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PubMed2001

Diffusive transport properties of some local anesthetics applicable for iontophoretic formulation of the drugs.

As part of a general study to improve the iontophoretic delivery of local anaesthetics of the amide type, the diffusion properties of the hydrochloride salts of bupivacaine, etidocaine, lidocaine, mepivacaine, prilocaine and ropivacaine, were studied in a 1% w/w agarose hydrogel. A source drug solution (25 mM) was placed in contact with the gel and, after an appropriate time, the drug concentration profile in the gel was analyzed to give a diffusion coefficient, D. The values of Dx10(10) expressed in m(2) s(-1) were: (bupi) 6.71, (eti) 6.71, (ropi) 6.39, (mepi) 7.31, (lido) 7.49 and (prilo) 7.76. For comparative reasons, the diffusion coefficient for LidHCl in an aqueous solution according to the Nernst-Hartley relation for the diffusion of ion-pairs was calculated, hereby taking into account ionic activity of LidH+ and Cl-. The diffusion coefficient thus obtained was 7.76x10(-10) m2 s-1 at infinite dilution. The relationship between the molecular weight of the compounds and the diffusion coefficient was investigated.

Anesthetics, Local↗

PubMed2003

Water self-diffusion in Chlorella sp. studied by pulse field gradient NMR.

The water self-diffusion behavior in chlorella water suspension was investigated by pulsed field gradient NMR technique. Three types of water was determined, which differs according to the self-diffusion coefficients; bulk water, extracellular and intracellular water. Intracellular and extracellular water self-diffusion were restricted, and the sizes of restriction regions were 3.4 microm and 17 microm, respectively. The water molecular exchange process between these three diffusion regions was investigated. The residence time and exchange rate constant for chlorella cells were obtained. The cell wall permeability determined from the rate constant as 3 x 10(-6) m/s agreed with the permeability 10(-6) m/s obtained from time dependence of intracellular water self-diffusion coefficient. The structural cluster model of chlorella cell is estimated to describe the extracellular water self-diffusion in chlorella water suspension.

Cell Membrane Permeability↗

PubMed1996

Self-diffusion in fluids in porous glass: confinement by pores and liquid adsorption layers.

Diffusion coefficients of 10 different polar and nonpolar liquids filled in porous glasses with mean pore diameters of 4 or 30 nm were determined with the aid of the NMR field-gradient technique. In the time scale of these experiments (0.3 to 500 ms) diffusion coefficients were found to be time independent. Within the experimental error, no influence of the polarity of the adsorbate can be stated. The diffusion coefficients of all investigated fluids in glass with 4 and 30 nm pores were reduced by factors of 0.17 and 0.63, respectively, relative to the bulk values. This relatively weak reduction can be explained by considering the known porosities of the adsorbents. The second objective of this study was to examine the diffusion behaviour below the melting point of adsorbates in porous glass. Fluids confined in pores do not freeze at the bulk freezing temperatures. In this respect, two phases must be distinguished. A maximal two monolayer thick film adsorbed on the inner surfaces does not crystallize at all, whereas the "free" fraction of the fluid in the pores freezes at reduced temperatures according to the Gibbs-Thompson relation. The nonfrozen surface layers form a network in which self-diffusion can be investigated. Experiments have been carried out with cyclohexane. A reduction factor of 0.06 was found relative to the extrapolated values of the entirely unfrozen fluid in porous glass with a mean pore diameter of 30 nm. It is, thus, demonstrated that molecules in adsorption layers virtually retain their translational degrees of freedom along the surfaces. The lowering of the diffusivity is mainly due to the geometric restriction rather than to the interaction with the surface.

Diffusion↗

PubMed1997

Fast T1 imaging of dual gel samples for diffusion measurements in NMR dosimetry gels.

Diffusion of iron is one of the major problems limiting the usefulness of NMR gel dosimetry. This was studied in dual gel samples using a 4.7T micro-imaging MR scanner and a fast T1 imaging sequence which allowed the acquisition of a 64 x 128 x 8 data sets (phase encoding x frequency encoding x number of inversion times) in less than 15 min. The procedure enabled us to obtain relative relaxation times for any region of interest within the sample. After the two differently doped gels were brought into contact in the dual gel samples (diameter 12 mm), the diffusion could be observed on subsequent images as a function of time. An inverse square root function was used to fit the change of 1/T1 across the junction between the two gel phases. A diffusion constant of 0.014 +/- 0.003 cm2/h was determined for Fe3+ in a typical dosimetry gel (1.5% agarose, 50 mM H2SO4). This could be lowered by adding a chelating agent such as xylenol orange to the gel. It was also found that diffusion was slower in gelatine gels, however these gels tended not to set properly when H2SO4 was added as required for NMR dosimetry. From the present results we propose that a gel consisting of 1.5% agarose (for stability), 3% gelatine and 0.1 mM xylenol orange (to combat diffusion and allow a visual evaluation) is a suitable base for NMR dosimetry gels. The use of a fast T1 imaging sequence reduces acquisition times and therefore the potential impact of diffusion.

Chelating Agents↗

PubMed1999

Mapping eddy current induced fields for the correction of diffusion-weighted echo planar images.

Diffusion-weighted echo-planar magnetic resonance imaging is potentially of great importance as a diagnostic imaging tool; however, the technique currently suffers a number of limitations, including the image distortion caused by the eddy current induced fields when the diffusion-weighting magnetic field gradient pulses are applied. The distortions cause mis-registration between images with different diffusion-weighting, that then results in artifacts in quantitative diffusion images. A method is presented to measure the magnetic fields generated from the eddy currents for each of three orthogonal gradient pulse vectors, and then to use these to ascertain the image distortion that occurs in subsequent diffusion-weighted images with arbitrary gradient pulse vector amplitude and direction, and image plane orientation. The image distortion can then be reversed. Both temporal and spatial dependence of the residual eddy current induced fields are included in the analysis. Image distortion was substantially reduced by the correction scheme, for arbitrary slice position and angulation. This method of correction is unaffected by the changes in image contrast that occur due to diffusion weighting, and does not need any additional scanning time during the patient scan. It is particularly suitable for use with single-shot echo planar imaging.

Anisotropy↗

PubMed2001

Automatic multicommmutated flow system for diffusion studies of pharmaceuticals through artificial enteric membrane.

An automatic flow procedure with spectrophotometric detection was developed for the study of pharmaceuticals diffusion through an artificial enteric membrane. The manifold comprised two independent flow pathways, gathered by a diffusion unit with two compartments and an enteric lipophilic membrane. The pathways were automatically filled with solutions simulating digestive and plasmatic conditions by means of four solenoid valves. The diffusion of pharmaceuticals from the enteric to the plasmatic compartment was performed in closed loop pathways, and was continuously monitored by a flow cell coupled to the acceptor solution pathway. The volumes of the digestive and plasmatic solutions were 6.0 and 3.6 ml, respectively, which comprised filling unit compartment, pumping tubing and connecting flow lines. Pumping flow rates of donor and acceptor solutions were maintained at 6.0 and 2.5 ml min(-1), respectively. The proposed system was employed in diffusion studies of caffeine and aminophylline, and in the evaluation of the influence of tensioactive agents on the diffusion process. After continuous solutions circulation for 60 min, caffeine concentration in the acceptor stream was ca. 18% of its initial concentration at the digestive compartment. The system could be programmed to perform several replicates, stopping them with different degrees of diffusion without operator assistance. The data generated by the spectrophotometer was read by the microcomputer as a time function, and stored for further mathematical treatment.

Aminophylline↗

PubMed2000

Lack of improvement of lung diffusing capacity following fluid withdrawal by ultrafiltration in chronic heart failure.

OBJECTIVES: We sought to investigate the possibility that lung diffusing capacity reduction observed in chronic heart failure is reversible in the short term. BACKGROUND: Mechanical properties of the lung usually ameliorate with antifailure treatment including drugs, ultrafiltration and heart transplantation, whereas lung diffusion rarely improves. METHODS: We studied the mechanical properties of the lung (pulmonary function tests with determination of alveolar volume, extravascular lung fluids and lung tissue), lung diffusion for carbon monoxide (DLco), including membrane diffusing capacity (Dm), pulmonary capillary blood volume (Vc) and pulmonary hemodynamics, in 28 patients with stable chronic heart failure, before a single session of extracorporeal ultrafiltration (3,973 +/- 2200 ml) and four days thereafter. Lung mechanics and diffusion were also evaluated in 18 normal subjects. RESULTS: Vital capacity, forced expiratory volume (1 s) and maximal voluntary ventilation were lower in patients when compared with normal subjects, and increased after ultrafiltration from 2.1 +/- 0.7 to 2.5 +/- 0.7(1)*, 1.7 +/- 0.5 to 2.0 +/- 0.6(1)* and 67 +/- 25 to 79 +/- 26 (1/min)*, respectively (* p < 0.02 vs. pre-ultrafiltration). Post-ultrafiltration alveolar volume was augmented, while lung tissue, body weight (approximately 6 kg), chest X-ray extravascular lung water score and pulmonary vascular pressure were reduced. Heart dimensions (echocardiography) remained unchanged. DLco, Dm and Vc were 29.0 +/- 5.0 ml/min/mm Hg, 47.0 +/- 11.0 ml/min/mm Hg, 102 +/- 20 ml in normal subjects and 17.1 +/- 4.0#, 24.1 +/- 6.5#, 113 +/- 38 and 17.0 +/- 5.0#, 24.8 +/- 7.9#, 100 +/- 39 in patients before and after ultrafiltration, respectively (# = p < 0.01 vs. controls). CONCLUSIONS: In chronic heart failure, ultrafiltration improves volumes and mechanical properties of the lung by reducing lung fluids. Diffusion is unaffected by ultrafiltration, suggesting that, in chronic heart failure, the alveolar-capillary membrane abnormalities are fluid-independent.

Chronic Disease↗

PubMed2003

Diffusion MRI findings in Wilson's disease.

Six patients having Wilson's disease were studied with diffusion MRI in order to characterize cerebral lesions. Diffusion MRI was obtained using the spin-echo, echo-planar sequence with a gradient strength of 30 mT/m. The trace protocol was used in the axial imaging plane. Heavily diffusion-weighted (b=1000s/mm(2)) images, and the ADC (apparent diffusion coefficient) values from automatically generated ADC maps were studied. The ADC values of the normal brain parenchyma were available in 17 age-matched cases for comparison (ADC values, 0.85+/-0.11 x 10(-3)mm(2)/s). In Wilson's disease two distinct diffusion MRI patterns were observed by quantitative evaluations of the ADC maps; cytotoxic edema-like (ADC values, 0.52+/-0.03 x 10(-3)mm(2)/s), and vasogenic edema-like (ADC values, 1.42+/-0.17 x 10(-3)mm(2)/s) patterns. Diffusion imaging appears to be a promising sequence to evaluate the changes in the brain tissue in Wilson's disease at least by revealing two different patterns.

Adolescent↗

PubMed2001

Drug diffusion mechanism through pH-sensitive hydrophobic/polyelectrolyte hydrogel membranes.

Methylmethacrylate/dimethylaminoethyl methacrylate cross-linked with divinylbenzene is a pH-sensitive hydrogel. We have studied the diffusion mechanisms of drugs with different water solubilities through this hydrogel. A water-soluble model drug (aminopyrine) was used to study the diffusion coefficient changes in different pHs. The results showed a water-content dependent diffusion for this pH-sensitive polycationic hydrogel. However, decreasing the solubility of the drug and increasing the hydrophobic character of the polymer by changing the pH caused a greater affinity (or partition coefficient) between the hydrogel and the drug. Aminopyrine diffusion was shown to follow the free-volume theory, suggesting the 'pore' type mechanism for water soluble drugs, while the 'partition' or 'solution-diffusion' mechanism better described the slow diffusion of water insoluble solutes through this pH-sensitive hydrogel. Comparing the swelling interface number for aminopyrine release through this pH-sensitive hydrogel showed a non-Fickian mechanism in the hydrated form of the hydrogel (pH 1.2), while Fickian in the dehydrated form (pH 7.4).

Aminopyrine↗

PubMed2002

Real-time and continuous monitoring of myocardial blood flow using a thermal diffusion method.

OBJECTIVES: There has been no method which can measure regional myocardial blood flow in real-time and continuously. The purpose of this study was to validate myocardial blood flow measurement using a thermal diffusion method. For this purpose, myocardial blood flow measurement was performed using the thermal diffusion method and the electrolytic hydrogen clearance method. METHODS: Seven pigs were used for this study, six were for comparison between the thermal diffusion and electrolytic hydrogen clearance methods, and one was for demonstration of myocardial blood flow measurement using the thermal diffusion method on a beating heart coronary artery bypass model with ischemic preconditioning. RESULTS: A good correlation was found between myocardial blood flow values obtained by the electrolytic hydrogen clearance method and 1/V values obtained by a thermal diffusion probe, the correlation coefficient was 0.841 (P<0.001). During the beating heart coronary artery bypass, the regional myocardial blood flow was recorded in real-time and continuously. CONCLUSIONS: This study demonstrated measurement of regional myocardial blood flow using the thermal diffusion method for the first time and simultaneous measurement using the electrolytic hydrogen clearance method for calibration. It provided a real-time and continuous myocardial blood flow measurement and has a potential to contribute to progress in beating-heart surgery.

Animals↗

PubMed2003

No barrier to diffusion between cell soma and neurite membranes in sympathetic neurons for a GPI-anchored glycoprotein.

As neurons extend their axons, it is thought that newly synthesised membrane components travel in vesicles along the axon, fuse with the growth cone membrane, and diffuse back along the axonal membrane. However, it is difficult to explain how axons continue to be populated with membrane proteins as they extend in length. To investigate this problem, we have used a CEPU-green fluorescent protein (GFP) chimeric protein to study the site of insertion of new glycosyl phosphatidyl inositol (GPI)-anchored glycoproteins and their subsequent behaviour in chick dorsal root ganglia (DRG) neurons. Infection of cultures grown for 24 h revealed rapid expression of CEPU-GFP over the whole surface of the neuron, more rapidly than could be accounted for by diffusion from the growth cone, and fluorescence intensity was uniform along the length of the neurite. Photobleaching experiments of neurite membrane revealed that recovery of fluorescence was due to diffusion from adjacent membranes and there was no evidence for membrane flow in either direction. Photobleaching of membrane adjacent to the cell body also showed rapid recovery, with chimera diffusing both from cell body membrane and the distal neurite membrane into the bleached area. These results suggest there is no barrier to diffusion between the cell body and neurite membrane in DRG and sympathetic neurons cultured for 1 or 2 days in vitro. We propose that the neurite is populated by newly synthesised chimera by diffusion from both regions. This situation may also occur in neurons in the early stages of extending axons in vivo prior to polarisation and the development of the dendritic field.

Adrenergic Fibers↗

PubMed2000

Diffusion of water molecules in crystalline beta-cyclodextrin hydrates.

To understand the rapid diffusion mechanism of water molecules in the crystal lattice of hydrated beta-cyclodextrin (beta-CD), molecular dynamics (MD) simulations of crystalline beta-CD were performed at five different relative humidities corresponding to hydration states ranging from beta-CD-9.4H2O to beta-CD-12.3H2O, and in aqueous solution. The trajectories for the crystalline beta-CD hydrates had lengths of 4 ns each, whereas the simulation in aqueous solution extended to 2 ns. Transport of water molecules in the crystal was characterized in terms of a spatially varying diffusion constant and the main direction of diffusion, which were evaluated using newly developed algorithms. The main diffusion pathway winds through the cavities of adjacent doughnut shaped beta-CD molecules and is slightly slanted with respect to the crystallographic b-axis. Water molecules outside the beta-CD cavities have access to the main diffusion pathway. The diffusion constant for transport of water molecules along the main pathway calculated from the MD simulation data adopts 1/30 of the value in bulk water at room temperature. This is in agreement with estimates that can be made from experimental data on the adjustment of a beta-CD crystal to changes in relative humidity.

Algorithms↗

PubMed2004

Influences upon the diffusion of thrombolysis for acute myocardial infarction in England: case study.

OBJECTIVES: To investigate the factors that influenced the adoption and diffusion of thrombolysis in acute myocardial infarction in England and to verify usage data from 1981 to 2001. METHODS: Survey of cardiologists in England using a pre-prepared time line of historical events and a plot of thrombolysis diffusion since 1981. The cardiologists were divided into three groups that were provided with (i) the time line only, (ii) the diffusion curve only, and (iii) the time line and the diffusion curve. RESULTS: The GISSI and ISIS-2 clinical trials were perceived to have had a significant influence upon the initial diffusion of thrombolysis in England occurring over the 3 years after launch. Other positive influences included the initial listing in the national formulary, the change to administration in emergency departments, the rise in evidence-based medicine, and production of national guidance. CONCLUSIONS: Although it is apparent that the overall influences on adoption and diffusion of thrombolysis were multiple; clinical trials, service developments, and national guidelines all were judged to have played a part. The GISSI and ISIS-2 clinical trials were confirmed as the major influence on initial adoption.

Attitude of Health Personnel↗

PubMed2003

Transferred nuclear overhauser effect in nuclear magnetic resonance diffusion measurements of ligand-protein binding.

The drug discovery process relies on characterizing structure-activity relationships, since specific ligand-target interactions often result in important biological functions. Measuring diffusion coefficients by nuclear magnetic resonance spectroscopy is a useful way to study binding, because changes can be detected when a small ligand interacts with a macromolecular target. Diffusion coefficients can be miscalculated, however, due to magnetization transfer between the receptor and ligand. This transferred nuclear Overhauser effect (trNOE) disrupts the observed signal decay due to diffusion as a function of the experimental diffusion time. Since longer diffusion times also selectively edit free ligand signal, the measured diffusion coefficients become biased toward the fraction of bound ligand. Despite this discrepancy, under these experimental conditions, the trNOE selectively influences the measured signals of binding ligands and can be used to gain insight into ligand-protein interactions. These phenomena have been studied for caffeine and L-tryptophan, which bind to human serum albumin, and the antimalarial agent trimethoprim, which interacts with dihydrofolate reductase. The results provide insight into the nature of ligand-protein binding and are thus useful for elucidating the molecular features of the ligand that interact with the protein.

Caffeine↗

PubMed1992

Lateral diffusion and percolation in two-phase, two-component lipid bilayers. Topology of the solid-phase domains in-plane and across the lipid bilayer.

Fluorescence recovery after photobleaching (FRAP) has recently been used to examine the percolation properties of coexisting phases in two-component, two-phase phosphatidylcholine bilayers [Vaz, W. L. C., Melo, E. C. C., & Thompson, T. E. (1989) Biophys. J. 56, 869-876]. We now report the use of FRAP to study two additional problems in similar systems. The first is the effect of solid-phase obstacles on the lateral diffusion in the fluid phase. The second is the question of whether or not, in a single bilayer, solid-phase domains in one monolayer are exactly superimposed on solid domains in the apposing monolayer. To address the first problem, the lateral diffusion of N-(7-nitrobenzoxa-2,3-diazol-4-yl)-1-palmitoyl-2-oleoylphosp hatidylethanolamine (NBD-POPE), a probe soluble only in the fluid phase when solid and fluid phases coexist, has been studied in the mixture N-lignoceroyldihydrogalactosylceramide (LigGalCer)/dipalmitoylphosphatidylcholine (DPPC). Percolation of the fluid phase occurs at a high mass fraction of solid phase. This indicates that the solid domains have a centrosymmetric shape, a characteristic which makes this a good experimental system to test theoretical simulations of diffusion in an archipelago. It is shown that agreement between theory and experiment is poor, a result that had already been observed when the obstacles were integral membrane proteins. We develop an effective-medium model for diffusion in two-phase systems which explains both our results and those obtained with integral proteins. The distinctive feature of the model is the consideration of an annular region around the obstacles where the lipids are more ordered than in the bulk fluid phase. The diffusion coefficient is then calculated by extending the free area model to two-phase systems, taking these annuli into account. The second question, the organization of the solid-phase domains across the lipid bilayer, is examined in the systems LigGalCer/DPPC and dimyristoylphosphatidylcholine (DMPC)/distearoylphosphatidylcholine (DSPC) by comparing the diffusion of a fluid-phase-soluble, gel-phase-insoluble lipid derivative which spans the two monolayers of a bilayer (NBD-membrane-spanning-phosphatidylethanolamine, NBD-msPE) with that of a probe which is restricted to a single monolayer. In LigGalCer/DPPC, 20:80, the distribution of solid domains in one of the monolayers is independent of the distribution in the apposing monolayer. In contrast, in DMPC/DSPC, 50:50, the solid domains in one monolayer are exactly superimposed upon the solid domains existing in the apposing monolayer.

Diffusion↗

PubMed1994

Pausing of the restriction endonuclease EcoRI during linear diffusion on DNA.

Linear diffusion is a mechanism to accelerate association rates beyond their three-dimensional diffusional limit. It is employed by the restriction endonuclease EcoRI as well as many other proteins interacting with specific DNA sequences to locate their target sites on the macromolecular substrate. In order to investigate biochemical and biophysical details of the linear diffusion process, we have developed a competitive cleavage assay which allows us to assess with great accuracy the influence of sequence, sequence context, and other structural features on the linear diffusion of EcoRI on DNA. We show here that linear diffusion is not a hopping but a sliding movement in which EcoRI follows the helical pitch of the DNA, because it does not "overlook" any cleavage site. Linear diffusion is slowed when EcoRI encounters sites on the DNA which resemble its recognition site ("star" sites). Pauses of up to 20 s are induced, depending on sequence and orientation of the star site. These data suggest that EcoRI can bind to DNA in two binding modes: one tight, specific, and immobile, leading to DNA cleavage, and another one loose and nonspecific, allowing for linear diffusion. Depending on the similarity between the recognition sequence and the DNA sequence being encountered by EcoRI, there will be a continuous transition between these binding modes. Other proteins bound to the DNA and irregular DNA structures such as bent DNA or a triple helix constitute a barrier that cannot easily be passed by EcoRI.

Base Sequence↗

PubMed1993

Solute diffusion in lipid bilayer membranes: an atomic level study by molecular dynamics simulation.

To elucidate the mechanism of solute diffusion through lipid bilayer membranes, nearly 4 ns of molecular dynamics simulations of solutes in phospholipid bilayers was conducted. The study, the first atomic level study of solute diffusion in a lipid bilayer, involved four simulations of an all-atom representation of a fully solvated dimyristoylphosphatidylcholine (DMPC) bilayer in the L alpha phase with benzene molecules as solutes, totaling over 7100 atoms. These simulations agree with experimental evidence that the presence of small solutes does not affect bilayer thickness but does result in slight perturbations in the ordering of the hydrocarbon chains. At room temperature, the benzene molecules have essentially isotropic motion and rotate freely. The rate of translational diffusion varies with position within the bilayer and is faster in the center than near the zwitterionic headgroups and is in excellent agreement with experimental values for the diffusion of small solutes in a bilayer. These simulations have elucidated the mechanism of diffusion in a bilayer to be similar to the "hopping" mechanism found for the diffusion of gases through soft polymers. Jumps of up to 8 A can occur in as little as 5 ps whereas average motions for that time period are only approximately 1.5 A. In many cases, the jumps are moderated by torsional changes in the hydrocarbon chains which serve as "gates" between voids through which the benzene molecules move. Comparison of these simulations with another 1000-ps simulation of benzene in a pure alkane provides evidence that lipid bilayers should not be treated as a homogeneous bulk hydrocarbon phase.

Alkanes↗

PubMed1982

Models for slow anisotropic rotational diffusion in saturation transfer electron paramagnetic resonance at 9 and 35 GHz.

Model systems of cholestane and 5-doxylstearic acid analogue spin probes in lipid bilayer dispersions of dipalmitoylphosphatidylcholine and cholesterol (9:1 w/w) are used to analyze saturation transfer electron paramagnetic resonance spectral behavior for slow rotational diffusion in an anisotropic medium. Measurements are made at both 9 and 35 GHz to provide enhanced spectral resolution for different types of motion. Parameter correlation plots of spectral parameters from different regions of the saturation transfer spectra appear to be potentially useful in characterizing different types of motion. Anisotropic rotational diffusion about a symmetry axis coincident with the nitroxide y principal axis is clearly distinguishable from isotropic rotational diffusion and may be distinguishable from rotational diffusion about the nitroxide z principal axis. Approximate anisotropic rotational diffusion about a symmetry axis coincident with the nitroxide z principal axis is distinguishable from isotropic rotational diffusion under some, but not all, conditions.

Cholestanes↗