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T Nelson

Publications and source records attributed to T Nelson.

At least 73 records · Page 4Linked to original sources
PubMed1989

Refinement of the kinetic model of the 2-[14C]deoxyglucose method to incorporate effects of intracellular compartmentation in brain.

A translocase to transport hexose phosphate formed in the cytosol into the cisterns of the endoplasmic reticulum, where the phosphatase resides, is absent in brain (Fishman and Karnovsky, 1986). 2-Deoxyglucose-6-phosphate (DG-6-P) may therefore have limited access to glucose-6-phosphatase (G-6-Pase), and transport of the DG-6-P across the endoplasmic reticular membrane may be rate limiting to its dephosphorylation. To take this compartmentation into account, a five-rate constant (5K) model was developed to describe the kinetic behavior of 2-deoxyglucose (DG) and its phosphorylated product in brain. Loss of DG-6-P was modeled as a two-step process: (a) transfer of DG-6-P from the cytosol into the cisterns of the endoplasmic reticulum; (b) hydrolysis of DG-6-P by G-6-Pase and subsequent return of the free DG to the precursor pool. Local CMRglc (LCMRglc) was calculated in the rat on the basis of this model and compared with values calculated on the basis of the three-rate constant (3K) and the four-rate constant (4K) models of the DG method. The results show that under normal physiological conditions all three models yield values of LCMRglc that are essentially equivalent for experimental periods between 25 and 45 min. Therefore, the simplest model, the 3K model, is sufficient. For experimental periods from 60 to 120 min, the 4K and 5K models do not correct completely for loss of product, but the 5K model does yield estimates of LCMRglc that are closer to the values at 45 min than those obtained with the 3K and 4K models.

Animals↗
PubMed1989

Direct chemical measurement of the lambda of the lumped constant of the [14C]deoxyglucose method in rat brain: effects of arterial plasma glucose level on the distribution spaces of [14C]deoxyglucose and glucose and on lambda.

The lumped constant in the operational equation of the 2-[14C]deoxyglucose (DG) method contains the factor lambda that represents the ratio of the steady-state tissue distribution spaces for [14C]DG and glucose. The lumped constant has been shown to vary with arterial plasma glucose concentration. Predictions based mainly on theoretical grounds have suggested that disproportionate changes in the distribution spaces for [14C]DG and glucose and in the value of lambda are responsible for these variations in the lumped constant. The influence of arterial plasma glucose concentration on the distribution spaces for DG and glucose and on lambda were, therefore, determined in the present studies by direct chemical measurements. The brain was maintained in steady states of delivery and metabolism of DG and glucose by programmed intravenous infusions of both hexoses designed to produce and maintain constant arterial concentrations. Hexose concentrations were assayed in acid extracts of arterial plasma and freeze-blown brain. Graded hyperglycemia up to 28 mM produced progressive decreases in the distribution spaces of both hexoses from their normoglycemic values (e.g., approximately -20% for glucose and -50% for DG at 28 mM). In contrast, graded hypoglycemia progressively reduced the distribution space for glucose and increased the space for [14C]DG. The values for lambda were comparatively stable in normoglycemic and hyperglycemic conditions but rose sharply (e.g., as much as 9-10-fold at 2 mM) in severe hypoglycemia.

Adrenal Glands↗
PubMed1988

Over-estimation of glucose-6-phosphatase activity in brain in vivo. Apparent difference in rates of [2-3H]glucose and [U-14C]glucose utilization is due to contamination of precursor pool with 14C-labeled products and incomplete recovery of 14C-labeled metabolites.

Significant dephosphorylation of glucose 6-phosphate due to glucose-6-phosphatase activity in rat brain in vivo was recently reported (Huang, M., and Veech, R.L. (1982) J. Biol. Chem. 257, 11358-11363). The evidence was an apparent more rapid 3H than 14C loss from the glucose pool and faster [2-3H]glucose than [U-14C]glucose utilization following pulse labeling of the brain with [2-3H,U-14C]glucose. Radiochemical purity of the glucose and quantitative recovery of the labeled products of glucose metabolism isolated from the brain were obviously essential requirements of their study, but no evidence for purity and recovery was provided. When we repeated these experiments with the described isolation procedures, we replicated the results, but found that: 1) the precursor glucose pool contained detritiated, 14C-labeled contaminants arising from glucose metabolism, particularly 2-pyrrolidone-5-carboxylic acid derived from [14C]glutamine; 2) [14C]glucose metabolite were not quantitatively recovered; 3) the procedure used to isolate the glucose itself produced detritiated, 14C-labeled derivatives of [2-3H,U-14C]glucose. These deficiencies in the isolation procedures could fully account for the observations that were interpreted as evidence of significant glucose 6-phosphate dephosphorylation by glucose-6-phosphatase activity. When glucose was isolated by more rigorous procedures and its purity verified in the present studies, no evidence for such activity in rat brain was found.

Animals↗
PubMed1988

Cell position and light influence C4 versus C3 patterns of photosynthetic gene expression in maize.

C4 plants such as maize partition photosynthetic activities in two morphologically distinct cell types, bundle sheath (BS) and mesophyll (M), which lie as concentric layers around veins. We show that both light and cell position relative to veins influence C4 photosynthetic gene expression. A pattern of gene expression characteristic of C3 plants [ribulose bisphosphate carboxylase (RuBPCase) and light-harvesting chlorophyll a/b binding protein in all photosynthetic cells] is observed in leaf-like organs such as husk leaves, which are sparsely vascularized. This pattern of gene expression reflects direct fixation of CO2 in the C3 photosynthetic pathway, as determined by O2 inhibition assays. Light induces a switch from C3-type to C4-type gene expression patterns in all leaves, primarily in cells that are close to a vein. We propose that light causes repression of RuBPCase expression in M cells, by a mechanism associated with the vascular system, and that this is an essential step in the induction of C4 photosynthesis.

Cell Compartmentation↗
PubMed1988

Isolation and characterization of a hydroxyacid-oxoacid transhydrogenase from rat kidney mitochondria.

A transhydrogenase that catalyzes the oxoacid-dependent oxidation of specific hydroxyacids has been found in rat kidney, liver, and brain. The hydroxyacids that have been found to be substrates for this enzyme are gamma-hydroxybutyrate, D-alpha-hydroxyglutarate, and L-beta-hydroxybutyrate. The oxoacids that are the best substrates for this enzyme are alpha-ketoglutarate and succinic semialdehyde; alpha-ketoadipate and oxalacetate are also substrates. This enzyme is located in the mitochondrial fraction of the cell and is not dependent on added NAD+ or NADP+.

3-Hydroxybutyric Acid↗
PubMed1988

Cellular pattern of photosynthetic gene expression in developing maize leaves.

Leaf development in C4 plants such as maize involves the differentiation of two photosynthetic cell types [bundle sheath (BS) and mesophyll (M)] to form Kranz-type leaf anatomy. This cellular dimorphism partitions photosynthetic activities so that each enzyme of the C4 pathway accumulates only in the appropriate cell type. We have exploited this property to study BS and M cell interactions in developing maize leaves. Our previous studies showed that C4 proteins appear concurrently with the appearance of Kranz anatomy. To look at earlier events in BS and M cell development we have used three of the corresponding C4 mRNAs as cell-specific markers. We have followed, in situ, the accumulation of malic enzyme (ME), phosphoenolpyruvate carboxylase (PEPCase), and ribulose bisphosphate carboxylase (RuBPCase) mRNAs in developing leaves of both normal and mutant argentia (ar) maize. We have isolated a partial cDNA clone for maize ME to examine ME mRNA expression. We show that throughout the development of light-grown seedlings, all three mRNAs accumulate in a cell-specific fashion in both normal and ar leaves. The pattern of C4 mRNA accumulation longitudinally along the veins, laterally across the leaf, and locally around individual veins reveals the spatial and temporal sequence of BS and M cell development. BS cell-specific mRNAs accumulate around developing veins before Kranz anatomy is evident morphologically. Our analysis of the ar mutant, in which C4 mRNA appearance is delayed relative to the appearance of Kranz anatomy, demonstrates first that BS and M cells develop in clusters across the leaf blade and second that BS cells surrounding any individual vein are activated asynchronously. We discuss our results in relation to models and mechanisms of BS and M cell development.

Cell Compartmentation↗
PubMed1988

Oxidation of gamma-hydroxybutyrate to succinic semialdehyde by a mitochondrial pyridine nucleotide-independent enzyme.

An antibody that inhibits over 95% of the cytosolic NADP+-dependent gamma-hydroxybutyrate (GHB) dehydrogenase activity of either rat brain or kidney was found to inhibit only approximately 50% of the conversion of [1-14C]GHB to 14CO2 by rat kidney homogenate. A similar result was obtained with sodium valproate, a potent inhibitor of GHB dehydrogenase. The mitochondrial fraction from rat brain and kidney was found to catalyze the conversion of [1-14C]GHB to 14CO2. The dialyzed mitochondrial fraction also catalyzed the oxidation of GHB to succinic semialdehyde (SSA) in a reaction that did not require added NAD+ or NADP+ and which was not inhibited by sodium valproate. The enzyme from the mitochondrial fraction which converts GHB to SSA appears to be distinct from the NADP+-dependent cytosolic oxidoreductase which catalyzes this reaction.

Animals↗
PubMed1987

The argentia mutation delays normal development of photosynthetic cell-types in Zea mays.

Photosynthesis in C4-type grasses such as maize involves the interaction of two cell types (bundle sheath (BS) and mesophyll (M)) which both contain cell-specific photosynthetic enzymes. Malate dehydrogenase, phosphoenolpyruvate carboxylase and pyruvate phosphate dikinase are located in the M cells and malic enzyme and ribulose bisphosphate carboxylase are found in the BS cells. We have studied photosynthetic development in leaves of the temperature-sensitive greening mutant argentia (ar). We have determined that with the exception of malate dehydrogenase, levels of C4 enzymes are lower in ar leaves than in normal leaves. Malate dehydrogenase accumulates identically in both. Using in situ immunolocalization techniques with normal and ar leaves, we have observed a developmental pattern of C4 protein accumulation. In normal leaves protein was detected first in cells surrounding the median vein, then in cells surrounding other major veins, and finally in cells surrounding minor veins. In ar leaves, C4 enzymes accumulate in the correct cell type and in this same order but their appearance is delayed. Furthermore, BS cell development is delayed with respect to M cell development. The observed pattern of photosynthetic development reflects an earlier manifested pattern of vascular development yet the timing of vascular differentiation in ar mutants appears to be normal.

Cell Differentiation↗
PubMed1987

Evidence for the participation of a cytosolic NADP+-dependent oxidoreductase in the catabolism of gamma-hydroxybutyrate in vivo.

The concentration of gamma-hydroxybutyrate (GHB) in brain, kidney, and muscle as well as the clearance of [1-14C]GHB in plasma have been found to be altered by the administration of a number of metabolic intermediates and drugs that inhibit the NADP+-dependent oxidoreductase, "GHB dehydrogenase," an enzyme that catalyzes the oxidation of GHB to succinic semialdehyde. Administration of valproate, salicylate, and phenylacetate, all inhibitors of GHB dehydrogenase, significantly increased the concentration of GHB in brain; salicylate increased GHB concentration in kidney, and alpha-ketoisocaproate increased GHB levels in kidney and muscle. The half-life of [1-14C]GHB in plasma was decreased by D-glucuronate, a compound that stimulates the oxidation of GHB by this enzyme and was increased by a competitive substrate of the enzyme, L-gulonate. The results of these experiments suggest a role for GHB dehydrogenase in the regulation of tissue levels of endogenous GHB.

Animals↗
PubMed1987

Auditory enhancement of changes in spectral amplitude.

An auditory enhancement effect occurs when one component of a harmonic series is omitted for a few hundred milliseconds and then reintroduced: The reintroduced harmonic stands out perceptually. Three experiments are reported that studied a version of this effect in which several components of a harmonic series are enhanced to define the formants of a vowel. Using the accuracy of vowel identification to measure the prominence of the formant peaks in the effective auditory representation, forms of the effect were identified that are qualitatively similar to the incremental and decremental responses seen in primary auditory-nerve fibers. These results are compatible with an origin for the enhancement effect in peripheral auditory adaptation. However, an additional mechanism is required to account for the demonstration [Viemeister and Bacon, J. Acoust. Soc. Am. 71, 1502-1507 (1982)] that enhancement can involve a true gain in the frequency region of the reintroduced component. These effects demonstrate one way in which the auditory system may attenuate the prominence of background noises while preserving the ability to represent changes in spectral amplitude produced by newly arriving signals.

Adult↗
PubMed1986

The Fate of Chloroplast Proteins during Photooxidation in Carotenoid-Deficient Maize Leaves.

Maize seedlings, treated with the herbicide norflurazon to produce a deficiency in carotenoid pigments, were grown in low-fluence-rate light. Under these conditions, which induced chlorophyll biosynthesis while minimizing photooxidation, carotenoid-deficient seedlings showed identical patterns of chloroplast protein accumulation compared with normal seedlings. Carotenoid pigments thus play no direct role in regulating the accumulation of chloroplast proteins. When shifted to high-fluence-rate light, chlorophyll was rapidly photooxidized in carotenoid-deficient seedlings. Chloroplast proteins showed varying degrees of sensitivity to photooxidation. The P-700 apoprotein of photosystem I was rapidly degraded. Most stromal and thylakoid proteins either decreased progressively in photooxidative conditions or appeared to be unaffected. The relative quantity of the light-harvesting chlorophyll a/b-binding protein of photosystem II increased significantly in the first few hours of high-fluence-rate light. It then appeared to be only minimally affected 18 hours after complete photooxidation of chlorophyll.

Journal Article↗
PubMed1986

Invalidity of criticisms of the deoxyglucose method based on alleged glucose-6-phosphatase activity in brain.

The observations made by Sacks et al. [Neurochem. Res. 8, 661-685 (1983)] on which they based their criticisms of the deoxyglucose method have been examined and found to have no relationship to the conclusions drawn by them. (1) The observations of Sacks et al. (1983) of constant concentrations of [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate, predominantly in the form of product, reflects only the postmortem phosphorylation of the precursor during the dissection of the brain in their experiments. When the brains are removed by freeze-blowing, the time courses of the [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate concentrations in brain during the 45 min after the intravenous pulse are close to those predicted by the model of the deoxyglucose method. (2) Their observation of a reversal of the cerebral arteriovenous difference from positive to negative for [14C]deoxyglucose and not for [14C]glucose after an intravenous infusion of either tracer is, contrary to their conclusions, not a reflection of glucose-6-phosphatase activity in brain but the consequence of the different proportions of the rate constants for efflux and phosphorylation for these two hexoses in brain and is fully predicted by the model of the deoxyglucose method. (3) It is experimentally demonstrated that there is no significant arteriovenous difference for glucose-6-phosphate in brain, that infusion of [32P]glucose-6-phosphate results in no labeling of brain, and that the blood-brain barrier is impermeable to glucose-6-phosphate. Glucose-6-phosphate cannot, therefore, cross the blood-brain barrier, and the observation by Sacks and co-workers [J. Appl. Physiol. 24, 817-827 (1968); Neurochem. Res. 8, 661-685 (1983)] of a positive cerebral arteriovenous difference for [14C]glucose-6-phosphate and a negative arteriovenous difference for [14C]glucose cannot possibly reflect glucose-6-phosphatase activity in brain as concluded by them. Each of the criticisms raised by Sacks et al. has been demonstrated to be devoid of validity.

Animals↗
PubMed1985

Reexamination of glucose-6-phosphatase activity in the brain in vivo: no evidence for a futile cycle.

Glucose-6-phosphatase activity in the rat brain in vivo was estimated by measuring the differential loss of tritium and carbon-14 from the glucose pool labeled by a mixture of [2-3H]glucose and [U-14C]glucose. The results provide no evidence of significant dephosphorylation of glucose-6-phosphate and do not support the hypothesis of a futile cycle involving glucose-6-phosphatase activity in the brain.

Animals↗
PubMed1985

Sequelae of left ventricular electrical endocardial ablation.

The endomyocardial residual effects of left ventricular endocardial electrical ablation utilizing unipolar and bipolar electrode catheters were studied in 15 dogs. Histopathologic techniques specific for contraction band necrosis revealed that the mean maximal depth and breadth of necrosis was 0.63 +/- 0.44 and 1.23 +/- 0.82 cm, respectively. The dimensions of necrosis were significantly increased when utilizing larger energy discharges, especially through unipolar electrodes. Four dogs died during the procedure, three from ventricular fibrillation and one from asystole, and two died suddenly within the succeeding 24 hours. Endocardial thrombi were noted at necropsy in two dogs. In conclusion, transcatheter endocardial electrical ablation may destroy a sufficient mass of myocardium to interrupt arrhythmogenic conduction tissue, especially when larger currents are delivered through unipolar electrodes. However, serious ventricular arrhythmias and endocardial thrombi should be anticipated.

Animals↗
PubMed1984

Light-regulated gene expression during maize leaf development.

We have established schedules of expression during maize leaf development in light and darkness for the messenger RNAs (mRNAs) and polypeptides for ribulose 1,5-bisphosphate carboxylase (RuBPCase) subunits, phosphoenolpyruvate carboxylase (PEPCase), and the light-harvesting chlorophyll a/b-binding protein (LHCP). Levels of mRNAs were measured by hybridization with cloned probes, and proteins were measured by immunodetection on protein gel blots. The initial synthesis in leaves of all four mRNAs follows a light-independent schedule; illumination influences only the level to which each mRNA accumulates. The synthesis of RuBPCase small and large subunits and of PEPCase polypeptides also follows a light-independent schedule which is modified quantitatively by light. However, the accumulation of LHCP polypeptides absolutely requires illumination. The accumulation of each protein closely follows the accumulation of its mRNA during growth in light. Higher ratios of PEPCase and RuBPCase protein to mRNA occur during dark growth.

Cloning, Molecular↗