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PubMed1990

Numerical investigation of a novel spiral wound membrane sandwich design for an implantable bioartificial pancreas.

A novel spiral wound membrane sandwich (SWMS) design for an implantable bioartificial pancreas is presented and is numerically evaluated using a comprehensive model of the glucose-insulin kinetics within the device. The spiral blood flow pattern in this design induces a convective flow of blood ultrafiltrate directly through the islet chamber. Use of ultrafiltration in addition to diffusion allows rapid transmission of blood glucose changes to the islet chamber and efficient transport of insulin from the islet chamber back to the blood stream. Simulation results suggest that the implantable bioartificial pancreas design presented in this paper may offer a means of improving the control of blood glucose levels in type I diabetics.

Blood Glucose↗
PubMed2006

Polyelectrolyte adsorption layers studied by streaming potential and particle deposition.

Adsorption of a cationic polyelectrolyte, polyallylamine hydrochloride (PAH), having a molecular weight of 70,000 on mica was characterized by the streaming potential method and by deposition of negative polystyrene latex particles. Formation of PAH layers was followed by determining the apparent zeta potential of surface zeta as function of bulk PAH concentration. The zeta potential was calculated from the streaming potential measured in the parallel-plate channel formed by two mica plates precovered by the polyelectrolyte. The experimental data were expressed as the dependence of the reduced zeta potential zeta/zeta0 on the PAH coverage Theta(PAH), calculated using the convective diffusion theory. It was found that for the ionic strength of 10(-2) M, the dependence of zeta/zeta0 on Theta(PAH) can be reflected by the theoretical model formulated previously for surfaces covered by colloid particles. The electrokinetic measurements were complemented by particle deposition experiments on PAH-covered mica surfaces. A direct correlation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For ThetaPAH > 0.3 the initial deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. The initial deposition rates for surfaces modified by PAH were compared with previous experimental and theoretical results obtained for heterogeneous surfaces formed by preadsorption of colloid particles. It was revealed that negative latex deposition occurred at surfaces exhibiting negative apparent zeta potential, which explained the anomalous deposition of particles observed in previous works. It was suggested that the combined electrokinetic and particle deposition methods can be used for detecting adsorbed polyelectrolytes at surfaces for coverage range of a percent. This enables one to measure bulk polyelectrolyte concentrations at the level of 0.05 ppm.

Journal Article↗
PubMedDate not supplied

Contrast constancy in natural scenes in shadow or direct light: A proposed role for contrast-normalisation (non-specific suppression) in visual cortex.

The range of contrasts in natural scenes is generally thought to far exceed the limited dynamic ranges of individual contrast-encoding neurons in the primary visual cortex. The visual system may employ gain-control mechanisms (Ohzawa et al. 1985) to compensate for the mismatch between the range of natural contrast energies and the limited dynamic range of visual neurons; one proposed mechanism is contrast normalisation or non-specific suppression (Heeger 1992a). This paper aims to evaluate the role of contrast normalisation in human contrast perception, using a computer model of primary visual cortex. The model uses orthogonal pairs of Gabor patches to simulate simple-cell receptive-fields to calculate local, band-limited contrast in a series of 50 digitised photographs of natural scenes. The average range of contrast energies in each image was 2.29 log units, while the "lifetime range" each model simple cell would see across all images was 2.98 log units. These ranges are greater than the dynamic range of real mammalian simple cells. Contrast normalisation (dividing contrast responses by the summed responses of all nearby neurons) reduces contrast ranges, perhaps sufficiently to match them to neurons' limited dynamic ranges. Comparison of images taken under diffuse and direct lighting conditions showed that contrast normalisation can sometimes match these conditions effectively. This may lead to perceptual contrast constancy in the face of spurious changes in contrast caused by natural environmental conditions.

Computer Simulation↗
PubMed1996

Platelet dense-granule centralization and the persistence of ADP secretion.

After activation of a human platelet, its adenosine diphosphate (ADP)-containing dense granules are moved toward the platelet's center and release their ADP into the channels of the open canalicular system (OCS). Mathematical modeling is used to investigate a possible role of this centralization in prolonging the duration of ADP secretion compared with direct release at the platelet's plasma membrane. A key parameter is the degree to which the diffusion of ADP through the narrow and tortuous channels of the OCS is slower than ADP diffusion in plasma. For small but physiologically plausible values of this parameter and with use of literature-based values for the amount and concentration of dense-granule, ADP, the platelet serves as a continuing source of ADP; the concentration of ADP in the immediate environment of the platelet remains high enough to activate nearby platelets for 5-13 s, many times longer than if ADP were released directly at the plasma membrane.

Adenosine Diphosphate↗
PubMed2001

Nutrient fluxes in the Po basin.

The nutrient load of the river Po needs to be reduced in order to bring the eutrophication problems in the Northwest Adriatic Sea under lasting control. In this paper we analyse the contribution of the different pollution sources to the nitrogen and phosphorus loads of the river Po (source apportionment). We also estimate the effects of measures that aim to reduce nitrogen and phosphorus pollution in the Po basin (scenario analyses). Using a model (PolFlow) that was previously applied to the Rhine and Elbe basins, we found that more than half of the nitrogen and phosphorus loads in the river Po originates from diffuse sources. The results of the scenario analyses indicate that the measures imposed by the EU Nitrates Directive and the EU Wastewater Treatment Directive may not be stringent enough to achieve a large reduction in the N and P loads in the river Po.

Agriculture↗
PubMed2005

Thermal diffusion probe analysis of perfusion changes in vascular occlusions of rabbit pedicle flaps.

BACKGROUND: The purpose of this study was to show the efficacy of a novel tissue blood flow measuring device in an animal model. Thermal diffusion technology evaluates changes in perfusion in small volumes of tissue. METHODS: The thermal diffusion probe device is a long, 0.9-mm-diameter flexible catheter with two thermistors, which are placed directly into the tissue; it excites an active thermistor to a constant temperature slightly above the tissue baseline and collects data on the power dissipated in the active thermistor. It also continuously monitors the baseline tissue temperature using an additional passive thermistor placed outside the heated field. In this study, rabbit epigastric pedicle flaps were instrumented with two thermal diffusion probes (peripheral and deep) to continuously monitor flap perfusion. RESULTS: Twenty-five vascular occlusion studies were performed in 16 flaps. Blood vessel occlusions (arterial, venous, and arteriovenous) were easily detectable with this system. Waveforms for arterial and arteriovenous occlusions differed from those for venous occlusions. Probes in both peripheral and deep tissue locations were sensitive to changes in tissue perfusion. CONCLUSION: Thermal diffusion probes may provide a useful clinical method for monitoring flap perfusion.

Animals↗
PubMed2005

Sedimentary microbial oxygen demand for laminar flow over a sediment bed of finite length.

Dead organic material accumulated on the bed of a lake, reservoir or wetland often provides the substrate for substantial microbial activity as well as chemical processes that withdraw dissolved oxygen (DO) from the water column. A model to estimate the actual DO profile and the "sedimentary oxygen demand (SOD)" must specify the rate of microbial or chemical activity in the sediment as well as the diffusive supply of DO from the water column through the diffusive boundary layer into the sediment. Most previous experimental and field studies have considered this problem with the assumptions that the diffusive boundary layer is (a) turbulent and (b) fully developed. These assumptions require that (a) the flow velocity above the sediment bed is fast enough to produce turbulent mixing in the boundary layer, and (b) the sediment bed is long. In this paper a model for laminar flow and SOD over a sediment bed of finite length is presented and the results are compared with those for turbulent flow. Laminar flow near a sediment bed is encountered in quiescent water bodies such as lakes, reservoirs, river backwaters, wetlands and ponds under calm wind conditions. The diffusive oxygen transfer through the laminar diffusive boundary layer above the sediment surface can restrict the microbial or chemical oxygen uptake inside the sediment significantly. The developing laminar diffusive boundary layer above the sediment/water interface is modeled based on the analogy with heat transfer, and DO uptake inside the sediment is modeled by Michaelis-Menten microbial growth kinetics. The model predicts that the rate of SOD at the beginning of the reactive sediment bed is solely dependent on microbial density in the sediment regardless of flow velocity and type. The rate of SOD, and the DO penetration depth into the sediment decrease in stream-wise direction over the length of the sediment bed, as the diffusive boundary layer above the sediment/water interface thickens. With increasing length of the sediment bed both SOD rate and DO penetration depth into the sediment tend towards zero if the flow is laminar, but tend towards a finite value if the flow is turbulent. That value can be determined as a function of both flow velocity and microbial density. The effect of the developing laminar boundary layer on SOD is strongest at the very lowest flow velocity and/or highest microbial density inside the sediment. Under quiescent conditions, the effective SOD exerted by a reactive sediment bed of a lake or wetland approaches zero, i.e. no or very little oxygen demand is exerted on the overlying water column, except at the leading edge.

Environmental Monitoring↗
PubMedDate not supplied

In situ analysis of doxorubicin uptake and cytotoxicity in a 3D culture model of human HT-1080 fibrosarcoma cells.

In solid tumors, chemotherapeutics must adequately diffuse through the extracellular compartment to achieve their cytotoxic effect. Using quantitative microspectrofluorometry, both Doxorubicin penetration through three-dimensional (3D) collagen I matrices and its subsequent intranuclear accumulation into HT-1080 cells cultured in this microenvironment were directly assessed. Evidence that collagen delayed the Doxorubicin penetration for 1 h is presented. During that period, drug concentrations were lower in the nuclei in 3D compared to 2D matrices. Anthracyclines were also found to exhibit similar cytotoxicity in 2D and 3D after long term incubation. In conclusion, in this 3D culture model, collagen type I matrices delayed the early distribution of low molecular weight therapeutics and failed to affect their long-term cytotoxic effects, as previously reported. This model may provide a rationale for avoiding the emergence of intrinsic chemoresistance in tissue.

Antineoplastic Agents↗
PubMed2004

Method for determining oxygen consumption rates of static cultures from microplate measurements of pericellular dissolved oxygen concentration.

We describe a simple protocol for determining the oxygen consumption of cells in static culture. The protocol is based on a noninvasive oxygen-sensing microplate and a simple mathematical model derived from Fick's Law. The applicability of the model is confirmed by showing the correlation of computed oxygen consumption rate (OCR) values to actual cell densities ascertained by direct cell counting and/or MTT for HL60 and U937 cells cultured in suspension. Correlation between computed OCR and these other indications of cell number was quite good, as long as the cultures were not diffusion-limited for oxygen. The impact of the geometric factors of media depth and well size were confirmed to be consistent with the model. Based on this demonstrated correlation, we also developed a simple, completely noninvasive algorithm for ascertaining the per-cell oxygen utilization rate (OUR), which is the ratio of OCR to cell number, and a fundamental cell characteristic. This is accomplished by correlating the known seed densities to extrapolated determinations of OCR at time zero. Such determinations were performed for numerous cell types, in varying well sizes. Resulting OUR values are consistent with literature values acquired by far more painstaking methods, and ranged from <0.01 fmol.min(-1).cell(-1) for bacteria to 0.1-10 fmol.min(-1).cell(-1) for immortalized mammalian and insect cell lines to >10 fmol.min(-1).cell(-1) for primary hepatocytes. This protocol for determining OCR and OUR is extremely simple and broadly applicable and can afford rapid, informative, and noninvasive insight into the state of the culture.

Animals↗
PubMed1996

Kinetics of cortical bone demineralization: controlled demineralization--a new method for modifying cortical bone allografts.

We investigated the kinetics of hydrochloric acid demineralization of human cortical bone with the objective of developing a method of controlled demineralization for structural bone allografts. It is known that the demineralization of cortical bone is a diffusion rate limited process with a sharp advancing reaction front. The demineralization kinetics of human cortical bone, described as the advance of the reaction front versus immersion time, were determined by measuring extraction of bone mineral in both planar and cylindrical geometries. Mathematical models based on diffusional mass transfer were developed to predict this process. The experimental data fit well with the behavior predicted by the model. The model for planar geometry is applicable to controlled demineralization of cortical bone allografts of irregular shapes such as cortical struts. The model for cylindrical geometry is appropriate when curved surfaces are involved such as in diaphyseal bone allografts. This method of demineralization has direct application to clinical modification of cortical bone allografts to potentially enhance their osteoinductive properties.

Adult↗
PubMed1997

Selective association of phospholipids as a clue for the passive flip-flop diffusion through bilayer lipid membranes.

We showed that the investigation of the selective association of phospholipids might contribute to the insight of the flip-flop diffusion processes. The process of selective association was studied quantitatively by testing the association probabilities for both parallel and anti-parallel orientations of the polar headgroups. The model of double chain binary mixture confirms a high capacity of phospholipids for self-association in parallel configuration of the electric dipole moments whether the cross-sectional area of the polar headgroups are in an usual range of 25-55 A2. It is demonstrated that the aggregation of a class of phospholipids from a binary mixture is strongly dependent on the dipole-dipole interaction between the same phospholipids and is modulated by the magnitude of the electric dipole moment of the other phospholipids from that binary mixture. There are a great number of mechanisms involved in the transbilayer movement of phospholipids. We referred here only to the passive transport of lipids from one monolayer to the other. The flip-flop mechanisms raised in this paper are the breakdown of bilayer due to the increase of the packing density and the inversion of the coupled phospholipids from the opposite monolayers of the same bilayer. Thus, the pair formation promoting a drop in occupied volume decreases the packing pressure in the respective monolayer and consequently triggers a flip-flop into the other direction since the packing pressure in the other monolayer has not dropped. According to the present model for the binary mixtures of double-chain lipids, the rate of the flip-flop diffusion decreased by increasing the number of the methylene groups added to the acyl chain. This dependence may be perturbed whether the phospholipids possesses a very high cross-section area of the polar headgroups (a > 55 A2). We think that the selective association of phospholipids is neither exclusively, nor only involved in promoting the transbilayer diffusion of phospholipids. Most probably, the selective association determines some phospholipid domains that attract certain particular proteins so that it can modulate the protein activity.

Diffusion↗
PubMed2004

A Lagrangian model of the evolution of the particulate size distribution of vehicular emissions.

The emission inventory for London indicates that nearly 80% of the particulate emissions derive from vehicular sources. Most of this mass is in the form of ultrafine submicrometer particles which are of concern because of their influence on lung function. The prediction of their dispersion in the atmosphere coupled to the physical and chemical transformations which affect their size distribution and concentration are of great importance. This paper reports the first results from a new meso-scale Lagrangian model which follows the particulate emissions and the evolution of their size distribution across the city. The vehicular emissions are based on the published inventory, corrected to time of day, while other emissions are assumed steady. The initial size distributions of background and emitted particles are represented by the sum of three lognormal distributions. Meteorological data are derived from Meteorological Office reports and are preprocessed to obtain the hourly values of boundary layer depth, Monin-Obukov (MO) length, friction velocity, etc., needed for the computation of the vertical dispersion process via eddy diffusivities and the aerodynamic component of the dry deposition process. In the vertical direction, three layers are assumed-surface layer (typically 50 m), canopy layer and one further layer up to the prevailing boundary layer depth. Currently, the model includes wet and dry deposition and coagulation but not chemical reaction, nucleation or deliquescence. Trajectories are evolved for several hours across the city and the number size distributions and mass concentrations (PM10, PM2.5, PM1 and PM0.1) output at each step. This enables the vehicular contributions over and above the background concentration in each size range to be studied in detail. Data from the model have been compared with experimental data for one of the London background sites where particle number size distribution up to 450 nm (SMPS), plus PM10 and PM2.5 (TEOM) data are available.

Journal Article↗
PubMed2005

Molecular basis for the effect of urea and guanidinium chloride on the dynamics of unfolded polypeptide chains.

Chemical denaturants are frequently used to unfold proteins and to characterize mechanisms and transition states of protein folding reactions. The molecular basis of the effect of urea and guanidinium chloride (GdmCl) on polypeptide chains is still not well understood. Models for denaturant--protein interaction include both direct binding and indirect changes in solvent properties. Here we report studies on the effect of urea and GdmCl on the rate constants (k(c)) of end-to-end diffusion in unstructured poly(glycine-serine) chains of different length. Urea and GdmCl both lead to a linear decrease of lnk(c) with denaturant concentration, as observed for the rate constants for protein folding. This suggests that the effect of denaturants on chain dynamics significantly contributes to the denaturant-dependence of folding rate constants for small proteins. We show that this linear dependency is the result of two additive non-linear effects, namely increased solvent viscosity and denaturant binding. The contribution from denaturant binding can be quantitatively described by Schellman's weak binding model with binding constants (K) of 0.62(+/-0.01)M(-1) for GdmCl and 0.26(+/-0.01)M(-1) for urea. In our model peptides the number of binding sites and the effect of a bound denaturant molecule on chain dynamics is identical for urea and GdmCl. The results further identify the polypeptide backbone as the major denaturant binding site and give an upper limit of a few nanoseconds for residence times of denaturant molecules on the polypeptide chain.

Guanidine↗
PubMed1983

Entropy effects on the ion-diffusion rate in transmembrane protein channels.

We treat the transport of univalent cations through pore-like protein channels in biological membranes analytically, using two models (A + B) for the channel and the ion-channel interaction. A Lennard-Jones-type repulsion between the ions and the pore wall is introduced. We also include Van der Waals- and coulomb-type interactions between polar ligands of the pore-forming protein (e.g., carbonyl groups directed towards the axis of the channel) and the migrating particles. In model A, the polar groups are assumed to occur in pairs of dipoles pointing in opposite directions (as in the gramicidin A channel), while in model B the channel is treated as a pore with a radially isotropic charge distribution. In both models the ion-channel interaction leads to the occurrence of periodic potentials, corresponding to quasi-equilibrium and transition state sites of the ion in the pore. The diffusion rate can be calculated employing rate-theoretical concepts on the basis of microscopic parameters. It is demonstrated that the anomaly (inversion of the normal mass effect) for the transport rates of different ions can be related to differences in the activation entropy. The latter quantity is estimated analytically for both models. As a test, we performed numerical calculations with parameters based on the gramicidin A model. The results are in good agreement with experimental data and data from computer simulations. This shows that simple analytic expressions are well suited for predicting trends in the ionic conductivity of protein channels on the basis of microscopic interactions.

Diffusion↗
PubMed1997

Flow cytometric measurement of kinetic and equilibrium binding parameters of arginine-glycine-aspartic acid ligands in binding to glycoprotein IIb/IIIa on platelets.

Antagonists of platelet glycoprotein IIb/IIIa (GPIIb/IIIa) represent a new therapeutic approach in inhibiting platelet aggregation, thus providing a powerful form of antithrombotic therapy. The measurement of binding of arginine-glycine-aspartic acid (RGD) peptidomimetics to GPIIb/IIIa on platelets is a key for the further understanding of ligand-receptor interactions and, thus, the design of new antagonists. The flow cytometric measurement of dynamic and equilibrium binding parameters of two new potent RGD peptidomimetics, L-762,745 and L-769,434, containing a fluorescein moiety is described in this paper. Kinetic binding measurements with these fluorescent ligands indicate a two-step binding mechanism that involves a conformational rearrangement of the receptor-ligand complex. The overall second-order binding constants are for both fluorescent ligands several orders of magnitude slower than for diffusion-controlled processes. The values of k(-1) and K(D) obtained by fitting the kinetic binding data in a two-step model are in good agreement with directly detected values of k(off)(L-762,745) = (1.9 +/- 0.6) 10(-3) s(-1), k(off)(L-769,434) = (5.1 +/- 0.7) 10(-3) s(-1), KD(L-762,745) = 12 +/- 0.5 nM, and K(D)(L-769,434) = 8 +/- 0.3 nM. Equilibrium binding measurements of fluorescent ligands with an orally active nonfluorescent antagonist, L-738,167, provided apparent dissociation binding constant K(D) of this ligand in the range from 0.1 to 0.2 nM. The kinetic dissociation measurement of L-738,167 using the binding of the fluorescent ligand L-762,745 as a reporting method yielded a k(off) for L-738,167 of (4.1 +/- 0.1) x 10(-4) s(-1) (t1/2 = 28 min).

Binding, Competitive↗
PubMed1984

Mechanisms affecting gas transport during high-frequency oscillation.

Traditional concepts of gas transport in the lung cannot explain the adequate alveolar ventilation observed during high-frequency ventilation (HFV) with tidal volumes (VT) less than the anatomic dead space (VD). Different physical mechanisms enhance and limit gas exchange during HFV. Mechanisms enhancing transport include: diffusion, the spontaneous intermingling of the molecules of 2 gases due to Brownian motion; convective mechanisms which depend on bulk flow, such as direct alveolar ventilation, pendelluft, and streaming; and the combined effects of convection and diffusion, often termed augmented transport. Equations describing some of these mechanisms have been incorporated into theorectical models of gas transport which predict that the efficiency of HFV is proportional to faVTb, where f is frequency and b is greater than a. These predictions generally agree with experimental results obtained in healthy animals and humans. However, experimental results in subjects with lung disease show that, at a fixed VT, gas transport efficiency plateaus as f increases. One explanation for this plateau is that in subjects with increased peripheral airway resistance, the upper airways act as a shunt compliance, absorbing a fraction of the delivered VT. Experimental results to date have not confirmed or refuted any specific theory, but it seems likely that gas mixing during HFV is enhanced by a number of the mechanisms mentioned above and that the mechanical properties of the lungs might limit gas transport, especially in patients with peripheral airway obstruction.

Airway Resistance↗
PubMed2004

Density and temperature effects on the orientational and dielectric properties of supercritical water.

The influence of temperature and density changes on the reorientational motions and dielectric properties of supercritical water is investigated by molecular dynamics simulations. A rigid simple point charged potential has been used to model water-water interactions. This model has revealed to be very satisfactory to reproduce thermodynamic, diffusive, and static dielectric properties of supercritical water. Reorientation times of water molecules have been computed along specific molecular directions and compared with available experimental data. The imaginary part of the frequency dependent dielectric constant, spectral densities of states, and residence times of water molecules in their first hydration shells are also evaluated.

Journal Article↗
PubMed2000

HyAlx, an aristaless-related gene, is involved in tentacle formation in hydra.

Developmental gradients are known to play important roles in axial patterning in hydra. Current efforts are directed toward elucidating the molecular basis of these gradients. We report the isolation and characterization of HyAlx, an aristaless-related gene in hydra. The expression patterns of the gene in adult hydra, as well as during bud formation, head regeneration and the formation of ectopic head structures along the body column, indicate the gene plays a role in the specification of tissue for tentacle formation. The use of RNAi provides more direct evidence for this conclusion. The different patterns of HyAlx expression during head regeneration and bud formation also provide support for a recent version of a reaction-diffusion model for axial patterning in hydra.

Amino Acid Sequence↗