25,706 research outputs found
Comment on "Absence of Compressible Edge Channel Rings in Quantum Antidots"
In a recent article, Karakurt et al. [I. Karakurt et al., Phys. Rev. Lett.
89, 226803 (2002)] reported the absence of compressible regions around antidots
in the quantum Hall regime. We wish to point out a significant flaw in their
analysis, which invalidates their claim.Comment: 1 page 1 figure, to be published in Phys. Rev. Let
Neuregulin 1-Beta cytoprotective role in AML 12 mouse hepatocytes exposed to pentachlorophenol.
Neuregulins are a family of growth factor domain proteins that are structurally related to the epidermal growth factor. Accumulating evidence has shown that neuregulins have cyto- and neuroprotective properties in various cell types. In particular, the neuregulin-1 Beta (NRG1-Beta) isoform is well documented for its antiinflammatory properties in rat brain after acute stroke episodes. Pentachlorophenol (PCP) is an organochlorine compound that has been widely used as a biocide in several industrial, agricultural, and domestic applications. Previous investigations from our laboratory have demonstrated that PCP exerts both cytotoxic and mitogenic effects in human liver carcinoma (HepG2) cells, primary catfish hepatocytes and AML 12 mouse hepatocytes. We have also shown that in HepG2 cells, PCP has the ability to induce stress genes that may play a role in the molecular events leading to toxicity and tumorigenesis. In the present study, we hypothesize that NRG1-Beta will exert its cytoprotective effects in PCP-treated AML 12 mouse hepatocytes by its ability to suppress the toxic effects of PCP. To test this hypothesis, we performed the MTT-cell respiration assay to assess cell viability, and Western-blot analysis to assess stress-related proteins as a consequence of PCP exposure. Data obtained from 48 h-viability studies demonstrated a biphasic response; showing a dose-dependent increase in cell viability within the range of 0 to 3.87 microg/mL, and a gradual decrease within the concentration range of 7.75 to 31.0 microg/mL in concomitant treatments of NRG1-Beta+PCP and PCP. Cell viability percentages indicated that NRG1-Beta+PCPtreated cells were not significantly impaired, while PCP-treated cells were appreciably affected; suggesting that NRG1-Beta has the ability to suppress the toxic effects of PCP. Western Blot analysis demonstrated the potential of PCP to induce oxidative stress and inflammatory response (c-fos), growth arrest and DNA damage (GADD153), proteotoxic effects (HSP70), cell cycle arrest as consequence of DNA damage (p53), mitogenic response (cyclin- D1), and apoptosis (caspase-3). NRG1-Beta exposure attenuated stress-related protein expression in PCP-treated AML 12 mouse hepatocytes. Here we provide clear evidence that NRG1-Beta exerts cytoprotective effects in AML 12 mouse hepatocytes exposed to PCP
Lightcone fluctuations in flat spacetimes with nontrivial topology
The quantum lightcone fluctuations in flat spacetimes with compactified
spatial dimensions or with boundaries are examined. The discussion is based
upon a model in which the source of the underlying metric fluctuations is taken
to be quantized linear perturbations of the gravitational field. General
expressions are derived, in the transverse trace-free gauge, for the summation
of graviton polarization tensors, and for vacuum graviton two-point functions.
Because of the fluctuating light cone, the flight time of photons between a
source and a detector may be either longer or shorter than the light
propagation time in the background classical spacetime. We calculate the mean
deviations from the classical propagation time of photons due to the changes in
the topology of the flat spacetime. These deviations are in general larger in
the directions in which topology changes occur and are typically of the order
of the Planck time, but they can get larger as the travel distance increases.Comment: 25 pages, 5 figures, some discussions added and a few typos
corrected, final version to appear in Phys. Rev.
Multi-scale Renormalisation Group Improvement of the Effective Potential
Using the renormalisation group and a conjecture concerning the perturbation
series for the effective potential, the leading logarithms in the effective
potential are exactly summed for scalar and Yukawa theories.Comment: 19 pages, DIAS STP 94-09. Expanded to check large N limit, typo's
corrected, to appear in Phys Rev
Cosmological and Black Hole Horizon Fluctuations
The quantum fluctuations of horizons in Robertson-Walker universes and in the
Schwarzschild spacetime are discussed. The source of the metric fluctuations is
taken to be quantum linear perturbations of the gravitational field. Lightcone
fluctuations arise when the retarded Green's function for a massless field is
averaged over these metric fluctuations. This averaging replaces the
delta-function on the classical lightcone with a Gaussian function, the width
of which is a measure of the scale of the lightcone fluctuations. Horizon
fluctuations are taken to be measured in the frame of a geodesic observer
falling through the horizon. In the case of an expanding universe, this is a
comoving observer either entering or leaving the horizon of another observer.
In the black hole case, we take this observer to be one who falls freely from
rest at infinity. We find that cosmological horizon fluctuations are typically
characterized by the Planck length. However, black hole horizon fluctuations in
this model are much smaller than Planck dimensions for black holes whose mass
exceeds the Planck mass. Furthermore, we find black hole horizon fluctuations
which are sufficiently small as not to invalidate the semiclassical derivation
of the Hawking process.Comment: 22 pages, Latex, 4 figures, uses eps
Semiclassical Gravity Theory and Quantum Fluctuations
We discuss the limits of validity of the semiclassical theory of gravity in
which a classical metric is coupled to the expectation value of the stress
tensor. It is argued that this theory is a good approximation only when the
fluctuations in the stress tensor are small. We calculate a dimensionless
measure of these fluctuations for a scalar field on a flat background in
particular cases, including squeezed states and the Casimir vacuum state. It is
found that the fluctuations are small for states which are close to a coherent
state, which describes classical behavior, but tend to be large otherwise. We
find in all cases studied that the energy density fluctuations are large
whenever the local energy density is negative. This is taken to mean that the
gravitational field of a system with negative energy density, such as the
Casimir vacuum, is not described by a fixed classical metric but is undergoing
large metric fluctuations. We propose an operational scheme by which one can
describe a fluctuating gravitational field in terms of the statistical behavior
of test particles. For this purpose we obtain an equation of the form of the
Langevin equation used to describe Brownian motion.Comment: In REVTEX. 20pp + 4 figures(not included, available upon request)
TUTP-93-
A quantum weak energy inequality for the Dirac field in two-dimensional flat spacetime
Fewster and Mistry have given an explicit, non-optimal quantum weak energy
inequality that constrains the smeared energy density of Dirac fields in
Minkowski spacetime. Here, their argument is adapted to the case of flat,
two-dimensional spacetime. The non-optimal bound thereby obtained has the same
order of magnitude, in the limit of zero mass, as the optimal bound of Vollick.
In contrast with Vollick's bound, the bound presented here holds for all
(non-negative) values of the field mass.Comment: Version published in Classical and Quantum Gravity. 7 pages, 1 figur
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