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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↗
PubMed1982

Use of a fluorescent cholesterol derivative to measure lateral mobility of cholesterol in membranes.

N1-Cholesterylcarbamoyl-N8-(4-nitrobenzo-2-oxa-1,3-diazole)-3,6-dioxaoctyl-1,8-diamine (NBD-Chol), a new fluorescent derivative of cholesterol, was incorporated into L-alpha-dimyristoylphosphatidylcholine (Myr2PtdCho)-based liposomes. The lateral mobility of this derivative, as well as that of N-(4-nitrobenzo-2-oxa-1,3-diazole)phosphatidylethanolamine (NBD-PtdEtn), was measured by fluorescence recovery after photobleaching techniques. In Myr2PtdCho liposomes, the diffusion coefficients (D) of the two probes are the same within experimental error below (D, approximately equal to 2 X 10(-10) cm2 X sec-1) and above (D, approximately equal to 2 X 10(-8) cm2 X sec-1) the main phase transition temperature of the bulk lipid (Tm). There is, however, a distinct difference between the mobilities of the derivatives at concentrations of added cholesterol between 5 and 20 mol % at temperatures below the main phase transition. Under these conditions, the diffusion coefficient of NBD-Chol is approximately twice that of NBD-PtdCho, a result consistent with the idea that cholesterol undergoes a lateral phase separation in these membranes at concentrations less than 20 mol %. At cholesterol concentrations greater than 20 mol % or temperatures above the Tm, the D values of the two probes are identical. The lateral mobility of a cholesterol derivative has thus been monitored directly in cholesterol-containing membranes.

4-Chloro-7-nitrobenzofurazan↗
PubMed2004

Effects of synaptic conductance on the voltage distribution and firing rate of spiking neurons.

A neuron in an active cortical circuit is subject to a fluctuating synaptic drive mediated by conductance changes. It was recently demonstrated that synaptic conductance effects in vivo significantly alter the integrative properties of neurons. These effects are missed in models that approximate the synaptic drive as a fluctuating current. Here the membrane-potential distribution and firing rate are derived for the integrate-and-fire neuron with delta correlated conductance-based synaptic input using the Fokker-Planck formalism. A number of different input scenarios are examined, including balanced drive and fluctuation changes at constant conductance, the latter of which corresponds to shifts in synchrony in the presynaptic population. This minimal model captures many experimentally observed conductance-related effects such as reduced membrane-potential fluctuations in response to increasing synaptic noise. The solvability of the model allows for a direct comparison with current-based approaches, providing a basis for assessing the validity of existing approximation schemes that have dealt with conductance change. In particular, a commonly used heuristic approach, whereby the passive membrane time constant is replaced by a drive-dependent effective time constant, is examined. It is demonstrated that this approximation is valid in the same limit that the underlying diffusion approximation holds, both for delta correlated as well as filtered synaptic drive.

Animals↗
PubMed2005

In situ measurement of solute transport in the bone lacunar-canalicular system.

Solute transport through the bone lacunar-canalicular system is believed to be essential for osteocyte survival and function but has proved difficult to measure. We report an approach that permits direct measurement of real-time solute movement in intact bones. By using fluorescence recovery after photobleaching, the movement of a vitally injected fluorescent dye (sodium fluorescein) among individual osteocytic lacunae was visualized in situ beneath the periosteal surface of mouse cortical bone at depths up to 50 microm with laser scanning confocal microscopy. Transport was analyzed by using a two-compartment mathematical model of solute diffusion that accounted for the characteristic anatomical features of the lacunar-canalicular system. The diffusion coefficient of fluorescein (376 Da) was determined to be 3.3 +/- 0.6 x 10(-6) cm2/sec, which is 62% of its diffusion coefficient in water and is similar to diffusion coefficients measured for comparably sized molecules in cartilage. The diffusion of fluorescein in bone is also consistent with the presence of an osteocyte pericellular matrix whose structure resembles that proposed for the endothelial glycocalyx [Squire, J. M., Chew, M., Nneji, G., Neal, C., Barry, J. & Michel, C. (2001) J. Struct. Biol. 136, 239-255]. To our knowledge, this is the first instance where the dynamics of molecular movement has been measured directly in the bone lacunar-canalicular system. This in situ imaging approach should also facilitate the analysis of convection-based transport mechanisms in bones of living animals.

Animals↗
PubMed2005

In silico stochastic network models that emulate the molecular sieving characteristics of bone.

Recent studies implicate bone's extracellular matrix as a "living electrophoresis and ion exchange column" with low pass filter function at the matrix level; whereas small molecules pass through the matrix microporosity, larger molecules penetrate the tissue through the pericellular space. In this study, stochastic network modeling principles were applied, for the first time to our knowledge, to build in silico, nano- to microscale models of bone. Small volumes of bone were modeled to include hierarchical levels of porosity comprising the bone matrix microporosity and the pericellular network. Flow and transport through the network was calculated for molecules from 1,000 to 100,000 datons (Da). On the basis of this study, two contrasting effects determine the rate and direction of transport of different size molecules through the hierarchical porous network of bone. Whereas diffusivity of a given molecule decreases with increasing molecular size, the size exclusion effects of bone's low pass molecular sieve translate into increasing flow velocities for large molecular species along transport paths located in the immediate vicinity of the cells. Both phenomena are expected to have a profound effect on the formation of molecular gradients at a tissue level, providing cues for tissue generation and repair by cellular "micromachines," i.e., osteoclasts and osteoblasts.

Animals↗
PubMed1999

Experimental test of the Warren-Langer model in nematic-isotropic planar interfaces.

In a directional solidification apparatus, the recoil of the nonsteady planar nematic-isotropic interface of the liquid crystal 8CB doped with hexachloroethane was measured, for different pulling velocities. Results agree very well with the predictions of our two-sided extension of Warren and Langer's one-sided model [Phys. Rev. E 47, 2702 (1993)], therefore supporting the validity of their ansatz about the evolution of the dopant concentration field. From the comparison between experiment and theory we obtain values for the segregation and diffusion coefficients of hexachloroethane in 8CB comparable to those found in the literature and measured by other methods. Using the same procedure, we measured the value of the segregation coefficient of 8CB doped with water as a function of applied sinusoidal electric field perpendicular to the sample, along the homeotropic direction. The segregation coefficient increases with electric field. In addition, preliminary results on the cellular instability in this system show that the capillary length of the pattern also increases with electric field. To our knowledge, this is the first binary system with continuously tunable segregation coefficient and capillary length.

Journal Article↗
PubMed2002

Surface electromigration patterns in a confined adsorbed metal film: Ga on GaN.

The mass transport of gallium adatoms in a confined gallium bilayer on GaN(0001) is studied with photoelectron spectromicroscopy with the goal to identify the diffusing species and their lateral distribution during directional surface electromigration and/or "random" thermal diffusion. It has been found that only the gallium atoms from the second layer undergo biased diffusion involving formation of three-dimensional islands. The development of different gallium concentration patterns is described by means of a general model, considering the presence of vacancies and trapping centres for the diffusing atoms.

Journal Article↗
PubMed2004

Biexponential diffusion tensor analysis of human brain diffusion data.

Several studies have shown that in tissues over an extended range of b-factors, the signal decay deviates significantly from the basic monoexponential model. The true nature of this departure has to date not been identified. For the current study, line scan diffusion images of brain suitable for biexponential diffusion tensor analysis were acquired in normal subjects on a clinical MR system. For each of six noncollinear directions, 32 images with b-factors ranging from 5 to 5000 s/mm2 were collected. Biexponential fits yielded parameter maps for a fast and a slow diffusion component. A subset of the diffusion data, consisting of the images obtained at the conventional range of b-factors between 5 and 972 s/mm2, was used for monoexponential diffusion tensor analysis. Fractional anisotropy (FA) of the fast-diffusion component and the monoexponential fit exhibited no significant difference. FA of the slow-diffusion biexponential component was significantly higher, particularly in areas of lower fiber density. The principal diffusion directions for the two biexponential components and the monoexponential solution were largely the same and in agreement with known fiber tracts. The second and third diffusion eigenvector directions also appeared to be aligned, but they exhibited significant deviations in localized areas.

Adult↗
PubMed2004

Current and future challenges in school-based prevention: the researcher perspective.

During the next decade we will see broad dissemination of a growing number of empirically validated school-based prevention programs. The processes of effectiveness research, broad program diffusion, and program integration at the school and community level will become a central focus of research activity. The paper presents six future directions for research in the field of school-based prevention and health promotion. The directions include developing new programs and models, developing standards and accountability systems related to school success, moving from efficacy to effectiveness research, understanding factors influencing program integration, broad dissemination of programs and policies, and the sustainability of programs, policies, and community partnerships. These future directions are driven by three significant research-to-service challenges faced both by practitioners and researchers that involve systems integration across developmental stages, levels of care, and institutional structures.

Humans↗
PubMed2001

Anisotropy of thermal diffusivity in the upper mantle.

Heat transfer in the mantle is a key process controlling the Earth's dynamics. Upper-mantle mineral phases, especially olivine, have been shown to display highly anisotropic thermal diffusivity at ambient conditions, and seismic anisotropy data show that preferred orientations of olivine induced by deformation are coherent at large scales (>50 km) in the upper mantle. Thus heat transport in the upper mantle should be anisotropic. But the thermal anisotropy of mantle minerals at high temperature and its relationship with deformation have not been well constrained. Here we present petrophysical modelling and laboratory measurements of thermal diffusivity in deformed mantle rocks between temperatures of 290 and 1,250 K that demonstrate that deformation may induce a significant anisotropy of thermal diffusivity in the uppermost mantle. We found that heat transport parallel to the flow direction is up to 30 per cent faster than that normal to the flow plane. Such a strain-induced thermal anisotropy implies that the upper-mantle temperature distribution, rheology and, consequently, its dynamics, will depend on deformation history. In oceans, resistive drag flow would result in lower vertical diffusivities in both the lithosphere and asthenosphere and hence in less effective heat transfer from the convective mantle. In continents, olivine orientations frozen in the lithosphere may induce anisotropic heating above mantle plumes, favouring the reactivation of pre-existing structures.

Journal Article↗
PubMed2001

Measuring shear-induced self-diffusion in a counterrotating geometry.

The novel correlation method to measure shear-induced self-diffusion in concentrated suspensions of noncolloidal hard spheres which we developed recently [J. Fluid Mech. 375, 297 (1998)] has been applied in a dedicated counterrotating geometry. The counterrotating nature of the setup enables experiments over a wider range of well-controlled dimensionless time (gamma;Deltat in the range 0.03-3.5, compared to 0.05-0.6 in previous experiments; here gamma; denotes the shear rate and Deltat the correlation time). The accessible range of timescales made it possible to study the nature of the particle motion in a more detailed way. The wide radius geometry provides a well-defined flow field and was designed such that there is optical access from different directions. As a result, shear-induced self-diffusion coefficients could be determined as a function of particle volume fraction straight phi (0.20-0.50) in both the vorticity and velocity gradient direction. A transition could be observed to occur for gamma;Deltat of O(1), above which the particle motion is diffusive. The corresponding self-diffusion coefficients do not increase monotonically with particle volume fraction, as has been suggested by numerical calculations and theoretical modeling of Brady and Morris [J. Fluid Mech. 348, 103 (1997)]. After an exponential growth up to straight phi=0.35, the diffusion coefficients level off. The experiments even suggest the existence of a maximum around straight phi=0.40. The results are in good agreement with experimental literature data of Phan and Leighton [J. Fluid Mech. (submitted)], although these measurements were performed for much larger values of the dimensionless time gamma;Deltat.

Journal Article↗
PubMed1984

Effect of extracellular calcium on contractile activation in guinea-pig ventricular muscle.

The problem of whether or not the alterations in twitch tension of mammalian cardiac muscle induced by changes in extracellular Ca concentration ([Ca]o) are due to the Na-Ca exchange process across the surface membrane and/or the changes in the amount of Ca in the sarcoplasmic reticulum has been re-examined by using thin bundles (70-120 micron diameter) dissected from guinea-pig papillary muscle. The observed time course of the change in the twitch tension due to a step change in [Ca]o was compared with that computed on a basis of the diffusion process of Ca ions in a circular cylinder and of the steady-state relation between [Ca]o and twitch tension. After a sudden decrease in [Ca]o from 2 mM to various lower concentrations, the isometric twitch tension of the thin bundles first fell rapidly and monotonically and then showed a much smaller and slower secondary fall. The correspondence of the observed time course of the rapid phase with the predicted time course and the observed half-time of the rapid phase ranging from 1.0 to 2.5 s indicate that the rapidity of the twitch response may be dominated by simple diffusion of Ca ions through the extracellular space. If so, the effective diffusion constant of Ca ions inside the bundles was 1.4 +/- 0.2 X 10(-6) cm2/s (mean +/- S.E., n = 9). The magnitude and direction of the step change in [Ca]o or the change at different stimulus frequencies gave rise to dissimilar time courses of the contractile change; the difference in the rapid time courses due to these factors could be explained by the simple diffusion models, but not in the much slower phase. The half-time for the Ca effect was the same as that for the rapid effect of Na ions in the external solutions. The time course of twitch decline due to [Ca]o decrease in the Na-free (Li) solution was identical to that predicted from the time course measured in the Na-rich solution and the steady-state relation between [Ca]o and tension in the Na-free solution. The half-time of Ca leak from the sarcoplasmic reticulum in the skinned cardiac muscle was 40-60 s in the presence of 10 mM-EGTA, much shorter than that of the Ca leak in the skinned amphibian skeletal muscle, but much longer than that of twitch responses due to step changes in [Ca]o in the intact cardiac muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2003

Influence of liver disease on phenolsulfonphthalein absorption from liver surface to examine possibility of direct liver surface application for drug targeting.

We have examined the influence of liver disease on drug absorption from the liver surface membrane, regarded as the first barrier for drug targeting to the liver. The main purpose of this study is to examine the possibility of direct liver surface application as a drug targeting method. We employed rats intoxicated with carbon tetrachloride (CCl(4)) or D-galactosamine (GAL) as the liver disease model, and examined drug absorption characteristics after application to the liver surface, by utilizing a cylindrical diffusion cell. In the liver-intoxicated rats, about 90% of a low molecular weight drug, phenolsulfonphthalein (PSP), as a model was absorbed from the liver surface in 6 h, similar to the normal rats (no treatment). Although the absorption rate was increased in the CCl(4) group, whereas slightly retarded absorption was observed in GAL group, there should be no serious problem for the clinical use of liver surface application. The PSP absorption from the liver surface in the CCl(4) group was indicated to obey first-order kinetics by elimination profile from the diffusion cell. The first-order absorption rate constant K(a) values of PSP from the liver surface, obtained by a compartment model and elimination profile, were increased 1.3-fold in the CCl(4) group compared to the control. Moreover, we performed drug application to the liver surface in the peritoneal cavity to assume clinical use. The K(a) of PSP in the CCl(4) group was about 4-fold larger than in the normal group, implying the importance of estimating changes in peritoneal drug absorption as a result of liver disease. Consequently, it is expected that there will be no marked decline in the absorption rate from the liver surface in a liver disease state, leading us to apply this administration method for liver targeting.

Absorption↗
PubMed1998

Antibody transport in cultured tumor cell layers.

This review summarizes our recent in vitro studies of the factors affecting the tumor penetration of immunoconjugates. The studies were designed to probe the mechanisms of diffusion and convection, using a cultured layer of mouse melanoma cells as a model tumor cell layer and an antibody to the murine transferrin receptor as a model ligand. Transport of the binding antibody was observed to be slower than that of a non-binding control, a result that is consistent with the "binding site barrier" hypothesis (Fujimori et al., J. Nucl. Med., 31: 1191-1198, 1990). Internalization of the antibody/receptor complex was necessary for this effect to be observed, implying that intracellular trafficking is a determinant of net tumor transport rates. Convective fluid flow exhibited a dependence on cell density that is consistent with a Poiseuille flow model, suggesting that convective transport occurs as laminar flow in tortuous channels. Implications for immunoconjugate therapy, limitations of the approach, and future directions of the research program are discussed.

Animals↗
PubMedDate not supplied

The effects of nest temperature, nest substrate, and clutch size on the oxygenation of embryos and larvae of the Australian moss frog, Bryobatrachus nimbus.

The jelly around amphibian eggs presents a formidable barrier to oxygen diffusion. Therefore, egg capsules must be thin enough, and the dimensions of globular egg masses small enough, to avoid oxygen limitation leading to developmental retardation or death. The eggs of the Australian moss frog, Bryobatrachus nimbus, have the thickest jelly capsule known for any anuran amphibian. Laboratory measurements of respirometric variables predict that single prehatching embryos should be normoxic between 5 degrees and 20 degrees C, with Po(2 in) maintained above critical levels (10.2-17.0 kPa). However, numerical models of embryos amid larger egg masses (13-20 eggs) predict hypoxia at temperatures above 5 degrees C. Contrary to model predictions, however, B. nimbus embryos rarely experience hypoxia in natural nests, because embryos occur in one or two layers and the moss substrate permits aeration of the lower surface while photosynthesis probably supplies oxygen directly. After hatching, larvae move to oxygen-rich regions of the jelly mass and disperse more widely within the mass as temperatures increase. Although nest characteristics relieve diffusive constraints, small clutch sizes, low rates of embryonic and larval respiration, and the cool climate occupied by B. nimbus are the main characteristics that prevent hypoxia.

Animals↗