2,300 research outputs found
Effects of xanthine amine congener on hypoxic coronary resistance and venous and epicardial adenosine concentrations
Objective: The aim was to define the contributions of interstitial and vascular adenosine in regulating coronary vascular resistance during hypoxia. To help in the assessment of adenosine in the vasodilator response, a potent adenosine receptor antagonist, xanthine amine congener (XAC), was used to block adenosine receptors. Methods: Seven isolated guinea pig hearts were perfused at constant flow with Krebs buffer. Coronary vascular resistance was determined during normoxia (95% O2) and mild hypoxia (60% O2) in the absence or presence of 200 or 400 nM XAC. Interstitial fluid was sampled by the epicardial disc technique and the interstitial concentration of XAC (ISF[XAC]) was determined directly by a radioreceptor assay or as tritiated XAC. Venous and epicardial concentrations of adenosine were determined by high performance liquid chromatography. In six additional experiments, the vasodilator effect of 1 ÎŒM intracoronary adenosine was measured in the absence or presence of 100 or 200 nM XAC. Results: Mild hypoxia decreased coronary resistance by 37(SEM 4)% in the absence of XAC and 26(5)% or 17(4)% in the presence of 200 or 400 nM XAC, respectively. ISF[XAC] rapidly equilibrated with [XAC] in the arterial perfusate or venous effluent. XAC 400 nM markedly increased (p<0.05) the hypoxic levels of venous and epicardial fluid adenosine from 49(19) and 251(42) nM to 75(11) and 495(48) nM, respectively. XAC 100-200 nM almost completely prevented the vasodilatation induced by 1 ÎŒM intracoronary adenosine. Conclusions: Adenosine mediates at least 54% of hypoxic vasodilatation. XAC rapidly equilibrates within the myocardial interstitial space and, as a result of blocking adenosine receptors, increases interstitial and venous adenosine concentrations. Increases in interstitial adenosine may partially overcome the adenosine receptor blockade by XAC, thereby reducing the effectiveness of XAC in attenuating the hypoxic vasodilatation. XAC attenuates intracoronary adenosine induced vasodilatation (mediated by endothelial adenosine receptors) much more effectively than it attenuates hypoxic vasodilatation, underscoring the minimal role played by the endothelial receptors in hypoxic vasodilatation. Cardiovascular Research 1994;28:604-60
Statistical Properties of the Interbeat Interval Cascade in Human Subjects
Statistical properties of interbeat intervals cascade are evaluated by
considering the joint probability distribution for two interbeat increments and of
different time scales and . We present evidence that the
conditional probability distribution
may obey a Chapman-Kolmogorov equation. The corresponding Kramers-Moyal (KM)
coefficients are evaluated. It is shown that while the first and second KM
coefficients, i.e., the drift and diffusion coefficients, take on well-defined
and significant values, the higher-order coefficients in the KM expansion are
very small. As a result, the joint probability distributions of the increments
in the interbeat intervals obey a Fokker-Planck equation. The method provides a
novel technique for distinguishing the two classes of subjects in terms of the
drift and diffusion coefficients, which behave differently for two classes of
the subjects, namely, healthy subjects and those with congestive heart failure.Comment: 5 pages, 6 figure
Editorial: The Impact of AI-Enabled Technologies in E-commerce and Omnichannel Retailing
[No abstract available
Dynamic correlations in stochastic rotation dynamics
The dynamic structure factor, vorticity and entropy density dynamic
correlation functions are measured for Stochastic Rotation Dynamics (SRD), a
particle based algorithm for fluctuating fluids. This allows us to obtain
unbiased values for the longitudinal transport coefficients such as thermal
diffusivity and bulk viscosity. The results are in good agreement with earlier
numerical and theoretical results, and it is shown for the first time that the
bulk viscosity is indeed zero for this algorithm. In addition, corrections to
the self-diffusion coefficient and shear viscosity arising from the breakdown
of the molecular chaos approximation at small mean free paths are analyzed. In
addition to deriving the form of the leading correlation corrections to these
transport coefficients, the probabilities that two and three particles remain
collision partners for consecutive time steps are derived analytically in the
limit of small mean free path. The results of this paper verify that we have an
excellent understanding of the SRD algorithm at the kinetic level and that
analytic expressions for the transport coefficients derived elsewhere do indeed
provide a very accurate description of the SRD fluid.Comment: 33 pages including 16 figure
Stochastic Feedback and the Regulation of Biological Rhythms
We propose a general approach to the question of how biological rhythms
spontaneously self-regulate, based on the concept of ``stochastic feedback''.
We illustrate this approach by considering the neuroautonomic regulation of the
heart rate. The model generates complex dynamics and successfully accounts for
key characteristics of cardiac variability, including the power spectrum,
the functional form and scaling of the distribution of variations, and
correlations in the Fourier phases. Our results suggest that in healthy systems
the control mechanisms operate to drive the system away from extreme values
while not allowing it to settle down to a constant output.Comment: 15 pages, latex2e using rotate and epsf, with 4 ps figures. Submitted
to PR
Quantum free energy differences from non-equilibrium path integrals: I. Methods and numerical application
The imaginary-time path integral representation of the canonical partition
function of a quantum system and non-equilibrium work fluctuation relations are
combined to yield methods for computing free energy differences in quantum
systems using non-equilibrium processes. The path integral representation is
isomorphic to the configurational partition function of a classical field
theory, to which a natural but fictitious Hamiltonian dynamics is associated.
It is shown that if this system is prepared in an equilibrium state, after
which a control parameter in the fictitious Hamiltonian is changed in a finite
time, then formally the Jarzynski non-equilibrium work relation and the Crooks
fluctuation relation are shown to hold, where work is defined as the change in
the energy as given by the fictitious Hamiltonian. Since the energy diverges
for the classical field theory in canonical equilibrium, two regularization
methods are introduced which limit the number of degrees of freedom to be
finite. The numerical applicability of the methods is demonstrated for a
quartic double-well potential with varying asymmetry. A general parameter-free
smoothing procedure for the work distribution functions is useful in this
context.Comment: 20 pages, 4 figures. Added clarifying remarks and fixed typo
Displacement field and elastic constants in non-ideal crystals
In this work a periodic crystal with point defects is described in the
framework of linear response theory for broken symmetry states using
correlation functions and Zwanzig-Mori equations. The main results are
microscopic expressions for the elastic constants and for the coarse-grained
density, point-defect density, and displacement field, which are valid in real
crystals, where vacancies and interstitials are present. The coarse-grained
density field differs from the small wave vector limit of the microscopic
density. In the long wavelength limit, we recover the phenomenological
description of elasticity theory including the defect density.Comment: Phys Rev. B, in print (2010
Polycyclic aromatic hydrocarbons with SPICA
Thanks to high sensitivity and angular resolution and broad spectral
coverage, SPICA will offer a unique opportunity to better characterize the
nature of polycyclic aromatic hydrocarbons (PAHs) and very small grains (VSGs),
to better use them as probes of astrophysical environments. The angular
resolution will enable to probe the chemical frontiers in the evolution process
from VSGs to neutral PAHs, to ionized PAHs and to "Grand-PAHs" in
photodissotiation regions and HII regions, as a function of G/n (UV
radiation field / density). High sensitivity will favor the detection of the
far-IR skeletal emission bands of PAHs, which provide specific fingerprints and
could lead to the identification of individual PAHs. This overall
characterization will allow to use PAH and VSG populations as tracers of
physical conditions in spatially resolved protoplanetary disks and nearby
galaxies (using mid-IR instruments), and in high redshift galaxies (using the
far-IR instrument), thanks to the broad spectral coverage SPICA provides. Based
on our previous experience with ISO and Spitzer we discuss how these goals can
be reached.Comment: To appear in Proc. Workshop "The Space Infrared Telescope for
Cosmology & Astrophysics: Revealing the Origins of Planets and Galaxies".
Eds. A.M. Heras, B. Swinyard, K. Isaak, and J.R. Goicoeche
The first CO+ image: Probing the HI/H2 layer around the ultracompact HII region Mon R2
The CO+ reactive ion is thought to be a tracer of the boundary between a HII
region and the hot molecular gas. In this study, we present the spatial
distribution of the CO+ rotational emission toward the Mon R2 star-forming
region. The CO+ emission presents a clumpy ring-like morphology, arising from a
narrow dense layer around the HII region. We compare the CO+ distribution with
other species present in photon-dominated regions (PDR), such as [CII] 158 mm,
H2 S(3) rotational line at 9.3 mm, polycyclic aromatic hydrocarbons (PAHs) and
HCO+. We find that the CO+ emission is spatially coincident with the PAHs and
[CII] emission. This confirms that the CO+ emission arises from a narrow dense
layer of the HI/H2 interface. We have determined the CO+ fractional abundance,
relative to C+ toward three positions. The abundances range from 0.1 to
1.9x10^(-10) and are in good agreement with previous chemical model, which
predicts that the production of CO+ in PDRs only occurs in dense regions with
high UV fields. The CO+ linewidth is larger than those found in molecular gas
tracers, and their central velocity are blue-shifted with respect to the
molecular gas velocity. We interpret this as a hint that the CO+ is probing
photo-evaporating clump surfaces.Comment: The main text has 4 pages, 2 pages of Appendix, 4 figures, 1 table.
Accepted for publication in Astronomy and Astrophysics letter
Influence of primary particle density in the morphology of agglomerates
Agglomeration processes occur in many different realms of science such as
colloid and aerosol formation or formation of bacterial colonies. We study the
influence of primary particle density in agglomerate structure using
diffusion-controlled Monte Carlo simulations with realistic space scales
through different regimes (DLA and DLCA). The equivalence of Monte Carlo time
steps to real time scales is given by Hirsch's hydrodynamical theory of
Brownian motion. Agglomerate behavior at different time stages of the
simulations suggests that three indices (fractal exponent, coordination number
and eccentricity index) characterize agglomerate geometry. Using these indices,
we have found that the initial density of primary particles greatly influences
the final structure of the agglomerate as observed in recent experimental
works.Comment: 11 pages, 13 figures, PRE, to appea
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