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Biomedical subjects

R Lima

Publications and source records attributed to R Lima.

At least 19 recordsLinked to original sources
PubMed2006

Chaotic advection and targeted mixing.

The advection of passive tracers in an oscillating vortex chain is investigated. It is shown that by adding a suitable perturbation to the ideal flow, the induced chaotic advection exhibits two remarkable properties which do not hold in the case of a generic perturbation: Particles remain trapped within a specific domain bounded by two oscillating barriers (suppression of chaotic transport along the channel), and the stochastic sea seems to cover this whole bounded domain (enhancement of mixing within the rolls).

Journal Article↗
PubMed2006

Discrete time piecewise affine models of genetic regulatory networks.

We introduce simple models of genetic regulatory networks and we proceed to the mathematical analysis of their dynamics. The models are discrete time dynamical systems generated by piecewise affine contracting mappings whose variables represent gene expression levels. These models reduce to boolean networks in one limiting case of a parameter, and their asymptotic dynamics approaches that of a differential equation in another limiting case of this parameter. For intermediate values, the model present an original phenomenology which is argued to be due to delay effects. This phenomenology is not limited to piecewise affine model but extends to smooth nonlinear discrete time models of regulatory networks. In a first step, our analysis concerns general properties of networks on arbitrary graphs (characterisation of the attractor, symbolic dynamics, Lyapunov stability, structural stability, symmetries, etc). In a second step, focus is made on simple circuits for which the attractor and its changes with parameters are described. In the negative circuit of 2 genes, a thorough study is presented which concern stable (quasi-)periodic oscillations governed by rotations on the unit circle - with a rotation number depending continuously and monotonically on threshold parameters. These regular oscillations exist in negative circuits with arbitrary number of genes where they are most likely to be observed in genetic systems with non-negligible delay effects.

Algorithms↗
PubMed2006

Anomalous transport fluctuations in a model of irregular media.

We consider a diffusion model with stochastic porosity for which the average solution exhibits an abnormal transport. In this paper we investigate the relation of such an anomalous diffusive property of the mean value with the behavior of the solution corresponding to each realization of the stochastic porosity. Such a solution will correspond to the actual measurements in an experiment made on a particular tube. The most relevant result of our work is that, although the concentration corresponding to each realization diffuses normally for large times, it experiments on large deviations from the mean value during intermediate times.

Journal Article↗
PubMed2006

Mean value and fluctuations in a model of diffusion in porous media.

We consider a stochastic model for the diffusion in a porous media. For a case where the average satisfies an anomalous diffusion equation, we investigate the behavior of the realizations around the mean value. The most relevant result of our work is that, although the concentration corresponding to each realization diffuses normally for large times, it experiences large deviations from the mean value during intermediate times. As a consequence, the experimental measurements will always depart from the average value of the realizations (with respect to the stochastic process) for unpredictable times.

Algorithms↗
PubMed2006

Role of ATP-sensitive potassium channels in normal and hypertension-associated pregnancy in rats.

1. Activation of vascular ATP-sensitive K(+) (K(ATP)) channels has been implicated in vasodilator responses to pregnancy. 2. The effect of glibenclamide, a K(ATP) channel inhibitor, on systolic blood pressure (SBP) and renal function was evaluated in pregnant and non-pregnant spontaneously hypertensive rats, as well as in normotensive and hypertensive Wistar rats that had been made hypertensive by simultaneous treatment with N(G)-nitro-l-arginine methyl ester (0.4 mg/mL) and indomethacin (2 mg/kg, i.p.) from Day 1 of gestation. Pregnant animals received 10 mg/kg glibenclamide for 12 days starting at Day 7. In addition, the mRNA levels of the vascular K(ATP) channel (Kir6.2) were estimated in aorta and kidney using real-time reverse transcription-polymerase chain reaction on Day 19 of pregnancy. 3. The decreased SBP observed in pregnant Wistar rats was paralleled by an increase in Kir6.2 mRNA levels. Glibenclamide blunted systemic vasodilation and reduced the mRNA expression of Kir6.2. There was no pregnancy induced vasodilation and no change in Kir6.2 mRNA expression in SHR. Glibenclamide had no effect on pregnant SHR. Hypertensive Wistar rats exhibited high SBP, followed by increased Kir6.2 mRNA levels. The effects of glibenclamide were not evaluated in this group because glibenclamide induced intense vaginal bleeding. 4. The results of the present study suggest that K(ATP) channels may be involved in pregnancy induced vasodilation during normotensive pregnancy, but not in pregnant SHR. Glibenclamide may have an abortive effect if administered during the early phases of gestation or in association with nitric oxide and prostaglandin inhibitors.

Adenosine Triphosphate↗
PubMed2005

Channeling chaos by building barriers.

Chaotic diffusion often represents a severe obstacle for the setup of experiments, e.g., in fusion plasmas or particle accelerators. We present a complete test of a method of control of Hamiltonian chaos, with both its numerical test and its first experimental realization on a paradigm for wave-particle interaction, i.e., a travelling wave tube. The core of our approach is a small apt modification of the system which channels chaos by building barriers to diffusion. Its experimental realization opens the possibility to practically achieve the control of a wide range of systems at a low additional cost of energy.

Journal Article↗
PubMed2001

Instanton solutions in the problem of wrinkled flame-front dynamics.

The statistics of the slopes of wrinkling flames propagating through an infinitely wide channel is investigated by the quantum-field-theory methods. We dwell on the WKB approximation in the functional integral, which is analogous to the Wyld functional integral in turbulence. The main contribution to statistics is due to a coupled field-force configuration. This configuration is related to a kink between metastable exact pole solutions of the Sivashinsky equation. These kinks are responsible for both the formation of new cusps and the rapid power-law acceleration of the mean flame front. The problem of asymptotic stability of the solutions is discussed.

Journal Article↗
PubMed2001

Effect of cyclosporin A on nitric oxide production in cultured LLC-PK1 cells.

The effect of Cyclosporin A on nitric oxide production was studied in cultured LLC- PK1 cells. For this purpose the cells were incubated with vehicle (olive oil, 10 microg/ml in DMSO), Cyclosporin A (CsA, 10 microg/ml), tumor necrosis factor (TNF-alpha, 150 U/ml) + interferon (IFN-gamma, 500 U/ml) to upregulate NOS synthesis, and therefore NO production (used as a positive control), or CsA + TNF-alpha + IFN-gamma. After 72 hours the culture medium was collected and nitrite was determined by the Griess method. The results were normalized to the protein harvested from these cells as measured by the Lowry method. Viability was determined by the exclusion of the fluorescent dyes (acridine orange and ethidium bromide). Intracellular calcium was measured spectrophotometrically using the fluorescent calcium indicator fura-2 AM. In CsA treated cells, the nitrite (pmoles/mg of protein) was decreased when compared to control (12.8 +/- 0.5 vs. 18.3 +/- 0.6; p < 0.05; both n = 8). TNF-alpha + IFN-gamma increased the nitrite synthesis (52.0 +/- 0.2; p < 0.05 vs. control; n = 6). This effect was decreased significantly by the simultaneous treatment with CsA (38.8 +/- 0.3; p < 0.05; n = 6). Cell viability in CsA group was decreased when compared to the control (84.7 +/- 0.2% vs. 93.6 +/- 0.1%; p < 0.05; both n = 10). TNF-alpha + IFN-gamma had no effect on viability (93.0 +/- 0.3%; n = 10). However, when combined with CsA, viability was decreased relative to the control (85.0 +/- 0.2%; p < 0.05; n = 10). Acute (1 h) or chronic (72 h) treatment of LLC- PK1 cells with CsA had no effect on basal calcium levels. Our results demonstrate a reduced level of nitric oxide production in LLC-PK1 cells treated with CsA. There was no effect of the drug on intracellular calcium levels, however CsA treatment did reduce cellular viability. We suggest that, in part, the decreased levels of NO production are a secondary consequence of direct cell damage. However, CsA may also be exerting direct effects on NO synthesis through its interactions with both iNOS and cNOS. These results also provide a dual mechanism of action for CsA induced nephrotoxicity, that is, direct cell damage and interference with the NO system within the nephron.

Animals↗
PubMed2000

Presence of human papillomavirus and Epstein-Barr virus in the cervix of women infected with the human immunodeficiency virus.

The presence of human papillomavirus (HPV) and Epstein-Barr virus (EBV) was sought in cervical scrapings from 110 human immunodeficiency virus (HIV)-infected women to evaluate the role of these viruses as risk factors for squamous intraepithelial lesions of the cervix. By using PCR, presence of HPV-DNA and EBV-DNA was found in 60.9% (67/110) and in 10% (11/110) of clinical samples, respectively. Identification of oncogenic group of HPV by hybrid capture (HC II, Murex-Digene) indicated the presence of low-risk HPV in 13 (19.4%) patients, high-risk HPV in 28 (41.8%), and both types of HPV in 26 (38.8%) patients. Squamous intraepithelial lesions were present in 59 cases, being low-grade (n = 52) and high-grade (n = 7) lesions. HPV was detected in 84.7% of patients with lesions, in association with low-grade (43/52) and high-grade lesions (7/7), and in 33% of patients without lesions. EBV-DNA was detected in 8 patients with low-grade lesions and in 3 patients without lesions. Concurrent genital HPV and EBV infection was observed in 9 cases. HPV was associated with detection of squamous intraepithelial lesions [OR = 3.55; 95% CI = (1.96; 6.48)]. No significant association was found between presence of EBV and detection of lesions, both in case of EBV infection alone [OR = 1.4; 95% CI = (0. 93; 2.12)] and in case of HPV/EBV combined infection [OR = 0.87; 95%CI = (0.54; 1.42)]. These data confirm the significant role of HPV as risk factor for squamous intraepithelial lesions and suggest that EBV could not be involved in the pathogenesis of the lesions that arise in the cervix of HIV-positive women.

Adolescent↗
PubMed2000

On-off intermittency at the onset of the ion-acoustic instability in a laboratory plasma

The ion-ion beam instability is experimentally studied just above threshold in a laboratory double-plasma device. Intermittent bursts of unstable waves are recorded in the target plasma and the distribution of the recurrence times of the bursts is estimated. At the onset of instability, the measurements are in agreement with the expected evolution deduced from a theoretical model that combines the normal form of a supercritical Hopf-Andronov bifurcation and the parametric noisy deviations from threshold typical of on-off intermittency. In both the experiment and the model, the distribution of the recurrence times of the bursts decays as an inverse power law and the evolution of the mean laminar length when the control parameter is increased beyond threshold exhibits a power law of exponent -1.

Journal Article↗
PubMed1999

Effects of choline and other nicotinic agonists on the tectum of juvenile and adult Xenopus frogs: a patch-clamp study.

We have used anatomical methods and whole-cell patch-clamp recording to assess the distribution of nicotinic receptors in the tectum of Xenopus frogs and to measure effects of nicotinic ligands (carbachol, cytisine and nicotine) on glutamatergic spontaneous miniature excitatory postsynaptic currents. Our results confirm that retinotectal axons account for the majority of nicotinic receptors in the tectum and that nicotinic agonists exert presynaptic effects that increase the rate of transmitter release on to tectal cells. The nicotinic blockers mecamylamine and methyllycaconitine reduced responses to carbachol and cytisine. A small percentage of cells also showed postsynaptic responses. We have assessed whether there are developmental changes in the frequency of occurrence of spontaneous miniature excitatory postsynaptic currents. The first three months post-metamorphosis fall within the critical period for the dramatic plasticity displayed by binocular inputs during development in Xenopus. During this period, visual activity governs the formation of orderly maps relayed from the ipsilateral eye via the cholinergic projection from the nucleus isthmi to the tectum. In this study, we have found that critical-period tecta (two to 12 weeks postmetamorphosis) tend to have higher spontaneous activity than do older tecta (two to 69 weeks postmetamorphosis), and that nicotinic agonists increase that activity in both groups, with the result that the peak rates in response to nicotinic agonists are higher during the critical period than later. We also investigated the possible role of choline as an agonist of nicotinic receptors in the tectum. We have found that choline, as well as carbachol and cytisine, can cause a reversible increase in the rate of miniature excitatory postsynaptic currents. This result may help to explain how the isthmotectal projection, which accounts for the overwhelming majority of cholinergic input to the tectum, can exert effects on retinotectal terminals even though there are no morphologically identifiable synapses between the two populations. We have examined the morphology of cells filled with biocytin during the patch-clamp experiments, and we find that cells with dendrites in the stratum zonale, a layer with particularly dense input from the contralateral nucleus isthmi, have higher spontaneous activity than cells with dendrites that do not extend into that layer. Nicotinic agonists increased the activity recorded in both classes of cells. In addition, four pretectal cells were identified. Nicotinic agonists increased the rate of spontaneous activity recorded in that population. The results indicate that retinotectal transmission in the superior colliculus can be increased presynaptically by activity of the cholinergic projections of the nucleus isthmi. This modulation may be the basis for observations that blocking of cholinergic input disrupts the formation of topographic retinotectal projections. Moreover, the ability of choline to activate these receptors suggests that this metabolite of acetylcholine may permit paracrine activation of presynaptic receptors even though the tectum contains high acetylcholinesterase activity.

Aging↗
PubMed1996

[Cardiac granulomas due to Schistosoma eggs and endomyocardial fibrosis].

Evidences of a possible association between endomyocardial fibrosis and schistosomiasis have been recently investigated. We describe the finding of cardiac schistosomal granulomas in a 14 year-old-girl presenting symptoms and signals of right side endomyocardial fibrosis. Refractory ascistis and progressive atrioventricular block were observed during follow-up. Endomyocardial biopsy and post mortem specimens showed inflammatory infiltrates, schistosomal granuloma and fibrosis.

Adolescent↗