1,426 research outputs found
Optical properties of an ensemble of G-centers in silicon
We addressed the carrier dynamics in so-called G-centers in silicon
(consisting of substitutional-interstitial carbon pairs interacting with
interstitial silicons) obtained via ion implantation into a
silicon-on-insulator wafer. For this point defect in silicon emitting in the
telecommunication wavelength range, we unravel the recombination dynamics by
time-resolved photoluminescence spectroscopy. More specifically, we performed
detailed photoluminescence experiments as a function of excitation energy,
incident power, irradiation fluence and temperature in order to study the
impact of radiative and non-radiative recombination channels on the spectrum,
yield and lifetime of G-centers. The sharp line emitting at 969 meV (1280
nm) and the broad asymmetric sideband developing at lower energy share the same
recombination dynamics as shown by time-resolved experiments performed
selectively on each spectral component. This feature accounts for the common
origin of the two emission bands which are unambiguously attributed to the
zero-phonon line and to the corresponding phonon sideband. In the framework of
the Huang-Rhys theory with non-perturbative calculations, we reach an
estimation of 1.60.1 \angstrom for the spatial extension of the
electronic wave function in the G-center. The radiative recombination time
measured at low temperature lies in the 6 ns-range. The estimation of both
radiative and non-radiative recombination rates as a function of temperature
further demonstrate a constant radiative lifetime. Finally, although G-centers
are shallow levels in silicon, we find a value of the Debye-Waller factor
comparable to deep levels in wide-bandgap materials. Our results point out the
potential of G-centers as a solid-state light source to be integrated into
opto-electronic devices within a common silicon platform
Metformin reduces left ventricular eccentric remodeling in experimental volume overload in the rat
Left ventricular hypertrophy (LVH) is often associated with a change in myocardial energy substrate preference
from fatty acids to glucose. A possible anti hypertrophic treatment strategy could aim at stimulating or restoring
normal myocardial energy metabolism. Metformin, an adenosine monophosphate-activated protein kinase (AMPK)
activator used in the management of glucose metabolism in diabetes, is also a fatty acid oxidation stimulator. The
effect of metformin on the development of eccentric LVH and ventricular function in chronic left ventricular (LV)
volume overload (VO) is unknown. This study was designed to study this question in a VO rat model caused by
severe aortic valve regurgitation (AR). Male Wistar rats were divided in four groups (13-15 animals / group): Shams
(S) treated or not (C) with metformin (M; 100 mg/kg/d PO) and severe ARreceiving or not metformin. Treatment was
started one week before surgery and the animals were sacrificed 9 weeks later. As expected AR rats developed
severe eccentric LVH during the course of the protocol. Metformin treatment did not influence the total heart weight.
However, LV remodeling associated with the severe VO was severe in ARM than in ARC. Fractional shortening, a
marker of systolic function, was significantly higher in ARM compared to ARC group. Metformin also increased the
activity of enzymes associated with fatty acid oxidation while inhibiting phosphofructokinase, a glycolytic enzyme. A
2 month treatment with metformin reduced LV eccentric remodeling associated with severe VO and helped maintain
a better systolic function
Endurance training or beta-blockade can partially block the energy metabolism remodeling taking place in experimental chronic left ventricle volume overload.
BACKGROUND:
Patients with chronic aortic valve regurgitation (AR) causing left ventricular (LV) volume overload can remain asymptomatic for many years despite having a severely dilated heart. The sudden development of heart failure is not well understood but alterations of myocardial energy metabolism may be contributive. We studied the evolution of LV energy metabolism in experimental AR.
METHODS:
LV glucose utilization was evaluated in vivo by positron emission tomography (microPET) scanning of 6-month AR rats. Sham-operated or AR rats (nâ=â10-30 animals/group) were evaluated 3, 6 or 9 months post-surgery. We also tested treatment intervention in order to evaluate their impact on metabolism. AR rats (20 animals) were trained on a treadmill 5 times a week for 9 months and another group of rats received a beta-blockade treatment (carvedilol) for 6 months.
RESULTS:
MicroPET revealed an abnormal increase in glucose consumption in the LV free wall of AR rats at 6 months. On the other hand, fatty acid beta-oxidation was significantly reduced compared to sham control rats 6 months post AR induction. A significant decrease in citrate synthase and complex 1 activity suggested that mitochondrial oxidative phosphorylation was also affected maybe as soon as 3 months post-AR.Moderate intensity endurance training starting 2 weeks post-AR was able to partially normalize the activity of various myocardial enzymes implicated in energy metabolism. The same was true for the AR rats treated with carvedilol (30 mg/kg/d). Responses to these interventions were different at the level of gene expression. We measured mRNA levels of a number of genes implicated in the transport of energy substrates and we observed that training did not reverse the general down-regulation of these genes in AR rats whereas carvedilol normalized the expression of most of them.
CONCLUSION:
This study shows that myocardial energy metabolism remodeling taking place in the dilated left ventricle submitted to severe volume overload from AR can be partially avoided by exercise or beta-blockade in rats
Angiotensin II converting enzyme inhibition improves survival, ventricular remodeling and myocardial energetics in experimental aortic regurgitation.
Backgroundâ Aortic valve regurgitation (AR) is a volume-overload disease causing severe eccentric left ventricular (LV) hypertrophy and eventually heart failure. There is currently no approved drug to treat patients with AR. Many vasodilators including angiotensin-converting enzyme inhibitors have been evaluated in clinical trials, but although some results were promising, others were inconclusive. Overall, no drug has yet been able to improve clinical outcome in AR and the controversy remains. We have previously shown in an animal model that captopril (Cpt) reduced LV hypertrophy and protected LV systolic function, but we had not evaluated the clinical outcome. This protocol was designed to evaluate the effects of a long-term Cpt treatment on survival in the same animal model of severe aortic valve regurgitation.
Methods and ResultsâForty Wistar rats with AR were treated or untreated with Cpt (1 g/L in drinking water) for a period of 7 months to evaluate survival, myocardial remodeling, and function by echocardiography as well as myocardial metabolism by ” positron emission tomography scan. Survival was significantly improved in Cpt-treated animals with a survival benefit visible as soon as after 4 months of treatment. Cpt reduced LV dilatation and LV hypertrophy. It also significantly improved the myocardial metabolic profile by restoring the level of fatty acids metabolic enzymes and use.
ConclusionsâIn a controlled animal model of pure severe aortic valve regurgitation, Cpt treatment reduced LV remodeling and LV hypertrophy and improved myocardial metabolic profile and survival. These results support the need to reevaluate the role of angiotensin-converting enzyme inhibitors in humans with AR in a large, carefully designed prospective clinical trial
Transcriptional changes associated with long-term left ventricle volume overload in rats : impact on enzymes related to myocardial energy metabolism.
Patients with left ventricle (LV) volume overload (VO) remain in a compensated state for many years although severe dilation is present. The myocardial capacity to fulfill its energetic demand may delay decompensation. We performed a gene expression profile, a model of chronic VO in rat LV with severe aortic valve regurgitation (AR) for 9 months, and focused on the study of genes associated with myocardial energetics. Methods. LV gene expression profile was performed in rats after 9 months of AR and compared to sham-operated controls. LV glucose and fatty acid (FA) uptake was also evaluated in vivo by positron emission tomography in 8-week AR rats treated or not with fenofibrate, an activator of FA oxidation (FAO). Results. Many LV genes associated with mitochondrial function and metabolism were downregulated in AR rats. FA ÎČ-oxidation capacity was significantly impaired as early as two weeks after AR. Treatment with fenofibrate, a PPARα agonist, normalized both FA and glucose uptake while reducing LV dilation caused by AR. Conclusion. Myocardial energy substrate preference is affected early in the evolution of LV-VO cardiomyopathy. Maintaining a relatively normal FA utilization in the myocardium could translate into less glucose uptake and possibly lesser LV remodeling
The fables of pity: Rousseau, Mandeville and the animal-fable
Copyright @ 2012 Edinburgh University PressPrompted by Derridaâs work on the animal-fable in eighteenth-century debates about political power, this article examines the role played by the fiction of the animal in thinking of pity as either a natural virtue (in Rousseauâs Second Discourse) or as a natural passion (in Mandevilleâs The Fable of the Bees). The war of fables between Rousseau and Mandeville â and their hostile reception by Samuel Johnson and Adam Smith â reinforce that the animal-fable illustrates not so much the proper of man as the possibilities and limitations of a moral philosophy that is unable to address the political realities of the state
Boron and nitrogen isotope effects on hexagonal boron nitride properties
The unique physical, mechanical, chemical, optical, and electronic properties
of hexagonal boron nitride (hBN) make it a promising two-dimensional material
for electronic, optoelectronic, nanophotonic, and quantum devices. Here we
report on the changes in hBN's properties induced by isotopic purification in
both boron and nitrogen. Previous studies on isotopically pure hBN have focused
on purifying the boron isotope concentration in hBN from its natural
concentration (approximately 20 at B, 80 at B) while
using naturally abundant nitrogen (99.6 at N, 0.4 at N),
i.e. almost pure N. In this study, we extend the class of
isotopically-purified hBN crystals to N. Crystals in the four
configurations, namely hBN, hBN, hBN,
and hBN, were grown by the metal flux method using boron and
nitrogen single isotope () enriched sources, with nickel plus chromium
as the solvent. In-depth Raman and photoluminescence spectroscopies demonstrate
the high quality of the monoisotopic hBN crystals with vibrational and optical
properties of the N-purified crystals at the state of the art of
currently available N-purified hBN. The growth of high-quality
hBN, hBN, hBN, and hBN
opens exciting perspectives for thermal conductivity control in heat
management, as well as for advanced functionalities in quantum technologies.Comment: 13 pages, 7 figure
Image-derived input function in dynamic human PET/CT: methodology and validation with 11C-acetate and 18F-fluorothioheptadecanoic acid in muscle and 18F-fluorodeoxyglucose in brain
International audienc
- âŠ