Biomedical subjects

A Venkatesan

Publications and source records attributed to A Venkatesan.

11 recordsLinked to original sources

PubMed2006

Experimental realization of strange nonchaotic attractors in a quasiperiodically forced electronic circuit.

We have identified the three prominent routes, namely Heagy-Hammel, fractalization, and intermittency routes, and their mechanisms for the birth of strange nonchaotic attractors (SNAs) in a quasiperiodically forced electronic system constructed using a negative conductance series LCR circuit with a diode both numerically and experimentally. The birth of SNAs by these three routes is verified from both experimental and their corresponding numerical data by maximal Lyapunov exponents, and their variance, Poincaré maps, Fourier amplitude spectrum, spectral distribution function, and finite-time Lyapunov exponents. Although these three routes have been identified numerically in different dynamical systems, the experimental observation of all these mechanisms is reported here in a single second order electronic circuit.

Journal Article↗

PubMed2006

Magnetization of a strongly interacting two-dimensional electron system in perpendicular magnetic fields.

We measure the thermodynamic magnetization of a low-disordered, strongly correlated two-dimensional electron system in silicon in perpendicular magnetic fields. A new, parameter-free method is used to directly determine the spectrum characteristics (Landé g factor and the cyclotron mass) when the Fermi level lies outside the spectral gaps and the interlevel interactions between quasiparticles are avoided. Intralevel interactions are found to strongly modify the magnetization, without affecting the determined g* and m*.

Journal Article↗

PubMed2001

Spatially resolved spin-injection probability for gallium arsenide.

We report a large spin-polarized current injection from a ferromagnetic metal into a nonferromagnetic semiconductor, at a temperature of 100 Kelvin. The modification of the spin-injection process by a nanoscale step edge was observed. On flat gallium arsenide [GaAs(110)] terraces, the injection efficiency was 92%, whereas in a 10-nanometer-wide region around a [111]-oriented step the injection efficiency is reduced by a factor of 6. Alternatively, the spin-relaxation lifetime was reduced by a factor of 12. This reduction is associated with the metallic nature of the step edge. This study advances the realization of using both the charge and spin of the electron in future semiconductor devices.

Journal Article↗

PubMed2001

Interruption of torus doubling bifurcation and genesis of strange nonchaotic attractors in a quasiperiodically forced map: mechanisms and their characterizations.

A simple quasiperiodically forced one-dimensional cubic map is shown to exhibit very many types of routes to chaos via strange nonchaotic attractors (SNAs) in a two-parameter (A-f) space. The routes include transitions to chaos via SNAs from both a one-frequency torus and a period-doubled torus. In the former case, we identify the fractalization and type-I intermittency routes. In the latter case, we point out that at least four distinct routes for the truncation of the torus-doubling bifurcation and the creation of SNAs occur in this model. In particular, the formation of SNAs through Heagy-Hammel, fractalization, and type-III intermittent mechanisms is described. In addition, it has been found that in this system there are some regions in the parameter space where a dynamics involving a sudden expansion of the attractor, which tames the growth of period-doubling bifurcation, takes place, creating the SNA. The SNAs created through different mechanisms are characterized by the behavior of the Lyapunov exponents and their variance, by the estimation of the phase sensitivity exponent, and through the distribution of finite-time Lyapunov exponents.

Journal Article↗

PubMed2001

Prevalence of migraine in patients with systemic lupus erythematosus.

OBJECTIVE: To determine the prevalence of migraine in patients with systemic lupus erythematosus (SLE), and to examine the relationships between headache type and other clinical, serologic, and treatment features of the disease. BACKGROUND: Headaches are common in SLE and are a significant source of patient disability. The exact prevalence of headaches in patients with SLE is unknown. The classification of headache syndromes in SLE is also unclear. Previous studies were based on small numbers of patients and the headache types and criteria to define headache types varied widely. METHODS: Four hundred fourteen patients meeting American College of Rheumatology criteria for the diagnosis of SLE were sent the University of California, San Diego Migraine Questionnaire. Patients who completed the questionnaire had their medical records reviewed for constitutional, respiratory, cardiac, vascular, skin, musculoskeletal, other neuropsychiatric, hematologic, renal, and immunologic manifestations of the disease. Recent corticosteroid, nonsteroidal anti-inflammatory drug, antimalarial, and immunosuppressive medications were also recorded. RESULTS: One hundred eighty-six patients completed the questionnaire. Sixty-two percent of patients reported headaches: 39% met diagnostic criteria for migraine and 23% met criteria for nonmigrainous headache. Of the patients with migraine, 56% met criteria for migraine without aura and 44% met criteria for migraine with aura. There were no significant associations between headache type and other clinical, serologic, or treatment features of the disease. CONCLUSIONS: There is a high prevalence of migraine in patients with SLE, and patients should be routinely evaluated for migraine symptoms.

Adolescent↗

PubMed2001

Novel fluorescence-based screen to identify small synthetic internal ribosome entry site elements.

We report here a novel fluorescent protein-based screen to identify small, synthetic internal ribosome entry site (IRES) elements in vivo. A library of bicistronic plasmids encoding the enhanced blue and green fluorescent proteins (EBFP and EGFP) separated by randomized 50-nucleotide-long sequences was amplified in bacteria and delivered into mammalian cells via protoplast fusion. Cells that received functional IRES elements were isolated using the EBFP and EGFP reporters and fluorescence-activated cell sorting, and several small IRES elements were identified. Two of these elements were subsequently shown to possess IRES activity comparable to that of a variant of the encephalomyocarditis virus IRES element in a context-independent manner both in vitro and in vivo, and these elements functioned in multiple cell types. Although no sequence or structural homology was apparent between the synthetic IRES elements and known viral and cellular IRES elements, the two synthetic IRES elements specifically blocked poliovirus (PV) IRES-mediated translation in vitro. Competitive protein-binding experiments suggested that these IRES elements compete with PV IRES-mediated translation by utilizing some of the same factors as the PV IRES to direct translation. The utility of this fluorescent protein-based screen in identifying IRES elements with improved activity as well as in probing the mechanism of IRES-mediated translation is discussed.

Base Sequence↗

PubMed2000

Intermittency transitions to strange nonchaotic attractors in a quasiperiodically driven duffing oscillator

Different mechanisms for the creation of strange nonchaotic attractors (SNAs) are studied in a two-frequency parametrically driven Duffing oscillator. We focus on intermittency transitions in particular, and show that SNAs in this system are created through quasiperiodic saddle-node bifurcations (type-I intermittency) as well as through a quasiperiodic subharmonic bifurcation (type-III intermittency). The intermittent attractors are characterized via a number of Lyapunov measures including the behavior of the largest nontrivial Lyapunov exponent and its variance, as well as through distributions of finite-time Lyapunov exponents. These attractors are ubiquitous in quasiperiodically driven systems; the regions of occurrence of various SNAs are identified in a phase diagram of the Duffing system.

Journal Article↗

PubMed2000

A clinical profile to assess the potential risk factors for cleft lip and palate.

Cleft lip and palate is one of the most common malformations in man. Although the subject of a great deal of investigation, etiologic factors contributing to these congenital deformities remain largely unknown. Various factors like race, sex, parental age at the time of conception etc. have been listed as the exogenous factors. This article strives to project any significant relationship of these factors to cleft lip and cleft palate.

Adolescent↗

PubMed1999

Structure and function of a small RNA that selectively inhibits internal ribosome entry site-mediated translation.

A 60 nt long RNA termed IRNA, isolated from the yeast Saccharomyces cerevesiae, was previously shown to selectively block internal ribosome entry site (IRES)-mediated translation without interfering with cap-dependent translation of cellular mRNAs both in vivo and in vitro. IRNA specifically bound cellular proteins believed to be important for IRES-mediated translation. We demonstrate here that a complementary copy of IRNA (cIRNA) is also active in blocking IRES-mediated translation and that it binds many of the same cellular proteins that IRNA does. We have probed the secondary structure of both IRNA and cIRNA using single-strand- and double-strand-specific nucleases as well as using oligonucleotide hybridization followed by RNase H digestion. Both IRNA and cIRNA share secondary structural homology, although distinct differences do exist between the two structures. Mutational analysis of IRNA shows that sequences that form both the main stem and one loop are critical for its translation inhibitory activity. Maintenance of the established secondary structure appears to be required for both IRNA's ability to bind cellular trans -acting proteins believed to be required for IRES-mediated translation and its ability to block IRES-mediated translation.

Computer Simulation↗

PubMed1998

Inhibition of internal entry site (IRES)-mediated translation by a small yeast RNA: a novel strategy to block hepatitis C virus protein synthesis.

The observation that poliovirus mRNA is not translated in the yeast Saccharomyces cerevisiae has led to the discovery of a small RNA (60 nt, called IRNA, inhibitor RNA) which was later shown to specifically inhibit internal ribosome entry site (IRES)-mediated translation of naturally uncapped mRNAs. Translation of cellular capped mRNAs was not significantly inhibited by IRNA. IRNA also specifically inhibited hepatitis C virus (HCV) IRES-mediated translation in vitro and in vivo. A hepatoma cell line constitutively expressing IRNA was refractory to infection by a chimeric poliovirus (PV/HCV) in which PV IRES is replaced by HCV-IRES. In contrast, a PV/EMCV chimeric virus containing the EMCV IRES was not significantly inhibited in the IRNA-hepatoma cell line compared to the control hepatoma cells. UV-crosslinking studies showed that the IRNA binds a number of cellular proteins that appear to be important for IRES-mediated translation. Interaction of these proteins with the viral IRES elements is believed to be important in recruiting ribosomes to the 5( UTR of viral RNAs. The binding of the purified La autoantigen to the HCV IRES element was efficiently and specifically competed by IRNA. These results provide a basis for development of novel drugs effective against HCV infection.

Animals↗

PubMedDate not supplied

Effect of smoking on lipid profile and lipid peroxidation in normal subjects.

The aim of the present study was to assess the association between smoking and the alteration in plasma concentration of lipid profile and lipid peroxides. Fourteen smokers and 11 age matched control were enrolled. Plasma levels of fasting cholesterol, triglycerides, lipoprotein cholesterol and malondialdehyde were estimated. In smokers the levels of total cholesterol, LDL cholesterol, Non-HDL cholesterol and MDA were significantly elevated when compared with the controls. The atherogenic index as indicated by various risk ratios were also found to be increased in smokers as compared to controls. These findings indicate that current smokers are at a pro- atherogenic state and as in other countries, in India smokers require particular attention in terms of public health interventions.

Adult↗