289 research outputs found
General relativistic gravitational field of a rigidly rotating disk of dust: Solution in terms of ultraelliptic functions
In a recent paper we presented analytic expressions for the axis potential,
the disk metric, and the surface mass density of the global solution to
Einstein's field equations describing a rigidly rotating disk of dust. Here we
add the complete solution in terms of ultraelliptic functions and quadratures.Comment: 5 pages, published in 1995 [Phys. Rev. Lett. 75 (1995) 3046
Phylogeography of the veined squid, Loligo forbesii, in European waters
The veined squid, Loligo forbesii Steenstrup, 1856, occurs at the European Shelf areas including the
Azores and represents a valuable resource for the European commercial fishery in the North East
Atlantic. However, very little is known about its population structure and phylogeography. This lack
of knowledge also impedes the development of sustainable fishery management for this species. The
present study combined the use of two types of markers that retrieve patterns of gene flow in different
time spans; the analysis of 16 nuclear microsatellites and sequencing of the mitochondrial cytochrome
oxidase subunit I (COI). Whereas the high mutation rate of microsatellites allows the description of
recent patterns of connectivity in species, the lower mutation rate of COI provides phylogeographic
patterns on a longer timescale. A total of 347 individuals of L. forbesii were investigated from nearly
the entire distribution range of the species, including the North East Atlantic Shelf, the Azores and
the Mediterranean. Individuals from the Western and Eastern Mediterranean Sea have never been
included in a genetic study before. We were able to analyse COI sequences from all 12 sampling areas
and define three clades of L. forbesii. Due to our large sampling area, we are presenting 13 COIhaplotypes
that were previously unknown. The microsatellite analysis does not include the Azores
but three main clades could be identified at the remaining 11 sampling sites. Low FST
values indicate
gene flow over large geographical distances. However, the genetically significant differences and an
additional slight grouping in the microsatellite structure reveal that geographical barriers seem to
influence the population structure and reduce gene flow. Furthermore, both markers provide strong
evidence that the observed phylogeographic pattern reflects the geographical history of the Azores
and the Mediterranean Sea.Postprin
Loss of mTORC1 signaling alters pancreatic α cell mass and impairs glucagon secretion
Glucagon plays a major role in the regulation of glucose homeostasis during fed and fasting states. However, the mechanisms responsible for the regulation of pancreatic α cell mass and function are not completely understood. In the current study, we identified mTOR complex 1 (mTORC1) as a major regulator of α cell mass and glucagon secretion. Using mice with tissue-specific deletion of the mTORC1 regulator Raptor in α cells (αRaptorKO), we showed that mTORC1 signaling is dispensable for α cell development, but essential for α cell maturation during the transition from a milk-based diet to a chow-based diet after weaning. Moreover, inhibition of mTORC1 signaling in αRaptorKO mice and in WT animals exposed to chronic rapamycin administration decreased glucagon content and glucagon secretion. In αRaptorKO mice, impaired glucagon secretion occurred in response to different secretagogues and was mediated by alterations in KATP channel subunit expression and activity. Additionally, our data identify the mTORC1/FoxA2 axis as a link between mTORC1 and transcriptional regulation of key genes responsible for α cell function. Thus, our results reveal a potential function of mTORC1 in nutrient-dependent regulation of glucagon secretion and identify a role for mTORC1 in controlling α cell-mass maintenance
Analysis of the noise-induced bursting-spiking transition in a pancreatic beta-cell model
A stochastic model of the electrophysiological behavior of the pancreatic
ÎČ
cell is studied, as a paradigmatic example of a bursting biological cell embedded in a noisy environment. The analysis is focused on the distortion that a growing noise causes to the basic properties of the membrane potential signals, such as their periodic or chaotic nature, and their bursting or spiking behavior. We present effective computational tools to obtain as much information as possible from these signals, and we suggest that the methods could be applied to real time series. Finally, a universal dependence of the main characteristics of the membrane potential on the size of the considered cell cluster is presented.This work has been supported by the Spanish Ministry of Science and Technology under Project Nos. BFM2000-0967 and BFM2003-03081 by a scholarship from the Spanish Ministry of Foreign Affaires (2001), and by Universidad Rey Juan Carlos under Project Nos. PGRAL-2001-02, PIGE-02-04, and GCO-2003â16. J.A. acknowledges support from the Danish Natural Science Foundation.Peer reviewe
Electron Beam-Induced Writing of Nanoscale Iron Wires on a Functional Metal Oxide
Electron beam-induced surface activation (EBISA) has been used to grow wires of iron on rutile TiO2(110)-(1 Ă 1) in ultrahigh vacuum. The wires have a width down to âŒ20 nm and hence have potential utility as interconnects on this dielectric substrate. Wire formation was achieved using an electron beam from a scanning electron microscope to activate the surface, which was subsequently exposed to Fe(CO)5. On the basis of scanning tunneling microscopy and Auger electron spectroscopy measurements, the activation mechanism involves electron beam-induced surface reduction and restructuring
Minimal state models for ionic channels involved in glucagon secretion
Pancreatic alpha cells synthesize and release glucagon. This hormone along with insulin, preserves blood glucose levels within a physiological range. During low glucose levels, alpha cells exhibit electrical activity related to glucagon secretion. In this paper, we introduce minimal state models for those ionic channels involved in this electrical activity in mice alpha cells. For estimation of model parameters, we use Monte Carlo algorithms to fit steadystate channel currents. Then, we simulate dynamic ionic currents following experimental protocols. Our aims are 1) To understand the individual ionic channel functioning and modulation that could affect glucagon secretion, and 2) To simulate ionic currents actually measured in voltage-clamp alpha-cell experiments in mice. Our estimations indicate that alpha cells are highly permeable to sodium and potassium which mainly manage action potentials. We have also found that our estimated N-type calcium channel population and density in alpha cells is in good agreement to those reported for L-type calcium channels in beta cells. This finding is strongly relevant since both, L-type and N-type calcium channels, play a main role in insulin and glucagon secretion, respectively
Frame dragging and bending of Light in Kerr and Kerr-(anti) de Sitter spacetimes
The equations of general relativity in the form of timelike and null
geodesics that describe motion of test particles and photons in Kerr spacetime
are solved exactly including the contribution from the cosmological constant.
We then perform a systematic application of the exact solutions obtained to the
following cases. The exact solutions derived for null, spherical, polar and
non-polar orbits are applied for the calculation of frame dragging
(Lense-Thirring effect) for the orbit of a photon around the galactic centre,
assuming that the latter is a Kerr black hole for various values of the Kerr
parameter including those supported by recent observations. Unbound null polar
orbits are investigated, and an analytical expression for the deviation angle
of a polar photon orbit from the gravitational Kerr field is derived. In
addition, we present the exact solution for timelike and null equatorial
orbits. In the former case, we derive an analytical expression for the
precession of the point of closest approach (perihelion, periastron) for the
orbit of a test particle around a rotating mass whose surrounding curved
spacetime geometry is described by the Kerr field. In the latter case, we
calculate an exact expression for the deflection angle for a light ray in the
gravitational field of a rotating mass (the Kerr field). We apply this
calculation for the bending of light from the gravitational field of the
galactic centre for various values of the Kerr parameter and the impact factor.Comment: LaTeX file, 45 pages 1 figure, typos fixed, v3 published in Classical
and Quantum Gravity 22 (2005) 4391-442
Odorant binding proteins : a biotechnological tool for odour control
The application of an odorant binding protein for odour control and fragrance delayed release from a textile surface was first explored in this work. Pig OBP-1 gene was cloned and expressed in Escherichia coli , and the purified protein was biochemically characterized. The IC50 values(concentrations of competitor that caused a decay of fluorescence to half-maximal intensity) were determined for four distinct fragrances, namely, citronellol, benzyl benzoate,citronellyl valerate and ethyl valerate. The results showed a strong binding of citronellyl valerate,citronellol and benzyl benzoate to the recombinant protein, while ethyl valerate displayed weaker binding. Cationized cotton substrates were coated with porcine odorant binding protein and tested for
their capacity to retain citronellol and to mask the smell of
cigarette smoke. The immobilized protein delayed the release
of citronellol when compared to the untreated cotton. According to a blind evaluation of 30 assessors, the smell of cigarette smoke, trapped onto the fabricsâ surface, was successfully attenuated by porcine odorant binding protein (more than 60 % identified the weakest smell intensity after protein exposure compared to ÎČ-cyclodextrin-treated and untreated cotton fabrics). This work demonstrated that porcine odorant binding protein can be an efficient solution to prevent and/orremove unpleasant odours trapped on the large surface of textiles. Its intrinsic properties make odorant binding proteins excellent candidates for controlled release systems which constitute a new application for this class of proteins.This work was co-funded by the European Social Fund through the management authority POPH and FCT. The authors Carla Silva and Teresa Matama would like to acknowledge their post-doctoral fellowships: SFRH/BPD/46515/2008 and SFRH/BPD/47555/2008, respectively
Sol-gel derived mesoporous Pt and Cr-doped WO(3) thin films: the role played by mesoporosity and metal doping in enhancing the gas sensing properties
Mesoporous Cr or Pt-doped WO(3) thin films to be employed as ammonia gas sensors were prepared by a fast one-step sol-gel procedure, based on the use of triblock copolymer as templating agent. The obtained films were constituted by aggregates of interconnected WO(3) nanocrystals (20-50 nm) separated by mesopores with dimensions ranging between 2 and 15 nm. The doping metals, Pt and Cr, resulted differently hosted in the WO(3) mesoporous matrix. Chromium is homogeneously dispersed in the oxide matrix, mainly as Cr(III) and Cr(V) centers, as revealed by EPR spectroscopy; instead platinum segregated as Pt (0) nanoparticles (4 nm) mainly included inside the WO(3) nanocrystals. The semiconductor layers containing Pt nanoclusters revealed, upon exposure to NH(3), remarkable electrical responses, much higher than Cr-doped and undoped layers, particularly at low ammonia concentration (6.2 ppm). This behavior was attributed to the presence of Pt nanoparticles segregated inside the semiconductor matrix, which act as catalysts of the N-H bond cleavage, decreasing the activation barrier in the ammonia dissociation. The role of the mesoporous structure in influencing the chemisorption and the gas diffusion in the WO(3) matrix appeared less decisive than the electronic differences between the two examined doping metals. The overall results suggest that a careful combination between mesoporous architecture and metal doping can really promote the electrical response of WO(3) toward ammonia
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