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Biomedical subjects

R G Shulman

Publications and source records attributed to R G Shulman.

At least 19 recordsLinked to original sources
PubMed1992

Dynamic mapping of the human visual cortex by high-speed magnetic resonance imaging.

We report the use of high-speed magnetic resonance imaging to follow the changes in image intensity in the human visual cortex during stimulation by a flashing checkerboard stimulus. Measurements were made in a 2.1-T, 1-m-diameter magnet, part of a Bruker Biospec spectrometer that we had programmed to do echo-planar imaging. A 15-cm-diameter surface coil was used to transmit and receive signals. Images were acquired during periods of stimulation from 2 s to 180 s. Images were acquired in 65.5 ms in a 10-mm slice with in-plane voxel size of 6 x 3 mm. Repetition time (TR) was generally 2 s, although for the long flashing periods, TR = 8 s was used. Voxels were located onto an inversion recovery image taken with 2 x 2 mm in-plane resolution. Image intensity increased after onset of the stimulus. The mean change in signal relative to the prestimulation level (delta S/S) was 9.7% (SD = 2.8%, n = 20) with an echo time of 70 ms. Irrespective of the period of stimulation, the increase in magnetic resonance signal intensity was delayed relative to the stimulus. The mean delay measured from the start of stimulation for each protocol was as follows: 2-s stimulation, delay = 3.5 s (SD = 0.5 s, n = 10) (the delay exceeds stimulus duration); 20- to 24-s stimulation, delay = 5 s (SD = 2 s, n = 20).

Brain Mapping↗
PubMed1992

1H-[13C] NMR measurements of [4-13C]glutamate turnover in human brain.

A limitation of previous methods for studying human brain glucose metabolism, such as positron emission tomography, is that metabolic steps beyond glucose uptake cannot be studied. Nuclear magnetic resonance (NMR) has the advantage of allowing the nondestructive measurement of 13C distribution in specific carbon positions of metabolites. In this study 1H-[13C] NMR spectroscopy in conjunction with volume localization was used to measure the rate of incorporation of 13C isotope from infused enriched [1-13C]glucose to human brain [4-13C]glutamate. In three studies C4 glutamate turnover time constants of 25, 20, and 17 min were measured in a 21-cm3 volume centered in the region of the visual cortex. Based on an analysis of spectrometer sensitivity the spatial resolution of the method can be improved to < 4 cm3. In conjunction with metabolic modeling and other NMR measurements this method can provide a measure of regional rates of the brain tricarboxylic acid cycle and other metabolic pathways.

Brain↗
PubMed1992

Co2+ as a shift reagent for 35Cl NMR of chloride with vesicles and cells.

Applications of high-resolution 35Cl NMR to the study of chloride in vivo and in vesicles have hitherto been limited by problems of NMR detectability and of resolving internal from external signals. We have characterized the effects of Co2+ on the 35Cl resonance of Cl- in solution and have shown that when added to suspensions of lipid vesicles, Co2+ shifts the 35Cl signal of the extravesicular Cl-, allowing clear resolution and quantitation of two peaks. We have assigned these signals to chloride inside and outside the vesicles. The spectra do not change over a 90-min period, demonstrating the stability of the vesicles in the presence of Co2+. This technique is shown to be applicable to red blood cell ghosts, where intravesicular and extravesicular chloride signals were separated and measured and chloride/sulfate exchange through the band 3 anion transport protein A was followed. In two plant species (an alga and a higher plant), an intracellular Cl- signal can be observed and resolved from the extracellular signal. The intracellular transportable chloride was found to be fully NMR-visible (+/- 5%) in the algal cells. The high steady-state levels of Cl- seen in the alga were consistent with previous work using 36Cl- labeling on a related species [Doblinger, R., & Tromballa, H.W. (1982) Planta 156, 10-15]. Successive spectra acquired after adding Co2+ to Chlorella cells under deenergizing conditions allow us to follow the time course of movement of Cl- out of the cells.

Cell Membrane↗
PubMed1992

Direct measurement of brain glucose concentrations in humans by 13C NMR spectroscopy.

Glucose is the main fuel for energy metabolism in the normal human brain. It is generally assumed that glucose transport into the brain is not rate-limiting for metabolism. Since brain glucose concentrations cannot be determined directly by radiotracer techniques, we used 13C NMR spectroscopy after infusing enriched D-[1-13C]glucose to measure brain glucose concentrations at euglycemia and at hyperglycemia (range, 4.5-12.1 mM) in six healthy children (13-16 years old). Brain glucose concentrations averaged 1.0 +/- 0.1 mumol/ml at euglycemia (4.7 +/- 0.3 mM plasma) and 1.8-2.7 mumol/ml at hyperglycemia (7.3-12.1 mM plasma). Michaelis-Menten parameters of transport were calculated to be Kt = 6.2 +/- 1.7 mM and Tmax = 1.2 +/- 0.1 mumol/g.min from the relationship between plasma and brain glucose concentrations. The brain glucose concentrations and transport constants are consistent with transport not being rate-limiting for resting brain metabolism at plasma levels greater than 3 mM.

Adolescent↗
PubMed1992

Localized 1H NMR spectra of glutamate in the human brain.

Localized 1H NMR spectra at TE = 12 ms were obtained from cerebral cortex of human subjects using ISIS with surface suppression. The 2.29-ppm resonance was assigned to C4 glutamate with contributions from C4 glutamine and GABA using in vivo spectral editing and comparison of chemical shift with pure compounds. The measured intensity ratio between the 2.29 resonance and the creatine resonance at 3.03 ppm was in good agreement with the ratio predicted from previously reported measurements of glutamate, glutamine, and GABA concentrations in biopsied human brain tissue.

Aspartic Acid↗
PubMed1992

Localized 13C NMR spectroscopy of myo-inositol in the human brain in vivo.

Natural abundance 13C NMR spectra obtained from 144-cm3 volumes in the human brain contained well-resolved resonances of myo-inositol after 60 min of data accumulation. A mean concentration of 7.2 +/- 0.5 mumol/g (+/- SE, n = 7) was calculated from the comparison with phantoms. 13C NMR spectroscopy thus provides a complementary role in the quantitation of metabolites also observed in the crowded 1H spectrum.

Adult↗
PubMed1992

Validation of 13C NMR measurement of human skeletal muscle glycogen by direct biochemical assay of needle biopsy samples.

Recent developments in 13C nuclear magnetic resonance (NMR) spectroscopy have permitted noninvasive assessment of glycogen concentration in human skeletal muscle. Before these indirect measurements could be accepted as accurate, it was essential that validation should be carried out by comparing the widely used method of muscle biopsy and direct biochemical assay for glycogen concentration with measurement by NMR. Eight normal subjects underwent six NMR scans of gastrocnemius and three biopsies of the same muscle on the same day. The overall mean for muscle glycogen concentration was 87.4 mM by NMR and 88.3 mM by biopsy. There was a close correlation between the pairs of observations on each subject (R = 0.95; P less than 0.0001). The mean coefficient of variation for NMR measurement was 4.3 +/- 2.1% and that for biopsy was 9.3 +/- 5.9%. The performance of the muscle biopsies was accompanied by a small but significant rise in plasma-free fatty acids (529 +/- 157 to 667 +/- 250; P less than 0.01), epinephrine (17 +/- 6 to 25 +/- 8 pg/ml; P less than 0.02), and norepinephrine (318 +/- 119 to 400 +/- 140 pg/ml; P less than 0.02) but no change in plasma glucose, plasma insulin, nor muscle glycogen concentration assessed by NMR. The study demonstrates that in vivo 13C NMR measurement of human muscle glycogen can be regarded as accurate, and the technique is associated with a higher precision that biopsy with direct biochemical assessment.

Biopsy, Needle↗
PubMed1992

NMR determination of the TCA cycle rate and alpha-ketoglutarate/glutamate exchange rate in rat brain.

A mathematical model of cerebral glucose metabolism was developed to analyze the isotopic labeling of carbon atoms C4 and C3 of glutamate following an intravenous infusion of [1-13C]glucose. The model consists of a series of coupled metabolic pools representing glucose, glycolytic intermediates, tricarboxylic acid (TCA) cycle intermediates, glutamate, aspartate, and glutamine. Based on the rate of 13C isotopic labeling of glutamate C4 measured in a previous study, the TCA cycle rate in rat brain was determined to be 1.58 +/- 0.41 mumol min-1 g-1 (mean +/- SD, n = 5). Analysis of the difference between the rates of isotopic enrichment of glutamate C4 and C3 permitted the rate of exchange between alpha-ketoglutarate (alpha-KG) and glutamate to be assessed in vivo. In rat brain, the exchange rate between alpha-KG and glutamate is between 89 +/- 35 and 126 +/- 22 times faster than the TCA cycle rate (mean +/- SD, n = 4). The sensitivity of the calculated value of the TCA cycle rate to other metabolic fluxes and to concentrations of glycolytic and TCA cycle intermediates was tested and found to be small.

Animals↗
PubMed1992

NMR determination of intracerebral glucose concentration and transport kinetics in rat brain.

The concentration of intracerebral glucose as a function of plasma glucose concentration was measured in rats by 13C NMR spectroscopy. Measurements were made in 20-60 min periods during the infusion of [1-13C]D-glucose, when intracerebral and plasma glucose levels were at steady state. Intracerebral glucose was found to vary from 0.7 to 19 mumol g-1 wet weight as the steady-state plasma glucose concentration was varied from 3 to 62 mM. A symmetric Michaelis-Menten model was fit to the brain and plasma glucose data with and without an unsaturable component, yielding the transport parameters Km, Vmax, and Kd. If it is assumed that all transport is saturable (Kd = 0), then Km = 13.9 +/- 2.7 mM and Vmax/Vgly = 5.8 +/- 0.8, where Vgly is the rate of brain glucose consumption. If an unsaturable component of transport is included, the transport parameters are Km = 9.2 +/- 4.7 mM, Vmax/Vgly = 5.3 +/- 1.5, and Kd/Vgly = 0.0088 +/- 0.0075 ml mumol-1. It was not possible to distinguish between the cases of Kd = 0 and Kd greater than 0, because the goodness of fit was similar for both. However, the results in both cases indicate that the unidirectional rate of glucose influx exceeds the glycolytic rate in the basal state by 2.4-fold and as a result should not be rate limiting for normal glucose utilization.

Animals↗
PubMed1992

Role of nucleoside uptake in renal postischemic ATP synthesis.

The role of nucleoside uptake in the enhanced metabolic recovery seen with postischemic ATP.MgCl2 was assessed by determining the effect of S-(p-nitrobenzyl)-6-thioinosine (NBTI) on postischemic ATP recovery in rats given normal saline (NS), ATP.MgCl2, or adenosine after 45 min of bilateral renal ischemia. In NS-infused animals, postischemic administration of NBTI (250 nmol) had no significant effect on the pattern of ATP recovery. In animals given 50 mumol ATP.MgCl2, coinfusion of NBTI significantly reduced the renal ATP content 2 h after reperfusion but blocked only one-half of the enhancement in renal ATP content compared with animals given ATP.MgCl2 alone. In animals postischemically infused with [2,5,8-3H]ATP.MgCl2 (50 mumol) there was significant labeling of nucleotides, nucleosides, and bases after 2 h of reperfusion. The specific activity of the adenosine pool was consistent with significant label uptake in the form of adenosine. Coinfusion of NBTI led to a significant reduction in label incorporation into renal ATP and total adenine nucleotide pools. These data are consistent with an important role for an NBTI-sensitive nucleoside uptake mechanism in the enhanced metabolic and functional recovery observed in ischemically injured kidney treated by postischemic infusion of ATP.MgCl2.

Adenosine↗
PubMed1992

Proton magnetic resonance spectroscopy of cerebral lactate and other metabolites in stroke patients.

BACKGROUND AND PURPOSE: Proton magnetic resonance spectroscopy can measure in vivo brain lactate and other metabolites noninvasively. We measured the biochemical changes accompanying stroke in 16 human subjects with cortical or deep cerebral infarcts within the first 3 weeks after symptom onset, and performed follow-up studies on six. METHODS: One-dimensional proton spectroscopic imaging encompassing the infarct region was performed with a 2.1-T whole-body magnet using the stimulated echo pulse sequence and an echo time of 270 msec. RESULTS: All but one of the cortical stroke patients had increased lactate within or near the infarct. Persistently elevated cerebral lactate was documented in five of six cases studied serially as long as 251 days after infarction. N-acetylaspartate levels were decreased in most cortical strokes. Elevated lactate, accompanied by minimal reduction in N-acetylaspartate, was recorded in two of four patients in the first week following a small subcortical infarct. CONCLUSIONS: Long-term elevation of lactate commonly occurs after stroke. This lactate may arise from ongoing ischemia or infiltrating leukocytes, or it may be a residual of the lactate formed during the initial insult. The ability to observe stroke-elevated lactate pools at any time after lesion onset provides an approach to distinguishing among these possibilities in the future.

Aspartic Acid↗
PubMed1992

31P nuclear magnetic resonance measurements of muscle glucose-6-phosphate. Evidence for reduced insulin-dependent muscle glucose transport or phosphorylation activity in non-insulin-dependent diabetes mellitus.

To assess the rate-limiting step in muscle glycogen synthesis in non-insulin-dependent diabetes mellitus (NIDDM), the concentration of glucose-6-phosphate (G6P) was measured by 31P nuclear magnetic resonance (NMR) during a hyperglycemic-hyperinsulinemic clamp. Six subjects with NIDDM and six age weight-matched controls were studied at similar steady-state plasma concentrations of insulin (approximately 450 pmol/liter) and glucose (11 mmol/liter). The concentration of G6P in the gastrocnemius muscle was measured by 31P NMR. Whole-body oxidative and nonoxidative glucose metabolism was determined by the insulin-glucose clamp technique in conjunction with indirect calorimetry. Nonoxidative glucose metabolism which under these conditions is a measure of muscle glycogen synthesis (1990. N. Engl. J. Med. 322:223-228), was 31 +/- 7 mumol/(kg body wt-min) in the normal subjects and 13 +/- 3 mumol/(kg body wt-min) in the NIDDM subjects (P less than 0.05). The concentration of G6P was higher (0.24 +/- 0.02 mmol/kg muscle) in the normal subjects than in the NIDDM subjects (0.17 +/- 0.02, P less than 0.01). Increasing insulin concentrations to insulin 8,500 pmol/liter in four NIDDM subjects restored the glucose uptake rate and G6P concentrations to normal levels. In conclusion, the lower concentration of G6P in the diabetic subjects despite a decreased rate of nonoxidative glucose metabolism is consistent with a defect in muscle glucose transport or phosphorylation reducing the rate of muscle glycogen synthesis.

Aged↗
PubMed1992

Increased rate of gluconeogenesis in type II diabetes mellitus. A 13C nuclear magnetic resonance study.

UNLABELLED: To quantitate hepatic glycogenolysis, liver glycogen concentration was measured with 13C nuclear magnetic resonance spectroscopy in seven type II diabetic and five control subjects during 23 h of fasting. Net hepatic glycogenolysis was calculated by multiplying the rate of glycogen breakdown by the liver volume, determined from magnetic resonance images. Gluconeogenesis was calculated by subtracting the rate of hepatic glycogenolysis from the whole body glucose production rate, measured using [6-3H]glucose. Liver glycogen concentration 4 h after a meal was lower in the diabetics than in the controls; 131 +/- 20 versus 282 +/- 60 mmol/liter liver (P < 0.05). Net hepatic glycogenolysis was decreased in the diabetics, 1.3 +/- 0.2 as compared to 2.8 +/- 0.7 mumol/(kg body wt x min) in the controls (P < 0.05). Whole body glucose production was increased in the diabetics as compared to the controls, 11.1 +/- 0.6 versus 8.9 +/- 0.5 mumol/(kg body wt x min) (P < 0.05). Gluconeogenesis was consequently increased in the diabetics, 9.8 +/- 0.7 as compared to 6.1 +/- 0.5 mumol/(kg body wt x min) in the controls (P < 0.01), and accounted for 88 +/- 2% of total glucose production as compared with 70 +/- 6% in the controls (P < 0.05). IN CONCLUSION: increased gluconeogenesis is responsible for the increased whole body glucose production in type II diabetes mellitus after an overnight fast.

Aged↗
PubMed1992

Spectroscopic imaging of stroke in humans: histopathology correlates of spectral changes.

Previous studies of human stroke by 1H nuclear magnetic resonance spectroscopy have shown elevation of lactate lasting 3 to 6 months. Complete metabolic turnover of the elevated lactate pool has been demonstrated 5 weeks after a stroke. Its cellular localization is among the first questions requiring clarification. Information pertinent to this question came to us from a patient with a 2-week-old stroke by 1H nuclear magnetic resonance spectroscopic imaging 1 week before his death led to neuropathologic examination of the brain. 1H spectra from voxels including the infarcts showed increased lactate and decreased N-acetylaspartate. Histopathology showed sheets of foamy macrophages in the infarct, but few neurons. Macrophage density ranged from 196 cells/mm2 near the surface of the infarct to 788 near its medial margin. Glial density was 500 to 800 cells/mm2. Lactate concentration in voxels including portions of the infarct was estimated at 7 to 14 mM. Voxels showing low N-acetylaspartate and high lactate on spectroscopic imaging were associated with histopathologic sections containing foamy macrophages. Brain macrophages--which begin to appear 3 days after infarction and gradually disappear over several months--could be a major source of elevated lactate signals that persist for months after stroke.

Brain↗
PubMed1991

Assignment of the 1H chemical shifts of glycogen.

Assignments of nearly all the 1H chemical shifts of glycogen are made by 2-D 1H-1H homonuclear and 13C-1H heteronuclear COSY. We demonstrated that it is possible to obtain well-resolved 2-D n.m.r. spectra for a large molecule like glycogen. The seven nonequivalent protons of the glucose residues in the alpha-(1----4)-linked chains, and of those at the nonreducing ends, were completely assigned. Distinct chemical shifts for H-1 and H-2 immediately adjacent to the alpha-(1----6) bonds at the branch points were also determined. Several modifications of previous 13C chemical shift assignments were made from the heteronuclear 2-D n.m.r. data.

Animals↗
PubMed1991

Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR.

The rate of net hepatic glycogenolysis was assessed in humans by serially measuring hepatic glycogen concentration at 3- to 12-hour intervals during a 68-hour fast with 13C nuclear magnetic resonance spectroscopy. The net rate of gluconeogenesis was calculated by subtracting the rate of net hepatic glycogenolysis from the rate of glucose production in the whole body measured with tritiated glucose. Gluconeogenesis accounted for 64 +/- 5% (mean +/- standard error of the mean) of total glucose production during the first 22 hours of fasting. In the subsequent 14-hour and 18-hour periods of the fast, gluconeogenesis accounted for 82 +/- 5% and 96 +/- 1% of total glucose production, respectively. These data show that gluconeogenesis accounts for a substantial fraction of total glucose production even during the first 22 hours of a fast in humans.

Adult↗