Showing papers by "University of Los Andes published in 2000"

Measuring the impact of minimum wages: evidence from latin america

Abstract: Simple numerical measures of the minimum wage may offer deceptive indications of its impact. Alternative measures, such as kernel density or cumulative distribution plots, are more reliable, and highlight influences higher in the wage distribution or on the informal sector. Panel employment data from Colombia - where minimum wages seem high and binding - show that the minimum wage can have important impacts on wages and unemployment across the wage distribution. Maloney, Nunez, and colleagues provide an overview of minimum wage levels in Latin America and their true impact on the distribution of wages, using both numerical measures and kernel density plots for eight countries (Argentina, Bolivia, Brazil, Chile, Colombia, Honduras, Mexico, and Uruguay). They especially try to identify "numeraire" effects - where the minimum is used as a reference higher in the wage distribution - and "lighthouse" effects - where it influences wage setting in the unregulated or "informal" sector. Their main findings: First, statutory minimum wages are often misleading, and graphical methods may be more reliable. Second, the minimum wage's effect on wage setting extends far beyond what is usually considered and probably beyond the effect in industrial countries. Using panel employment data from Colombia, where minimum wages seem high and binding, the authors quantify the minimum wage's effects on wages and on the probability of becoming unemployed. The Colombian case confirms the evidence offered by kernel density estimates: The minimum wage can have an important impact on wage distribution in the neighborhood of the minimum wage. The effects echo up the wage distribution in a clear demonstration of the "numeraire" effect. That this effect is stronger in Latin America than in the United States suggests that the minimum wage induces further-reaching rigidities in the labor market. The tradeoff between any possible effect on poverty and reduced flexibility is likely to be more severe in countries where this is the case. The effects on employment, and unemployment, are substantial. Informal salaried wages are also affected, confirming the graphical evidence of strong lighthouse effects. Self-employment earnings are not, however, confirming that the minimum wage is not simply serving as a measure of inflationary expectations. This paper - a product of the Poverty Sector Unit and the Office of the Chief Economist, Latin America and the Caribbean Region - is part of a larger effort in the region to measure and understand the impact of labor market rigidities on employment and poverty.

Sliding mode control: an approach to regulate nonlinear chemical processes

Abstract: A new approach for the design of sliding mode controllers based on a first-order-plus-deadtime model of the process, is developed. This approach results in a fixed structure controller with a set of tuning equations as a function of the characteristic parameters of the model. The controller performance is judged by simulations on two nonlinear chemical processes

Influence of Fruit Availability on Ecological Overlap among Four Neotropical Primates at Tinigua National Park, Colombia1

Abstract: Data on fruit abundance and ecological overlap among Ateles belzebuth, Lagothrix lagothricha, Cebus apella, and Alouatta seniculus were gathered during 13 months at Tinigua National Park (Colombia), in an effort to test the following hypotheses concerning competition for fruits. Coexistence is possible because: (1) during periods when fruit availability is limited, the species utilize different resources; and (2) the species have different fruit preferences independent of fruit production in the forest. Differences were found in resource use (diet and habitat) for all four species. Despite these differences, all four devoted large proportions of time feeding on fruit. Fruit abundance influenced their activity patterns. Ninety percent of all interspecific aggressive interactions (N = 69) were seen in fruiting trees. The first hypothesis was best supported, given that all species significantly increased their intake of the vegetative parts of plants during periods of fruit scarcity. Fruit partition...

Role of 11β-Hydroxysteroid Dehydrogenase in Nongenomic Aldosterone Effects in Human Arteries

Abstract: The aim of the present study was to demonstrate rapid effects of aldosterone on the Na(+)-H(+) exchanger in strips of human vascular vessels and to determine whether 11beta-hydroxysteroid dehydrogenase enzyme (11beta-HSD) could play a protective role in this response, such as that described for the classic type I mineralocorticoid receptor (MR). The activity of 11beta-HSD isoforms 1 and 2 were measured in fetal and adult arteries. Both isoforms are present in adult and fetal vessels. However, a significant difference in the proportion of each isoform was found. Isoform 1 activity (in pmol x min(-1) x 100 mg(-1) protein) was 42+/-5 in fetal vessels and 29+/-2 in adult arteries, and isoform 2 activity was 78+/-7 in fetal and 12+/-2 in adult tissue. The nongenomic effect of aldosterone on Na(+)-H(+) exchanger activity was measured in strips of chorionic and radial uterine arteries loaded with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein. Recordings of intracellular pH (pH(i)) were made by videofluorescence microscopy. Aldosterone (0.5 nmol/L) rapidly increased pH(i), with a half-maximal effect between 2 and 3 nmol/L in both fetal and adult vessels. Ethylisopropylamiloride, a specific inhibitor of the Na(+)-H(+) exchanger, inhibited this effect. The hormone-mediated increase in pH(i) was unaffected by spironolactone, a classic antagonist of MR, but was completely blocked by RU28318. Cortisol (up to 1 micromol/L) had no effect on pH(i), but when applied in the presence of carbenoxolone, a dramatic increase in Na(+)-H(+) exchanger activity was evident. The increments on pH(i) for each cortisol concentration were similar to those observed for aldosterone. These findings suggest that vascular 11beta-HSD plays an active role in maintaining the specificity of the rapid effects of aldosterone.

Effects of Rote, Context, Keyword, and Context/Keyword Methods on Retention of Vocabulary in EFL Classrooms

Abstract: In the present study, the effects of rote rehearsal, context, keyword, andcontext/keyword methods on immediate and long-term retention of English as a foreign language(EFL) vocabulary in natural classroom settings was examined. 8 intact 9th-grade EFL classes were randomly assigned to 1 of 4 learning conditions: rote rehearsal, context, keyword, and context/keyword condition. Cued recall was assessed either immediately or after a 1-week delay. Results showed that the context/keyword method produced superior recall to any of the other 3 methods after 1 week, suggesting a very promising educational value for this method.

Current Phenomenological Know-How and Modeling of Emulsion Inversion

Abstract: This paper encompasses classic trends as well as recent advances in the understanding of emulsion inversion phenomena. The generalized formulation issue is first discussed from hydrophilic−lipophilic balance to the most recent concepts. The so-called standard inversion line on the formulation−composition map exhibits several branches, referred to as transitional and catastrophic inversions, that bound normal and abnormal emulsion regions. Dynamic inversion is also discussed with its hysteresis zones, where both types of emulsions may be attained, depending upon the system's previous history of the formulation−composition map. Recent findings are reported concerning the effect of variables with practical relevance (i.e., stirring energy, viscosity of phases, surfactant concentration, and partitioning) on the standard and dynamic inversion patterns. State-of-the-art emulsion inversion modeling is briefly discussed.

Cattle and Weedy Shrubs as Restoration Tools of Tropical Montane Rainforest

Abstract: "En los ultimos 150 anos, una gran proporcion de los bosques de America Latina se han convertido en pastizales. Cuando se abandonan estos pastos, los pastos pueden retrasar el restablecimiento de especies lenosas y limitar la regeneracion forestal. En este estudio, exploramos el uso del ganado para facilitar el establecimiento de vegetacion lenosa en pastos de montana colombianos, dominados por los pastos africanos Pennisetum clandestinum (Kikuyo) y Melinis minutiflora (Yaragua). Primero, describimos la vegetacion lenosa y herbacea en pastizales y pastos sin pastoreo. En segundo lugar, probamos el efecto del pastoreo y la adicion de semillas en el establecimiento y crecimiento de especies lenosas. Tambien determinamos si el efecto del pastoreo era diferente en P. clandestinum y M. minutiflorapastizales Descubrimos que la baja densidad de ganado bovino aumento considerablemente la densidad, el numero de ramas por individuo (una medida de ""arbusto"") y el area basal de especies lenosas, pero tambien redujo la riqueza y diversidad de especies de plantas lenosas. En la zona de pastoreo, los arbustos Baccharis latifolia (Chilca) y Salvia sp. (Salvia) fueron los mas abundantes. El efecto combinado del pastoreo y la sombra de los arbustos redujo la vegetacion herbacea en un 52 a 92%. En el experimento de pastoreo / adicion de semillas, el pastoreo aumento el establecimiento de plantulas lenosas, particularmente del arbusto Verbesina arborea(camargo), pero el mayor efecto fue la adicion de semillas. Cuando los pastos son una barrera importante para la regeneracion, el pastoreo puede facilitar el establecimiento de arbustos que crean un microhabitat mas adecuado para el establecimiento de especies de arboles forestales montanos."

Quantum entanglement and information processing via excitons in optically driven quantum dots

Abstract: We show how optically driven coupled quantum dots can be used to prepare maximally entangled Bell and Greenberger-Horne-Zeilinger states. Manipulation of the strength and duration of the selective light pulses needed for producing these highly entangled states provides us with crucial elements for the processing of solid-state-based quantum information. Theoretical predictions suggest that several hundred single quantum bit rotations and controlled-NOT gates could be performed before decoherence of the excitonic states takes place.

Expression of α2-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP) in rat microglial cells

Abstract: Low density lipoprotein receptor-related protein (LRP) participates in the uptake and degradation of several ligands implicated in neuronal pathophysiology including apolipoprotein E (apoE), activated a2 -macroglobulin (a2M*) and b-amyloid precursor protein (APP). The receptor is expressed in a variety of tissues. In the brain LRP is present in pyramidal-type neurons in cortical and hippocampal regions and in astrocytes that are activated as a result of injury or neoplasmic transformation. As LRP is expressed in the monocyte/macrophage cell system, we were interested in examining whether LRP is expressed in microglia. We isolated glial cells from the brain of neonatal rats and LRP was immunodetected both in microglial cells and in astrocytes expressing glial fibrillar acidic protein (GFAP). Microglial cells were able to bind and internalize LRP-specific ligand, a2M*. The internalization was inhibitable by RAP, with a Kd of 1.7 nM. The expression of LRP was upregulated by dexamethasone, and down-regulated by lipopolysaccharide (LPS), gamma interferon (IFN-g )o r ac ombination of both. LRP was less sensitive to dexamethasone in activated astrocytes than in microglia. We provided the first analysis of LRP expression and regulation in microglia. Our results open the possibility that microglial cells could be related to the participation of LRP and its ligands in different pathophysiological states in brain. J. Neurosci. Res. 60:401‐411, 2000. © 2000 Wiley-Liss, Inc.

Studies of the relation between phase behavior and emulsification methods with nanoemulsion formation

Abstract: The main aim of this work was to study the relationship between the type of phases present during the emulsification process, the order of addition of components and the droplet size of the resulting emulsions. In this study, a pseudoternary water/poly(oxyethylene) nonionic surfactant/decane system was chosen as a model system to form oil-in-water emulsions at 25 °C. The phase behavior of the model system was determined at constant temperature in order to know the equilibrium phases and also those involved in the emulsification process. The low-energy emulsification methods studied were A. Addition of oil to an aqueous surfactant dispersion. B. Addition of water to a surfactant solution in oil. C. Mixing preequilibrated samples of the components.

Extraction of the width of the W boson from measurements of σ(pp̄→W+X)×B(W→eν) and σ(pp̄→Z+X)×B(Z→ee) and their ratio

Abstract: We report on measurements on inclusive cross sections times branching fractions into electrons for W and Z bosons produced in p (p) over bar collisions at root s = 1.8 TeV. From an integrated luminosity of 84.5 pb(-1) recorded in 1994-1995 using the D empty set detector at the Fermilab Tevatron, we determine sigma(p (p) over bar--> W + X) x B(W-->e nu) = 2310 +/- 10(stat) +/- 50(syst) +/- 100(lum)pb and sigma(p (p) over bar-->Z + X) x B(Z-->ee)=221+/-3(stat)+/-4(syst)+/-10(lum)pb. From these, we derive sigma(p (p) over bar-->W + X) x B(N-->e nu)/sigma(p (p) over bar-->Z + X) x B(Z-->ee) = 10.43+/-0.15(stat) +/- 0.20(syst) +/- 0.10(NLO), B(W-->e nu) = 0.1044+/-0.0015(stat)+/-0.0020(syst)+/-0.0017(theory)+/-0.0010(NLO). and Gamma(w) = 2169 +/- 0.03 l(stat) +/- 0.042(syst) +/- 0.041(theory)+/- 0.022(NLO)GeV. We use the latter to set a 95% confidence level upper limit on the partial decay width of the W boson into nonstandard model final states, Gamma(W)(inv), of 0.213 GeV. Combining these results with those from the 1992-1993 data gives sigma(p (p) over bar-->W+X) x B(W-->e nu)/sigma(p (p) over bar-->Z+X) x B(Z-->ee) = 10.51+/-0.25, Gamma(W) = 2.152 +/- 0.066 GeV, and a 95% C.L. upper limit an Gamma(W)(inv) of 0.191 GeV. Using a sample with a luminosity of 505 nb(-1) taken at root s = 630 GeV, we measure sigma(p (p) over bar--> W+ X) x B(W --> e nu) = 658 +/- 67 pb.

Chagas disease and human migration.

Abstract: Human Chagas disease is a purely accidental occurrence. As humans came into contact with the natural foci of infection might then have become infected as a single addition to the already extensive host range of Trypanosoma cruzi that includes other primates. Thus began a process of adaptation and domiciliation to human habitations through which the vectors had direct access to abundant food as well as protection from climatic changes and predators. Our work deals with the extraction and specific amplification by polymerase chain reaction of T. cruzi DNA obtained from mummified human tissues and the positive diagnosis of Chagas disease in a series of 4,000-year-old Pre-Hispanic human mummies from the northern coast of Chile. The area has been inhabited at least for 7,000 years, first by hunters, fishers and gatherers, and then gradually by more permanent settlements. The studied specimens belonged to the Chinchorro culture, a people inhabiting the area now occupied by the modern city of Arica. These were essentially fishers with a complex religious ideology, which accounts for the preservation of their dead in the way of mummified bodies, further enhanced by the extremely dry conditions of the desert. Chinchorro mummies are, perhaps, the oldest preserved bodies known to date.

Gas exchange and low temperature resistance in two tropical high mountain tree species from the Venezuelan Andes

Abstract: Temperature may determine altitudinal tree distribution in different ways: affecting survival through freezing temperatures or by a negative carbon balance produced by lower photosynthetic rates. We studied gas exchange and supercooling capacity in a timberline and a treeline species (Podocarpus oleifoliusand Espeletia neriifolia, respectively) in order to determine if their altitudinal limits are related to carbon balance, freezing temperature damage, or both. Leaf gas exchange, leaf temperature-net photosynthesis curves and leaf temperature at which ice formation occurred were measured at two sites along an altitudinal gradient. Mean CO2 assimilation rates for E. neriifolia were 3.4 and 1.3 µmol·m -2 ·s -1 , at 2 400 and 3 200 m, respectively. Mean night respiration was 2.2 and 0.9 µmol·m -2 ·s -1 for this species at 2 400 and 3 200 m, respectively. Mean assimilation rates for P. oleifolius were 3.8 and 2.2 µmol·m -2 ·s -1 at 2 550 and 3 200 m, respectively. Night respiration was 0.8 µmol·m -2 ·s -1 for both altitudes. E. neriifolia showed similar optimum temperatures for photosynthesis at both altitudes, while a decrease was observed in P. oleifolius. E. neriifolia and P. oleifolius presented supercooling capacities of -6.5 and -3.0 °C, respectively. For E. neriifolia, freezing resistance mechanisms are sufficient to reach higher altitudes; however, other environmental factors such as cloudiness may be affecting its carbon balance. P. oleifolius does not reach higher elevations because it does not have the freezing resistance mechanisms. © 2000 Editions scientifiques et medicales Elsevier SAS

Influence of H2O2 and Fe(III) in the photodegradation of nitrobenzene

Abstract: The influence of H 2 O 2 and Fe(III) in the degradation of nitrobenzene in aqueous solutions combined with UV radiation has been studied. A major efficiency is achieved when UV is combined with H 2 O 2 or Fe(III). Likewise, H 2 O 2 and Fe(III) concentration has an important effect on the degradation. Nitrophenol isomers are detected as the major intermediate products.

Measurement of the W boson mass using large rapidity electrons

Abstract: \We present a measurement of the W boson mass using data collected by the DO experiment at the Fermilab Tevatron during 1994-1995. We identify W bosons by their decays to ev final states where the electron is detected in a forward calorimeter. We extract the W boson mass M-W by fitting the transverse mass and transverse electron and neutrino momentum spectra from a sample of 11 089 W-->e nu decay candidates. We use a sample of 1687 dielectron events, mostly due to Z-->ee decays, to constrain our model of the detector response. Using the forward calorimeter data, we measure M-w=80.691+/-0.227 GeV. Combining the forward calorimeter measurements with our previously published central calorimeter results, we obtain M-W= 80.482 +/-0.091 GeV.

Effects of water deficit on the dynamics of flowering and fruit production in Capsicum chinense Jacq in a tropical semiarid region of Venezuela.

Abstract: The degree of crop sensitivity to water deficit, during the different developmental stages of a plant, is an important aspect to consider in tropical semiarid regions. The effect of water deficit on flowering dynamics and fruit production of sweet pepper (Capsicum chinense) was evaluated under several water regimes, during three consecutive years. A complete randomized block design with three replicates was employed. Flowering dynamics and fruit production were measured weekly, and water potentials were obtained twice during the experiments. For all years, flowering began at approximately 70 days after transplanting. In the most favourable year (trial 1996), there were no differences in total flower and fruit production in terms of irrigation frequency. For the other less favourable years, flowering and fruit production were significantly different between irrigation frequencies. Our results show that low water availability, prior to flowering, reduces the number of flowers produced and retards the occurrence of maximum flowering. On the other hand, a water deficit during the period between flowering and fruit development reduces final fruit production. This suggests that a water deficit during this last stage affects final yield to a greater extent.

Nonsperm cells in human semen and their relationship with semen parameters.

Abstract: The prevalence and clinical significance of leukocytes (WBC) and immature germ cells in semen is currently a matter of controversy. The aim of this work was to assess the prevalence of leukocytospermia in semen samples from Venezuelan men and its possible effects on sperm parameters. The concentration of WBC and round cells (RC) was evaluated in 118 semen samples from 19 fertile subjects (group 1), 62 infertile patients (group II), and 37 men with varicocele (group III). Semen WBC concentration was assessed by peroxidase assay. Twenty-six (22%) of the total samples had more than 10 WBC/mL semen. Twenty of the infertile men had leukocytospermia (32%) compared with 16% in the fertile group and 8% in the varicocele group. Semen RC concentration was lower than 5 x 10(6)/mL in all groups but, in groups II and III was significantly higher compared with group I. Infertile men had the highest WBC concentration. WBC concentration was negatively correlated with progressive motility, percentage of morphologically normal sperm, and hypoosmotic swelling test in infertile men but not in the varicocele group. In this group a negative correlation was obtained between immature germ cells and normal sperm morphology. The data show that leukcytospermia occurs frequently in infertile patients and is associated with poor semen quality parameters. In contrast, in men with varicocele, the increased number of immature germ cells might play a pivotal role in the pathogenesis of abnormal spermatozoa.

DNA damage and repair in cells of lead exposed people.

Abstract: Background One of the main sources of occupational exposure to lead in Colombia is in workers of battery industries and lead smelters. Genotoxic studies in human populations exposed to this metal have had conflicting results; this type of study has not been reported in Colombia. Methods Genotoxic effects of lead were studied in blood cell samples from workers of electric battery factories exposed to lead compounds. Single strand breaks and interference with DNA repair processes after an in vitro exposure of x-rays (300 cGy) were analyzed using the Comet Assay. The battery workers (n = 43) and 13 people not occupationally exposed to lead compounds who were selected as a control group, were classified into four categories according to their blood lead level. Results A significant difference was observed in DNA damage before the x-rays exposure (basal) between the lowest and highest categories of lead (mean DNA migration 55.6 μ and 85.9 μ, respectively). Additionally, a significant difference in DNA migration was also found immediately after irradiation between the lowest and highest lead categories (mean DNA migration. 199.8 μ and 317.8 μ respectively). The DNA migrations at different times after irradiation did not show a significant difference among the different lead levels. Conclusions We concluded that although the single strand breaks following irradiation were not affected by blood lead concentration, the metal seems to sensitize the cells to damage induced by other genotoxicants.

Magnetoacoustic Waves of Small Amplitude in Optically Thin Quasi-isentropic Plasmas

Abstract: The evolution of quasi-isentropic magnetohydrodynamic waves of small but finite amplitude in an optically thin plasma is analyzed. The plasma is assumed to be initially homogeneous, in thermal equilibrium and with a straight and homogeneous magnetic field frozen in. Depending on the particular form of the heating/cooling function, the plasma may act as a dissipative or active medium for magnetoacoustic waves, while Alfven waves are not affected directly. An evolutionary equation for fast and slow magnetoacoustic waves in the single wave limit has been derived and solved that allows us to analyze the wave modification by competition of weakly nonlinear and quasi-isentropic effects. It was shown that the sign of the quasi-isentropic term determines the scenario of the evolution, either dissipative or active. In the dissipative case, when the plasma is first-order isentropically stable, the magnetoacoustic waves are damped and the time for shock wave formation is delayed. However, in the active case when the plasma is isentropically overstable, the wave amplitude grows, the strength of the shock increases, and the breaking time decreases. The magnitude of the above effects depends on the angle between the wavevector and the magnetic field. For hot (T > 104 K) atomic plasmas with solar abundances either in the interstellar medium or in the solar atmosphere, as well as for the cold (T < 103 K) ISM molecular gas, the range of temperatures in which the plasma is isentropically unstable and the corresponding timescale and length scale for wave breaking have been found.

Isolated photon cross section in p p̄ collisions at √s = 1.8 TeV

Abstract: We report a new measurement of the cross section for the production of isolated photons, with transverse energies (ET) above 10 GeV and pseudorapidities |eta| 36 GeV.