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At least 649 records · Page 36Linked to original sources
PubMed2006

Kinetics of finite dose absorption through skin 2: volatile compounds.

A diffusion model to account for the disposition of an arbitrary dose of a (potentially) volatile compound applied to skin from a volatile vehicle is presented. In its most general form, the model allows for variable diffusivity of the permeant in the stratum corneum (SC) and must be solved numerically. However, for permeants having a constant diffusivity, absorption, and evaporation is characterized in terms of four dimensionless parameters-a reduced time tau, a fractional deposition depth in the SC f, a ratio of membrane capacity for the permeant to the applied dose beta, and a ratio of evaporative mass transfer coefficient to diffusive permeability chi. An important combination of these parameters arises as the reduced dose M(r) = (fbeta)(-1). Two cases are distinguished. In Case 1, corresponding to M(r) < or = 1, the dose is less than that required to saturate the upper layers of the SC, and the shape of the absorption and evaporation profiles is independent of the dose. Analytical solutions to Case 1 may be derived for arbitrary initial distributions of the permeant; the solution for a square wave is presented. In Case 2, corresponding to M(r) > 1, absorption and evaporation approach steady-state values as the dose is increased. Numerical evaluations of this behavior are shown. Limiting behavior for the case of a highly volatile solvent applied to skin is discussed. A companion paper discusses the application of the model to the absorption and evaporation of benzyl alcohol from human skin in vitro.

Administration, Cutaneous↗
PubMed2005

Catalase activity and thiobarbituric acid reactive substances (TBARS) production in a rat model of diffuse axonal injury. Effect of gadolinium and amiloride.

This study evaluated the effect of mechanogated membrane ion channel blockers on brain catalase (CAT) activity and thiobarbituric acid reactive substances (TBARS) production after traumatic brain injury (TBI). A weight drop trauma model was used. Controls were sham-operated and received no weight drop. Gadolinium (GAD) or amiloride (AMI) were administered to control and experimental rats (30 min after TBI). Brain CAT activity and TBARS production were measured. When blood vessels were washed out with saline perfusion brain CAT activity significantly increased up to 6 h after trauma, decreasing significantly by 24 h; GAD or AMI administration preserved CAT activity 24 h after TBI. TBARS production increased after trauma, this effect being significantly reversed by GAD or AMI administration. GAD significantly decreased TBARS production in control animals. Mechanogated membrane ion channels may be involved in the genesis of the ionic disruption leading to oxidative stress and other secondary injury processes in head trauma.

Amiloride↗
PubMedDate not supplied

Chaos and order in plankton dynamics. Complex behavior of a simple model.

The role of the diffusive interaction between fish-populated and fish-free habitats in a patchy environment in plankton pattern formation is studied by means of a minimal reaction-diffusion model of the nutrient-plankton-fish food chain. It is shown that such interaction can give rise to spatio-temporal plankton patterns. The fractal dimension of the patterns is shown dependent on the fish predation rate. The spatially averaged plankton dynamics depending on both fish predation rate and distance between fish-populated habitats can exhibit chaotic and regular behavior. The chaotic plankton dynamics is characteristic of a wide parameter range.

Animals↗
PubMed1993

Van Gieson's picrofuchsin. The staining mechanisms for collagen and cytoplasm, and an examination of the dye diffusion rate model of differential staining.

The staining mechanism of van Gieson's picrofuchsin was studied by use of simple protein model systems and tissue sections, and by spectrophotometry and dialysis experiments. At the endpoint of the staining reaction (equilibrium) cytoplasm is yellow. Dye dilution experiments demonstrated that the highest affinity in the tissue section--picrofuchsin system is between binding sites in cytoplasmic protein and acid fuchsin. Nevertheless sections that were first stained in acid fuchsin (AcF) and then in picrofuchsin ended up with cytoplasm stained yellow. It was concluded that differences in the dye diffusion rates and differences in the permeability of tissue components cannot be invoked to explain the differential staining result. Model experiments with dissolved proteins demonstrated a positive relationship between protein concentration and uptake of picric acid (PA) from picrofuchsin. From this and experiments with additives (sodium dodecylsulphate, urea etc.) and organic solvents, it is proposed that coagulant interchain cross-linking at the high protein concentration of the cytoplasm masks potential dye-binding sites. This affects high affinity dyes with multiple binding sites more than small dyes, and so puts AcF at a disadvantage compared to PA. Staining of non-collagen proteins is mainly by hydrophobic bonding, involving ionic attractions, apolar bonds, and release of water. This mode of binding is relatively strong, decreases swelling and leads to slow dye exchange. Dye binding to collagen is mostly by hydrogen bonds, but in aqueous dye solvent nonpolar residues and charged residues may also participate. This structure remains relatively open during and after dye-binding, and the bound dye ions are therefore easily exchanged for other dye ions.

Ammonium Sulfate↗
PubMed2005

Modeling of immunoglobulin uptake by N,N,N',N'-ethylenediaminetetramethylenephosphonic acid-modified zirconia particles under static and dynamic conditions.

A matrix developed from N,N,N',N'-ethylenediaminetetramethylenephosphonic acid-modified zirconia beads (further referred to as r_PEZ); 25-38 microm in diameter and with a pore size of 22+/-3 nm, was utilized for the separation of immunoglobulins (Igs). r_PEZ has been shown to bind to various Igs originating from a wide variety of species. To understand the mechanisms controlling the uptake of Igs by r_PEZ, static protein uptake experiments were carried out. The protein uptake profiles were further modeled with a kinetic rate constant model. Individual studies were undertaken for human immunoglobulin A, G and M (HIgA, HIgG and HIgM). The kinetic rate constant model indicated that HIgG binding to r_PEZ was more favorable than its disassociation. The equilibrium rate constants were found to decrease with increasing concentration. The effect of continuous loading in a packed bed system utilizing r_PEZ matrix was evaluated by carrying out frontal studies, using different feed concentrations and linear velocities. The breakthrough profiles obtained for the uptake of HIgG were modeled with the pore diffusion model. The model was found to best describe the breakthrough profiles obtained at a feed concentration of 2.0 mg of HIgG per milliliter. The NTU for the packed bed was found to be equal to 2.

Adsorption↗
PubMed1988

Longitudinal mixing in dog lungs during high-frequency forced flow oscillation.

Longitudinal mixing in the conducting airways of eight intubated anesthetized beagles (10.8 +/- 0.9 kg) was studied at functional residual capacity in the presence of forced sinusoidal flow oscillations and in the absence of fresh air bias flow. The ranges of oscillation conditions were: frequencies, f, from 3 to 18 Hz and minute volumes, Vosc, from 50 to 150 ml/sec, corresponding to tidal volumes, Vosc/f, from 0.3 to 4.5 ml/kg body mass. Oscillations were imposed during a breath holding interval incorporated into a modified single-breath nitrogen (N2) washout maneuver. The expired N2 fraction curves were analyzed with a Fickian diffusion model by adjusting the value of a global mixing parameter, (DA2), to achieve an optimal fit of the model to the data. The mixing parameter was an increasing function of minute volume and a decreasing function of frequency, which is well represented by the equation: (DA2) = 2.72 Vosc 1.74 f-1.57 By comparison to available theory and previous measurements in physical systems, this formula implies that Taylor-type dispersion is the dominant mixing mechanism in the conducting airways. Also, the diffusion model predicted, and the data verified, the existence of a mouth-ward 'diffusion flow' during breath holding. This effect, caused by the non-uniform nature of the summed airway cross-section, is directly correlated with the value of (DA2).

Animals↗
PubMed2001

A new model for diffuse brain injury by rotational acceleration: II. Effects on extracellular glutamate, intracranial pressure, and neuronal apoptosis.

The aim of this study is to monitor excitatory amino acids (EAAs) in the extracellular fluids of the brain and to characterize regional neuronal damage in a new experimental model for brain injury, in which rabbits were exposed to 180-260 krad/s2 rotational head acceleration. This loading causes extensive subarachnoid hemorrhage, focal tissue bleeding, reactive astrocytosis, and axonal damage. Animals were monitored for intracranial pressure (ICP) and for amino acids in the extracellular fluids. Immunohistochemistry was used to study expression of the gene c-Jun and apoptosis with the terminal deoxynucleotidyl transferase nick-end labeling (TUNEL) technique. Extracellular glutamate, glycine, and taurine increased significantly in the hippocampus within a few hours and remained high after 24 h. Neuronal nuclei in the granule layers of the hippocampus and cerebellum were positive for c-Jun after 24 h. Little immunoreactivity was detected in the cerebral cortex. c-Jun-positive neuronal perikarya and processes were found in granule and pyramidal CA4 layers of the hippocampus and among the Purkinje cells of the cerebellum. Also some microglial cells stained positively for c-Jun. TUNEL reactivity was most intense at 10 days after trauma and was extensive in neurons of the cerebral cortex, hippocampus, and cerebellum. The initial response of the brain after rotational head injury involves brain edema after 24 h and an excitotoxic neuronal microenvironment in the first hour, which leads to extensive delayed neuronal cell death by apoptosis necrosis in the cerebral cortex, hippocampus and cerebellum.

Acceleration↗
PubMed2005

The effect of stimulus strength on the speed and accuracy of a perceptual decision.

Both the speed and the accuracy of a perceptual judgment depend on the strength of the sensory stimulation. When stimulus strength is high, accuracy is high and response time is fast; when stimulus strength is low, accuracy is low and response time is slow. Although the psychometric function is well established as a tool for analyzing the relationship between accuracy and stimulus strength, the corresponding chronometric function for the relationship between response time and stimulus strength has not received as much consideration. In this article, we describe a theory of perceptual decision making based on a diffusion model. In it, a decision is based on the additive accumulation of sensory evidence over time to a bound. Combined with simple scaling assumptions, the proportional-rate and power-rate diffusion models predict simple analytic expressions for both the chronometric and psychometric functions. In a series of psychophysical experiments, we show that this theory accounts for response time and accuracy as a function of both stimulus strength and speed-accuracy instructions. In particular, the results demonstrate a close coupling between response time and accuracy. The theory is also shown to subsume the predictions of Piéron's Law, a power function dependence of response time on stimulus strength. The theory's analytic chronometric function allows one to extend theories of accuracy to response time.

Decision Making↗
PubMedDate not supplied

[Water exchange in human crystalline lens studied by combined dispersion confocal microspectroscopy].

We have employed the technique of the confocal Raman microspectroscopy to monitor water exchange in human eye lens in vitro. Heavy water (D2O) was used as a marker of the exchange. Raman spectra in the high frequency range (2200-3500 cm-1) were successively recorded at several locations inside the lens in order to register dynamics of the replacement of H2O by D2O. The intensities of the Raman peaks at 2450 cm-1 (OD stretch) and 3390 cm-1 (OH stretch) were used as indicators of H2O/D2O exchange, whereas the Raman peak at 2935 cm-1 (CH stretch of protein) served as an internal standard. The ratios of the Raman intensities I2450/I2935 and I3390/I2935 determined the relative concentrations of D2O and H2O, respectively. For the quantitative description of the exchange, we proposed a diffusion model assuming a constant diffusion coefficient over the volume of the lens. We report the results of experiments performed on four fresh and one fixed lenses. The diffusion coefficient of D2O in the human eye lens was estimated using the least-squares fit of the experimental data.

Deuterium Oxide↗
PubMed2006

Kinetic modeling of adsorption of di-2-pyridylketone salicyloylhydrazone on silica gel.

The kinetics of DPKSH (di-2-pyridylketone salicyloylhydrazone) adsorption onto silica gel has been investigated at (25+/-1) degrees C and pH 1 and 4.7. The kinetics of adsorption of DPKSH is discussed using three kinetic models, the first-order Lagergren model, the pseudo-second-order model, and the intraparticle diffusion model. The adsorption of DPKSH, at pH 1 and 4.7, onto silica gel proceeds according to the pseudo-second-order model and the correlation coefficients were very close to 1. The intraparticle diffusion of DPKSH molecules within the silica gel particles was identified as the rate-limiting step. The parameters of the pseudo-second-order model are q(max,calc)=1.02 x 10(-4) and 1.5 x 10(-4) g DPKSH/g silica; k(2)=3.01 x 10(4) and 9.67 x 10(4) h(-1)g silica/g DPKSH, respectively, for pH 1 and 4.7.

Adsorption↗
PubMedDate not supplied

Prediction of multicomponent adsorption behavior in activated carbon adsorbers: kinetic aspects.

Although the concentrations of solutes are very low in water treatment, it cannot be expected that the film-diffusion model predicts breakthrough behavior satisfactorily, for the following reasons: most of the solutes have less favorable isotherms than p-nitrophenol or p-chlorophenol; many solutes are much larger molecules and hence have a much higher internal diffusion resistance than p-nitrophenol or p-chlorophenol; and displacement effects cause a much higher internal resistance than expected from single-solute data. Therefore, internal diffusion resistance has to be incorporated into the film-homogeneous diffusion model. All parameters needed in this model can be obtained from batch reactor tests. Multi-solute systems may be regarded as a single-solute system or a bi-solute system, respectively, if all solutes except one or two are present in very different concentrations; and/or have a comparatively small affinity to activated carbon; and/or have a comparatively small internal diffusion coefficient.

Adsorption↗
PubMed2006

Use of a parallel artificial membrane system to evaluate passive absorption and elimination in small fish.

A parallel artificial lipid membrane system was developed to mimic passive mass transfer of hydrophobic organic chemicals in fish. In this physical model system, a membrane filter-supported lipid bilayer separates two aqueous phases that represent the external and internal aqueous environments of fish. To predict bioconcentration kinetics in small fish with this system, literature absorption and elimination rates were analyzed with an allometric diffusion model to quantify the mass transfer resistances in the aqueous and lipid phases of fish. The effect of the aqueous phase mass transfer resistance was controlled by adjusting stirring intensity to mimic bioconcentration rates in small fish. Twenty-three simple aromatic hydrocarbons were chosen as model compounds for purposes of evaluation. For most of the selected chemicals, literature absorption/elimination rates fall into the range predicted from measured membrane permeabilities and elimination rates of the selected chemicals determined by the diffusion model system.

Absorption↗
PubMed2004

Modeling the backbone dynamics of reduced and oxidized solvated rat microsomal cytochrome b5.

In this article, a description of the statistics and dynamics of cytochrome b(5) in both reduced and oxidized forms is given. Results of molecular dynamics computer simulations in the explicit solvent have been combined with mode-coupling diffusion models including and neglecting the molecule-solvent correlations. R(1) and R(1 rho) nuclear magnetic relaxation parameters of (15)N in the protein backbone have been calculated and compared with experiments. Slight changes in charge density in the heme upon oxidation produces a cascade of changes in charge distributions from heme propionates up to charged residues approximately 1.5 nm from Fe. These changes in charge distributions modify the molecular surface and the water shell surrounding the protein. The statistical changes upon oxidation can be included in diffusive models that physically explain the upper and lower limits of R(1 rho) relaxation parameters at high off-resonance fields.

Animals↗
PubMed2001

Dynamics of spatially nonuniform patterning in the model of blood coagulation.

We propose a reaction-diffusion model that describes in detail the cascade of molecular events during blood coagulation. In a reduced form, this model contains three equations in three variables, two of which are self-accelerated. One of these variables, an activator, behaves in a threshold manner. An inhibitor is also produced autocatalytically, but there is no inhibitor threshold, because it is generated only in the presence of the activator. All model variables are set to have equal diffusion coefficients. The model has a stable stationary trivial state, which is spatially uniform and an excitation threshold. A pulse of excitation runs from the point where the excitation threshold has been exceeded. The regime of its propagation depends on the model parameters. In a one-dimensional problem, the pulse either stops running at a certain distance from the excitation point, or it reaches the boundaries as an autowave. However, there is a parameter range where the pulse does not disappear after stopping and exists stationarily. The resulting steady-state profiles of the model variables are symmetrical relative to the center of the structure formed. (c) 2001 American Institute of Physics.

Journal Article↗
PubMed2006

The antimicrobial effect of MTAD, sodium hypochlorite, doxycycline, and citric acid on Enterococcus faecalis.

This study compared the antimicrobial effect of MTAD, two of its components, doxycycline and citric acid, and sodium hypochlorite (NaOCl) in two in vitro models on Enterococcus faecalis. In the bovine tooth model, the lumens of 30 bovine dentin discs were infected with E. faecalis for 2 weeks before treating with either one of the experimental irrigants or saline. Bacteria in the shavings were collected with two sizes of burs and enumerated after overnight culturing. Zones of inhibition were recorded in the agar diffusion model for each irrigant. In the tooth model, NaOCl and doxycycline were more effective than control in killing E. faecalis at the shallow bur depth, but at the deeper bur depth only NaOCl was superior. In the agar diffusion model, NaOCl produced less inhibition than MTAD or doxycycline.

Animals↗
PubMed2002

Cluster approximation solution of a two-species annihilation model.

A two-species reaction-diffusion model, in which particles diffuse on a one-dimensional lattice and annihilate when meeting each other, has been investigated. Mean-field equations for general choice of reaction rates have been solved exactly. The cluster mean-field approximation of the model is also studied. It is shown that the general form of large time behavior of one- and two-point functions of the number operators are determined by the diffusion rates of the two type of species, and is independent of annihilation rates.

Journal Article↗
PubMed2003

Modelling the distribution of plutonium in the Pacific Ocean.

An Oceanic General Circulation Model (OGCM) including a plutonium scavenging model as well as an advection-diffusion model has been developed for modelling the distribution of plutonium in the Pacific Ocean. Calculated 239, 240Pu water profile concentrations and 239, 240Pu inventories in water and sediment of the Pacific Ocean have showed a reasonable agreement with the experimental results. The presence of local fallout plutonium in central North Pacific waters has been confirmed. The observed 240Pu/239Pu mass ratios confirm that plutonium originating from local fallout from nuclear weapons tests carried out at Bikini and Enewetak Atolls is more rapidly removed from surface waters to deeper waters than plutonium originating from global fallout. The developed OGCM can be used for modelling the dispersion of other non-conservative tracers in the ocean as well.

Diffusion↗
PubMed2006

Spatially distributed stochastic systems: Equation-free and equation-assisted preconditioned computations.

Spatially distributed problems are often approximately modeled in terms of partial differential equations (PDEs) for appropriate coarse-grained quantities (e.g., concentrations). The derivation of accurate such PDEs starting from finer scale, atomistic models, and using suitable averaging is often a challenging task; approximate PDEs are typically obtained through mathematical closure procedures (e.g., mean field approximations). In this paper, we show how such approximate macroscopic PDEs can be exploited in constructing preconditioners to accelerate stochastic computations for spatially distributed particle-based process models. We illustrate how such preconditioning can improve the convergence of equation-free coarse-grained methods based on coarse timesteppers. Our model problem is a stochastic reaction-diffusion model capable of exhibiting Turing instabilities.

Algorithms↗