Dyn Cm-2 Research Articles

Short-term cell/substrate contact dynamics of subconfluent endothelial cells following exposure to laminar flow.

The manner in which fluid stresses are transmitted from the apical to the basal surface of the endothelium will influence the dynamics of cell/substrate contacts. Such dynamics could be important in the design of synthetic vascular grafts to promote endothelial cell adhesion. To examine whether the initial response of cell/substrate contact sites to flow depends on the magnitude of the applied shear stress, subconfluent monolayers of endothelial cells were exposed to flow at 10, 20, and 30 dyn cm-2 wall shear stresses for 20 min. Cell/substrate contact sites were visualized with total internal reflection fluorescence microscopy. Flow induced a rapid fluctuation in the membrane topography, which was reflected in dynamic changes in cell/substrate contacts. Exposure to flow caused marked changes in contact area. Contact movement occurred normal and parallel to the direction of flow. Contact sites demonstrated significant variability in contact area at 30 dyn cm-2 during the experiment but no significant movement of the contact sites in flow direction after 20 min of flow. Mean square displacements of the contact center of mass were described in terms of a directed diffusion model. Prior to onset of flow, contact movement was random. Flow induced a significant convective component to contact movement for 300-600 s, followed by reestablishment of diffusive growth and movement of contacts. These results suggest that fluid stresses are rapidly transmitted from the apical to the basal surface of the cell via the cytoskeleton.

Thermodynamics of the first transition in writhe of a small circular DNA by Monte Carlo simulation

Monte Carlo simulations are employed to investigate the thermodynamics of the first transition in writhe of a circular model filament corresponding to a 468 base-pair DNA. Parameters employed in these simulations are the torsional rigidity, C = 2.0 x 10(-19) dyne cm2, and persistence length, P = 500 A. Intersubunit interactions are modeled by a screened Coulomb potential. For a straight line of subunits this accurately approximates the nonlinear Poisson-Boltzmann potential of a cylinder with the linear charge density of DNA. Curves of relative free energy vs writhe at fixed linking difference (delta l) exhibit two minima, one corresponding to slightly writhed circles and one to slightly underwrithed figure-8's, whenever delta l lies in the transition region. The free energies of the two minima are equal when delta lc = 1.35, which defines the midpoint of the transition. At this midpoint, the free energy barrier between the two minima is found to be delta Gbar = (0.20) kBT at 298 K. Curves of mean potential energy vs writhe at fixed linking difference similarly exhibit two minima for delta l values in the transition region, and the two minimum mean potential energies are equal when delta l = 1.50. At the midpoint writhe, delta lc = 1.35, the difference in mean potential energy between the minimum free energy figure-8 and circle states is (1.3) kBT, and the difference in their entropies is 1.3 kB. Thus, the entropy of the minimum free energy figure-8 state significantly exceeds that of the circle at the midpoint of the transition. The first transition in writhe is found to occur over a rather broad range of delta l values from 0.85 to 1.85. The twist energy parameter (ET), which governs the overall free energy of supercoiling, undergoes a sigmoidal decrease, while the translational diffusion coefficient undergoes a sigmoidal increase, over this same range. The static structure factor exhibits an increase, which reflects a decrease in radius of gyration associated with the circle to figure-8 transition.

Effects of red blood cell hyperaggregation on the rat microcirculation blood flow.

This study presents the effects of red blood cell (RBC) hyperaggregation on the blood flow and pressure in the rat mesentery and cremaster network. We exclusively studied in situ non-vasodilated organs, in order to maintain the physiological regulation mechanisms. Dextran 500 was injected at different concentrations to increase RBC aggregation. The aggregation rate was measured on very small blood samples with an erythroaggregameter (SEFAM) which evaluated the disaggregating shear stress (tau D) needed to break the RBC aggregates. Microscopic observations and laser Doppler velocimetry were used to quantify the flow rate. The plasmatic dextran concentration (C) increase had different correlated effects: for example, tau D increased from 3 dynes cm-2 (for the control sample) to 14 dynes cm-2 (for C = 75 microM L-1); the flow rate was reduced threefold and very large aggregates were observed in the venules; the arteriolar pressure increased while venular pressure decreased. In order to differentiate the effects of RBC hyperaggregation from those of plasma hyperviscosity (due to dextran 500) on microcirculatory blood flow, we injected an RBC antiaggregating drug (troxerutine) (50 or 100 mg kg-1 i.v.). The consequences were a high reduction for (tau D) (from 14 dynes cm(-2)-9 dynes cm-2), smaller aggregates and higher blood flow in the venules. No effect of troxerutine was observed on plasma viscosity (plasma control: 1.9 cP with or without troxerutine; plasma with dextran at C = 75 microM L-1: 2.45 cP with or without troxerutine). The results strongly suggest that RBC aggregation has a significant influence on blood flow rate in the microcirculatory network.

Improving endothelial cell adhesion to vascular graft surfaces: Clinical need and strategies

Synthetic vascular grafts do not spontaneously endothelialize in humans and require some form of anticoagulation to maintain patency. Preseeding synthetic graft materials such as expanded polytetrafluoroethylene (ePTFE) and polyethylene terephthalate (PET) with endothelial cells (EC) has been examined in various in vitro and in vivo models. Although various studies provide encouraging results, clinical trials for EC seeding on synthetic grafts have not been equally successful. This paper provides a brief review of the various reports on EC seeding in animal and clinical studies. We discuss the inefficiencies associated with the EC seeding process and examine plasma protein treatment of the graft surfaces as a viable option for improving EC attachment, retention and spreading. As an alternative to exsisting therapies we present data on a heterogeneous ligand treatment of fibronectin (Fn) and avidin-biotin for enhanced human umbilical vein endothelial cell (HUVEC) adhesion to ePTFE graft surfaces. Control consisted of HUVECs seeded on Fn treated ePTFE graft surfaces. Functionality of HUVECs was assessed by measuring prostacyclin production of cells on both homogeneous and heterogeneous ligand treated surfaces. Laminar flow studies with a variable width flow chamber and scanning electron microscopy were used to measure initial cell retention and observe initial cell spreading on ePTFE surfaces, respectively. HUVEC retention on heterogeneous ligand treated graft surface was significantly (p < 0.001) higher compared to homogeneous ligand treated surfaces for shear stress in the range of 10-30 dyn cm-2. HUVEC showed more cellular spreading on the heterogeneous ligand treated surface after seeding for 1-2 h. In vivo experimentation was performed in immune deficient (nude) rats by replacing a section of both the femoral arteries with 8 mm long, 1 mm internal diameter denucleated ePTFE grafts treated with homogeneous and heterogeneous ligands respectively. Both grafts were seeded with similar cell density for 15 min prior to implantation. EC attachment and retention was measured by staining EC with hematoxylin and counting the cells before and after flow using light microscopy. The results indicate that a heterogeneous ligand treatment of graft surfaces using avidin-biotin and Fn-integrin attachment mechanisms increase cell seeding efficiency, initial cell retention and cellular spreading.

Vascular cell attachment and procoagulant activity on metal alloys

The attachment and growth of vascular smooth muscle cells on biomaterials used as components of devices implanted in the vascular space may influence the biocompatibility of such materials. The nature of the materials may affect the attachment and/or the activation of these cells' procoagulant responses. Therefore, the main objective of this study was to measure the strength of adhesion of these vascular cells to potential biomaterials (titanium, zirconium alloys, and stainless steel) by exposing them to a range of shear stresses (50-300 dyn cm-2) in a parallel plate flow chamber. The procoagulant responses of the cells were evaluated by measuring the tissue factor (TF) activity promoted by the different materials under flow conditions. The materials supported distinctly different levels of initial cell adhesion in static culture. However, the fraction of adherent cells did not decline significantly with incrementally increasing shear stress within the range tested. TF expression, as measured by factor Xa (FXa) production, was material-dependent. For example, cells cultured on Ti1313 exhibited more FXa production (13.2 nM 10-5 cells) than Ti1313(DH) (8.5 nM 10-5 cells) or stainless steel (2 nM 10-5 cells). Thus, our studies indicate that the level of adhesion, strength of attachment and the expression of procoagulant activity of adherent vascular cells depend strongly on the nature of the underlying biomaterial.

New carbon chains in the laboratory and in interstellar space

Twenty seven new carbon-chain molecules have been detected over the past two years with a Fourier-transform microwave (FTM) spectrometer, 13 of which are reported here for the first time. Of the 27, 11 are closed shell polyynes, 9 are free radicals, 2 are cumulene carbenes, and 5 are carbenes formed by substituting carbon chains for one of the hydrogen atoms of the three member carbene ring C3H2. All the astronomically interesting rotational transitions [including hyperfine structure (hfs) for the radicals] of the entire set have either been measured to high precision, or are readily calculated to comparable accuracy from the spectroscopic constants derived from the laboratory data. On the basis of this data, 4 of the chains (C7H, C8H, H2C6, and HC11N) have already been detected in astronomical sources and, with large radio telescopes under construction or the discovery of better astronomical sources, it is possible that nearly all can be found. Astronomical detection is aided by the apparent high polarity of all unsymmetrical chains. The sensitivity of the present liquid-nitrogen-cooled spectrometer is far from fundamental limits; an increase by one to two orders or magnitude is possible with liquid helium cooling and other refinements, and better precursor gases may be found. Many of the chains here probably have low-lying isomers, and ionized and radical variations, which may be detected by the present techniques. Carbon chains are hard to stretch but easy to bend: centrifugal distortion is well described by a classical model according to which all chains distort under rotation like classical thin rods with the same Young's modulus: E=1.7×1013 dyn cm-2, larger than that of diamond; the longest two chains we have detected, HC15N and HC17N, have low frequency bending vibrations which lie within the range of existing high altitude radio telescopes. Finally, it is pointed out that the density of the chains at the limit of detection in our spectrometer, ca. 108 cm-3, is high by the standards of modern laser spectroscopy, so that optical detection of many should be possible.

An in vitro traumatic injury model to examine the response of neurons to a hydrodynamically-induced deformation.

A novel in vitro system was developed to examine the effects of traumatic mechanical loading on individual cells. The cell shearing injury device (CSID) is a parallel disk viscometer that applies fluid shear stress with variable onset rate. The CSID was used in conjunction with microscopy and biochemical techniques to obtain a quantitative expression of the deformation and functional response of neurons to injury. Analytical and numerical approximations of the shear stress at the bottom disk were compared to determine the contribution of secondary flows. A significant portion of the shear stress was directed in the r-direction during start-up, and therefore the full Navier-Stokes equation was necessary to accurately describe the transient shear stress. When shear stress was applied at a high rate (800 dyne cm-2 sec-1) to cultured neurons, a range of cell membrane strains (0.01 to 0.53) was obtained, suggesting inhomogeneity in cellular response. Functionally, cytosolic calcium and extracellular lactate dehydrogenase levels increased in response to high strain rate (> 1 sec-1) loading, compared with quasistatic (< 1 sec-1) loading. In addition, a subpopulation of the culture subjected to rapid deformation subsequently died. These strain rates are relevant to those shown to occur in traumatic injury, and, as such, the CSID is an appropriate model for studying the biomechanics and pathophysiology of neuronal injury.

Supernova-relevant Hydrodynamic Instability Experiments on the Nova Laser

Supernova 1987A focused attention on the critical role of hydrodynamic instabilities in the evolution of supernovae. To test the modeling of these instabilities, we are developing laboratory experiments of hydrodynamic mixing under conditions relevant to supernovae. The target consists of a two-layer planar package composed of 85 μm Cu backed by 500 μm CH2, having a single-mode sinusoidal perturbation at the interface, with λ = 200 μm and η0 = 20 μm. The Nova Laser is used to generate a 10-15 Mbar [(10-15) × 1012 dyne cm2] shock at the interface, which triggers perturbation growth as a result of the Richtmyer-Meshkov instability, followed by the Rayleigh-Taylor instability as the interface decelerates. This resembles the hydrodynamics of the He-H interface of a Type II supernova at intermediate times, up to a few times 103 s. The experiment is modeled using the hydrodynamics codes HYADES and CALE and the supernova code PROMETHEUS. Results of the experiments and simulations are presented.

Single-monolayer in situ modulus measurements using a SAW device Photocrosslinking of a diacetylenic thiol-based monolayer

We report direct measurement of the modulus change that accompanies the crosslinking of a single molecular monolayer. We measured a change in elastic modulus of 5×1010 dyn cm-2 as a result of UV-induced photocrosslinking of a single surface-confined monolayer of the conjugated diacetylenic thiol HS(CH2)10Câ–·CCâ–·C(CH2)10CO2H (DAT). The modulus measurement was made on a monolayer of DAT chemisorbed upon a gold film on the surface of a 97-MHz ST-quartz surface acoustic wave (SAW) delay line. The ratio of the changes recorded in SAW velocity and attenuation, ca. 4:1, suggests that the measured effect is mainly a change in the elastic (real) component of the complex shear modulus, viscous changes playing a lesser role. In relation to typical polymer modulus values, the change of 5×1010 dyn cm-2 is consistent with a change from a rubbery material (G′≈107–108 dyn cm-2) to a fairly rigid, glassy material (G′≈1010 dyn cm-2), reasonable for comparison of the monolayer in its as-adsorbed and crosslinked forms. This report of the direct measurement of the modulus change that accompanies the crosslinking of a single molecular monolayer is consistent with previous insitu measurements of this process using thickness-shear mode resonators

Viscoelastic characterization of electroactive polymer films at the electrode/solution interface

We describe and explore a new general transmission line model that describes the viscoelastic characteristics of thin films in terms of their shear moduli. The model contains within it the commonly used lumped-elementmodel as a limiting case, and we delineate the conditions under which thesimpler lumped-element model can be applied without significant error. We apply our analysis to the interpretation of crystal impedance spectra acquired dynamically during the electrodeposition of poly(2,2′-bithiophene) (PBT) conducting polymer films. In the very early stages of deposition, PBTentrapped within surface features behaves as a rigidly coupled mass. Beyondthis, it is a viscoelastic material behaving as a Maxwell fluid. PBT storage and loss moduli are initially polymer coverage dependent, rising to limiting values G′≈G″≈5×106 dyn cm-2. These values imply an acoustic decay length of ca. 0.3 µm. Accordingly, films significantly thicker than this behave as bulk material. For thick films there is also evidence for resonance effects.

From Ultracompact to Extended H II Regions

view Abstract Citations (208) References (73) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS From Ultracompact to Extended H II Regions Garcia-Segura, Guillermo ; Franco, Jose Abstract The dynamical evolution of H II regions and wind-driven bubbles in dense clouds is studied. In particular, we address two different issues: (1) the conditions under which ultracompact H II (UCHII) regions can reach pressure equilibrium with their surrounding medium (and thereby stall their expansion) and (2) the appearance of a powerful dynamic instability in expanding H II regions. At pressure equilibrium, the ionized regions become static, and as long as the ionization sources and the ambient gas densities remain about constant, the resulting UCHII regions are stable and long-lived. The equilibrium sizes and densities, Rs,eq ∼3 X 10-2F⅓48T⅔H II, 4P-⅔7 pc and ni,eq ∼4 × 104P7T-1H II, 4 cm-3 (where Fβ8 is the photoionizing flux in units of 1048 s-11, P7 is the pressure in units of 10-7 dyne cm-2, and TH II,4 is the ion temperature in units of 104 K), are similar to those actually observed in UCHII regions. Similarly, ultra- compact wind-driven bubbles can reach pressure equilibrium, and the resulting final sizes are similar to those of UCHII'S. The same is true for a combined ultracompact structure consisting of an interior wind- driven cavity and an external H II region. For nonmoving stars in a constant-density medium, the lifetimes for all types of ultracompact objects only depend on the stellar lifetimes. For cases with a density gradient, depending on the core size and slope of the density distribution, some regions never reach the static equilibrium condition. A powerful dynamic instability appears when cooling is included in the neutral gas swept up by an H II region or a combined wind-H II region structure. This instability was first studied by Giuliani and is associated with the thin-shell instability described by Vishniac. The internal ionization front exacerbates the growth of the thin-shell instability, creating a rapid shell fragmentation, and our numerical simulations confirm the linear analysis of Giuliani. The fragments tend to merge as the evolution proceeds, creating dense and more massive clumps, and are slowly eroded by ionization fronts. Thus the resulting structures have a variety of shapes, sizes, densities, and lifetimes. Intriguing features such as "elephant trunks" and cometary-like globules can easily be explained as a result of this instability. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177769 Bibcode: 1996ApJ...469..171G Keywords: ISM: H II REGIONS; HYDRODYNAMICS; ISM: BUBBLES; ISM: CLOUDS full text sources ADS | data products SIMBAD (4)

A quantitative analysis of the surge of March 19, 1989

A large surge event appearing in AR 5395 was observed at the Yunnan Observatory on March 19, 1989. Hα spectral profiles of the event are interpreted by using a two-cloud model and the contours of three parameters: excitation temperature, T exc, microturbulent velocity, V t ,and column density of hydrogen atoms at the second level, N 0,2, are obtained, respectively. The question about the unique feature of the solution obtained by the fitting method is also discussed. The results show that the surge is composed of some conglomerated materials with higher temperature and density; the mass ejection is probably intermittent. Neither T exc nor N 0,2 vary with the height over the solar limb but decrease from the center to the periphery of the surge. V t varies from 10 to 30 km s-1 and decreases with height. Some other important parameters, such as electron density, n e ,and electron pressure, P e ,etc., have also been estimated. In the surge, with N 0,2 about 2.0 × 1012 cm-2 and T exc about 8500 K on average, we obtained n e = 1.80 × 1010 cm-3 and P e = 0.023 dyn cm-2. The energy variations of the surge during the ascending phase are estimated.

An Alternative Explanation for the Observed Variation of Solar [ITAL]p[/ITAL]-Modes with the Solar Cycle

By mimicking the effects of a magnetic field on the interior structure of a solar model, observed changes in p-mode frequencies over the course of the solar cycle are explained in terms of a change in the intensity and distribution of a magnetic field near the top of the solar convection zone; previous attempts of explain these observations have concentrated on magnetic fields extending into the solar atmosphere. Specifically, the observed frequency changes for the 5 ≤ l ≤ 60 modes between 1986 and 1989 can be accounted for by a change in magnetic field strength of 400 G approximately 320 km below the solar surface; in a standard solar model, this depth corresponds to a temperature of 10,800 K and a pressure of 3.63 × 105 dyn cm-2. These results are discussed in light of a measured change in the solar radius of 4 km between 1992 and 1994 by the Solar Disk Sextant Experiment.

Scaling Law of Solar Coronal Loops Obtained with YOHKOH

view Abstract Citations (102) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Scaling Law of Solar Coronal Loops Obtained with YOHKOH Kano, Ryouhei ; Tsuneta, Saku Abstract We find a clear correlation among the maximum temperature Tmax (K), the pressure p (dyn cm-2) and the length L (cm) of 32 steady coronal loop structures observed with Yohkoh Soft X-Ray Telescope: Tmax = 3.8 × 104 (pL)1/(5.1±0.5). The best-fit equation, however, systematically deviates from the theoretical scaling law Tmax = 1.4 × 103 (pL)1/3 derived by Rosner, Tucker, & Vaiana. We examine various possibilities for the cause of the discrepancy. Publication: The Astrophysical Journal Pub Date: December 1995 DOI: 10.1086/176547 Bibcode: 1995ApJ...454..934K Keywords: MAGNETOHYDRODYNAMICS: MHD; SUN: CORONA; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Structural cardiac changes in relation to 24‐h ambulatory blood pressure levels in borderline hypertension

To investigate left ventricular hypertrophy (LVH) in relation to 24-h ambulatory blood pressure (24-ABPM) and insulin levels in borderline hypertension. A case-control study. Borderline hypertensive men (diastolic blood pressure (DBP) 85-94 mmHg, n = 69) and age-matched normotensive controls (DBP < or = 80 mmHg, n = 69) from a population screening programme. Echocardiography (M-mode), insulin (RIA) and 24-APBM (Del Mar P-IV) levels. The borderline group showed a significant increase in septal thickness (10.4 +/- 1.5 vs. 9.7 +/- 1.5 mm, P < 0.01), peak systolic wall stress (218 +/- 38 vs. 202 +/- 38 10(3) dynes cm-2, P < 0.05) and a decrease in LV ejection time (28.4 +/- 2.5 vs. 29.5 +/- 2.1s, P < 0.01). The septum vs. posterior wall thickness ratio was significantly higher in the borderline group (1.13 +/- 0.14 vs. 1.06 +/- 0.14, P < 0.01). Casual BP levels did not correlate with LVH indices, while 24-ABPM systolic levels correlated strongly with LVH indices in the borderline group (r = 0.22-0.52, P < 0.05) but not in the normotensive group. Insulin levels correlates strongly with LVH indices in the normotensive group (r = 0.34-0.47, P < 0.01) but not the borderline, group. Signs of asymmetric LVH and altered ventricular function are already detectable in borderline hypertension. The data also suggest that early structural cardiac changes are related to ambulatory blood pressure profile, but not to casual blood pressure or trophic factors such as insulin.

A model cosmology based on gravity-electromagnetism unification

In this article the GEM (Brandenburg, 1992; Brandenburg, 1988) theory is applied to the problem of the cosmos in which most of the matter is hydrogen, spacetime is flat, and a Cosmic Background Radiation CBR field exists. Using the two postulates of the GEM theory: 1. That gravity fields are equivalent to an array of E × B drifts or a spacially varying Poynting field, such that spacetime is determined by EM fields so that the stress tensor of ultrastrong fields is self-canceling; 2. That EM and gravity fields and protons and electrons are unified at the Planck scale of lengths and energies and split apart to form distinct fields and separate particles at the “Mesoscale” of normal particle rest energies and classical radii. A new derivation is made of the formula for G found previously: G = e 2/(m p m e )α exp(−2R 1/2) = 6.668 × 10−8 dynes cm2 g−2where m p and m e are the proton and electron masses respectively, R = m p /m e and α is the fine structure constant, shows that quantum processes may occur which make the vacuum unstable to appearance of hydrogen thus allowing matter creation and a steady state universe to occur. The value for the Hubble Time calculated from this model is T o = (3/((2α)(αR1/2)4))1/3(r e /c)(e 2/Gm p m e ) = 19 Gyr where r e = e 2/(m e c 2) and follows the form first hypothesized by Dirac(1937). The CBR is traced to this process of matter creation and its temperature is calculated as being T CBR = ((3/4)Gm e 2 c/(σ2σo))1/4 = 2.66K where σ is the Thomson cross section of the electron and σo is the Stefan-Boltzman constant.

Lifetime of the P-selectin-carbohydrate bond and its response to tensile force in hydrodynamic flow.

Selectins tether to the blood vessel wall leukocytes that are flowing in the bloodstream and support subsequent labile rolling interactions as the leukocytes are subjected to hydrodynamic drag forces. To support this rolling, selectins have been proposed to have rapid bond association and dissociation rate constants, and special mechanical properties linking tensile forces and bond dissociation. We have visualized transient tethering and release of neutrophils in hydrodynamic flow on lipid bilayers containing densities of P-selectin below those required to support rolling. We report here that transient tethers had first-order kinetics and other characteristics suggesting a unimolecular interaction between P-selectin and its glycoprotein ligand (PSGL-1). The unstressed dissociation constant (off rate) was 1 s-1. Hydrodynamic shear stresses of up to 1.1 dyn cm-2, corresponding to a force on the bond of up to 110 pN, increased the off rate only modestly, to 3.5 s-1. The data was adequately matched by a proposed equation relating off rate to the exponential of tensile force on the bond and the bond interaction distance, and gave a bond interaction distance of 0.5 A. This distance is compatible with hydrogen and metal coordination bonds between P-selectin and PSGL-1. Fast on and off rates, together with the high tensile strength of the selectin bond, appear necessary to support rolling at physiological shear stresses.

Monte Carlo simulations of supercoiling free energies for unknotted and trefoil knotted DNAs

A new Monte Carlo (MC) algorithm is proposed for simulating inextensible circular chains with finite twisting and bending rigidity. This new algorithm samples the relevant Riemann volume elements in a uniform manner, when the constraining potential vanishes. Simulations are performed for filaments comprising 170 subunits, each containing approximately 28 bp, which corresponds to a DNA length of 4770 bp. The bending rigidity is chosen to yield a persistence length, P = 500 A, and the intersubunit potential is taken to be a hard-cylinder potential with diameter d = 50 A. This value of d yields the same second virial coefficient as the electrostatic potential obtained by numerical solution of the Poisson-Boltzmann equation for 150 mM salt. Simulations are performed for unknotted circles and also for trefoil knotted circles using two different values of the torsional rigidity, C = (2.0 and 3.0) x 10(-19) dyne cm2. In the case of unknotted circles, the simulated supercoiling free energy varies practically quadratically with linking difference delta l. The simulated twist energy parameter ET, its slope dET/dT, and the mean reduced writhe <w>/delta l for C = 3 x 10(-19) dyne cm2 all agree well with recent simulations for unknotted circles using the polygon-folding algorithm with identical P, d, and C. The simulated ET vs. delta l data for C = 2.0 x 10(-19) dyne cm2 agree rather well with recent experimental data for p30 delta DNA (4752 bp), for which the torsional rigidity, C = 2.07 x 10(-19) dyne cm2, was independently measured. The experimental data for p30 delta are enormously more likely to have arisen from C = 2.0 x 10(-19) than from C = 3.0 x 10(-19) dyne cm2. Serious problems with the reported experimental assessments of ET for pBR322 and their comparison with simulated data are noted. In the case of a trefoil knotted DNA, the simulated value, (ET)tre, exceeds that of the unknotted DNA, (ET)unk, by approximately equal to 1.40-fold at magnitude of delta l = 1.0, but declines to a plateau about 1.09-fold larger than (ET)unk when magnitude of delta l > or = 15. Although the predicted ratio, (ET)tre/(ET)unk approximately equal to 1.40, agrees fairly well with recent experimental measurements on a 5600-bp DNA, the individual measured ET values, like some of those reported for pBR322, are so large that they cannot be simulated using P = 500 A, d = 50 A, and any previous experimental estimate of C.

Physiology of primary saccular afferents of goldfish: implications for Mauthner cell response.

Mauthner cells receive neurally coded information from the otolith organs in fishes, and it is most likely that initiation and directional characteristics of the C-start response depend on this input. In the goldfish, saccular afferents are sensitive to sound pressure (< -30 dB re: 1 dyne cm-2) in the most sensitive frequency range (200 to 800 Hz). This input arises from volume fluctuations of the swimbladder in response to the sound pressure waveform and is thus nondirectional. Primary afferents of the saccule, lagena, and utricle of the goldfish also respond with great sensitivity to acoustic particle motion (< 1 nanometer between 100 and 200 Hz). This input arises from the acceleration of the fish in a sound field and is inherently directional. Saccular afferents can be divided into two groups based on their tuning: one group is tuned at about 250 Hz, and the other tuned between 400 Hz and 1 kHz. All otolithic primary afferents phaselock to sinusoids throughout the frequency range of hearing (up to about 2 kHz). Based on physiological and behavioral studies on Mauthner cells, it appears that highly correlated binaural input to the M-cell, from the sacculi responding to sound pressure, may be required for a decision to respond but that the direction of the response is extracted from small deviations from a perfect interaural correlation arising from the directional response of otolith organs to acoustic particle motion.

Interactions of plasma proteins with a novel polysaccharide surfactant physisorbed to polyethylene.

A polysaccharide surfactant, dextran-[1,6 bis(2-hydroxypropyl-1-amine)hexane]-dextran, (D-H-D) was prepared by reacting dextran (Mw = 8200) with epichlorohydrin followed by reaction with 1,6-hexanediamine. The D-H-D polymer product was characterized by gel permeation chromatography (GPC), and 13C-nuclear magnetic resonance spectroscopy (13C-NMR). D-H-D was physisorbed on polyethylene (PE) from aqueous solution, and the adhesion stability and resistance to protein adsorption was examined under static and dynamic flow conditions, using a modified rotating disk system. Modified surfaces were characterized by attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR), electron spectroscopy for chemical analysis (ESCA) and by water contact angles. Under applied shear stresses of up to 73 dyn cm-2, the adhesion of D-H-D on PE was sufficient to inhibit desorption by water (> 90% D-H-D on PE was retained) and 5% SDS surfactant solution (approximately 83% D-H-D retained), as determined by ATR-FTIR. Under similar shear stress conditions, albumin adsorption on D-H-D modified PE was reduced by over 90%, and protein adsorption from fresh human plasma was reduced by approximately 70% compared with unmodified PE. The results are discussed in terms of interfacial forces, and the suitability of this approach for studying protein-surface interactions and for developing a novel class of protein-resistant biomaterials.