and peaked at t = 110-140 min. Multiplets due to homonuclear 13C-13C coupling between the C3 and C4 peaks of the glutamate molecule were observed in vivo. Isotopomer analysis of spectra acquired between 120 and 180 min yielded a 13C isotopic fraction at C4 glutamate of 27 +/- 2% (n = 4), which was slightly less than one-half the enrichment of the C1 position of plasma glucose (63 +/- 1%), p < 0.05. By comparison with an external standard the total amount of [4-13C]glutamate was directly quantified to be 2.4 +/- 0.1 mumol/ml-brain. Together with the isotopomer data this gave a calculated brain glutamate concentration of 9.1 +/- 0.7 mumol/ml, which agrees with previous estimates of total brain glutamate concentrations. The agreement suggests that essentially all of the brain glutamate is derived from glucose in health human brain.

Adult↗
PubMed1994

NMR investigation of the futile cycling of ethanol in chronic alcoholic rats.

Weight gain efficiency differences previously reported between alcohol-fed rats and their controls were investigated. Additionally, the futile cycling of ethanol proposed to explain such differences was studied by NMR spectroscopy. Male Sprague-Dawley rats were fed a nutritionally adequate diet containing 36% of the calories as alcohol, and their paired controls were fed an isocaloric diet for 11 weeks to establish conditions of chronic alcohol feeding. Normalized metabolic efficiencies varied significantly during the initial 2-week period (6.86 +/- 0.51 vs. 2.83 +/- 0.18 g/kcal x 10(-2) for control and alcohol-fed groups, respectively, and to a lesser extent over the entire feeding period (6.41 +/- 0.78 vs. 4.60 +/- 0.27 g/kcal x 10(-2) for control and alcohol-fed groups, respectively. Alcohol-induced weight gain inefficiency in metabolism has previously been studied and explained by a variety of different biochemical and physiological mechanisms. One possible pathway of energy wastage may occur due to ethanol futile cycling from ethanol to acetaldehyde through the microsomal ethanol oxidation system pathway, and simultaneously from acetaldehyde to ethanol via the ADH pathway. This futile cycle represents a net loss of 6 ATP/cycle, corresponding to the loss of two reducing equivalents (NADH and NADPH). 1H NMR spectroscopy was used to test for this cycling in blood extracts after administration of 1,1-2H2 ethanol. No futile cycling was detected either during the initial 2 weeks of feeding or after the entire feeding period.

Acetaldehyde↗
PubMed1994

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.

Animals↗
PubMed1994

Liver glycogen turnover in fed and fasted humans.

Whether liver glycogen synthesis and breakdown occur simultaneously during net glycogen synthesis was assessed in fed and fasted healthy humans. The peak intensity of the carbon-1 (C1) resonance of the glycosyl units of glycogen was monitored with 13C nuclear magnetic resonance spectroscopy during [1-13C]glucose infusion followed by unlabeled glucose infusion. The C1 peak intensity increased almost linearly during the [1-13C]glucose infusion, reflecting a near linear rate of glycogen synthesis. When switched to unlabeled glucose, the C1 peak intensity reached a plateau in the fasted subjects and declined in the fed subjects, reflecting active glycogenolysis during a time of net glycogen synthesis. We conclude that liver glycogen synthesis and degradation occur simultaneously in humans under conditions of net glycogen synthesis. The relative turnover rate was significantly higher in the fed (57 +/- 3%) than in the fasted state (31 +/- 8%; P < 0.01). The results indicate that glycogen may regulate its rate of breakdown and that liver glycogen turnover may be an important factor in limiting the accumulation of liver glycogen in humans.

Adult↗
PubMed1994

Human muscle glycogen resynthesis after exercise: insulin-dependent and -independent phases.

To study the effects of glycogen depletion and insulin concentration on glycogen synthesis, gastrocnemius glycogen was measured with 13C-nuclear magnetic resonance at 4.7 T after exercise. Subjects performed single-leg toe raises to deplete gastrocnemius glycogen to 75, 50, or 25% of resting concentration (protocol I). Insulin dependence of glycogen synthesis was assessed after depletion to 25% with (protocol II) and without (protocol III) infusion of somatostatin to inhibit insulin secretion. After depletion to 75 and 50%, glycogen resynthesis rates were similar (2.4 +/- 0.7 and 2.8 +/- 0.6 mM/h, respectively). When glycogen was depleted to 25% (< 30 mM), the resynthesis rate was significantly higher (P < 0.02) at 33 +/- 7 mM/h, and it declined to 3.5 +/- 0.9 mM/h at > 35 mM glycogen. At < 35 mM glycogen, synthesis was not affected by low insulin (24 +/- 4 mM/h, protocol vs. 19 +/- 3 mM/h, protocol III), whereas at > 35 mM glycogen, synthesis ceased without insulin (-0.07 +/- 0.19 mM/h, protocol II). After depletion to 25% (protocol III), plasma lactate transiently increased (0.81 mM at rest, 1.82 mM 0 h after exercise, and 0.76 mM 2 h after exercise), whereas other plasma constituents did not significantly change. We conclude that after depletion to < 30 mM initial glycogen resynthesis is insulin independent and glycogen dependent, which suggests local control.

Adult↗
PubMed1994

Turnover of human muscle glycogen with low-intensity exercise.

To determine whether glycogen turnover occurs during prolonged low-intensity exercise, five subjects performed plantar flexion of the right leg at 15% MVC for 5 h. At rest and during the initial 2.5 h of exercise gastrocnemius glycogen was monitored in both legs with natural abundance 13C NMR. At 2.5 h exercise, a step-up infusion of 99% enriched 1-13C glucose was begun and maintained over the next 1.5 h of continued exercise to monitor 1-13C glucose incorporation into the exercising muscle's glycogen pool. Exercise was continued for an hour following the infusion, and NMR scans were performed throughout the session. During the first 2 h of exercise, glycogen 1-13C signal amplitudes dropped approximately 30% and remained there at 2.5 h, indicating that glycogen concentration had leveled. Following infusion, glycogen signal amplitudes rose to 123% of resting values, remaining there during an hour of subsequent exercise. There was no change of glycogen 1-13C signal in the nonexercising leg. Venous glucose levels remained stable until the infusion was begun and then rose < 7% (5.57-5.96 mmol.l-1) during the infusion. Venous insulin and C-peptide levels did not change during the infusion. We conclude that the human gastrocnemius can degrade and synthesize glycogen simultaneously during prolonged low-intensity exercise.

Adult↗
PubMed1993

NMR studies of 1H NOEs in glycogen.

We have examined the cross-relaxation behavior among the protons of oyster glycogen using nuclear Overhauser enhancement (NOE). Steady-state and transient NOEs were generated using low-power CW irradiation and frequency-selective inversions. In D2O, saturation of glycogen H2 and H4' at 3.64 ppm gave a strong negative NOE (eta = -0.74) at H1. The NOE was similar to the value predicted by the correlation time (tau c) calculated from the T1 and T2 of glycogen H1 in D2O assuming an isotropic rigid motor dipole-dipole model. Selective inversion of H2 and H4' gave a transient NOE at H1. In D2O, selective inversion of H1 also led to negative transient NOEs in the H2 + H4', H3, and H5 resonances. The magnitude and rates of appearance of the NOEs in H3 and H5 were too large to arise from direct H1-H3 and H1-H5 dipolar interactions, but were consistent with very efficient cross-relaxation leading to large second-order NOEs. The glycogen H1 NOE in H2O was also studied. Replacement of D2O with H2O as solvent significantly reduced the steady-state NOE at H1 following saturation of H2 + H4'. Saturation of the water resonance caused a large negative NOE at H1 (eta = -0.55) consistent with our earlier study which indicated that there was no direct dipolar interaction between H1 and free H2O.

Animals↗
PubMed1993

Localized 1H NMR measurement of glucose consumption in the human brain during visual stimulation.

Spatially localized 1H NMR spectroscopy has been applied to measure changes in brain glucose concentration during 8-Hz photic stimulation. NMR spectroscopic measurements were made in a 12-cm3 volume centered on the calcarine fissure and encompassing the primary visual cortex. The average maximum change in glucose levels was 0.34 mumol.g-1 (n = 5) at 15 min; glucose level had turned toward resting level at 25 min. The glucose change was used to calculate the increase of glucose cerebral metabolic rate in the visual cortex region for individual subjects by using the Michaelis-Menten model of glucose transport on the assumption of constant transport kinetics. The glucose cerebral metabolic rate was calculated to increase over the nonstimulated rate by 22% during the first 15 min of photic stimulation. A model in which the glucose metabolic rate gradually decreases during stimulation was proposed as a possible explanation for the recovery of brain glucose and previously measured lactate concentrations to prestimulus values after 15 min.

Blood Glucose↗
PubMed1993

Nuclear magnetic resonance relaxation of glycogen H1 in solution.

The NMR relaxation properties of the H1 proton of oyster glycogen in D2O and H2O solutions have been studied using nonselective, semiselective, and selective inversion recovery and Hahn spin-echo pulse sequences. The data were analyzed in terms of an isotropic, rigid-rotor dipole-dipole model including cross-relaxation. At 8.4 T in D2O, p = 5.4 +/- 0.4 s-1 and sigma = -4.5 +/- 0.4 s-1. The large, negative sigma value is consistent with strong cross-relaxation and a long correlation time. The relaxation data can be explained by a single correlation time, tau c = 2.7 x 10(-9) s, indicating significant internal mobility. With this value of tau c, and assuming that the structure of the glucose moieties was the same as in alpha-D-glucose crystals, the dipole sum contributing to T1 relaxation was calculated. The intra-ring relaxation was dominated by dipole fields from the H2 proton, but these only accounted for approximately 18% of the total relaxation. Most of the relaxation comes from inter-glucose relaxation. From modeling, this is dominated by the H4' across the alpha-1,4-glycosidic bond. The H1 longitudinal relaxation rates were significantly enhanced in H2O compared with D2O. This enhancement is not due to direct dipolar interaction between H1 and bulk water. Transverse relaxation rates were not significantly enhanced in H2O.

Animals↗