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At least 1,081 records · Page 60Linked to original sources
PubMed2006

Velocity-selective arterial spin labeling.

In pathologies in which slow or collateral flow conditions may exist, conventional arterial spin labeling (ASL) methods that apply magnetic tags based on the location of arterial spins may not provide robust measures of cerebral blood flow (CBF), as the transit delay for the delivery of blood to target tissues may far exceed the relaxation time of the tag. Here we describe current methods for ASL with velocity-selective (VS) tags (termed VSASL) that do not require spatial selectivity and can thus provide quantitative measures of CBF under slow and collateral flow conditions. The implementation of a robust multislice VSASL technique is described in detail, and data obtained with this technique are compared with those obtained with conventional pulsed ASL (PASL). The technical considerations described here include the design of VS pulses, background suppression, anisotropy with respect to velocity-encoding directions, and CBF quantitation issues.

Adult↗
PubMed1984

Subcellular distribution of spin-labeled bithiazoles and bleomycin in living KB cells: an ESR study.

Spin-labeled derivatives of bithiazole and bleomycin were studied with respect to their uptake and localization in KB cells in vivo. It was found that the amidification of the C-terminal carboxylic group of bleomycinic acid was essential for the penetration of the probes in the cells and that the subcellular localization depended on the number and spacing of positively charged groups.

Bleomycin↗
PubMed1992

Binding of endotoxin to macrophages; interactions of spin-labelled saccharide residues.

The molecular mechanisms of endotoxin action are poorly understood. A prerequisite to cellular activation by this agent must be interaction (binding) with the plasma membrane. In this study we have investigated the role of the polysaccharide region of endotoxin (LPS) in binding to macrophages and macrophage-like cell lines. The LPS molecules, from Escherichia coli O111.B4, J5 and the lipid-A, were spin labelled with 2,2,6,6-tetramethylpiperidine-N-oxyl] (Tempo) free radical in their sugar residues, and examined by electron spin resonance spectroscopy. This is the first report of the synthesis of spin-labelled endotoxins. Measurement of the rotational correlation times (Tc) indicated that the saccharide resides do not bind to membrane surface structures and suggests that the binding of LPS to macrophages is mediated by the lipid acyl chains. Anti-sera to LPS from E. coli O111.B4 was effective in binding to the polysaccharide of the same LPS bound to the cell surface.

Animals↗
PubMedDate not supplied

Effect of inhalation anesthetics on spin-labeled cholesterol containing DPPC vesicles.

We have investigated by means of electron spin resonance (ESR) spectroscopy the influence of three inhalation anesthetics, i.e. halothane, chloroform and diethyl ether, on the interfacial and hydrophobic region as well of 38 mol% cholesterol containing DPPC unilamellar vesicles. The study has been carried out in the temperature range 25-45 degrees C. The variation of the order parameter, S, vs temperature of the lipid phase indicates that with this content of cholesterol the characteristic gel----liquid crystalline main phase transition of DPPC, normally occurring at Tt approximately 41 degrees C, disappears. When halothane and chloroform are added to the vesicles suspension up to [DPPC]/[anesthetic] molar ratio of 1:1 the main phase transition, as detected with the stearic acid spin label I (12,3), reappears again and it results down shifted at Tt approximately 35 and 39 degrees C, respectively. In presence of diethyl ether, instead, the main phase transition is not observable also at the highest concentration of anesthetic used. Moreover, halothane and chloroform affect similarly the hydrophobic core of cholesterol + DPPC vesicles which, in turn, results to be different from the action exerted by diethyl ether in the same region. The ESR findings are discussed in terms of competitive effects shown by cholesterol and inhalation anesthetics. Moreover, the interfacial region of CHOL + DPPC vesicles results to be the target of anesthetics.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1983

[1H-NMR study of the Naja naja oxiana neurotoxin II and its spin-labeled derivatives. Conformation of "short" neurotoxins].

In 1H NMR spectra of neurotoxin II N. n. oxiana the chemical shift pH-dependences in H2O and 2H2O solutions were studied, and also the deuterium exchange rates and chemical shift temperature gradients were measured for the amide protons. The spin probe method was applied to assess the degree of exposure into solvent of the amide and side chain protons. With the purpose of establishing mutual disposition of certain neurotoxin II groupings, nuclear Overhauser effect was studied in the 1H NMR spectra, along with the broadening of proton resonances induced by spin labels selectively attached to epsilon-amino groups of Lys26, Lys27, Lys45 or Lys47. The mobility of these labels was determined from the EPR spectra. The methyl resonances of Val and Leu residues were assigned to a definite position in the amino acid sequence. The following pKa were determined: alpha-NH2 Leu1 (9,2), gamma-COOH Glu2 (3,7), alpha-COOH Asn62 (1,3). The protonation of a carboxyl group(s) in neurotoxin II (alpha-COOH Asn62 seems to be involved) decreases the temperature stability of the neurotoxin II conformation. On the basis of studies on neurotoxin II and some other homologous neurotoxins, the model for the "short" neurotoxin folding in solution was proposed. Comparison of experimental data for the disposition of equivalent groups in homologous neurotoxins and in the X-ray structure of erabutoxin b Laticauda semifasciata revealed that the Val46 side chain in solution might change its orientation by 180 degrees with respect to polypeptide backbone. Binding of spin labeled neurotoxin II derivatives to the acetylcholine receptor was discussed in light of the obtained data.

Amino Acid Sequence↗
PubMed2003

Probing the binding pocket and endocytosis of a G protein-coupled receptor in live cells reported by a spin-labeled substance P agonist.

To probe the molecular nature of the binding pocket of a G protein-coupled receptor and the events immediately following the binding and activation, we have modified the substance P peptide, a potent agonist for the neurokinin-1 receptor, with a nitroxide spin probe specifically attached at Lys-3. The agonist properties and binding affinity of the spin-labeled substance P are similar to the native peptide. Using electron paramagnetic resonance (EPR) spectroscopy, the substance P analogue is capable of reporting the microenvironment found in the binding pocket of the receptor. The EPR spectrum of bound peptide indicates that the Lys-3 portion of the agonist is highly flexible. In addition, we detect a slight increase in the mobility of the bound peptide in the presence of a non-hydrolyzable analogue of GTP, indicative of the alternate conformational states described for this class of receptor. The down-regulation of neurokinin-tachykinin receptors is accomplished by a rapid internalization of the activated protein. Thus, it was also of interest to establish whether spin-labeled substance P could serve as a real time reporter for endocytosis. Our findings show the receptor agonist is efficiently endocytosed and the loss of EPR signal upon internalization provides a real time monitor of endocytosis. The rapid loss of signal suggests that endosomal trafficking vesicles maintain a reductive environment. Whereas the reductive capacity of the lysosome has been established, our findings indicate this capacity in early endosomes as well.

Animals↗
PubMed2003

Evaluation of systematic quantification errors in velocity-selective arterial spin labeling of the brain.

Velocity-selective (VS) sequences potentially permit arterial spin labeling (ASL) perfusion imaging with labeling applied very close to the tissue. In this study the effects of cerebrospinal fluid (CSF) motion, radiofrequency (RF) field imperfections, and sequence timing parameters on the appearance and quantitative perfusion values obtained with VS-ASL were evaluated. Large artifacts related to CSF motion were observed with moderate velocity weighting, which were removed by inversion recovery preparation at the cost of increased imaging time. Imperfect refocusing and excitation pulses resulting from nonuniform RF fields produced systematic errors in the ASL subtraction images. A phase cycling scheme was introduced to eliminate these errors. Quantitative perfusion images were obtained with CSF suppression and phase cycling. Gray matter blood flow of 27.7 ml 100 g(-1) min(-1), approximately half the value reported in studies using spatially-selective ASL, was measured. Potential sources for this underestimation are discussed.

Adult↗
PubMed2005

Spin-labeling coronary MR angiography with steady-state free precession and radial k-space sampling: initial results in healthy volunteers.

The purpose of this study was to prospectively compare free-breathing navigator-gated cardiac-triggered three-dimensional steady-state free precession (SSFP) spin-labeling coronary magnetic resonance (MR) angiography performed by using Cartesian k-space sampling with that performed by using radial k-space sampling. A new dedicated placement of the two-dimensional selective labeling pulse and an individually adjusted labeling delay time approved by the institutional review board were used. In 14 volunteers (eight men, six women; mean age, 28.8 years) who gave informed consent, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel sharpness, vessel length, and subjective image quality were investigated. Differences between groups were analyzed with nonparametric tests (Wilcoxon, Pearson chi2). Radial imaging, as compared with Cartesian imaging, resulted in a significant reduction in the severity of motion artifacts, as well as an increase in SNR (26.9 vs 12.0, P < .05) in the coronary arteries and CNR (23.1 vs 8.8, P < .05) between the coronary arteries and the myocardium. A tendency toward improved vessel sharpness and vessel length was also found with radial imaging. Radial SSFP imaging is a promising technique for spin-labeling coronary MR angiography.

Adult↗
PubMed1992

Partitional and motional properties of a spin-labeled daunomycin in lipid bilayers. An ESR study.

The partition coefficient of a spin-labeled daunomycin (DAU-SL) in dimyristoylphosphatidylcholine membrane has been determined using the electron spin resonance (ESR) method. The experiment was carried out as a function of temperature between 5 degrees C and 35 degrees C, giving partition coefficients between 2 and 6 without abrupt change at the phase transition. The thermodynamic parameters on transferring the DAU-SL from the aqueous phase to the lipid bilayer were also calculated. The calculated values are: delta H = 6.11 kcal/mol and delta S = 23 cal/K mol. The partitioning of the DAU-SL and its motion in the membrane were investigated in a wide range of pH (4-10.3). The data show that pH has no effect on partitioning of the DAU-SL which suggest that the drug exists in the uncharged form in the bilayer.

Chemical Phenomena↗
PubMed2004

High-resolution myocardial perfusion mapping in small animals in vivo by spin-labeling gradient-echo imaging.

An ECG and respiration-gated spin-labeling gradient-echo imaging technique is proposed for the quantitative and completely noninvasive measurement and mapping of myocardial perfusion in small animals in vivo. In contrast to snapshot FLASH imaging, the spatial resolution of the perfusion maps is not limited by the heart rate. A significant improvement in image quality is achieved by synchronizing the inversion pulse to the respiration movements of the animals, thereby allowing for spontaneous respiration. High-resolution myocardial perfusion maps (in-plane resolution=234 x 468 microm2) demonstrating the quality of the perfusion measurement were obtained at 4.7 T in a group of seven freely breathing Wistar-Kyoto rats under isoflurane anesthesia. The mean perfusion value (group average +/- SD) was 5.5 +/- 0.7 ml g(-1)min(-1). In four animals, myocardial perfusion was mapped and measured under cardiac dobutamine stress. Perfusion increased to 11.1 +/- 1.9 ml g(-1)min(-1). The proposed method is particularly useful for the study of small rodents at high fields.

Animals↗
PubMed2005

Quantifying CBF with arterial spin labeling.

The basic principles of measuring cerebral blood flow (CBF) using arterial spin labeling (ASL) are reviewed. The measurement is modeled by treating the ASL method as a magnetic resonance imaging (MRI) version of a microsphere study, rather than a diffusible tracer study. This approach, particularly when applied to pulsed ASL (PASL) experiments, clarifies that absolute calibration of CBF primarily depends on global properties of blood, rather than local tissue properties such as the water partition coefficient or relaxation time. However, transit delays from the tagging region to the image voxel are a potential problem in all standard ASL methods. The key to quantitative CBF measurements that compensate for this systematic error is to create a well-defined bolus of tagged blood and to ensure that all of the bolus has been delivered to an imaging voxel at the time of measurement. Two practical technical factors considered here are 1) producing a tagged bolus with a well-defined temporal width and 2) accounting for reduction in magnitude of the tagged magnetization due to relaxation. The ASL approach has the potential to provide a robust estimation of CBF, although the timing of water exchange into tissue and the effects of pulsatile flow require further investigation.

Blood Physiological Phenomena↗
PubMed1983

Spin-label detection of sickle hemoglobin--membrane interaction at physiological pH.

The spin-label electron paramagnetic resonance technique has been used to compare the interactions of normal and sickle hemoglobin molecules with human erythrocyte membranes. The sickle hemoglobin molecules show an enhanced binding to membranes when compared to normal hemoglobin (HbA) molecules. Using a simple equilibrium model for hemoglobin--membrane interactions, we obtain an equilibrium dissociation constant for sickle hemoglobin of about half that of HbA at pH 7.4 in 5 mM phosphate at 20 degrees C. The interactions are very low affinity in nature and are stronger at lower pH than at pH 7.4 (Fung, 1981a). The difference between normal and sickle hemoglobin persists at both high (pH 7.4) and low (pH 6.7) pH values. The concentrations of hemoglobin at the saturation level are close to physiological concentrations. Removal of spectrin--actin protein molecules from the membranes causes little change in the interactions, indicating that the remaining membrane proteins play the primary role in hemoglobin--membrane interactions. This observation is further supported by data of spectrin--actin-depleted inside-out vesicle samples. The stronger interaction of sickle hemoglobin than normal hemoglobin with membranes is discussed in relation to the formation of irreversibly sickled cells.

Actins↗
PubMed2005

Input parameter sensitivity analysis and comparison of quantification models for continuous arterial spin labeling.

The regional cerebral blood flow (rCBF) values determined using continuous arterial spin labeling (CASL) are subject to several sources of variability, including natural physiologic variations, sensitivity to the input parameters, and the use of different quantification models. To date, a thorough analysis of the impact of input parameters and the choice of quantification model has not been performed. These sources of variability were investigated through computer simulations using bootstrap techniques on actual CASL data. Coefficients of variation for representative single voxels were 6.7% for gray matter and 29% for white matter, and for eight-voxel regions of interest they were 4.5% for gray matter and 23% for white matter. Comparison of nine CASL quantification models showed differences in gray matter rCBF values of up to 42%. An analysis of the sensitivity of the rCBF to input parameters for each of the nine quantification models demonstrated that accurate quantification of the inversion efficiency, tissue and arterial blood longitudinal relaxation times, and transit times were critical in calculating precise rCBF values. The large potential variations in rCBF and the effect of the choice of quantification model suggest that interpreting absolute rCBF values in CASL studies can be challenging and requires great care.

Adult↗
PubMed1979

A spin-label study of sciatic nerves from quaking, jimpy and trembler mice.

The spin labels, 5-nitroxide stearic acid and 16-nitroxide stearic acid were incorporated into whole sciatic nerves dissected from normal, quaking, jimpy and trembler mice. With 5-nitroxide stearic acid, we have studied the thermal variation of the maximal apparent coupling constant (T) between 0 degrees C and 50 degrees C. Within this range of temperatures, we obtained identical values of 2 T for nerves from normal and jimpy mice, whereas 2 T was smaller for nerves from quaking and trembler mice. With 16-nitroxide stearic acid, composite spectra were recorded, particularly in the high-field range. A line characteristic of myelin was clearly observed in the spectra of nerves from normal and jimpy mice; its intensity was somewhat less in nerves from quaking mice and much less in spectra from trembler mice. A shoulder in the principal highfield line of the spectrum is modified only with nerves from jimpy mice. The results agree well with those obtained by electron microscopy, which reveal normal myelination in nerves from jimpy mice, a slight modification of the myelin from those of quaking mice and a practically complete demyelination in peripheral nerves from trembler mice. However, the structure of the nerves of jimpy mice also seems to be modified at an, as yet, undetermined level.

Animals↗
PubMed2006

Pulsed arterial spin labeling parameter optimization for an elderly population.

PURPOSE: To optimize pulsed arterial spin labeling (PASL) parameters for the elderly to take into account possible perfusion changes that occur in the brain with age. MATERIALS AND METHODS: Healthy young (N = 14, age range = 21-27 years) and elderly (N = 12, age range = 61-67 years) subjects were scanned using Q2TIPS (QUIPSS II with thin-slice TI, periodic saturation) with varying inversion times (TI(2)) at 1.5T. The difference signal (DeltaM), transit time (deltat), and cerebral blood flow (CBF) were calculated in segmented gray matter (GM). RESULTS: The young displayed more perfusion-weighted signal difference than the elderly at all TI(2)'s. The peak DeltaM occurred at TI(2) approximately 1300 msec and 1500 msec in the young and elderly groups, respectively. Qualitatively, intravascular signal was minimal in the younger group by TI(2) = 1500 msec, whereas a longer TI(2) of 1800 msec was needed to minimize this signal in the elderly. The transit time was approximately 100 msec longer in the elderly, and CBF was in the range of literature values. CONCLUSION: For acquiring perfusion-weighted images with minimal intravascular signal and adequate tissue signal for PASL studies of cerebral perfusion in the elderly, a longer inversion time is advantageous.

Adult↗
PubMed1988

Sequences of 12 monoclonal anti-dinitrophenyl spin-label antibodies for NMR studies.

Eleven monoclonal antibodies specific for a spin-labeled dinitrophenyl hapten (DNP-SL) have been produced for use in NMR studies. They have been named AN01 and AN03-AN12. The stability constants for the association of these antibodies with DNP-SL and related haptens were measured by fluorescence quenching and ranged from 5.0 X 10(4) M-1 to greater than 1.0 X 10(8) M-1. cDNA clones coding for the heavy and light chains of each antibody and of an additional anti-DNP-SL monoclonal antibody, AN02, have been isolated. The nucleic acid sequence of the 5' end of each clone has been determined, and the amino acid sequence of the variable regions of each antibody has been deduced from the cDNA sequence. The sequences are relatively heterogeneous, but both the heavy and the light chains of AN01 and AN03 are derived from the same variable-region gene families as those of the AN02 antibody. AN07 has a heavy chain that is related to that of AN02, and AN09 has a related light chain. AN05 and AN06 are unrelated to AN02 but share virtually identical heavy and light chains. Preliminary NMR difference spectra comparing related antibodies show that sequence-specific assignment of resonances is possible. Such spectra also provide a measure of structural relatedness.

Amino Acid Sequence↗
PubMed1975

An ESR Study of the mobility of the cholestane spin label in oriented lecithin-cholesterol multibilayers.

The motion of the cholestane spin label in oriented lecithin-cholesterol multibilayers is described in terms of a rotational diffusion about the long molecular axis with diffusion coefficient D parrell and a restricted random librational motion about axes perpendicular to the long axis with diffusion coefficient D1. The diffusion coefficients have been determined from the angular dependence of the ESR line shape at various temperatures and cholesterol contents. The temperature dependence of D parrell and D1 clearly shows the transition from the gel to liquid crystalline phase. Increasing amounts of cholesterol reduce the transition temperature. A strong reduction is found from o to 10 mole % cholesterol. At 50 mole % no longer a sharp transition is observed. In the temperature range from 40 to 80 degrees C the range of D is about 10 times larger than the range of D parrell, indicating a high activation energy for the librational motion arising from a strong hindrance by interaction with surrounding molecules. Cholesterol contents up to 10-20 mole % give an increase of D parrell and D1, arising from strong decrease of the transition temperature in this range. Above 10-20 mole % a reduction of D parrell and D1 is found. However, the effect of cholesterol is much stronger on D1 than on D parrell. In the liquid crystalline phase at about 60 degrees C the effect of cholesterol on D parrell is even negligible, while D1 strongly changes. This indicates that in the liquid crystalline phase only the librational motion is influenced by cholesterol, due to a denser packing of the molecules in the bilayer.

Binding Sites↗
PubMed2004

Membrane penetration of nitric oxide and its donor S-nitroso-N-acetylpenicillamine: a spin-label electron paramagnetic resonance spectroscopic study.

S-nitroso-N-acetylpenicillamine (SNAP) is a pharmacological agent with diverse biological effects that are mainly attributable to its favorable characteristics as a nitric oxide (NO)-evolving agent. It is found that SNAP incorporates readily into dimyristoyl phosphatidylcholine (DMPC) bilayer membranes; and an approximate penetration profile was obtained from the depth dependence of the perturbation that it exerts on spin-labeled lipid chains. The profile of SNAP locates it deep in the hydrophobic core of both fluid- and gel-phase membranes. The spin relaxation enhancement of spin-labeled phospholipids with nitroxide group located at different depths in DMPC membranes was determined for nitric oxide (NO) and molecular oxygen (O(2)), at close to atomic spatial resolution. The relaxation enhancement, which is proportional to the corresponding vertical membrane profile of the concentration-diffusion product, was measured in the gel and fluid phases of the lipid bilayer. No significant membrane penetration was observed in the gel phase for the two water-dissolved gases. In the fluid phase, the transmembrane profiles of NO and O(2) are similar and could be well described by a sigmoidal function with a maximum in the center of the bilayer, but that of NO is less steep and is shifted toward the center of the membrane, relative to that of O(2). These differences can be attributed mainly to the difference in hydrophobicity between the two gases and the presence of the donor in the NO experiments. The biological implications of the above results are discussed.

Cell Membrane↗