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PubMed2000

Freezing by heating in a driven mesoscopic system

We investigate a simple model corresponding to particles driven in opposite directions and interacting via a repulsive potential. The particles move off-lattice on a periodic strip and are subject to random forces as well. We show that this model-which can be considered as a continuum version of some driven diffusive systems-exhibits a paradoxical, new kind of transition called here "freezing by heating." One interesting feature of this transition is that a crystallized state with a higher total energy is obtained from a fluid state by increasing the amount of fluctuations.

Journal Article↗

PubMed2005

Effect of oxygen consumption by measuring method on PO2 transients associated with the passage of erythrocytes in capillaries of rat mesentery.

Mathematical models have predicted the existence of Po(2) gradients between erythrocytes in capillaries in the usual case where plasma contributes substantial resistance to oxygen diffusion. According to theoretical predictions, these gradients could be detected as rapid Po(2) fluctuations (erythrocyte-associated transients, EATs) along the capillary. However, verification of a model and correct choice of its parameters can be made only on the basis of direct experimental measurements. We used phosphorescence quenching microscopy to measure Po(2) in 52 capillaries of rat mesentery to obtain plasma Po(2) values 100 times/s at a given point along a capillary. A 532-nm laser generated 10-mus pulses of light, concentrated by a x100 objective, onto a spot 0.9 mum in diameter. The presence of erythrocytes in the excitation region was detected on the basis of phosphorescence amplitude (PA), proportional to the amount of plasma encountered by the laser beam, and on the basis of the intensity of transmitted laser light (LT), detected by a photodiode placed under the capillary. The data revealed correlated waveforms in PA, LT, and Po(2) in capillaries. The magnitude of the Po(2) gradients between erythrocytes and plasma was correlated with average capillary Po(2). EATs in Po(2) were more readily detected in capillaries with relatively low oxygenation. The correlation coefficients between PA and Po(2) for the half of the capillaries (n = 26) below the median Po(2) (mean Po(2) = 17 mmHg; R = -0.72) was higher than that for the other half (mean Po(2) = 39 mmHg; R = -0.38). These results support the theoretical predictions of EATs and plasma Po(2) gradients in capillaries.

Animals↗

PubMed2005

Erythrocyte-associated transients in PO2 revealed in capillaries of rat mesentery.

Mathematical models have predicted the existence of Po(2) gradients between erythrocytes in capillaries in the usual case where plasma contributes substantial resistance to oxygen diffusion. According to theoretical predictions, these gradients could be detected as rapid Po(2) fluctuations (erythrocyte-associated transients, EATs) along the capillary. However, verification of a model and correct choice of its parameters can be made only on the basis of direct experimental measurements. We used phosphorescence quenching microscopy to measure Po(2) in 52 capillaries of rat mesentery to obtain plasma Po(2) values 100 times/s at a given point along a capillary. A 532-nm laser generated 10-micros pulses of light, concentrated by a x100 objective, onto a spot 0.9 microm in diameter. The presence of erythrocytes in the excitation region was detected on the basis of phosphorescence amplitude (PA), proportional to the amount of plasma encountered by the laser beam, and on the basis of the intensity of transmitted laser light (LT), detected by a photodiode placed under the capillary. The data revealed correlated waveforms in PA, LT, and Po(2) in capillaries. The magnitude of the Po(2) gradients between erythrocytes and plasma was correlated with average capillary Po(2). EATs in Po(2) were more readily detected in capillaries with relatively low oxygenation. The correlation coefficients between PA and Po(2) for the half of the capillaries (n = 26) below the median Po(2) (mean Po(2) = 17 mmHg; R = -0.72) was higher than that for the other half (mean Po(2) = 39 mmHg; R = -0.38). These results support the theoretical predictions of EATs and plasma Po(2) gradients in capillaries.

Animals↗

PubMed1999

Quantum proteolysis by neutrophils: implications for pulmonary emphysema in alpha 1-antitrypsin deficiency.

Traditional enzyme kinetics provide a poor explanation for the increased risk of lung injury in alpha 1-antitrypsin (AAT) deficiency. Millimolar concentrations of leukocyte elastase, when released from single azurophil granules of activated neutrophils, lead to evanescent quantum bursts of proteolytic activity before catalysis is quenched by pericellular inhibitors. Herein, we tested the possibility that quantum proteolytic events are abnormal in AAT deficiency. We incubated neutrophils on opsonized fluoresceinated fibronectin in serum from individuals with various AAT phenotypes, and then measured and modeled quantum proteolytic events. The mean areas of the events in serum from heterozygous individuals (Pi MZ and Pi SZ) were slightly, but significantly, larger than those in serum from normal patients (Pi M). In marked contrast, mean areas of events in serum from AAT-deficient individuals were 10-fold larger than those in serum from normal patients. Diffusion modeling predicted that local elastase concentrations exceed AAT concentrations for less than 20 milliseconds and for more than 80 milliseconds in Pi M and Pi Z individuals, respectively. Thus, quantum proteolytic events are abnormally large and prolonged in AAT deficiency, leading directly to an increased risk of tissue injury in the immediate vicinity of activated neutrophils. These results have potentially important implications for the pathogenesis and prevention of lung disease in AAT deficiency.

Cytoplasmic Granules↗

PubMed1997

[New aspects of the mechanism of Ca2+ entry in non-excitable cells].

The capacitative Ca2+ entry has been accepted to play a crucial role in Ca2+ signaling in non-excitable cells, and the mechanisms linking the depletion of intracellular Ca2+ stores to the activation of Ca2+ entry are presently the subject of intensive investigation. There are two hypotheses to explain the molecular mechanism of capacitative Ca2+ entry. One is that a diffusible second messenger released from intracellular Ca2+ stores may activate a Ca2+ channel at the plasma membrane. Numerous molecules, including CIF, cGMP, arachidonic acid, and a small G-protein, are proposed as the candidates for the diffusible messenger. In addition, the capacitative Ca2+ entry has been suggested to be modulated by protein phosphorylation/dephosphorylation. Another hypothesis is termed the "conformational coupling model". This model suggests that information between the IP3 receptor and a plasma membrane Ca2+ channel may be transferred directly through conformational protein-protein interactions. The plasma membrane Ca2+ channel involved in the capacitative Ca2+ entry is still neither purified nor cloned, but recent studies suggest that the mammalian homologue of the Drosophila protein Trp may function as a capacitative Ca2+ entry channel.

Animals↗

PubMed2006

Single-cell characterization of retrograde signaling by brain-derived neurotrophic factor.

Brain-derived neurotrophic factor (BDNF) is a key regulator of hippocampal synaptic plasticity in the developing and adult nervous system. It can be released from pyramidal neuron dendrites in an activity-dependent manner and has therefore been suggested to serve as a signal that provides the retrograde intercellular communication necessary for Hebbian plasticity and hippocampal-dependent learning. Although much has been learned about BDNF function by field stimulation of hippocampal neurons, it is not known whether moderate action potential-independent depolarization of single cells is capable of releasing sufficient BDNF to influence transmission at individual synapses. In this study, we show directly at the single-cell level that such modulation can occur. By using K-252a, anti-BDNF antibody, and interruption of regulated release, we confirm a model in which postsynaptic depolarization elicits calcium-dependent release of BDNF that diffuses retrogradely and enhances presynaptic transmitter release.

Animals↗

PubMed2003

On the progressive enrichment of the oxygen isotopic composition of water along a leaf.

A model has been derived for the enrichment of heavy isotopes of water in leaves, including progressive enrichment along the leaf. In the model, lighter water is preferentially transpired leaving heavier water to diffuse back into the xylem and be carried further along the leaf. For this pattern to be pronounced, the ratio of advection to diffusion (Péclet number) has to be large in the longitudinal direction, and small in the radial direction. The progressive enrichment along the xylem is less than that occurring at the sites of evaporation in the mesophyll, depending on the isolation afforded by the radial Péclet number. There is an upper bound on enrichment, and effects of ground tissue associated with major veins are included. When transpiration rate is spatially nonuniform, averaging of enrichment occurs more naturally with transpiration weighting than with area-based weighting. This gives zero average enrichment of transpired water, the modified Craig-Gordon equation for average enrichment at the sites of evaporation and the Farquhar and Lloyd (In Stable Isotopes and Plant Carbon-Water Relations, pp. 47-70. Academic Press, New York, USA, 1993) prediction for mesophyll water. Earlier results on the isotopic composition of evolved oxygen and of retro-diffused carbon dioxide are preserved if these processes vary in parallel with transpiration rate. Parallel variation should be indicated approximately by uniform carbon isotope discrimination across the leaf.

Journal Article↗

PubMed1988

Induction of B-cell lymphomas in athymic NIH Swiss nu/nu mice.

A B-cell lymphoma was induced in athymic NIH Swiss nu/nu mice by challenging the animals with NIH3T3 cells, previously transfected with a recombinant DNA carrying a human oncogene hhcM, ligated to an SV40 promoter with a neomycin-resistance marker. The gross pathology of the tumor-bearing animal revealed generalized lymphadenopathy and the histopathology indicated widespread infiltration of lymphocytes into organs, such as brain, liver, kidney and lung, in addition to the lymphoid tissues and spleen; the appearance was consistent with diffuse histiocytic lymphoma. Direct immunofluorescence assays with specific typing anti-sera on live cells prepared from spleen and various lymph nodes in short-term culture, suggested the cells were B-cells. This B-cell lymphoma provides an experimental model, not only for studying possible oncogene activation but also for studying the various interactions involved in signal transduction essential for the activation of B-cell proliferation and differentiation.

Animals↗

PubMed1998

[Gene transduction for experimental brain tumors using recombinant adenovirus vector].

Recent advances in molecular biology have permitted significant progress toward the treatment of malignant brain tumors using gene transduction methods. Adenovirus vectors have recently been shown to transduce genes successfully into brain tumor cells both in vitro and in vivo. We have investigated the feasibility of gene transduction for brain tumors using adenovirus vectors. To evaluate in vitro transduction rate by adenovirus vectors, rat 9L gliosarcoma cells or human glioblastoma cells were infected with recombinant replication-deficient adenovirus vectors containing the E. coli beta-galactosidase gene (Adex-CALacZ) and stained with X-Gal. We observed a multiplicity of infection (MOI)-dependent rate. Approximately 100% transgene expression was achieved at a MOI of 5 after seven days of incubation. To evaluate transgene expression in a rat brain tumor model, AdexCALacZ was stereotactically injected into established rat 9L brain tumors. Intratumoral injection of AdexCALacZ resulted in high transgene expression in tumor cells. Although injection of AdexCALacZ in the normal basal ganglia resulted in broad and diffuse transduction into endogenous neural cells, direct intratumoral injection resulted in transduction that was relatively restricted to the tumor cells as well as some neighboring normal cells. Transduction rates were relatively elevated at the margin of the tumor. Our results suggest that adenovirus vectors might be a feasible method to transfer therapeutic genes into malignant brain tumors.

Adenoviridae↗

PubMed1986

Simulation of biomolecular diffusion and complex formation.

Diffusion is a phenomenon of very widespread importance in molecular biophysics. Diffusion can determine the rates and character of the assembly of multisubunit structures, the binding of ligands to receptors, and the internal motions of molecules and assemblies that involve solvent surface displacements. Current computer simulation techniques provide much more detailed descriptions of diffusional processes than have been available in the past. Models can be constructed to include such realistic features as structural subunits at the submolecular level (domains, monomers, or atoms); detailed electrostatic charge distributions and corresponding solvent-screened inter- and intramolecular interactions; and hydrodynamic interactions. The trajectories can be analyzed either to provide direct information on biomolecular function (e.g., the bimolecular rate constant for formation of an electron-transfer complex between two proteins), or to provide or test models for the interpretation of experimental data (e.g., the time dependence of fluorescence depolarization for segments of DNA). Here, we first review the theory of diffusional simulations, with special emphasis on new techniques such as those for obtaining transport properties of flexible assemblies and rate constants of diffusion-controlled reactions. Then we survey a variety of recent applications, including studies of large-scale motion in DNA segments and substrate "steering" in enzyme-substrate binding. We conclude with a discussion of current work (e.g., formation of protein complexes) and possible areas for future work.

Biopolymers↗

PubMed2002

Fluorescence evidence that a phase transition causes the induction time in the reduction in dynamic tension during surfactant adsorption to a clean air/water interface and a kinetic-diffusive transport model for the phase-induced induction.

An aqueous soluble surfactant adsorbing from solution onto an initially clean air/water interface often exhibits an induction period in the surface tension relaxation in which, as the adsorption begins, the tension remains near the clean interface value for an extended period of time before decreasing rapidly to the equilibrium value. In this study, using a model nonionic soluble surfactant, C14E6(CH3(CH2)13-(OCH2CH2)6-OH), we present direct fluorescence evidence that this induction is due to a first-order phase transition from a gaseous (G) to a liquid expanded (LE) phase that the assembling monolayer undergoes at constant surface pressure. An open channel flow cell is initially filled with water, and onto its air/water interface is spread an insoluble amphiphilic dye that fluoresces upon irradiation in the LE phase and whose fluorescence is quenched in the G phase. An aqueous solution of C14E(6) is then allowed to flow through the channel. We observe the immediate appearance of bright islands of the LE phase growing in a dark (G) background, confirming the presence of the G/LE phase transition. These islands eventually occupy the entire surface, after which the interface remains uniformly bright. We correlate this phase transition to the induction period by simultaneously measuring the tension of the interface of the open channel, and verifying that as the islands grow the tension remains at the clean value until the bright LE phase occupies the entire surface, whereupon the tension rapidly decreases. We further develop a phase transition surfactant transport model for the induction period in which surfactant diffuses toward and kinetically adsorbs onto the surface, and then rapidly equilibrates between the G and LE phases. For our model surfactant C14E6, we independently measure the surface concentration of the nucleating LE phase, the LE phase surfactant equation of state, the kinetic rate constants for adsorption into the LE phase, and the bulk diffusion coefficient. Using these measurements, we predict induction times for adsorption onto a clean surface without convection. We also measure these induction times in tension relaxation for adsorption onto a pendant bubble using axisymmetric shape analysis, and demonstrate agreement with the simulations with no adjustable constants.

Journal Article↗

PubMed2006

Three-component competitive adsorption model for fixed-bed and moving-bed granular activated carbon adsorbers. Part II. Model parameterization and verification.

COMPSORB-GAC is a 3-component competitive adsorption kinetic model for granular activated carbon (GAC) adsorbers that was developed in Part I of this study, including a proposed procedure for determining model parameters in natural water applications with background natural organic matter (NOM). Part II of this study demonstrates the proposed parameterization procedure and validates the modeling approach by comparing predictions with experimental breakthrough curves at multiple empty-bed contact times for both fixed-bed and moving-bed reactors. The parameterization procedure consists of a set of independent, short-term experimental tests with fresh and batch preloaded adsorbents and then data fitting using both classic and recently developed theoretical expressions. The model and parameterization procedure simplifies NOM into two fictive fractions (pore-blocking and strongly competing) and incorporates three competitive effects that vary both temporally and axially in a GAC column (direct competition for sites, intraparticle pore blockage, and external surface pore blockage). With all three competitive mechanisms accounted for, the model could accurately predict breakthrough profiles for column lengths and durations that were much longer than those used for model parameterization. Model predictions that ignored one or more of the competitive mechanisms showed that each mechanism was important for different regions of the breakthrough curve. The external surface pore-blockage effect was predominant for the prediction of early breakthrough data, whereas direct competition for sites and intraparticle pore blockage were prevalent when predicting higher breakthrough levels and data later in the column run.

Adsorption↗

PubMed2001

Simulation of acute subdural hematoma and diffuse axonal injury in coronal head impact.

Coronal head impacts were simulated in a physical model, based on the hypothesis that acute subdural hematoma (ASDH) is related to cerebral vertex displacement and diffuse axonal injury (DAI) to local Green-Lagrange strain. The geometry of the 2D model was based on anatomical measurements taken from the MRI scans of 10 adult males. Silicone gel modelled the cerebrum, paraffin the CSF and elastic membranes the trabeculae of the sulci. Pendulum impacts gave peak angular acceleration of 7800 rad s(-2) in models with and without sulci. The motion of the gel and Green-Lagrange strain were calculated from tracked coordinates of Patrick markers. Worst-case bridging vein strains are produced on the contrecoup side and are approximately doubled by adding sulci. Given that axons in the corpus callosum are highly oriented, Green-Lagrange strain was resolved in the fibre direction. It is found to be close to the minimum principal strain, indicating a degree of natural, teleological protection for the axons. The data support the use of delta0peak as a suitable descriptor for the risk of DAI but not for ASDH.

Adult↗

PubMed2005

Labyrinthine versus straight-striped patterns generated by two-dimensional Turing systems.

Striped patterns are often observed on fish skin. Such patterns have been accounted for by reaction-diffusion (RD) Turing-type models, in which two substances can spontaneously form a spatially heterogeneous pattern in a homogeneous field. Among the striped patterns generated by Turing-type models, some are "straight-striped patterns," with many stripes running in parallel, while others are "labyrinthine patterns," in which the stripes often change direction, merge with each other, and frequently branch out. RD models differ in terms of their tendency to generate either labyrinthine or straight-striped patterns. Here, we studied the conditions under which either a labyrinthine or straight-striped pattern would emerge. First, we defined an index for stripe clearness, Sh. Straight-striped patterns (large Sh) are formed if only a narrow range of spatial periods corresponds to an unstable mode. Labyrinthine patterns (small Sh) are formed when a wide range of spatial periods is unstable. More specifically, labyrinthine patterns are formed when the maximum spatial period of unstable modes is more than twice that of the minimum spatial period of unstable modes; otherwise, straight-striped patterns are formed. We then examined RD models with nonlinear reaction terms, including both activator-inhibitor and substrate-depletion models, and we demonstrated that the same conclusions hold with respect to the conditions required for labyrinthine versus straight-striped patterns.

Animals↗

PubMed1986

Interpretation of steady-state current-voltage curves: consequences and implications of current subtraction in transport studies.

A problem often confronted in analyses of charge-carrying transport processes in vivo lies in identifying porter-specific component currents and their dependence on membrane potential. Frequently, current-voltage (I-V)--or more precisely, difference-current-voltage (dI-V)--relations, both for primary and for secondary transport processes, have been extracted from the overall membrane current-voltage profiles by subtracting currents measured before and after experimental manipulations expected to alter the porter characteristics only. This paper examines the consequences of current subtraction within the context of a generalized kinetic carrier model for Class I transport mechanisms (U.-P. Hansen, D. Gradmann, D. Sanders and C.L. Slayman, 1981, J. Membrane Biol. 63:165-190). Attention is focused primarily on dI-V profiles associated with ion-driven secondary transport for which external solute concentrations usually serve as the experimental variable, but precisely analogous results and the same conclusions are indicated in relation to studies of primary electrogenesis. The model comprises a single transport loop linking n (3 or more) discrete states of a carrier 'molecule.' State transitions include one membrane charge-transport step and one solute-binding step. Fundamental properties of dI-V relations are derived analytically for all n-state formulations by analogy to common experimental designs. Additional features are revealed through analysis of a "reduced" 2-state empirical form, and numerical examples, computed using this and a "minimum" 4-state formulation, illustrate dI-V curves under principle limiting conditions. Class I models generate a wide range of dI-V profiles which can accommodate essentially all of the data now extant for primary and secondary transport systems, including difference current relations showing regions of negative slope conductance. The particular features exhibited by the curves depend on the relative magnitudes and orderings of reaction rate constants within the transport loop. Two distinct classes of dI-V curves result which reflect the relative rates of membrane charge transit and carrier recycling steps. Also evident in difference current relations are contributions from 'hidden' carrier states not directly associated with charge translocation in circumstances which can give rise to observations of counterflow or exchange diffusion. Conductance-voltage relations provide a semi-quantitative means to obtaining pairs of empirical rate parameters.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1996

F-actin, a model polymer for semiflexible chains in dilute, semidilute, and liquid crystalline solutions.

Single actin filaments were analyzed in solutions ranging from dilute (0.2 microgram/ml), where filaments interact only with solvent, to concentrations (4.0 mg/ml) at which F-actin forms a nematic phase. A persistence length of approximately 1.8 microns and an average length of approximately 22 microns (Kaufmann et al., 1992) identify actin as a model for studying the dynamics of semiflexible polymers. In dilute solutions the filaments exhibit thermal bending undulations in addition to diffusive motion. At higher semidilute concentrations (1.4 mg/ml) three-dimensional reconstructions of confocal images of fluorescently labeled filaments in a matrix of unlabeled F-actin reveal steric interactions between filaments, which account for the viscoelastic behavior of these solutions. The restricted undulations of these labeled chains reveal the virtual tube formed around a filament by the surrounding actin. The average tube diameter scales with monomer concentration c as varies; is directly proportional to c-(0.5 +/- 0.15). The diffusion of filaments in semidilute solutions (c = (0.1-2.0) mg/ml) is dominated by diffusion along the filament contour (reptation), and constraint release by remodeling of the surrounding filaments is rare. The self-diffusion coefficient D parallel along the tube decreases linearly with the chain length for semidilute solutions. For concentrations > 2.5 mg/ml a transition occurs from an isotropic entangled phase to a coexistence between isotropic and nematic domains. Analysis of the molecular motions of filaments suggests that the filaments in the aligned domains are in thermal equilibrium and that the diffusion coefficient parallel to the director D parallel is nearly independent of filament length. We also report the novel direct observation of u-shaped defects, called hairpins, in the nematic domains.

Actins↗

PubMed2005

Measuring and mapping cardiac fiber and laminar architecture using diffusion tensor MR imaging.

The ventricular myocardium is known to exhibit a complex spatial organization, with fiber orientation varying as a function of transmural location. It is now well established that diffusion tensor magnetic resonance imaging (DTMRI) may be used to measure this fiber orientation at high spatial resolution. Cardiac fibers are also known to be organized in sheets with surface orientation varying throughout the ventricles. This article reviews results on use of DTMRI for measuring ventricular fiber orientation, as well as presents new results providing strong evidence that the tertiary eigenvector of the diffusion tensor is aligned locally with the cardiac sheet surface normal. Considered together, these data indicate that DTMRI may be used to reconstruct both ventricular fiber and sheet organization. This article also presents the large deformation diffeomorphic metric mapping (LDDMM) algorithm and shows that this algorithm may be used to bring ensembles of imaged and reconstructed hearts into correspondence (e.g., registration) so that variability of ventricular geometry, fiber, and sheet orientation may be quantified. Ventricular geometry and fiber structure is known to be remodeled in a range of disease processes; however, descriptions of this remodeling have remained subjective and qualitative. We anticipate that use of DTMRI for reconstruction of ventricular anatomy coupled with application of the LDDMM method for image volume registration will enable the detection and quantification of changes in cardiac anatomy that are characteristic of specific disease processes in the heart. Finally, we show that epicardial electrical mapping and DTMRI imaging may be performed in the same hearts. The anatomic data may then be used to simulate electrical conduction in a computational model of the very same heart that was mapped electrically. This facilitates direct comparison and testing of model versus experimental results and opens the door to quantitative measurement, modeling, and analysis of the ways in which remodeling of ventricular microanatomy influences electrical conduction in the heart.

Diffusion Magnetic Resonance Imaging↗

PubMed1993

The dynamics of innovation in minimally invasive therapy.

The emergence of minimally invasive therapy (MIT), which provides alternatives to major open-surgery procedures, is affecting all aspects of medical care delivery. In the present environment of resource and cost constraint in health services, an uncommon consensus among patients, physicians, providers, and payers has evolved regarding the rapid acceptance of this area of medical intervention, an acceptance that, in turn, is stimulating further innovation. This paper discusses the dynamics of medical innovation and analyzes these forces in the context of three minimally invasive therapies: percutaneous transluminal coronary angioplasty, extracorporeal shock wave lithotripsy, and laparoscopic cholecystectomy. The different experiences of the United States and Europe are used to illustrate how scientific, medical, economic, and regulatory factors affect both the rate and direction of technological change in minimally invasive therapy.

Angioplasty, Balloon, Coronary↗