Biomedical subjects

A Loiseau

Publications and source records attributed to A Loiseau.

At least 19 recordsLinked to original sources

PubMed2006

Raman spectroscopy of single-wall boron nitride nanotubes.

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A1 tangential mode at 1370 cm(-1). This is 5 cm(-1) higher than the E(2g) mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction.

Boron Compounds↗

PubMed2006

Spatial evolutions of Co and Ni atoms during single-walled carbon nanotubes formation: measurements and modeling.

Spatial investigations of nickel and cobalt atoms and of C2 and C3 radicals are performed by laser induced fluorescence (LIF) in a continuous CO2 laser-vaporization reactor during the synthesis of single-walled carbon nanotubes. The chemical composition of the gas vaporized from bimetallic Ni/Co catalysts-carbon targets is determined using a chemical kinetic model. In this model, the evolution of Ni and Co atoms is driven by kinetics of condensation/evaporation process of pure metal clusters. Metal-carbon clusters are assumed to form from soot particles (C80) and 128-atom metal clusters. Spatial profiles of Ni and Co atoms obtained by LIF are compared with the calculations to validate the modeling and to adjust the input data. The value of the initial molar fraction of carbon-metal mixture diluted in helium is determined through a parametric study. Good agreement is found between the measured and the calculated evolution of Ni for a molar fraction of the helium diluent ranging from 10 to 15%. To fit the spatial profile of Co, the activation energy is adjusted in the evaporation rate, changing the cobalt dimer bond energy. The latter is found to be largely uncertain; and three values are tested: 167, 208, and 230 kJ x mol(-1). From comparison, the activation energy is found to be 208 kJ x mol(-1). However, the C2 LIF profiles show that the depletion of C2 is accelerated when cobalt is present. The observed Co evolutions suggest that small Co-C clusters are easier and/or faster to form compared to Ni-C clusters.

Carbon↗

PubMed2005

Electron energy loss spectroscopy measurement of the optical gaps on individual boron nitride single-walled and multiwalled nanotubes.

Spatially resolved electron energy loss spectroscopy experiments have been performed in an electron microscope on several individual boron nitride (BN) single-, double-, and triple-walled nanotubes, whose diameters and number of shells have been carefully measured. In the low-loss region (from 2 to 50 eV) the spectra have been analyzed within the framework of the continuum dielectric theory, leading to the conclusion of a weak influence of out-of-plane contribution to the dielectric response of the tubes. The gap has been measured to be independent of the nanotubes geometry, and close to the in-plane gap value of hexagonal BN (5.8+/-0.2 eV).

Journal Article↗

PubMed2005

Optical transitions in single-wall boron nitride nanotubes.

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.

Journal Article↗

PubMed2004

In-situ optical analysis of the gas phase during the formation of carbon nanotubes.

A reactor has been developed at ONERA to investigate the gas phase during carbon nanotube formation by laser-induced fluorescence (LIF), Laser-induced incandescence (LII), coherent anti-Stokes Raman Scattering (CARS), and emission spectroscopy. Continuous vaporization is achieved with a continuous wave CO2 laser. Optimized conditions are used for single-walled nanotube growth, that is, a graphite target doped with 2 atom % Ni and 2 atom % Co, helium as buffer gas at a flow rate of 50 ml/s, and a pressure of 300 hPa. Temperature profiles are measured by CARS on H2, and soot images are obtained by LII in the hot carbonaceous flow. LIF and spontaneous emission of the C2 radical and Ni and Co atoms are presented. Spectral investigations are conducted at 3100 and 3200 K to have an optimized pair of excitation/detection wavelengths. Spatial investigations of the relative concentrations in the hot carbonaceous flow are performed up to 3500 K. The concentrations are measured as a function of target temperature. Two regimes of vaporization are observed. Vaporization is slow up to 3350 K and becomes much faster above this temperature. The fast regime in the 3350-3500 K range corresponds to the observed spatial extent of the metal vapors region. At 3500 K, the C2 profiles obtained with and without catalysts are very different as a result of carbon coalescence as well as carbon dissolution into the metal nanoparticles when these are present in the gas phase. The shape of the C2 profile can be related to nanotube formation and growth at a target temperature of 3500 K.

Carbon↗

PubMed2004

Nucleation and growth of single-walled nanotubes: the role of metallic catalysts.

We present a review of experimental and theoretical results on the nucleation and growth of single-walled nanotubes, with particular emphasis on the growth of nanotube bundles emerging from catalyst particles obtained from evaporation-based elaboration techniques. General results are first discussed. Experiments strongly suggest a root-growth process in which carbon, dissolved at high temperatures in catalytic particles, segregates at the surface at lower temperatures to form tube embryos and finally nanotubes through a nucleation and growth process. A theoretical analysis of the reasons carbon does not always form graphene sheets to wrap the particles suggests analogies with other surface or interface instabilities, in particular, with those found in epitaxial growth. In the second part, detailed experimental results for nickel-rare earth metal catalysts are presented. By using various electron microscopy techniques, it is shown that carbon and the rare earth metal co-segregate at the surface of the particle and form carbide platelets, providing nucleation sites for nanotubes growing in directions perpendicular to the surface. A simple theoretical model is then presented in which the role of the rare earth metal is just to transfer electrons from metal to carbon. The graphene sheet is shown to become unstable; pentagons and heptagons are favored, which can explain the occurrence of local curvatures and of tube embryos. Finally, a brief discussion of some recent atomistic models is given.

Carbon↗

PubMed2003

Synthesis of highly nitrogen-doped multi-walled carbon nanotubes.

We present a new synthesis route for nitrogen doped carbon nanotubes (CNx) based on the aerosol method. Tubes with a record high concentration of nitrogen (approximately 20 atom%) have been synthesized, confirmed by electron energy loss spectroscopy (EELS). A strong correlation between the N/C ratio and morphology of the tubes is observed and discussed.

Journal Article↗

PubMed2003

Interactions between verapamil and neutral and acidic liposomes: effects of the ionic strength.

Patients with cancer often develop major electrolyte disorders, which are aggravated by radiation therapy and chemotherapy and by the concomitant impairment of the renal function and the development of drug resistance. In addition, tumour cells have membranes with more negative charges than normal eukaryotic cells. This study was designed to test the hypothesis that the ability of the Ca(2+) blocker verapamil to mediate the reversal of multidrug resistance (MDR) by interacting with the membrane phospholipids may be correlated with the ionic strength and membrane surface potential in resistant tumours. The permeation properties of verapamil, which is the best-known MDR-modulator, were therefore studied by quantifying its ability to induce the leakage of carboxyfluorescein through unilamellar liposomes containing various mole fractions of phosphatidic acid (x(EPA)=0, 0.1 and 0.3), at four different ionic strengths (I=0.052, 0.124, 0.204 and 0.318 M). The dye leakage induced by verapamil varied greatly with I, depending on x(EPA). The permeation process was a co-operative one (1.3<Hill coefficient<3.5) and the permeation doses inducing 50% dye leakage (PD(50)) ranged between 0.2 and 1.8 mM. A highly significant multiple correlation was found to exist between the variations of log(1/PD(50)) with those of 1/ radical I and x(EPA) (dlog(1/PD(50))/d(1/ radical I)=0.15+/-0.01, dlog(1/PD(50))/dx(EPA)=2.07+/-0.08, y-intercept=2.46+/-0.03, P<0.000001). Kinetic studies on the permeation process showed that it involved two steps. The apparent rate constants of the slow and fast kinetic steps, which were driven by electrostatic and hydrophobic interactions, respectively, increased with the verapamil concentrations, depending on x(EPA). The results provide evidence that in resistant tumours (high negative membrane surface potential), the MDR reversal by verapamil might be enhanced by favourable drug-membrane interactions in patients with severe hypo-electrolytic (Na(+) and K(+)) disorders, whereas the MDR reversal might be reduced by unfavourable drug-membrane interactions in patients with severe hyper-electrolytic (Ca(2+), Na(+) and K(+)) disorders.

Calcium Channel Blockers↗

PubMed2002

Field evaporation behaviour in the gamma phase in Ti-Al during analysis in the tomographic atom probe.

A Ti-48 at% Al alloy has been successfully investigated, using atom probe field ion microscopy and transmission electron microscopy. After a specific heat treatment, this alloy has a (alpha2 + gamma) lamellar microstructure. Using the tomographic atom probe (TAP), it has been possible to image the stacking of superlattice planes of gamma and to identify titanium as the highest evaporation field element. In addition, the influence of analysis site on atom probe measurements has been estimated for this phase. A TAP analysis has also made it possible to observe an extremely thin step along a gamma/gamma interface at a near atomic scale.

Journal Article↗

PubMed2001

Root-growth mechanism for single-wall carbon nanotubes.

The catalytic growth of single-wall carbon nanotubes is investigated by high-resolution transmission electron microscopy. The similarities between the samples synthesized from different techniques suggest a common growth mechanism based on a vapor-liquid-solid model. Quantum-molecular-dynamics simulations support a root growth mechanism where carbon atoms are incorporated into the tube base by a diffusion-segregation process.

Journal Article↗

PubMed2001

Effects of acute and chronic hypertension on the labyrinthine barriers in rat.

Hearing loss, vertigo, and tinnitus have been related to arterial hypertension. The aim of the present work was to study the permeability of the blood-perilymph and of the labyrinthine barrier, between endolymph and perilymph, to small molecules during chronic and acute hypertension. Experiments were performed in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Acute hypertension was induced by phenylephrine. Perilymph was sampled from the first turn of the scala vestibuli and the Na, K, urea, and radioactive concentrations ((14)C-urea and (3)H-mannitol) were measured. In another experimental set, the endocochlear potential was recorded from the basal turn of scala media, before and after phenylephrine injection. The composition of the perilymph and the kinetic constants for (14)C-urea and (3)H-mannitol were similar in WKY and SHR, and not modified after acute hypertension. In endolymph, the endocochlear potential in SHR (+80+/-2.7 mV, n=24) was lower (P<0.001) than in WKY (+98+/-1.5 mV, n=29). The endocochlear potential was decreased by 40 mV during acute hypertensive peak in seven out of 19 WKY but not in SHR rats (n=13). In conclusion, chronic or acute hypertension did not severely alter the permeability to small molecules of the blood-perilymph barrier. The relationship between the low endocochlear potential and hypertension in SHR remains to be evaluated. After acute hypertensive peak, the presence of vascular protective mechanisms in the cochlea could account for the stable endocochlear potential recorded in SHR and 60% of normotensive rats.

Acute Disease↗

PubMed2001

Left ventricular mechanical efficiency in hypertensive patients with and without increased myocardial mass and with normal pump function.

Left ventricular hypertrophy (LVH) is a physiologic process of adaptation of the heart to mechanical load increase. Despite depression of left ventricular contractile performance, mechanical efficiency and ventriculoarterial coupling are preserved in hypertensive patients with LVH. To assess the differences between patients with and without LVH, left ventricular contractile performance and the ventriculoarterial coupling were compared in two groups of hypertensive patients with similar body surface area and arterial pressures, and normal pump function: 30 patients with LVH (group 1) and 23 without LVH (group 2). Left ventricular angiography coupled with simultaneous recording of pressures with a micromanometer were used to determine end-systolic stress-to-volume ratio (ESSVR), end-systolic elastance (Ees), effective arterial elastance (Ea), external work (EW), and pressure-volume area (PVA). Myocardial contractile performance, assessed by Ees normalized by myocardial mass and by ESSVR, was lower in group I than in group 2 (1.23 +/- 0.28 v 1.89 +/- 0.48 mm Hg/mL/100 g, and 3.85 +/- 0.99 v 5.13 +/- 0.56 g/cm2/mL, respectively, both P < .001). Ventriculoarterial coupling evaluated through Ea/Ees ratio, and mechanical efficiency evaluated through EW/PVA ratio, were similar in the two groups (0.53 +/- 0.08 v 0.51 +/- 0.05, and 0.78 +/- 0.03 v 0.80 +/- 0.02, respectively, NS). In conclusion, this study shows that ventriculoarterial coupling and mechanical efficiency are comparable in hypertensive patients with and without LVH. These results suggest that in hypertensive patients, the matching between left ventricular performance and arterial load and the energy transfer are preserved either through left ventricular hypertrophy with moderate depression of myocardial contractile performance or through enhancement of myocardial contractile performance in patients with normal left ventricular mass.

Adult↗

PubMed2001

Genetic diversity of the honeybee in Africa: microsatellite and mitochondrial data.

A total of 738 colonies from 64 localities along the African continent have been analysed using the DraI RFLP of the COI-COII mitochondrial region. Mitochondrial DNA of African honeybees appears to be composed of three highly divergent lineages. The African lineage previously reported (named A) is present in almost all the localities except those from north-eastern Africa. In this area, two newly described lineages (called O and Y), putatively originating from the Near East, are observed in high proportion. This suggests an important differentiation of Ethiopian and Egyptian honeybees from those of other African areas. The A lineage is also present in high proportion in populations from the Iberian Peninsula and Sicily. Furthermore, eight populations from Morocco, Guinea, Malawi and South Africa have been assayed with six microsatellite loci and compared to a set of eight additional populations from Europe and the Middle East. The African populations display higher genetic variability than European populations at all microsatellite loci studied thus far. This suggests that African populations have larger effective sizes than European ones. According to their microsatellite allele frequencies, the eight African populations cluster together, but are divided in two subgroups. These are the populations from Morocco and those from the other African countries. The populations from southern Europe show very low levels of 'Africanization' at nuclear microsatellite loci. Because nuclear and mitochondrial DNA often display discordant patterns of differentiation in the honeybee, the use of both kinds of markers is preferable when assessing the phylogeography of Apis mellifera and to determine the taxonomic status of the subspecies.

Africa↗

PubMed2001

Designing multidrug-resistance modulators circumventing the reverse pH gradient in tumours.

Multidrug-resistant tumours often exhibit a reverse pH gradient (acid outside), as they have an acid extracellular pH (pHe) and a neutral alkaline intracellular pH (pHi). This study was designed to test the hypothesis that the ability of lipophilic drugs to mediate multidrug resistance (MDR) reversal by interacting with the membrane phospholipids may be correlated with pH in resistant tumours. The permeation properties of five MDR modulators were therefore studied at 37 degrees C by quantifying their ability to induce the leakage of Sulfan blue through unilamellar anionic liposomes, over the range pH 6.5-7.7, and in the absence of any membrane potential (pHe = pHi). The dye leakage induced by two calcium blockers (diltiazem and verapamil) and two antiparasitic agents (thioacridine derivative and mepacrine) was found to significantly increase with the pH of the medium (P < 0.001), whereas that induced by a non-ionic detergent (Triton X-100) showed almost no pH-dependent variations. This process was a cooperative one (0.8 < Hill coefficient < 8.5) and the permeation doses inducing 50% dye leakage (PD50) ranged from 1.6 to 36.0 mM. The permeation ability of the MDR modulators (log(1/PD50)) significantly increased with their octanol-buffer distributions (logD) (slope = 0.35+/-0.06; y intercept = 1.65 +/- 0.14; P < 0.0001) and significantly decreased with their net electric charge (z) (slope = -0.48+/-0.07; y intercept = 2.85+/-0.08; P < 0.0001). A highly significant multiple correlation was found to exist between the variations of log(1/PD50) with those of logD and z (dlog(1/PD50)/dlogD = 0.21 +/- 0.05; dlog(1/PD50)/dz = -0.34+/-0.07; y intercept = 2.27+/-0.17; P < 0.000001). The results provide evidence that in resistant tumours (acid pHe and neutral alkaline pHi), the MDR reversal might be enhanced by favourable drug-membrane interactions if the modulators are designed in the form of highly lipophilic (logP approximately equals 4) mono-basic drugs with a near neutral pKa (pKa approximately equals 7-8).

Antineoplastic Agents↗

PubMed2000

Membrane permeation by multidrug-resistance-modulators and non-modulators: effects of hydrophobicity and electric charge.

This study was designed to test the hypothesis that lipophilic cationic drugs with only roughly similar structures mediate the reversal of multidrug-resistance (MDR) by interacting with membrane phospholipids. The permeation properties of MDR-modulators and non-modulators were studied by quantifying their ability to induce the leakage of Sulphan blue through the membrane of negatively charged unilamellar liposomes. Of the 22 compounds under investigation, only those bearing a net positive electric charge per molecule (z) > or = 0.2 induced dye leakage. All these efficient drugs are well-known MDR-modulators: calcium-channel blockers (propranolol, verapamil, diltiazem and dipyridamole), calmodulin antagonists (clomipramine and thioridazine) and antiparasitic agents (mepacrine, thioacridine derivatives and quinine). The non-modulators tested, including antineoplastic agents and steroids, did not induce any membrane permeation. The permeation process was a co-operative one (1.1 < Hill coefficient < 4.1) and the permeation doses inducing 50% dye leakage (PD50) were 1.9-11.2 mM. The permeation ability of the MDR-modulators (log(1/PD50)) increased significantly with octanol-buffer distributions per unit net electric charge ((logD)/z). The results provide evidence that a complex interplay occurs between the electric charge and the lipophilicity of the MDR-modulators when a dye leakage is induced through model membranes, and probably also when the MDR is reversed in leukaemic cells.

Anti-Inflammatory Agents↗

PubMed2000

Thermal dependence of multidrug-resistant-modulator efficiency: a study in anionic liposomes.

This study was designed to test the hypothesis that there exists a correlation between the ability of lipophilic drugs to mediate the reversal of multidrug-resistance (MDR) by interacting with the membrane phospholipids and the metabolic level in tissues. The permeation properties of five MDR-modulators were studied by quantifying their ability to induce the leakage of Sulphan blue through unilamellar liposomes, over the temperature range 27-42 degrees C. The dye leakage induced by a non-ionic detergent (Triton X-100), two calcium blockers (diltiazem and verapamil) and two antiparasitic agents (thioacridine derivative and mepacrine) was temperature-dependent. The permeation process was a co-operative one (1.1 < Hill coefficient < 7.5) and the permeation doses inducing 50% dye leakage (PD50) were 1.5 - 14.9 mM. The permeation ability of the MDR-modulators (log(1/PD50)) decreased significantly as the net electric charge (z) increased. The passive dye leakage (deltaG < 0) was found to be an endothermic process (deltaH > 0), favoured by an increase in the membrane disorder (deltaS > 0). The apparent enthalpy factor (deltaH50) associated with 50% dye leakage increased with the net electric charge of the compound, and this energetically non-favoured event was entirely offset by the concomitant increase in the entropy factor (deltaS50). The apparent permeation enthalpy (deltaH50) and entropy (deltaS50) showed the lowest values for Triton X-100 (deltaH50 = 7.1 +/- 0.53 kJ mol(-1), deltaS50 = 76.9 +/- 1.86 Jmol(-1) K(-1)), and the highest values for mepacrine (deltaH50 = 79.5 +/- 3.80 kJmol(-1), deltaS50 = 306.7 +/- 5-97 J mol(-1) K(-1)). When the temperature was increased from 27 to 42 degrees C, the apparent Gibbs free energy (deltaG50) of the dye leakage induced by Triton X-100 decreased by less than 10% of the initial value, and that induced by mepacrine decreased by nearly 40%. The results provide evidence that in tissues with high metabolic levels and therefore high temperatures, MDR-reversal is likely to be enhanced via favourable drug-membrane interactions controlled by the electric charge of the modulators.

Anions↗

PubMed1999

Evidence for apical K conductance and Na-K-2Cl cotransport in the endolymphatic sac of guinea pig.

The transepithelial potential in the endolymphatic sac (ESP) was recorded up to 60 min after apical injection of ouabain, bumetanide, quinine, barium, tetraethylammonium, and 4-aminopyridine. After control injection, ESP decreased by 74% and completely recovered at 30 min. After ouabain, barium, or quinine injection, the ESP time course was similar to that in the control group. After bumetanide, tetraethylammonium, or 4-aminopyridine injection, complete recovery was only observed at 60 min. These results suggest that apical K+ conductance and Na-K-2Cl cotransporter could be involved in the genesis of ESP.

Animals↗

PubMed1999

Halothane stimulates a Na+H+ antiporter involved in the regulation of intracellular pH in alveolar epithelial cells.

UNLABELLED: Changes in intracellular pH (pHi) of alveolar type II (ATII) cells have been involved in the pathophysiology of pulmonary edema. ATII cells have evolved several ions transporters to regulate their pHi, including a Na+H+ antiporter. Because halothane alters the activity of ion transporters in various cells types, it may also affect the activity of this Na+H+ antiporter. This study was performed 1) to characterize a Na+H+ antiporter in a model of ATII cells and 2) to investigate the effect of halothane on the activity of this antiporter. ATII cells were obtained from primary rat ATII cells transfected with a mutant of simian virus SV40 large T antigen (SV40-T2), and their pHi was monitored using the pH-sensitive fluorescent probe 2'-7' (bis carboxyethyl)-5(6')-carboxyfluorescein. We demonstrated in vitro that 1) a Na+H+ antiporter (apparent Km 6.8 +/- 3.4 mM, Vmax 0.0105 +/- 0.0013 delta UpHi/s) regulates the pHi of SV40-T2 cells and 2) at clinically relevant concentrations (10(-3) to 10(-5) M) and for a short exposure duration (60 min), halothane enhances the activity of this antiporter. Because ATII cell acidification has been associated with alterations in the alveolar epithelial barrier, halothane-induced intracellular alkalinization might exhibit some protective effect in clinical situations, such as aspiration pneumonia. IMPLICATIONS: In vitro, halothane induces an intracellular alkalinization of pneumocytes II via the activation of a Na+H+ antiporter. Because acidification of these cells has been associated with alterations in the alveolar epithelial barrier, halothane might exhibit some protective effect in clinical situations, such as aspiration pneumonia.

Anesthetics, Inhalation↗