Search PubMed⌕ Search

Biomedical subjects

D Needham

Publications and source records attributed to D Needham.

71 records · Page 4Linked to original sources
PubMed1989

Rapid deformation of "passive" polymorphonuclear leukocytes: the effects of pentoxifylline.

Entry times for spherical (no pseudopods) polymorphonuclear leukocytes (PMNs) into a 4 microns micropipet have been measured as a function of pipet suction pressure (2,500-20,000 dyn/cm2) and concentration of the drug pentoxifylline (PTX, 0.1-10.0 mM). For control cells (0 mM PTX), entry rates (reciprocal entry times) increased almost linearly with increasing suction pressure, indicating a Newtonian-like behavior. With incubation in PTX solutions, entry rate vs. suction pressure became increasingly non-linear, suggesting a shear-thinning effect for the dissipative structure. At a given suction pressure the rate of entry showed a dose-dependent increase with increasing PTX concentration, the effect being most pronounced at high suction pressures (20,000 dyn/cm2). Also, with increasing PTX concentration two other effects were observed: i) there was a decreased incidence of cells that displayed pseudopodia, and ii) there was an increased incidence of cells forming hernias and an increased streaming of cell cytoplasm during aspiration. The first observation points to a down-regulation of the cell's functional ability to "activate" in response to surface/chemical stimuli, and the second indicates that both the cortical and cytoskeletal networks are weakened either by disruption and/or reduction in density of the protein polymers. These observations are in line with other recently published experiments which suggest that the rheological effects of pentoxifylline on PMNs may be associated with the state of actin.

Actins↗
PubMed1989

Electro-mechanical permeabilization of lipid vesicles. Role of membrane tension and compressibility.

A simple micropipet technique was used to determine the critical electric field strength for membrane breakdown as a function of the applied membrane tension for three different reconstituted membranes: stearoyloleoylphosphatidylcholine (SOPC), red blood cell (RBC) lipid extract, and SOPC cholesterol (CHOL), 1:1. For these membranes the elastic area expansivity modulus increases from approximately 200 to 600 dyn/cm, and the tension at lysis increases from 5.7 to 13.2 dyn/cm, i.e., the membranes become more cohesive with increasing cholesterol content. The critical membrane voltage, Vc, required for breakdown was also found to increase with increasing cholesterol from 1.1 to 1.8 V at zero membrane tension. We have modeled the behavior in terms of the bilayer expansivity. Membrane area can be increased by either tensile or electrocompressive stresses. Both can store elastic energy in the membrane and eventually cause breakdown at a critical area dilation or critical energy. The model predicts a relation between tension and voltage at breakdown and this relation is verified experimentally for the three reconstituted membrane systems studied here.

Cell Membrane↗
PubMed1989

Relevance of kinetic and metabolic data to the interpretation of toxicological findings.

Metabolic and kinetic data on pesticides are now becoming an important part of the toxicological dossier required for product registration. In this paper the approach by one agrochemical company to generating such data is discussed. Metabolic data for the acaricide clofentezine are presented to illustrate why such data should be regarded as not only an important registration requirement, but also as integral to understanding the toxicological profile of pesticides.

Animals↗
PubMed1988

Structure and mechanical properties of giant lipid (DMPC) vesicle bilayers from 20 degrees C below to 10 degrees C above the liquid crystal-crystalline phase transition at 24 degrees C.

We have used micromechanical tests to measure the thermoelastic properties of the liquid and gel phases of dimyristoylphosphatidylcholine (DMPC). We have found that the rippled P beta' phase is only formed when a vesicle is cooled to temperatures below the main acyl chain crystallization transition, Tc, under zero or very low membrane tension. We also found that the P beta' surface ripple or superlattice can be pulled flat under high membrane tension into a planar structure. For a ripple structure formed by acyl chains perpendicular to the projected plane, the projected area change that results from a flattening process is a direct measure of the molecular crystal angle. As such, the crystal angle was found to increase from about 24 degrees just below Tc to about 33 degrees below the pretransition. It was also observed that the P beta' superlattice did not form when annealed L beta' phase vesicles were heated from 5 degrees C to Tc; likewise, ripples did not form when the membrane was held under large tension during freezing from the L alpha phase. Each of these three procedures could be used to create a metastable planar structure which we have termed L*beta' since it is lamellar and plane-crystalline with acyl chains tilted to the bilayer plane. However, we show that this structure is not as condensed as the L beta' phase below 10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization↗
PubMed1988

Thermomechanical and transition properties of dimyristoylphosphatidylcholine/cholesterol bilayers.

Mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol) have been used to examine the effects of cholesterol on the chain crystallization transitions and thermomechanical properties in phospholipid bilayer membranes. The mechanical properties--elastic moduli and level of tension at membrane rupture--were derived from micropipet pressurization of giant single-walled vesicles. Also, the micropipet method allowed temperature-dependent area transitions to be measured at constant membrane tension. X-ray diffraction measurements were made on selected lipid/cholesterol mixtures. Wide-angle patterns and electron density profiles were used to measure bilayer thickness as an indication of chain tilt and fluidity. Vesicle area versus temperature plots showed that the main acyl chain crystallization transition of DMPC broadened and shifted to higher temperatures. Both above and below the broad transition, the elastic area compressibility modulus, K, was greatly increased with cholesterol addition. The value for the 1:1 DMPC/Chol complex was found to be approximately 700 dyn/cm, comparable to that for DMPC in the L beta' phase. However, for all concentrations above 12.5 mol % (which was weakly solid), vesicle bilayers behaved as surface liquids with no surface shear rigidity even at temperatures well below the DMPC phase transition. Area changes over the broadened transitions were reduced by cholesterol and disappeared with the addition of 50 mol % to leave the thermal area expansivity at 1.3 X 10(-3)/degrees C. These area changes are consistent with separate formation of a 1:1 DMPC/Chol complex that does not condense plus residual free lipid and lipid loosely associated with the 1:1 complex that freezes normally.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol↗
PubMed1985

A quantitative explanation of the effects of some alcohols on gramicidin single-channel lifetime.

The effects of n-decanol, n-hexadecanol, n-octyl(oxyethylene)3 alcohol and cholesterol on gramicidin single-channel lifetime in planar lipid bilayers have been determined. The bilayers used were formed from a solution of monoolein in squalene. Measurements have also been made of the above compounds' effects on membrane thickness (as measured by electrical capacity and optical reflectance technique) and surface tension (as derived from bulk interfacial tension and bilayer-lens contact angle measurements). The reduction in single-channel lifetime caused by the n-alkanols may be accounted for quantitatively in terms of the effects of these compounds on bilayer thickness and surface tension. The n-octyl(oxyethylene)3 alcohol caused an increase in single-channel lifetime which is also consistent with the thickness/tension theory. The reduction in channel lifetime caused by cholesterol, however, was much larger than would be predicted from its effects on bilayer thickness and surface tension.

Alcohols↗
PubMed1985

Inactivation of the sodium current in squid giant axons by hydrocarbons.

The voltage dependence of the steady state inactivation parameter (h infinity) of the sodium current in the squid giant axon is known to be shifted in the hyperpolarizing direction by hydrocarbons and it has been suggested that the shifts arise from thickness changes in the axon membrane, analogous to those produced in lipid bilayers (Haydon, D. A., and J. E. Kimura, 1981, J. Physiol. [Lond.], 312:57-70; Haydon, D. A., and B. W. Urban, 1983, J. Physiol. [Lond.], 338:435-450; Haydon, D. A., J. R. Elliott, and B. M. Hendry, 1984, Curr. Top. Membr. Transp., 22:445-482). This hypothesis has been tested systematically by examining the effects of a range of concentrations of cyclopentane on the high-frequency capacitance per unit area both of the axonal membrane and of lipid bilayers formed from monoolein plus squalene. A similar comparison has been made for cyclopropane and n-butane, both at a pressure of 1 atm. The results are consistent with the notion that thickness increases in the axolemma produce the shifts in h infinity. Except at very high concentrations, however, the thickness changes in the lipid bilayer were too small to account for the h infinity shifts. A possible explanation of this finding is discussed.

Animals↗
PubMed1983

The effects of bilayer thickness and tension on gramicidin single-channel lifetime.

Measurements have been made of gramicidin single-channel lifetimes in monoacylglycerol bilayers chosen so that their thickness ranged from above to below the length of the gramicidin channel. Contact angles, electrical capacities and bulk-phase interfacial tensions have also been determined for these systems. The mean channel lifetime decreased with the hydrocarbon thickness of the membrane until the latter reached 2.2 nm, after which the lifetime was relatively constant. A theoretical model has been proposed which relates the mean channel lifetime (or dissociation constant) to both the thickness and the tension of the bilayers. The analysis of the present results and of those of previous studies has led to the idea that aggregates of water molecules may play an important rôle in the dissociation of the gramicidin channel.

Glycerides↗
PubMed1983

Tensions and free energies of formation of "solventless" lipid bilayers. Measurement of high contact angles.

A method is described for the accurate measurement of the interfacial tension of lipid bilayer membranes containing little or no solvent. The tensions were obtained from the interfacial tensions of the equilibrium film-forming solution in the Plateau-Gibbs border, measured by conventional techniques, and the contact angle between the border and the bilayer. The contact angles in these systems are large (greater than 10 degrees) and were estimated by a new method that involved the injection of small known volumes of lipid solution into the bilayer so as to form a lens. Results have been obtained for monoolein-triolein, monoolein-squalene, and monoolein-squalene-decane systems. Half bilayer tensions in these systems were up to approximately 1 mN m-1 less than the single interface tensions. Although bilayer tension tended to increase with bilayer thickness, the interdependence of these quantities varied with the alkane solvents present. In the monoolein-squalene-decane systems, small concentrations of decane have a larger effect on tension than on thickness. Free energies of formation of the near-solventless bilayers were much greater than estimated from the simple application of Lifshitz theory.

Lipid Bilayers↗
PubMedDate not supplied

Lysolipid exchange with lipid vesicle membranes.

While the aqueous solubility for bilayer phospholipids is less than 10(-10) M--keeping lipid membranes at essentially constant mass, single chain surfactants can have a significant aqueous solubility. Thus, in surfactant solutions, both monomer and micelles can interact with a lipid bilayer, and the mass and composition of the bilayer can be changed in seconds. These changes in composition are expected to have direct consequences on bilayer structure and material properties. We have found that the exchange of surfactants like lysolecithin can be described in terms of a kinetic model in which monomer and micelles are transported to the membrane from bulk solution. Molecular transport is considered at the membrane interfaces and across the midplane between the two monolayers of the bilayer. Using micropipet manipulation, single vesicles were transferred into lysolecithin solutions, and the measurement of vesicle area change gave a direct measure of lysolecithin uptake. Transfer back to lysolecithin-free media resulted in desorption. The rates of uptake and desorption could therefore be measured at controlled levels of membrane stress. With increasing lysolecithin concentration in the bulk phase, the amount of lysolecithin in the membrane reached saturation at approximately 3 mol% for concentrations below the critical micelle concentration (CMC) and at > 30 mol% for concentrations above the CMC. When convective transport was used to deliver lysolecithin, uptake occurred via a double exponential: initial uptake into the outer monolayer was fast (approximately 0.2 sec-1); transfer across the bilayer midplane was much slower (0.0019 sec-1).

Biological Transport↗